Patent Publication Number: US-2021171989-A1

Title: Microorganisms and process for producing glycolic acid from pentoses and hexoses

Description:
TECHNICAL FIELD 
     The present invention relates to the technical field of bioconversion of a carbon source into at least one metabolite of interest and in particular into glycolic acid. 
     More particularly, the present invention relates to a microorganism the central carbon metabolism of which has been rearranged so as to convert hexoses and pentoses, from a carbon source and, in particular, plant biomass into glycolic acid. The present invention is also concerned with a process for producing glycolic acid from hexoses and pentoses contained in plant biomass implementing such a microorganism. 
     STATE OF PRIOR ART 
     Glycolic acid or hydroxyacetic acid having the formula HO—CH 2 —C(═O)—OH finds many applications in different fields. Indeed, by way of illustrating purposes, it can be used as a pH regulating agent or keratolytic agent in cosmetics and pharmaceutics, as a dyeing and tanning agent in the textile industry or even as a disinfectant in the agro-food industry. Glycolic acid also enables polymers such as thermoplastic resins comprising poly(glycolic acid) to be produced. Such polymers exhibit outstanding gas barrier properties and have the capability of being hydrolysed in aqueous environments gradually and controllably, making these polymers good candidates for packaging materials or resorbable materials useful in the biomedical field. 
     Even if glycolic acid can be obtained from sugar cane, beet or grape extracts, its industrial scale production comes from chemical synthesis. Several of these syntheses use, as a starting reagent, formaldehyde which is an irritant and carcinogenic compound, which precludes any trace of this substance in preparations having a Marketing Authorisation. 
     Consequently, bio-sourced glycolic acid has an interest in pharmaceutical, cosmetic but also textile fields. Under a polymerised form, poly(glycolic acid) and polymers containing poly(glycolic acid) could represent a new generation of packaging bio-plastics and bio-resorbable polymers [1]. That is why for a few years, several research teams have been interested in its production by the microbial pathway. 
     Glycolic acid can be naturally produced in small amounts via the reduction of glyoxylate in bacteria and moulds, including yeast [2]. The former engineering projects have been inspired by the natural metabolism. The maximum theoretical yield which can be reached by this natural pathway is 2 moles glycolic acid (GA) per mole of hexose and 1.66 mole per mole of pentoses (or 0.84 g GA/g sugar). 
     Thus, a process for the bioproduction of glycolic acid based on the improvement of the natural metabolism in  Escherichia coli  has been the object of the international application WO 2007/141316 [3]. This pathway is called, in the following, “glyoxylate shunt (GS) pathway”. In the process described, the reactions consuming glyoxylate and the enzymes metabolising glycolate have been deleted, whereas NADPH glyoxylate reductase which converts glyoxylate into glycolate has been overexpressed in order to increase the glycolic acid yield. The regulation system of the glyoxylate cycle has also been modified to remove inhibitions and increase the carbon flow to glyoxylate. From the work described in [3], other modifications have been provided to improve glycolic acid production from (D)-glucose, which enabled 92.9% of the maximum theoretical yield to be reached [4]. 
     However, from the stoichiometric point of view, this synthetic pathway is not optimal, the theoretical yield is limited by the carbon loss due to decarboxylation of pyruvate into acetyl-CoA. In this context, the substitution and/or complementation of the natural metabolism by a synthetic pathway for sugar assimilation and glycolic acid production is a contemplatable strategy to remove this limitation. 
     For that purpose, a semi-synthetic pathway for pentose assimilation has been described in the international application WO 2017/059236 [5]. This pathway involves phosphorylation in position 1 of the (D)-ribulose or (L)-xylulose ring and is called, in the following, “ribulose-1-P pathway”. This synthetic pathway allows pentose assimilation, more particularly (D)-xylose assimilation which is the majority pentose of hemicellulose. The (D)-xylose yield is 0.44 g/g [6]. It is however to be noted that the pathway described in international application WO 2017/059236 [5] requires, to be functional, reducing or inactivating the xylulokinase activity and overexpressing the following four enzymes: an epimerase which interconverts xylulose and ribulose, a (D)-ribulose-phosphate aldolase, a D-ribulokinase and a glycolaldehyde dehydrogenase. 
     Another synthetic pathway for glycolic acid production has been developed and been the object of the international application WO 2016/079440 [7]. This synthetic pathway phosphorylates pentose sugars on the first carbon (C1) instead of the carbon 5 (C5) as is the case in the pentose phosphate pathway. This phosphorylation reaction catalysed by a heterologous ketohexokinase, from mammals, is followed by a cleavage reaction by an aldolase leading to a C3 molecule (dihydroxyacetone-P) and a C2 molecule (glycolaldehyde). The latter through an oxidation reaction catalysed by a glycoladehyde dehydrogenase leads to glycolic acid synthesis or through a reduction reaction catalysed by an endogenous reductase to ethylene glycol. This synthetic pathway, called in the following “xylulose-1-P pathway” involves three different enzymes and not five as in [5] and enables a molar theoretical production in glycolic acid and ethylene glycol (1 mole per mole of pentose sugars) to be achieved. 
     It should be noticed that the synthetic pathways independent of the glyoxylate pathway described in [5] and [7] do not have a decarboxylation step and thus CO 2  loss but both of them generate a C3 compound, that is dihydroxyacetone phosphate (DHAP), which limits the maximum theoretical yield in glycolic acid. 
     Thus, in order to be able to use DHAP for glycolic acid production, the synthetic pathways described in [5] and [7] have been coupled with the optimised natural pathway described in [3]. The combination of the pathways described in [7] and [3] resulted in a yield of 0.63 g glycolic acid per g of sugars but the maximum theoretical yield is still limited because of the CO 2  loss, a consequence of the use of the optimised natural pathway [8]. 
     In a correspondence addressed to Nature Biotechnology [9], Dugar and Stephanopolous have introduced the notion of “maximum energy yield (Y E )”. This yield is purely calculated based on the energy balance between the substrate and the product, which is practically reflected by taking the redox state of the substrate and of the product into account. Thus, Y E  is determined by the ratio γ s /γ p  where γ s  and γ p  are the reduction degrees of the substrate and of the product. For example, the reduction degree of glucose γ s  is equal to 24 whereas that of the product ethanol γ p  is equal to 12. Thus, Y E  is 2. On the other hand, the yield of the metabolic pathway (Y P ) depends on the pathway or the metabolic network and it is calculated from the stoichiometry of the pathway considered. Thus, for the case of fermentation of glucose into ethanol, Y P  is 2 and it is found that Y E  is Y P . This equality is not found for all the cases. For example, the conversion of glucose into acetate results in a yield Y P  of 2 but the maximum energy yield Y E  is 3. Thus, there is a potential material loss of 33% which is explained by the natural metabolic pathway. However, the Liao et al team has shown that it is possible to construct viable metabolic pathways leading to the production of 3 moles of acetate per mole of glucose [10]. According to the same principle, it can be calculated that the maximum energy yield of glycolic acid (γ p =6) produced from glucose (γ s =24) would be 4 and from pentose (γ p =20) would be 3.3. However, this yield can only be reached if the biological system is capable of uptaking a carbon mole as CO 2 . 
     Given this postulate and in view of the growing interest in products from renewable carbon resources, the inventors have set the goal to provide a microorganism and a process making it possible to produce in a simple and industrialisable way from plant biomass and in particular from hexoses and pentoses contained in the same, glycolic acid with improved yields with respect to processes of prior art. 
     To that end, Table 1 hereinafter gives theoretical yields of pathways of prior art for producing glycolic acid (mol/mol) depending on the carbon source used. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Xylose 
                 Glucose 
                 Reference 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Glyoxylate shunt  
                 1.66 
                 2 
                 [3] 
               
               
                   
                 (GS) pathway 
                   
                   
                   
               
               
                   
                 Ribulose-1P pathway 
                 1 
                 0 
                 [5] 
               
               
                   
                 Xylulose-1P pathway 
                 1 
                 0 
                 [7] 
               
               
                   
                 Ribulose-1P + GS pathways 
                 2 
                 2 
                 [6] 
               
               
                   
                 Xylulose-1P + GS pathways 
                 2 
                 2 
                 [8] 
               
               
                   
                   
               
            
           
         
       
     
     DISCLOSURE OF THE INVENTION 
     The present invention enables the technical problems of processes of prior art such as those previously defined to be solved and the goal set by the inventors to be reached. 
     To that end, the works of the inventors have identified three enzymes enabling the non-natural way for synthesising glycolic acid presented in  FIG. 1  to be constructed, said enzymes being: 
     i) D-arabinose-5-phosphate isomerase (KdsD), enzyme of the lipopolysaccharide synthetic pathway which converts D-ribulose-5-phosphate into D-arabinose-5-phosphate [11]; 
     ii) fructose-6-phosphate aldolase (FSA) which catalyses the cleavage of D-arabinose-5-phosphate into D-glyceraldehyde-3-phosphate and glycolaldehyde; it is to be noted that this enzyme had been initially identified as catalysing the aldolytic cleavage of fructose-6-phosphate into dihydroxyacetone and glyceraldehyde-3-phosphate, but has shown an aldolytic activity on D-arabinose-5-phosphate with an affinity 10 times higher than for fructose-6-phosphate [12] and 
     iii) aldehyde dehydrogenase (AldA) which oxidises glycolaldehyde into glycolate [13]. 
     It is to be noted that the first substrate of this new pathway, namely D-ribulose-5-phosphate, is naturally obtained from pentoses by the pentose phosphate (PP) pathway. 
     Even if these three enzymes are naturally expressed in microorganisms as  E. coli , they do not allow glycolic acid synthesis since the overexpression of kdsD, fsa and aldA genes and thus the overexpression of these enzymes are necessary to produce glycolic acid. In other words, the synthetic pathway according to the invention does not exist in the natural state in microorganisms as  E. coli . That can therefore be called a “non-natural pathway” or “synthetic metabolic pathway”. 
     Additionally, the FSA activity on D-arabinose-5-phosphate generates glyceraldehyde 3P (C3) which is supported by the oxidative glycolysis. As described in [8], this C3 molecule can give glycolaldehyde but with a CO 2  loss at the reaction catalysed by pyruvate dehydrogenase. To bypass this metabolic limit, the inventors have rearranged the central carbon metabolism in order to optimise carbon preservation by recovering this C3 in the PP pathway by attenuating or even inactivating the gapA gene coding glyceraldehyde-3-phosphate dehydrogenase. As a result of this attenuation/inactivation, glyceraldehyde-3-phosphate enters the PP pathway and participates in the synthesis of D-xylulose-5-phosphate, precursor of D-ribulose-5-phosphate. The maximum theoretical yield is thereby 2.5 moles of glycolic acid per mole of pentose. 
     Moreover, for a process to be economically viable, it should convert majority lignocellulosic sugars, namely pentoses as D-xylose and hexoses as D-glucose. The latter is naturally assimilated by the glycolysis pathway. Thus, by inactivating/deleting the gapA gene, the carbon flow is redirected to the PP pathway which makes the conversion of D-glucose into glycolic acid possible with a theoretical maximum yield of 3 moles of glycolic acid per mole of D-glucose. 
     Table 2 below resumes the theoretical yields of the pathways of prior art and gives the theoretical yield of the pathway according to the invention for producing glycolic acid (mol/mol) depending on the carbon source used. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                 Xylose 
                 Glucose 
                 Reference 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Glyoxylate shunt (GS) pathway 
                 1.66 
                 2 
                 [3] 
               
               
                 Ribulose-1P pathway 
                 1 
                 0 
                 [5] 
               
               
                 Xylulose-1P pathway 
                 1 
                 0 
                 [7] 
               
               
                 Ribulose-1P + GS pathways 
                 2 
                 2 
                 [6] 
               
               
                 Xylulose-1P + GS pathways 
                 2 
                 2 
                 [8] 
               
               
                 Pathway according to the present 
                 2.5 
                 3 
                   
               
               
                 invention 
               
               
                   
               
            
           
         
       
     
     Thus, the present invention enables glycolic acid synthesis to be made from hexoses and pentoses by a single non-natural pathway, which enables the renewable carbon to be valued at best. Finally, this approach removes the stoichiometric requirement made necessary by the  E. coli  metabolism. Further, calculations on the theoretical yields of the non-natural pathway of pentose and hexose assimilation described in the present invention, for producing glycolic acid, enable a significant improvement in the yields to be foreseen with respect to the biosynthesis processes based on the optimisation of natural and/or semi-synthetic pathways of prior art. 
     More particularly, the present invention relates to a recombinant microorganism which exhibits 
     i) a conversion activity from D-ribulose-5-phosphate into D-arabinose-5-phosphate, increased in comparison with the same, non-modified microorganism; 
     ii) an aldolic cleavage activity from D-arabinose-5-phosphate into D-glyceraldehyde-3-phosphate and glycolaldehyde, increased in comparison with the same, non-modified microorganism; 
     iii) an oxidation activity from glycolaldehyde into glycolate, increased in comparison with the same, non-modified microorganism; and 
     iv) an oxidation activity from glyceraldehyde-3-phosphate into 1,3-bisphosphoglycerate, decreased in comparison with the same, non-modified microorganism, 
     said recombinant microorganism producing glycolic acid from pentoses and hexoses. 
     By “microorganism”, it is meant any organism which exists in the form of a microscopic cell belonging both to the procaryote and the eukaryote fields. Consequently, the term “microorganism” encompasses procaryotic micro-algae, bacteria, archaea and eubacteria of all the species as well as eukaryotic microorganisms such as plant cells, eukaryotic micro-algae, yeasts and fungi. The term also comprises cell cultures of any species which can be cultured for glycolic acid production. 
     By way of examples of bacteria usable within the scope of the present invention, bacteria from the families Burkholderiaceae, Enterobacteriaceae, Brevibacteriaceae, Clostridiaceae, Bacillaceae, Moraxellaceae, Sphingomonadaceae, Lactobacillaceae, Streptomycetaceae, Streptococcaceae, Methylobacteriacae and Corynebacteriaceae can be mentioned. By way of more particular examples of bacteria usable within the scope of the present invention,  Brevibacterium flavum, Brevibacterium lactofermentum, Escherichia coli, Bacillus subtilis, Corynebacterium glutamicum, Corynebacterium efficiens, Zymomonas mobilis, Ralstonia eutropha, Clostridium acetobutylicum, Methylobacterium extorquens  and  Lactococcus lactis  can be mentioned. 
     By way of examples of yeasts usable within the scope of the present invention, yeasts from the families Saccharomycetaceae, Pichiaceae, Schizosaccharomycetaceae and  Yarrowia  can be mentioned. By way of more particular examples of yeasts usable within the scope of the present invention,  Saccharomyces cerevisiae, Schizosaccharomyces pombe, Candida albicans, Candida blankii, Candida rugosa, Kluyveromyces lactis, Kluyveromyces marxianus, Pichia jadinii, Scheffersomyces stipitis, Pichia pastoris  and  Yarrowia lipolytica  can be mentioned. 
     By way of examples of fungi usable within the scope of the present invention, fungi of the genera  Penicillium, Aspergillus, Chrysosporium  and  Trichoderma  can be mentioned. By way of more particular examples of fungi usable within the scope of the present invention,  Penicillium notatum, Penicillium chrysogenum, Aspergillus niger, Chrysosporium pannorum  and  Trichoderma reesei  can be mentioned. 
     More particularly, the microorganism implemented within the scope of the present invention is an  E. coli  type bacterium or a  Saccharomyces cerevisiae  type yeast. 
     By “recombinant microorganism”, it is intended a microorganism such as previously defined which is not found in nature and which is genetically different from its equivalent in nature. The terms “equivalent in the nature”, “non-modified microorganism”, “natural microorganism” and “wild type microorganism” are equivalent and usable interchangeably. The recombinant microorganism is modified by introduction, deletion and/or modification of genetic elements. 
     The recombinant microorganism usable in the present invention can be modified to modulate the expression level of an endogenous gene. By “endogenous gene”, it is intended a gene which was present in the microorganism before any genetic modification of the wild type microorganism. 
     Endogenous genes can be overexpressed by introducing additional heterologous sequences or by replacing endogenous regulatory elements, or by introducing one or more further copies of the gene in a chromosome or on one or more plasmid(s). Endogenous genes can also be modified to modulate their expression and/or their activity. For example, mutations can be introduced in the coding sequence to modify the gene product or heterologous sequences can be introduced in addition or to replace the endogenous regulatory elements. The modulation of an endogenous gene can cause a positive regulation and/or an increase in the activity of the endogenous gene product or, alternatively, downregulate and/or decrease the activity of the endogenous gene product. 
     Another way to modulate the expression of an endogenous gene is to exchange the endogenous promoter of the same as the wild type promoter, with a stronger or weaker promoter to upregulate or downregulate the expression of the endogenous gene. These promoters can be homologous or heterologous. 
     Alternatively, the recombinant microorganism usable in the present invention can also be modified to express an exogenous gene. The recombinant microorganism usable in the present invention can be modified to express exogenous genes if these genes are introduced with all the elements enabling them to be expressed in this microorganism. Those skilled in the art know different modification, transformation or transfection methods of a microorganism with an exogenous gene. By way of example and not exhaustively, this method can be a conjugation; an electroporation; a lipofection; a micro-injection; a particle bombardment (or biolistic); a biological transformation of a plant using  Agrobacterium tumefasciens ; a transformation by a chemical permeabilisation; a transformation by the DEAE-dextran method or an introduction via a virus, a virion or a viral particle. 
     By “exogenous gene”, it is intended a gene which has been introduced in a microorganism, by means well known to those skilled in the art whereas this gene is not naturally found in the microorganism. Exogenous genes can be integrated in one or the chromosome of the microorganism or be expressed extra-chromosonally by means of plasmids, vectors, cosmids, bacteriophages or viruses such as a baculovirus. An exogenous gene can be a homologous gene. 
     By “homologous gene”, it is intended a gene homologous to a gene coding a reference protein and which codes a protein homologous to this reference protein. By “protein homologous to a reference protein”, it is intended a protein having a similar function and/or a similar structure as the reference protein. Thus, when the reference protein is an enzyme, a protein homologous to this reference protein catalyses the same enzymatic reaction. 
     By using references given in databases of amino acid or nucleotide sequences such as Genbank or NCBI BioProject for known genes or proteins, those skilled in the art are capable of determining genes or proteins which are homologous i.e. equivalent in other organisms as bacterial strains, yeasts, fungi, mammals or even plants. This routine work is advantageously made by using consensus sequences identified by sequence alignments with genes or proteins, derived from other organisms. 
     A homologous of a gene A can also be a gene coding for a variant of the protein coded by the gene A. This homologous can be obtained by synthetic pathway. 
     Typically, a protein (or a gene) homologous to a reference protein (or a reference gene) has at least 40%, at least 45%, at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 95% and/or at least 99% identity respectively with the amino acid sequence of the reference protein (or the nucleotide sequence of the reference gene). By “identity percentage” between two amino acid sequences (or between two nucleotide sequences), it is intended, within the scope of the present invention, a percentage of amino acid residues (or nucleotides) that are identical between both compared sequences, this percentage being obtained after implementing the best alignment (optimum alignment) between both sequences. Those skilled in the art know different techniques enabling such an identity percentage to be obtained and involving homology algorithms or computer programs such as the BLAST program. 
     The recombinant microorganism according to the invention has one or more increased enzymatic activity(ies) in comparison with the same, non-modified microorganism and at least one enzymatic activity decreased in comparison with the same, non-modified microorganism. In other words, the intracellular activity of some enzymes is increased in comparison with the same, non-modified microorganism and the intracellular activity of at least one other enzyme is decreased in comparison with the same, non-modified microorganism. 
     In the present invention, the term “activity” of an enzyme is used interchangeably with the term “function” and designates the reaction which is catalysed by the enzyme. Those skilled in the art know different techniques to measure the enzymatic activity of a given enzyme. The experimental part hereinafter gives different enzymatic tests usable to measure the activity of enzymes involved in the present invention. 
     Within the scope of the present invention, the terms “increased activity” applied to an enzyme designate a catalytic activity specific to the enzyme which is increased and/or an amount or availability of the enzyme which is increased in the cell. 
     Within the scope of the present invention, an increased enzymatic activity in the recombinant microorganism should be understood as an enzymatic activity which is increased by a factor of at least 2, in particular at least 5, in particular at least 10 and, more particularly, at least 20 with respect to the enzymatic activity of the same, non-modified microorganism. 
     Those skilled in the art know different microbiology and molecular biology techniques usable to obtain, in a given microorganism, the increase in an enzymatic activity. 
     Indeed, an increase in the enzymatic activity can be achieved (i a ) by increasing the number of copies of the gene coding for the enzyme in the microorganism, (ii a ) by increasing the expression of the gene coding for the enzyme in the microorganism, for example, by modifying the promoter, the regulatory regions and/or the ribosome binding site, (iii a ) by modifying the sequence of the gene coding the enzyme so as to obtain a form which is more active or more resistant to inhibition and, optionally, (iv a ) by combining at least two of the alternatives (i a ), (ii a ) and (iii a ). 
     As regards the alternative (i a ) above, the gene can be coded chromosomally or extra-chromosomally. When the gene is located on the chromosome, several copies of the gene can be introduced on the chromosome by recombination methods, known to those skilled in the art (including via gene replacement). When the gene is located extra-chromosomally, it can be carried by a recombinant expression vector. 
     By “recombinant expression vector”, it is intended a nucleic acid adapted to express, in a microorganism, at least one enzyme coded by a nucleotide sequence contained in this vector. The expression vector according to the present invention comprises, in addition to the nucleotide sequence coding for an enzyme of interest, one (or more) element(s) which enable(s) this nucleotide sequence to be expressed i.e. transcripted and translated. 
     The expression vector implemented in the present invention is advantageously chosen from a plasmid, a cosmid, a bacteriophage and a virus such as a baculovirus. In particular, the vector of the invention is an autonomously replicating vector including elements enabling it to be maintained and replicated in the microorganism such as a replication origin. Further, the vector can include elements enabling it to be selected in the microorganism. These elements are also known as “selection markers”. Such expression vectors are well known to those skilled in the art and widely described in literature. 
     An expression vector can further have one or more element(s) chosen from a promoter, an enhancer, a 3′ UTR (“UnTranslated Region”) signal, an IRES (“Internal Ribosome Entry Site”) signal, a ribosome binding site (RBS), a transcription termination signal comprising a cleavage site and a polyA signal (“polyadenylation signal”). The expression vector according to the invention can comprise 2, 3 or 4 elements listed above. Those skilled in the art are capable of choosing, in this list, the additional element(s) that the expression vector can comprise depending on the microorganism in which the expression should be made. 
     By “selection marker”, it is intended a marker chosen from a selection marker usable in prokaryotes or in eukaryotes such as an antibiotic resistance bacterial gene and a metabolism gene to be used with an auxotrophic microorganism, i.e. a selection gene which ensures the complementation with the respective gene deleted at the genome of the host microorganism. Thus, the expression vector according to the present invention can contain a bacterial gene for resisting to an antibiotic such as amoxicillin, ampicillin, phleomycin, kanamycin, chloramphenicol, neomycin, hygromycin, geneticin (or G418), carboxin, nourseothricin or triclosan. By way of illustrating examples of metabolism genes, the gene trp1 to be used with a microorganism lacking the phosphoribosylanthranilate isomerase enzyme such as a yeast trp1 −  or the gene URA3 to be used with the eukaryotic organism lacking the orotidine 5-phosphate decarboxylase enzyme such as a yeast ura3 −  can be mentioned. 
     By “promoter”, it is intended, within the scope of the present invention, both a constitutive or inducible promoter, adapted to any microorganism such as previously defined and a constitutive or inducible promoter, specific to a group of particular microorganisms. The usable promoter can be homologous or heterologous. A constitutive promoter usable within the scope of the present invention is in particular chosen from promoter proD, promoter proC, promoter 35S, promoter 19S and promoter TEV (“Tobacco Etch Virus”). An inducible promoter usable within the scope of the present invention can be promoter GAL1 inducible by galactose, promoter AOX1 inducible by methanol, promoter PA1lac0-1 inducible by isopropyl β-D-1-thiogalactopyranoside (IPTG), the pTac hybrid promoter inducible by IPTG; promoter MET15 inducible by methionine depletion or promoter CUP1 inducible by cupper ions. In the vector implemented in the present invention, the promoter can be associated with one or more transcriptional regulatory sequences that are the enhancers. 
     Advantageously, the expression vector implemented in the present invention comprises, being operationally linked together, a promoter, a nucleotide sequence coding for an enzyme of interest and a transcription termination signal comprising a cleavage site and/or a polyA signal. By “operationally linked together” according to the invention, it is intended elements linked together such that the functioning of one of the elements is altered by that of another. By way of example, a promoter is operationally linked to a coding sequence when it is capable of altering the expression of the same. The elements with regulate transcription, translation and maturation of peptides that the vector can comprise are known to those skilled in the art and the latter is capable of choosing them depending on the host microorganism in which the expression or cloning should be made. 
     Furthermore, those skilled in the art know, as an expression vector, different plasmid types which differ by their replication origin and thus by their copy number in the cell. Typically, these plasmids are present in the microorganism as 10 to 15 copies, or about 30 to 50 copies, or even up to 100 copies, depending on the plasmid nature: plasmids with a low copy number with a narrow replication, plasmids with a medium copy number or plasmids with a high copy number. By way of examples of plasmids usable within the scope of the present invention, plasmids pSC101, RK2, pACYC, pRSF1010, pZ and pSK bluescript II can be mentioned. 
     Typically, the alternative (ii a ) above consists in using a promoter inducing a high expression level of the endogenous gene. Depending on the nature and properties of the endogenous promoter, those skilled in the art will be able to determine which promoter, being homologous or heterologous, inducible or constitutive, to use in order to replace said endogenous promoter. Alternatively, the alternative (ii a ) can consist in attenuating the activity or expression of a transcription repressor, which is specific or not to the endogenous gene. 
     Within the scope of the present invention, the terms “decreased activity” or “reduced activity” applied to an enzyme designates a reduced catalytic activity specific to the enzyme and/or a decreased amount or availability of the enzyme in the cell. 
     Within the scope of the present invention, a decreased enzymatic activity in the recombinant microorganism should be intended as an enzymatic activity which is reduced by a factor of at most 0.5, in particular at most 0.1, in particular, at most 0.01 and more particularly, at most 0.001 with respect to the enzymatic activity of the same, non-modified microorganism. 
     Those skilled in the art know different microbiology and molecular biology techniques usable to obtain, in a given microorganism, the decrease in an enzymatic activity. 
     Indeed, a decrease in the enzymatic activity can be achieved (i d ) by decreasing the expression of the gene coding the enzyme in the microorganism, for example, by modifying the promoter, the regulatory regions and/or the ribosome binding site, (ii d ) by modifying the sequence of the gene coding for the enzyme so as to achieve a reduced expression of the gene and/or the expression of an enzyme the activity of which is reduced, (iii d ) by using elements which destabilise the mRNA obtained after the gene transcription and, optionally, (iv d ) by inactivating the gene in particular by total or partial deletion of said gene, by total or partial deletion of the promoter preventing any expression of the gene and/or by inserting an external gene element in the coding region of the gene or in the promoter region. It is to be noted that, in some forms of the alternative (iv d ) and in particular in the case of a total deletion of the gene, it is possible to have no residual enzymatic activity. In other words, in the alternative (iv d ), the enzymatic activity is silenced. 
     The recombinant microorganism according to the invention has a conversion activity from D-ribulose-5-phosphate into D-arabinose-5-phosphate, increased in comparison with the same, non-modified microorganism. In other words, the intracellular activity of the enzyme which converts D-ribulose-5-phosphate into D-arabinose-5-phosphate in the recombinant microorganism is increased in comparison with the same, non-modified microorganism. 
     The enzyme (E i ) which converts D-ribulose-5-phosphate into D-arabinose-5-phosphate is in the form of a D-arabinose-5-phosphate isomerase (EC 5.3.1.13). Such an enzyme is also known as “D-arabinose-5-phosphate aldose-ketose-isomerase”, “arabinose phosphate isomerase”, “D-arabinose-5-phosphate ketol-isomerase” and “phosphoarabinoisomerase”. All these designations make reference to the same enzyme and are interchangeably usable. 
     Thus the recombinant microorganism according to the invention presents a conversion activity from D-ribulose-5-phosphate into D-arabinose-5-phosphate catalysed by an enzyme consisting in a D-arabinose-5-phosphate isomerase that converts D-ribulose-5-phosphate into D-arabinose-5-phosphate, increased in comparison with the same, non-modified microorganism 
     Advantageously, the enzyme (E i ) is coded by the  E. coli  kdsD gene or by a homologue of such a gene. Based on the previously provided “homologue” definition, a homologue of the  E. coli  kdsD gene encodes a protein capable of converting D-ribulose-5-phosphate into D-arabinose-5-phosphate. By way of particular example of  E. coli  kdsD gene, the gene from the strain K12 MG1655 the coding sequence of which is included between nucleotides 2829813 and 2830778 in the sequence NC_000913.3 accessible from the NCBI (“National Center for Biotechnology Information”) site https://www.ncbi.nlm.nih.gov/ and corresponding to the full genome of the strain K12 MG1655 can be mentioned. The protein sequence of 321 amino acids, which is coded by this gene is referenced, on the NCBI site, sequence NP_417188.4 and corresponds to the sequence SEQ ID NO: 1 in the appended sequence listing. 
     All the previously contemplated alternatives to increase an enzymatic activity are usable to increase the activity of the enzyme (E i ) which converts D-ribulose-5-phosphate into D-arabinose-5-phosphate. 
     Typically, in the recombinant microorganism according to the invention, the increase in the conversion activity from D-ribulose-5-phosphate into D-arabinose-5-phosphate catalysed by an enzyme consisting in a D-arabilose-5-phosphate isomerase is obtained by increasing the number of copies of the gene coding for this enzyme in the microorganism and/or by increasing the expression of the gene coding for this enzyme in the microorganism. In particular, the recombinant microorganism subject matter of the present invention exhibits an overexpression of the gene coding for a D-arabilose-5-phosphate isomerase which converts D-ribulose-5-phosphate into D-arabinose-5-phosphate. 
     More particularly, the recombinant microorganism subject matter of the present invention exhibits an overexpression of the  E. coli  kdsD gene or a homologue thereof. 
     The recombinant microorganism according to the invention exhibits an aldolic cleavage activity from D-arabinose-5-phosphate into D-glyceraldehyde-3-phosphate and glycolaldehyde, increased in comparison with the same, non-modified microorganism. In other words, the intracellular activity of the enzyme which catalyses the cleavage of D-arabinose-5-phosphate into D-glyceraldehyde-3-phosphate and glycolaldehyde in the recombinant microorganism is increased in comparison with the same, non-modified microorganism. 
     The enzyme (E ii ) which catalyses the cleavage of D-arabinose-5-phosphate into D-glyceraldehyde-3-phosphate and glycolaldehyde is in the form of a fructose-6-phosphate aldolase (EC 4.1.2.-) [14]. 
     Thus, the recombinant micro-organism according to the invention exhibits an aldolic cleavage activity from D-arabinose-5-phosphate into D-glyceraldehyde-3-phosphate and glycolaldehyde, catalysed by an enzyme consisting in a fructose-6-phosphate aldolase which catalyses the cleavage of D-arabinose-5-phosphate into D-glyceraldehyde-3-phosphate and glycolaldehyde, increased in comparison with the same, non-modified microorganism. 
     Advantageously, the enzyme (E ii ) is coded by the  E. coli  fsa gene or a homologue of such a gene. Based on the previously provided “homologue” definition, a homologue of the  E. coli  fsa gene encodes a protein capable of catalysing the cleavage of D-arabinose-5-phosphate into D-glyceraldehyde-3-phosphate and glycolaldehyde. By way of particular examples of the  E. coli  fsa gene, one can mention:
         the gene fsaA from the strain K12 MG1655 the coding sequence of which is included between the nucleotides 863642 and 864304 in the sequence NC_000913.3 accessible from the NCBI site. The protein sequence of 220 amino acids (FSAA), coded by this gene is referenced, on the NCBI site, sequence NP_415346.4 and corresponds to the sequence SEQ ID NO: 2 in the appended sequence listing, and   the gene fsaB from the strain K12 MG1655 the coding sequence of which is included, in the complementary strand, between the nucleotides 4139046 and 4139708 in the sequence NC_000913.3 accessible from the NCBI site. The protein sequence of 220 amino acids (FSAB), coded by this gene is referenced, on the NCBI site, sequence NP_418381.1 and corresponds to the sequence SEQ ID NO: 3 in the appended sequence listing.       

     As particular examples of homologous of the  E. coli  fsa gene, on can mention the genes coding for the fructose-6-phosphate aldolase variants FSAA L107Y/A129G and FSAA A129T/A165G disclosed by Szekrenyi et al, 2014 [15]. 
     All the previously contemplated alternatives to increase an enzymatic activity are usable to increase the activity of the enzyme (E ii ) which catalyses the cleavage of D-arabinose-5-phosphate into D-glyceraldehyde-3-phosphate and glycolaldehyde. 
     Typically, in the recombinant microorganism according to the invention, the increase in the aldolic cleavage activity from D-arabinose-5-phosphate into D-glyceraldehyde-3-phosphate and glycolaldehyde, catalysed by an enzyme consisting in a fructose-6-phosphate aldolase is obtained by increasing the number of copies of the gene coding for this enzyme in the microorganism and/or by increasing the expression of the gene coding for this enzyme in the microorganism. In particular, the recombinant microorganism subject matter of the present invention exhibits an overexpression of the gene coding for a fructose-6-phosphate aldolase which catalyses the cleavage of D-arabinose-5-phosphate into D-glyceraldehyde-3-phosphate and glycolaldehyde. 
     More particularly, the recombinant microorganism subject matter of the present invention exhibits an overexpression of the  E. coli  fsa gene or a homologue thereof. 
     The recombinant microorganism according to the invention exhibits an oxidation activity from glycolaldehyde into glycolate, increased in comparison with the same, non-modified microorganism. In other words, the intracellular activity of the enzyme which oxidises glycolaldehyde into glycolate in the recombinant microorganism is increased in comparison with the same, non-modified microorganism. 
     The enzyme (E iii ) which oxidises glycolaldehyde into glycolate is in the form of a glycolaldehyde dehydrogenase and in particular a glycolaldehyde dehydrogenase the activity of which requires the presence of the cofactor NAD +  (EC 1.2.1.21). 
     Thus, the recombinant microorganism according to the invention exhibits an oxidation activity from glycolaldehyde into glycolate catalysed by an enzyme consisting in a glycolaldehyde dehydrogenase which oxidises glycolaldehyde into glycolate, increased in comparison with the same, non-modified microorganism. 
     Advantageously, the enzyme (E iii ) is coded by the  E. coli  aldA gene or a homologue of such a gene. Based on the previously provided “homologue” definition, a homologue of the  E. coli  aldA gene encodes a protein capable of oxidising glycolaldehyde into glycolate in the presence of a cofactor as, for example, NAD + . By way of particular example of the  E. coli  aldA gene, the gene from the strain K12 MG1655 the coding sequence of which is included between nucleotides 1488232 and 1489671 in the sequence NC_000913.3 accessible from the NCBI site can be mentioned. The protein sequence of 479 amino acids, coded by this gene is referenced, on the NCBI site, sequence NP_415933.1 and corresponds to the sequence SEQ ID NO: 4 in the appended sequence listing. 
     All the previously contemplated alternatives to increase an enzymatic activity are usable to increase the activity of the enzyme (E iii ) which oxidises glycolaldehyde into glycolate. 
     Typically, in the recombinant microorganism according to the invention, the increase in the oxidation activity from glycolaldehyde into glycolate, catalysed by an enzyme consisting in a glycolaldehyde dehydrogenase is obtained by increasing the number of copies of the gene coding for this enzyme in the microorganism and/or by increasing the expression of the gene coding for this enzyme in the microorganism. In particular, the recombinant microorganism subject matter of the present invention exhibits an overexpression of the gene coding for a glycolaldehyde dehydrogenase which oxidises glycolaldehyde into glycolate. 
     More particularly, the recombinant microorganism subject matter of the present invention exhibits an overexpression of the  E. coli  aldA gene or a homologue thereof. 
     In particular, the recombinant microorganism according to the present invention comprises:
         either at least one plasmid having the sequence of the  E. coli  KdsD gene or a homologue thereof, the sequence of the  E. coli  fsa gene or a homologue thereof and the sequence of the  E. coli  aldA gene or a homologue thereof, said sequences being optionally cloned as an operon,   or at least two plasmids, with an identical nature but having replication origins compatible or of a different nature, one having two sequences, optionally cloned as an operon, chosen from the sequence of the  E. coli  kdsD gene or a homologue thereof, the sequence of the  E. coli  fsa gene or a homologue thereof and the sequence of the  E. coli  aldA gene or a homologue thereof and the other plasmid the third of these sequences. In this alternative, one can have (fsa+aldA) on a plasmid and kdsD on the other; (fsa+kdsD) on a plasmid and aldA on the other or even (kdsD+aldA) on a plasmid and fsa on the other;   or at least three plasmids, being identical or different, each having a different sequence chosen from the sequence of the  E. coli  kdsD gene or a homologue thereof, the sequence of the  E. coli  fsa gene or a homologue thereof and the sequence of the  E. coli  aldA gene or a homologue thereof.       

     All that has been previously described on plasmids is applicable to the plasmid(s) contained in the recombinant microorganism subject matter of the invention. 
     More particularly, the recombinant microorganism subject matter of the invention comprises: 
     α) a first plasmid in which the sequence of the  E. coli  kdsD gene or a homologue thereof and the sequence of the  E. coli  fsa gene or a homologue thereof lie, said sequences being cloned as an operon and under the control of a first inducible or constitutive promoter and 
     β) a second plasmid in which the sequence of the  E. coli  aldA gene or a homologue thereof under the control of a second inducible or constitutive promoter lies, 
     said first and second promoters being identical or different. 
     The recombinant microorganism according to the invention exhibits an oxidation activity from glyceraldehyde-3-phosphate into 1,3-bisphosphoglycerate, decreased in comparison with the same, non-modified microorganism. In other words, the intracellular activity of the enzyme which oxidises glyceraldehyde-3-phosphate into 1,3-bisphosphoglycerate in the recombinant microorganism is decreased in comparison with the same, non-modified microorganism. 
     The enzyme (E iv ) which oxidises glyceraldehyde-3-phosphate into 1,3-bisphosphoglycerate is in the form of a glyceraldehyde-3-phosphate dehydrogenase and in particular of a glyceraldehyde-3-phosphate dehydrogenase the cofactor of which is NAD +  (EC 1.2.1.12). Such an enzyme is also known as “D-glyceraldehyde-3-phosphate:NAD +  oxydoreductase”. Both designations make reference to the same enzyme and are interchangeably usable. 
     Advantageously, the enzyme (E iv ) is coded by the  E. coli  gapA gene or by a homologue of such a gene. Based on the previously provided “homologue” definition, a homologue of the  E. coli  gapA gene encodes a protein capable of oxidising glyceraldehyde-3-phosphate into 1,3-bisphosphoglycerate in the presence of a cofactor as, for example, NAD + . By way of particular example of the  E. coli  gapA gene, the gene from the strain K12 MG1655 the coding sequence of which is included between nucleotides 1862771 and 1863766 in the sequence NC_000913.3 accessible from the NCBI site can be mentioned. The protein sequence of 331 amino acids, coded by this gene is referenced, on the NCBI site, sequence NP_416293.1 and corresponds to the sequence SEQ ID NO:4 in the appended sequence listing. 
     All the previously contemplated alternatives to decrease an enzymatic activity are usable to decrease the activity of the enzyme (E iv ) which oxidises glycolaldehyde into glycolate. 
     Typically, in the recombinant micro-organism according to the invention, the decrease in the oxidation activity from glyceraldehyde-3-phosphate into 1,3-bisphosphoglycerate catalysed by an enzyme consisting in a glyceraldehyde-3-phosphate dehydrogenase is obtained by decreasing the expression of the gene coding for this enzyme and/or by inactivating the gene coding for this enzyme. 
     In a particular implementation, in the recombinant microorganism subject matter of the present invention, the expression of the  E. coli  gapA gene or a homologue thereof is decreased but not inactivated with respect to the non-modified microorganism. In this case, the mitigation of the gapA activity enables the growth and, at the same time, the production of glycolic acid. However, maintaining a residual glycolysis does not enable the maximum yield to be reached. 
     In an alternative implementation, in the recombinant microorganism subject matter of the present invention, the expression of the  E. coli  gapA gene or a homologue thereof is inactivated with respect to the non-modified microorganism. In this case, the growth and the production of glycolic acid are decoupled. The gapA inactivation requires that the recombinant microorganism according to the invention has a growth substrate, containing, in addition to pentoses and/or hexoses, C2, C3 or C4 compounds such as acetate, pyruvate, malate or succinate or which enter the metabolism after the glyceraldehyde-3-phosphate (GAP). 
     Advantageously, the recombinant microorganism according to the present invention is modified in terms of glucose transport in that the phosphotransferase system (PTS), which depends on phosphoenolpyruvate (PEP) is inactivated, whereas a glucose transport activity coded by  E. coli  galP or  Zymomonas mobilis  glf or a homologue thereof and a transformation activity from glucose into glucose-6-phosphase are increased in comparison with the same, non-modified microorganism. This modification enables D-glucose and D-xylose to be assimilated [8, 16, 17]. 
     As a reminder, in  E. coli , the phosphotransferase system (PTS), which depends on phosphoenolpyruvate (PEP) is the most efficient system for glucose transport. The activity of the PTS system has an effect on carbon flow distribution and plays a key role in the carbon catabolism repression. The cytoplasmic PTS system is coded by the operon ptsHIcrr. The deletion of the ptsHIcrr operon in particular in  E. coli  is the most commonly employed strategy to inactivate the PTS system. 
     The PTS −  phenotype is characterised by a very limited glucose transport and phosphorylation capability. In a PTS −  strain, PEP is not necessary to glucose phosphorylation. Glucokinase catalyses ATP-dependent phosphorylation of glucose in the cytoplasm. The overexpression of galactose permease (GalP) for glucose and glucokinase (Glk) transport for phosphorylation restores the phenotype PTS −  [18]. Another strategy is to overexpress glf zm  and glk zm  genes respectively coding for a facilitated glucose transporter (Glf) and the glucokinase of  Zymomonas mobilis  in  E. coli  [19]. 
     Thus, in the recombinant microorganism according to the present invention, a glucose transport and phosphorylation activity from phosphoenolpyruvate by the phosphoenolpyruvate-dependent phosphotransferase system (PTS) is silenced in comparison with the same, non-modified microorganism. 
     By way of particular example, the inactivation of the cytoplasmic PTS system coded by the operon ptsHIcrr in  E. coli  results in deleting the genes:
         ptsH of the operon ptsHIcrr encoding the phosphate-bearing phosphohistidine protein Hpr, of the PTS system. By way of more particular example of the  E. coli  ptsH gene, the gene from the strain K12 MG1655 the coding sequence of which is included, on the complementary strand, between nucleotides 2533764 and 2534021 in the sequence NC_000913.3 accessible from the NCBI site can be mentioned. The protein sequence of 85 amino acids coded by this gene is referenced, on the NCBI site, sequence NP_416910.1 and corresponds to the sequence SEQ ID NO: 6 in the appended sequence listing.   ptsI of the operon ptsHIcrr encoding the enzyme I of the PTS system (EC 2.7.3.9). By way of more particular example of  E. coli  ptsI gene, the gene from the strain K12 MG1655 the coding sequence of which is included, on the complementary strand, between nucleotides 2534066 and 2535793 in the sequence NC_000913.3 accessible from the NCBI site can be mentioned. The protein sequence of 575 amino acids coded by this gene is referenced, on the NCBI site, sequence NP_416911.1 and corresponds to the sequence SEQ ID NO: 7 in the appended sequence listing.   crr of the operon ptsHIcrr encoding the enzyme IIA of the complex II of the PTS system (EC 2.7.1.69). By way of more particular example of the  E. coli  crr gene, the gene from the strain K12 MG1655 the coding sequence of which is included, on the complementary strand, between nucleotides 2535834 and 2536343 in the sequence NC_000913.3 accessible from the NCBI site can be mentioned. The protein sequence of 169 amino acids coded by this gene is referenced, on the NCBI site, sequence NP_416912.1 and corresponds to the sequence SEQ ID NO: 8 in the appended sequence listing.       

     Advantageously, the deletion of the ptsG gene of the PTS system, which does not belong to the operon ptsHIcrr, is also required, because PtsG is strongly involved in the catabolic repression in  E. coli . The PtsG −  mutants are capable of simultaneously consuming glucose, arabinose and xylose whereas a wild strain consumes glucose and then arabinose and finally xylose sequentially [20]. The ptsG gene encodes a wide IIB/C hydrophobic domain of the complex II of the PTS system. By way of more particular example of the  E. coli  ptsG gene, the gene from the strain K12 MG1655 strain the coding sequence of which is included, on the complementary strand, between nucleotides 1157869 and 1159302 in the sequence NC_000913.3 accessible from the NCBI site can be mentioned. The protein sequence of 477 amino acids coded by this gene is referenced, on the NCBI site, sequence NP_415619.1 and corresponds to the sequence SEQ ID NO: 9 in the appended sequence listing. 
     The  E. coli  galP gene encodes a galactose permease. Based on the previously provided “homologue” definition, a homologue of the  E. coli  galP gene encodes a galactose permease type protein. By way of particular example of the  E. coli  galP gene, the gene from the strain K12 MG1655 the coding sequence of which is included between nucleotides 3088284 and 3089678 in the sequence NC_000913.3 accessible from the NCBI site can be mentioned. The protein sequence of 464 amino acids, coded by this gene is referenced, on the NCBI site, sequence NP_417418.1 and corresponds to the sequence SEQ ID NO: 10 in the appended sequence listing. 
     The  Z. mobilis  gf gene codes for a glucose transporter. Based on the previously provided “homologue” definition, a homologue of the  Z. mobilis  glf gene codes for a glucose transporter. By way of particular example of the  Z. mobilis  glf gene, the gene from the ATCC 31821/ZM4/CP4 strain which encodes the protein of 473 amino acids referenced, in the UniProtKB site (http://www.uniprot.org/), sequence P21906 and corresponding to the sequence SEQ ID NO: 11 in the appended sequence listing can be mentioned. 
     Advantageously, the recombinant microorganism according to the invention exhibits a transformation activity from glucose into glucose-6-phosphase, increased in comparison with the same, non-modified microorganism. In other words, the intracellular activity of the enzyme which transforms glucose into glucose-6-phosphase in the recombinant microorganism is increased in comparison with the same, non-modified microorganism. 
     The enzyme which transforms glucose into glucose-6-phosphase is in the form of a glucokinase (EC 2.7.1.2). 
     Thus the recombinant micro-organism according to the invention exhibits a transformation activity from glucose into glucose-6-phosphase catalysed by an enzyme consisting in a glucokinase which transforms glucose into glucose-6-phosphase, increased in comparison with the same, non-modified microorganism. 
     Advantageously, this enzyme is coded by the  E. coli  glK gene or by a homologue of such a gene. Based on the previously provided “homologue” definition, a homologue of the  E. coli  glK gene encodes a protein capable of transforming glucose into glucose-6-phosphase. By way of particular example of the  E. coli  glK gene, the gene from the strain K12 MG1655 the coding sequence of which is included between nucleotides 2508461 and 2509426 in the sequence NC_000913.3 accessible from the NCBI site can be mentioned. The protein sequence of 321 amino acids, coded by this gene is referenced, on the NCBI site, sequence NP_416889.1 and corresponds to the sequence SEQ ID NO: 12 in the appended sequence listing. 
     All the previously contemplated alternatives to increase an enzymatic activity are usable to increase the glucose transporter activity and the transformation activity from the glucose into glucose-6-phosphase. 
     Typically, in the recombinant microorganism according to the invention, the increase in the glucose transport catalysed by an enzyme consisting in a glucose transporter or a galactose permease is obtained by increasing the number of copies of a gene coding for this enzyme in the microorganism and/or by increasing the expression of a gene coding for this enzyme in the microorganism. In particular, the recombinant microorganism subject matter of the present invention exhibits an overexpression of a gene coding for a glucose transporter and/or an overexpression of a gene coding for a galactose permease which catalyses the glucose transport. Moreover, in the recombinant microorganism according to the invention, the increase in the transformation activity from the glucose into glucose-6-phosphase catalysed by an enzyme consisting in a glucokinase which transforms the glucose into glucose-6-phosphase is obtained by increasing the number of copies of a gene coding for this enzyme in the microorganism and/or by increasing the expression of a gene coding for this enzyme in the microorganism. In particular, the recombinant microorganism subject matter of the present invention exhibits an overexpression of a gene coding for a glucokinase which transforms the glucose into glucose-6-phosphase. 
     More particularly, the recombinant microorganism subject matter of the present invention exhibits an overexpression of the  E. coli  aldA gene or the  Z. mobilis  g/f gene or one of their homologues and an overexpression of the  E. coli  g/K gene or a homologue thereof. To that end, in the recombinant microorganism according to the invention, the genes listed above are under the dependence of a strong constitutive promoter like the promoter proD. 
     Advantageously, in order to further optimise glycolic acid production via the non-natural synthetic pathway according to the invention, the recombinant microorganism subject matter of the present invention can exhibit at least one of the following characteristics:
         v) an oxidation activity from glycolate into glyoxylate, decreased in comparison with the same, non-modified microorganism;   vi) a repression of the genes involved in regulating the aerobic respiratory metabolism, decreased in comparison with the same, non-modified microorganism;   vii) a glycolate internalisation, decreased in comparison with the same, non-modified microorganism;   viii) an irreversible formation activity of methylglyoxal from dihydroxyacetone, decreased in comparison with the same, non-modified microorganism;   ix) a conversion activity from fructose-6-phosphate into fructose-1,6-biphosphate, decreased in comparison with the same, non-modified microorganism;   x) a production activity of D-ribose-1-phosphate from dihydroxyacetone phosphate and glycolaldehyde, decreased in comparison with the same, non-modified microorganism; and   xi) an oxidation activity from D-glucose-6-phosphate into 6-phospho D-glucono-1,5-lactone, modified in comparison with the same, non-modified microorganism.       

     The recombinant microorganism according to the invention exhibits an oxidation activity from glycolate to glyoxylate, decreased in comparison with the same, non-modified microorganism. In other words, the intracellular activity of the enzyme which oxidises glycolate into glyoxylate in the recombinant microorganism is decreased in comparison with the same, non-modified microorganism. 
     The enzyme (E v ) which oxidises glycolate into glyoxylate is in the form of glycolate dehydrogenase and in particular a glycolate dehydrogenase the cofactor of which is NAD +  (EC 1.1.99.14). Such an enzyme is also known as “glycolate oxidoreductase” and “glycolate oxidase”. All these designations make reference to the same enzyme and are interchangeably usable. 
     Thus, the recombinant microorganism according to the invention exhibits an oxidation activity from glycolate to glyoxylate catalysed by an enzyme consisting in a glycolate dehydrogenase which oxidises glycolate into glyoxylate, decreased in comparison with the same, non-modified microorganism. 
     Advantageously, the  E. coli  glycolate dehydrogenase activity is coded by the glcDEF genes or a homologue of such genes. Based on the previously provided “homologue” definition, a homologue of the  E. coli  glcD, glcE and glcF genes encodes a protein capable of oxidising glycolate into glyoxylate in the presence of a cofactor as, for example, NAD + . 
     By way of particular example of  E. coli  glcD gene, the gene from the strain K12 MG1655 the coding sequence of which is included, on the complementary strand, between nucleotides 3126522 and 3128021 in the sequence NC_000913.3 accessible from the NCBI site can be mentioned. The protein sequence of 499 amino acids, coded by this gene is referenced, on the NCBI site, sequence NP_417453.1 and corresponds to the sequence SEQ ID NO: 13 in the appended sequence listing. 
     By way of particular example of  E. coli  glcE gene, the gene from the strain K12 MG1655 the coding sequence of which is included, on the complementary strand, between nucleotides 3125470 and 3126522 in the sequence NC_000913.3 accessible from the NCBI site can be mentioned. The protein sequence of 350 amino acids, coded by this gene is referenced, on the NCBI site, sequence Y P _026191.1 and corresponds to the sequence SEQ ID NO: 14 in the appended sequence listing. 
     By way of particular example of  E. coli  glcF gene, the gene from the strain K12 MG1655 the coding sequence of which is included, on the complementary strand, between nucleotides 3124236 and 3125459 in the sequence NC_000913.3 accessible from the NCBI site can be mentioned. The protein sequence of 407 amino acids, coded by this gene is referenced, on the NCBI site, sequence Y P _026190.1 and corresponds to the sequence SEQ ID NO: 15 in the appended sequence listing. 
     All the previously contemplated alternatives to decrease an enzymatic activity are usable to decrease the glycolate dehydrogenase activity. 
     Typically, in the recombinant microorganism according to the invention, the decrease of the oxidation activity from glycolate to glyoxylate catalysed by an enzyme consisting in a glycolate dehydrogenase is obtained by decreasing the expression of the gene coding for this enzyme and/or by inactivating the gene coding for this enzyme. More particularly, in the recombinant microorganism subject matter of the present invention, the expression of  E. coli  glcD, glcE and glcF genes or homologues thereof is decreased with respect to the non-modified microorganism. In a particular implementation, the  E. coli  glcD, glcE and glcF genes or homologues thereof are deleted in the recombinant microorganism subject matter of the present invention in order to enable glycolate to be accumulated [21]. 
     The recombinant microorganism according to the invention exhibits an aerobic respiratory activity increased in comparison with the same, non-modified microorganism, by decreasing the repression of the genes involved in regulating the aerobic respiratory metabolism. This repression does not involve an enzyme but a transcriptional regulator. 
     Thus, the recombinant microorganism according to the invention exhibits a repression of the genes involved in the regulation of the aerobic respiratory metabolism induced by a transcriptional regulator capable of repressing the genes involved in the regulation of the aerobic respiratory metabolism, decreased in comparison with the same, non-modified microorganism. 
     Advantageously, this transcriptional regulator is coded by the  E. coli  arcA gene or a homologue of such a gene. Based on the previously provided “homologue” definition, a homologue of the  E. coli  arcA gene encodes a protein capable of repressing the genes involved in the aerobic respiratory metabolism [22]. 
     By way of particular example of  E. coli  arcA gene, the gene from the strain K12 MG1655 the coding sequence of which is included, on the complementary strand, between nucleotides 4639590 and 4640306 in the sequence NC_000913.3 accessible from the NCBI site can be mentioned. The protein sequence of 238 amino acids, coded by this gene is referenced, on the NCBI site, sequence NP_418818.1 and corresponds to the sequence SEQ ID NO: 16 in the appended sequence listing. 
     All the previously contemplated alternatives to decrease an enzymatic activity are usable, mutatis mutandis, to decrease the transcriptional regulator activity repressing the genes involved in the aerobic respiratory metabolism. 
     Typically, in the recombinant microorganism according to the invention, the decrease in the repression of the genes involved in the regulation of the aerobic respiratory metabolism induced by a transcriptional regulator is obtained by decreasing the expression of the gene coding for this regulator and/or by inactivating the gene coding for this regulator. 
     More particularly, in the recombinant microorganism subject matter of the present invention, the expression of the  E. coli  arcA gene or a homologue thereof is decreased with respect to the non-modified microorganism. In a particular implementation, the  E. coli  arcA gene or homologue thereof is deleted in the subject matter recombinant microorganism of the present invention. 
     The recombinant microorganism according to the invention exhibits a glycolate internalisation, decreased in comparison with the same, non-modified microorganism. This internalisation involves proteins importing glycolate [23, 24, 25]. 
     Thus, the recombinant microorganism according to the invention exhibits a glycolate internalisation induced by at least one protein importing the glycolate, decreased in comparison with the same, non-modified microorganism. 
     Advantageously, proteins are coded by the  E. coli  glcA, lldP or yjcG genes or homologues of such genes. Based on the previously provided “homologue” definition, homologues of the  E. coli  glcA, lldP or yjcG genes encode proteins capable of internalising glycolate. 
     By way of particular example of  E. coli  glcA gene, the gene from the strain K12 MG1655 the coding sequence of which is included between nucleotides 1157869 and 1159302 in the sequence NC_000913.3 accessible from the NCBI site can be mentioned. The protein sequence of 477 amino acids, coded by this gene is referenced, on the NCBI site, sequence NP_415619.1 and corresponds to the sequence SEQ ID NO: 17 in the appended sequence listing. 
     By way of particular example of  E. coli  lldP gene, the gene from the strain K12 MG1655 the coding sequence of which is included between nucleotides 3777399 and 3779054 in the sequence NC_000913.3 accessible from the NCBI site can be mentioned. The protein sequence of 551 amino acids, coded by this gene is referenced, on the NCBI site, sequence NP_418060.1 and corresponds to the sequence SEQ ID NO: 18 in the appended sequence listing. 
     By way of particular example of  E. coli  yjcG gene, the gene from the strain K12 MG1655 the coding sequence of which is included, on the complementary strand, between nucleotides 4283253 and 4284902 in the sequence NC_000913.3 accessible from the NCBI site can be mentioned. The protein sequence of 549 amino acids, coded by this gene is referenced, on the NCBI site, sequence NP_418491.1 and corresponds to the sequence SEQ ID NO: 19 in the appended sequence listing. 
     All the previously contemplated alternatives to decrease an enzymatic activity are usable, mutatis mutandis, to decrease the activity of proteins importing glycolate. 
     Typically, in the recombinant microorganism of the invention, the decrease in the glycolate internalization induced by one or more protein(s) importing the glycolate is obtained by decreasing the expression of the gene(s) coding this or these protein(s) and/or by inactivating the gene(s) coding this or these protein(s). 
     More particularly, in the recombinant microorganism subject matter of the present invention, the expression of the  E. coli  glcA, lldP or yjcG genes or homologues thereof is decreased with respect to the non-modified microorganism. Alternatively, in the recombinant microorganism subject matter of the present invention, the  E. coli  glcA, lldP or yjcG genes or homologues thereof are deleted with respect to the non-modified microorganism. 
     The recombinant microorganism according to the invention exhibits an irreversible formation activity of methylglyoxal from dihydroxyacetone phosphate (DHAP), decreased in comparison with the same, non-modified microorganism. In other words, the intracellular activity of the enzyme which irreversibly forms methylglyoxal, a cytotoxic compound, from DHAP in the recombinant microorganism is decreased in comparison with the same, non-modified microorganism. 
     The enzyme (E viii ) which irreversibly forms methylglyoxal from DHAP is in the form of a methylglyoxal synthase (EC 4.2.3.3) [26]. 
     Thus, the recombinant microorganism according to the invention exhibits an irreversible formation activity of methylglyoxal from DHAP catalysed by an enzyme consisting in a methylglyoxal synthase which irreversibly forms methylglyoxal from DHAP, decreased in comparison with the same, non-modified microorganism. 
     Advantageously, the enzyme (E viii ) is coded by the  E. coli  mgsA gene or by a homologue of such a gene. Based on the previously provided “homologue” definition, a homologue of the  E. coli  mgsA gene encodes a protein capable of irreversibly forming, methylglyoxal from DHAP. By way of particular example of  E. coli  mgsA gene, the gene from the strain K12 MG1655 the coding sequence of which is included, on the complementary strand, between nucleotides 1026557 and 1027015 in the sequence NC_000913.3 accessible from the NCBI site can be mentioned. The protein sequence of 152 amino acids, coded by this gene is referenced, on the NCBI site, sequence NP_415483.2 and corresponds to the sequence SEQ ID NO: 20 in the appended sequence listing. 
     All the previously contemplated alternatives to decrease an enzymatic activity are usable to decrease the activity of the enzyme (E viii ) which irreversibly forms methylglyoxal from DHAP. 
     Typically, in the recombinant microorganism according to the invention, the decrease in the irreversible formation activity of methylglyoxal from DHAP catalysed by an enzyme consisting in a methylglyoxal synthase is obtained by decreasing the expression of the gene coding for this enzyme and/or by inactivating the gene coding for this enzyme. 
     More particularly, in the recombinant microorganism subject matter of the present invention, the expression of the  E. coli  mgsA gene or a homologue thereof is decreased with respect to the non-modified microorganism. In a particular implementation, the  E. coli  mgsA gene or homologue thereof is deleted in the recombinant microorganism subject matter of the present invention. 
     The recombinant microorganism according to the invention has a conversion activity from fructose-6-phosphate into fructose-1,6-biphosphate, decreased in comparison with the same, non-modified microorganism. In other words, the intracellular activity of the enzyme which converts fructose-6-phosphate into fructose-1,6-biphosphate, in the recombinant microorganism is decreased in comparison with the same, non-modified microorganism. 
     The enzyme (E ix ) which converts fructose-6-phosphate into fructose-1,6-biphosphate is in the form of a phosphofructokinase (EC 2.7.1.11). 
     Thus, the recombinant microorganism according to the invention exhibits a conversion activity from fructose-6-phosphate into fructose-1,6-biphosphate catalysed by an enzyme consisting in a phosphofructokinase which converts fructose-6-phosphate into fructose-1,6-biphosphate, decreased in comparison with the same, non-modified microorganism. 
     Advantageously, the enzyme (E ix ) is coded by the  E. coli  pfKA gene or a homologue of such a gene. Based on the previously provided “homologue” definition, a homologue of the  E. coli  pfKA gene encodes a protein capable of converting fructose-6-phosphate into fructose-1,6-biphosphate. By way of particular example of  E. coli  pfKA gene, the gene from the strain K12 MG1655 the coding sequence of which is included, on the complementary strand, between nucleotides 4107552 and 4108514 in the sequence NC_000913.3 accessible from the NCBI site can be mentioned. The protein sequence of 320 amino acids, coded by this gene is referenced, on the NCBI site, sequence NP_418351.1 and corresponds to the sequence SEQ ID NO: 21 in the appended sequence listing. 
     All the previously contemplated alternatives to decrease an enzymatic activity are usable to decrease the activity of the enzyme (E ix ) which converts fructose-6-phosphate into fructose-1,6-biphosphate. 
     Typically, in the recombinant microorganism according to the invention, the decrease in the conversion activity from fructose-6-phosphate into fructose-1,6-biphosphate catalysed by an enzyme consisting in a phosphofructokinase is obtained by decreasing the expression of the gene coding for this enzyme and/or by inactivating the gene coding for this enzyme. 
     More particularly, in the recombinant microorganism subject matter of the present invention, the expression of the  E. coli  pfKA gene or a homologue thereof is decreased with respect to the non-modified microorganism. In a particular implementation, the  E. coli  pfKA gene or homologue thereof is deleted in the recombinant microorganism subject matter of the present invention, in order to avoid a futile cycle with fructose-1,6-biphosphate [27]. 
     The recombinant microorganism according to the invention exhibits a production activity of D-ribose-1-phosphate from dihydroxyacetone phosphate and glycolaldehyde, decreased in comparison with the same, non-modified microorganism. In other words, the intracellular activity of the enzyme which produces D-ribose-1-phosphate from dihydroxyacetone phosphate and glycolaldehyde, in the recombinant microorganism is decreased in comparison with the same, non-modified microorganism. 
     The enzyme (Ex) which produces D-ribose-1-phosphate from dihydroxyacetone phosphate and glycolaldehyde is in the form of a L-fuculose-phosphate aldolase (EC 4.1.2.17) [28]. 
     Thus, the recombinant microorganism according to the invention exhibits a production activity of D-ribose-1-phosphate from dihydroxyacetone phosphate and glycolaldehyde catalysed by an enzyme consisting in a L-fuculose-phosphate aldolase which produces D-ribose-1-phosphate from dihydroxyacetone phosphate and glycolaldehyde, decreased in comparison with the same, non-modified microorganism. 
     Advantageously, the enzyme (Ex) is coded by the  E. coli  fucA gene or a homologue of such a gene. Based on the previously provided “homologue” definition, a homologue of the  E. coli  fucA gene encodes a protein capable of producing D-ribose-1-phosphate from dihydroxyacetone phosphate and glycolaldehyde. By way of particular example of the  E. coli  fucA gene, the gene from the strain K12 MG1655 the coding sequence of which is included, on the complementary strand, between nucleotides 2933041 and 2933688 in the sequence NC_000913.3 accessible from the NCBI site can be mentioned. The protein sequence of 215 amino acids, coded by this gene is referenced, on the NCBI site, sequence NP_417280.1 and corresponds to the sequence SEQ ID NO: 22 in the appended sequence listing. 
     All the previously contemplated alternatives to decrease an enzymatic activity are usable to decrease the activity of the enzyme (Ex) which catalyses the reversible interconversion from D-ribose-1-phosphate into dihydroxyacetone phosphate and glycolaldehyde. 
     Typically, in the recombinant microorganism according to the invention, the decrease in the production activity of D-ribose-1-phosphate from dihydroxyacetone phosphate and glycolaldehyde catalysed by an enzyme consisting in a L-fuculose-phosphate aldolase is obtained by decreasing the expression of the gene coding for this enzyme and/or by inactivating the gene coding for this enzyme. 
     More particularly, in the recombinant microorganism subject matter of the present invention, the expression of the  E. coli  fucA gene or a homologue thereof is decreased with respect to the non-modified microorganism. In a particular implementation, the  E. coli  fucA gene or homologue thereof is deleted in the recombinant microorganism subject matter of the present invention. 
     The recombinant microorganism according to the invention exhibits a production activity of 6-phospho-D-glucono-1,5-lactone from D-glucose-6-phosphate decreased or increased in comparison with the same, non-modified microorganism. 
     In a first embodiment, the recombinant microorganism according to the invention exhibits a production activity of 6-phospho-D-glucono-1,5-lactone from D-glucose-6-phosphate decreased in comparison with the same, non-modified microorganism. In other words, the intracellular activity of the enzyme (E xi ) which produces the 6-phospho-D-glucono-1,5-lactone from D-glucose-6-phosphate, in the recombinant microorganism is decreased in comparison with the same, non-modified microorganism. 
     The enzyme (E xi ) which produces 6-phospho-D-glucono-1,5-lactone from D-glucose-6-phosphate is in the form of a NADP + -dependent glucose-6-phosphate dehydrogenase (EC 1.1.1.49) [29]. 
     Thus, the recombinant microorganism according to the invention exhibits a production activity of 6-phospho-D-glucono-1,5-lactone from D-glucose-6-phosphate catalysed by an enzyme consisting in cofactor-dependent glucose-6-phosphate dehydrogenase which produces 6-phospho-D-glucono-1,5-lactone from D-glucose-6-phosphate, decreased in comparison with the same, non-modified microorganism. 
     Advantageously, the enzyme (E xi ) is coded by the  E. coli  zwf gene or a homologue thereof. Based on the previously provided “homologue” definition, a homologue of the  E. coli  zwf gene encodes a protein capable of producing 6-phospho-D-glucono-1,5-lactone from D-glucose-6-phosphate in the presence of a cofactor as, for example, NADP + . By way of particular example of the  E. coli  zwf gene, the gene from the strain K12 MG1655 the coding sequence of which is included, on the complementary strand, between nucleotides 1934839 and 1936314 in the sequence NC_000913.3 accessible from the NCBI site can be mentioned. The protein sequence of 491 amino acids, coded by this gene is referenced, on the NCBI site, sequence NP_416366 corresponding to the sequence SEQ ID NO: 23 in the appended sequence listing. 
     All the previously contemplated alternatives to decrease an enzymatic activity are usable to decrease the activity of the enzyme (E xi ) which produces 6-phospho-D-glucono-1,5-lactone from D-glucose-6-phosphate. 
     Typically, in the recombinant microorganism according to the invention, the decrease in the production activity of 6-phospho-D-glucono-1,5-lactone from D-glucose-6-phosphate catalysed by an enzyme consisting in a cofactor-dependent glucose-6-phosphate dehydrogenase is obtained by decreasing the expression of the gene coding for this enzyme and/or by inactivating the gene coding for this enzyme. 
     More particularly, in the recombinant microorganism subject matter of the present invention, the expression of the  E. coli  zwf gene or a homologue thereof is decreased with respect to the non-modified microorganism. In a particular implementation, the  E. coli  zwf gene or homologue thereof is deleted in the recombinant microorganism subject matter of the present invention. 
     In a second embodiment, the recombinant microorganism according to the invention exhibits a production activity of 6-phospho-D-glucono-1,5-lactone from D-glucose-6-phosphate increased in comparison with the same, non-modified microorganism. In other words, the intracellular activity of the enzyme (E xi ) which produces 6-phospho-D-glucono-1,5-lactone from D-glucose-6-phosphate, in the recombinant microorganism is increased in comparison with the same, non-modified microorganism. 
     Thus, the recombinant microorganism according to the invention exhibits a production activity of 6-phospho-D-glucono-1,5-lactone from D-glucose-6-phosphate catalysed by an enzyme consisting in cofactor-dependent glucose-6-phosphate dehydrogenase which produces 6-phospho-D-glucono-1,5-lactone from D-glucose-6-phosphate, increased in comparison with the same, non-modified microorganism. 
     All the previously contemplated alternatives to increase an enzymatic activity are usable to increase the activity of the enzyme (E xi ) which produces 6-phospho-D-glucono-1,5-lactone from D-glucose-6-phosphate. 
     Typically, in the recombinant microorganism according to the invention, the increase in the production activity of 6-phospho-D-glucono-1,5-lactone from D-glucose-6-phosphate catalysed by an enzyme consisting in a cofactor-dependent glucose-6-phosphate dehydrogenase is obtained by increasing the number of copies of the gene coding for this enzyme and/or by increasing the expression of the gene coding for this enzyme. In particular, the recombinant micro-organism subject matter of the present invention exhibits an overexpression of a gene coding for a cofactor-dependent glucose-6-phosphate dehydrogenase which produces 6-phospho-D-glucono-1,5-lactone from D-glucose-6-phosphate. 
     More particularly, the recombinant microorganism subject matter of the present invention exhibits an overexpression of the  E. coli  zwf gene or a homologue thereof. 
     In this second embodiment, the recombinant microorganism subject matter of the invention exhibits the following characteristics: 
     xi) an oxidation activity from D-glucose-6-phosphate into 6-phospho D-glucono-1,5-lactone, increased in comparison with the same, non-modified microorganism; 
     xii) a formation activity of 2-dehydroxy-3-deoxy-D-gluconate-6-phosphate from D-gluconate-6-phosphate, decreased in comparison with the same, non-modified microorganism; and 
     xiii) a formation activity of glyceraldehyde-3-phosphate and pyruvate from 2 dehydroxy-3-deoxy-D-gluconate-6-phosphate, decreased in comparison with the same, non-modified microorganism. 
     In this second embodiment, the recombinant microorganism according to the invention exhibits a formation activity of 2-dehydroxy-3-deoxy-D-gluconate-6-phosphate from D-gluconate-6-phosphate decreased in comparison with the same, non-modified microorganism. In other words, the intracellular activity of the enzyme which forms 2-dehydroxy-3-deoxy-D-gluconate-6-phosphate from D-gluconate-6-phosphate, in the recombinant microorganism is decreased in comparison with the same, non-modified microorganism. 
     The enzyme (E xii ) which produces 2-dehydroxy-3-deoxy-D-gluconate-6-phosphate from D-gluconate-6-phosphate is in the form of a phosphogluconate dehydratase (EC 4.2.1.12) [30]. 
     Thus, the recombinant micro-organism according to the invention exhibits a formation activity of 2-dehydroxy-3-deoxy-D-gluconate-6-phosphate from D-gluconate-6-phosphate catalysed by an enzyme consisting in a phosphogluconate dehydratase which produces 2-dehydroxy-3-deoxy-D-gluconate-6-phosphate from D-gluconate-6-phosphate, decreased in comparison with the same, non-modified microorganism. 
     Advantageously, the enzyme (E xii ) is coded by the  E. coli  edd gene or a homologue thereof. Based on the previously provided “homologue” definition, a homologue of the  E. coli  edd gene encodes a protein capable of producing 2-dehydroxy-3-deoxy-D-gluconate-6-phosphate from D-gluconate-6-phosphate. By way of particular example of the  E. coli  edd gene, the gene from the strain K12 MG1655 the coding sequence of which is included, on the complementary strand, between nucleotides 1932794 and 1934604 in the sequence NC_000913.3 accessible from the NCBI site can be mentioned. The protein sequence of 603 amino acids, coded by this gene is referenced, on the NCBI site, sequence CAA45221 and corresponds to the sequence SEQ ID NO: 24 in the appended sequence listing. 
     Typically, in the recombinant microorganism according to the invention, the decrease in the oxidation activity from glyceraldehyde-3-phosphate into 1,3-bisphosphoglycerate catalysed by an enzyme consisting in a glyceraldehyde-3-phosphate dehydrogenase is obtained by decreasing the expression of the gene coding for this enzyme and/or by inactivating the gene coding for this enzyme. 
     More particularly, in the recombinant microorganism subject matter of the present invention, the expression of the  E. coli  edd gene or of a homologue thereof is decreased in comparison with the non-modified microorganism. In a particular embodiment, the  E. coli  edd gene or the homologue thereof is deleted in the recombinant microorganism subject matter of the present invention. 
     In this second embodiment, the recombinant microorganism according to the invention exhibits a production activity of D-glyceraldehyde-3-phosphate and pyruvate from 2-dehydroxy-3-deoxy-D-gluconate-6-phosphate decreased in comparison with the same, non-modified microorganism. In other words, the intracellular activity of the enzyme which produces D-glyceraldehyde-3-phosphate and pyruvate from 2-dehydroxy-3-deoxy-D-gluconate-6-phosphate, in the recombinant microorganism is decreased in comparison with the same, non-modified microorganism. 
     The enzyme (E xiii ) which produces D-glyceraldehyde-3-phosphate and pyruvate from 2-dehydroxy-3-deoxy-D-gluconate-6-phosphate is in the form of a 2-dehydroxy-3-deoxy-phosphogluconate aldolase [31]. 
     Thus, the recombinant microorganism according to the invention exhibits a production activity of D-glyceraldehyde-3-phosphate and pyruvate from 2-dehydroxy-3-deoxy-D-gluconate-6-phosphate catalysed by an enzyme consisting of a 2-dehydroxy-3-deoxy-phosphogluconate aldolase which produces D-glyceraldehyde-3-phosphate and pyruvate from 2-dehydroxy-3-deoxy-D-gluconate-6-phosphate, decreased in comparison with the same, non-modified microorganism. 
     Advantageously, the enzyme (E xiii ) is coded by the  E. coli  eda gene or a homologue thereof. Based on the previously provided “homologue” definition, a homologue of the  E. coli  edd gene encodes a protein capable of producing D-glyceraldehyde-3-phosphate and pyruvate from 2-dehydroxy-3-deoxy-D-gluconate-6-phosphate. By way of particular example of  E. coli  eda gene, the gene from the strain K12 MG1655 the coding sequence of which is included, on the complementary strand, between nucleotides 1932115 and 1932756 in the sequence NC_000913.3 accessible from the NCBI site can be mentioned. The protein sequence of 213 amino acids, coded by this gene is referenced, on the NCBI site, sequence NP_416364.1 and corresponds to the sequence SEQ ID NO: 25 in the appended sequence listing. 
     Typically, in the recombinant microorganism according to the invention, the decrease in the production activity of D-glyceraldehyde-3-phosphate and pyruvate from 2-dehydroxy-3-deoxy-D-gluconate-6-phosphate catalysed by an enzyme consisting of a 2-dehydroxy-3-deoxy-phosphogluconate aldolase is obtained by decreasing the expression of the gene coding for this enzyme and/or by inactivating the gene coding for this enzyme. 
     More particularly, in the recombinant microorganism subject matter of the present invention, the expression of the  E. coli  eda gene or of a homologue thereof is decreased in comparison with the non-modified microorganism. In a particular embodiment, the  E. coli  eda gene or the homologue thereof is deleted in the recombinant microorganism subject matter of the present invention. 
     The glycolic acid synthetic cyclic pathway from main lignocellulosic monosaccharides, with an optimum carbon preservation, in accordance with the present invention is represented in  FIG. 2 . 
     The present invention also relates to the use of a recombinant microorganism as previously defined for producing glycolic acid from a culture medium comprising, as a carbon source, at least one pentose and/or at least one hexose. 
     Thus, the present invention relates to a process for producing glycolic acid comprising the steps of: 
     a) culturing a recombinant microorganism as defined previously in a culture medium comprising, as a carbon source, at least one pentose and/or at least one hexose; and 
     b) recovering glycolic acid from the microorganism and/or in the culture medium. 
     The production process subject matter of the present invention implements steps and devices conventionally used in the biofermentation field. 
     Thus, in step (a) of the process according to the present invention, the microorganism can be cultured in a culture medium according to the usual techniques used to culture this microorganism type. The culture medium can be a commercial medium or an extemporaneously prepared medium. 
     Advantageously, the culture medium implemented in the process of the invention is in the form of a sterile liquid containing a carbon source, a nitrogen source, a phosphate source, trace elements and optionally a sulphur source. 
     Within the scope of the present invention, the carbon source comprises at least one pentose and/or at least one hexose. By way of example, this carbon source can comprise at least two different pentoses and at least one hexose, in particular at least xylose, arabinose and glucose and, in particular, D-xylose, L-arabinose and D-glucose. These pentoses and hexose are typically from a renewable carbon material such as plant biomass and, in particular lignocellulosic biomass. 
     The plant biomass is chosen from the group consisting of agricultural productions such as so-called “energy” dedicated productions such as miscanthus, switchgrass ( Panicum virgatum ) and very short rotation coppices as, for example, poplar or willow; agricultural production residues such as cereal straws, maize canes and sugar cane rods; forest productions; forest production residues such as wood processing residues. 
     One of the main elements of the plant biomass is lignocellulose, which corresponds to a main component of the plant cellular wall. Lignocellulose is comprised of 75% carbohydrates and its hydrolysis releases fermentescible sugars, mainly D-glucose, D-xylose and L-arabinose [32]. 
     In a first implementation of step (a) of the process according to the invention, the cultured recombinant microorganism is a recombinant microorganism as previously defined and in which the expression of the  E. coli  gapA gene or a homologue thereof is decreased but not inactivated with respect to the non-modified microorganism. In this first implementation, the carbon source implemented can only comprise one element chosen from D-glucose, D-xylose, L-arabinose and a mixture thereof. By “mixture”, it is intended a mixture of D-glucose and D-xylose, a mixture of D-glucose and L-arabinose, a mixture of D-xylose and L-arabinose and a mixture of D-glucose, D-xylose and L-arabinose. 
     In a second implementation of step (a) of the process according to the invention, the cultured recombinant microorganism is a recombinant microorganism as previously defined and in which the expression of the  E. coli  gapA gene or a homologue thereof is inactivated with respect to the non-modified microorganism. In this second implementation, the carbon source implemented comprises, in addition to D-xylose and/or L-arabinose and/or D-glucose, one or more C2, C3 or C4 compounds chosen from malate, pyruvate, succinate, acetate and a mixture thereof. In this second implementation, the process for producing glycolic acid is made in two phases, with a first biomass production phase, followed by a production phase which is triggered when the C2, C3 or C4 compounds come to be depleted promoting bioconversion of hexoses and pentoses into glycolic acid. 
     In the first and second implementations of the process according to the invention, the carbon source implemented can further comprise at least one other carbon element such as galactose, xylose, fructose, lactose, sucrose, maltose, molasses, starch and a starch hydrolysate. 
     Examples of adapted nitrogen sources include ammonia, the ammonium salts such as ammonium chloride, ammonium sulphate, ammonium acetate and ammonium phosphate, other nitrogen-containing compounds; a peptone as tryptone; a meat extract, a yeast extract, a maize liquor; a casein hydrolysate; a soya flour and a soya flour hydrolysate. 
     Those skilled in the art know different examples of sulphur sources, phosphate sources and trace elements usable during step (a) of the process according to the invention. They will be able to choose, without inventive effort, the best adapted sources and trace elements as a function of the cultured recombinant microorganism and the culture conditions. 
     The temperature during culturing of step (a) is typically between 15 and 45° C., and the pH during this culture is typically maintained at a value between 3.0 and 9.0. The pH can be adjusted by using, for example, an inorganic or organic acid, an alkaline solution, urea or a calcium carbonate. 
     It is to be noted that, depending on the cultured microorganism, those skilled in the art will be able to adapt the culture medium and culture conditions to the particular requirements thereof, optionally by means of routine tests. Hence, the recombinant microorganism is, during step (a), in conditions under which there is a glycolic acid production from the carbon source as previously defined. Typically, this culturing is made under aerobic conditions i.e. in the presence of oxygen. 
     Advantageously, the recombinant microorganisms are cultured as a suspension. A cell “suspension” is generally intended as including all the types of suspended or dispersed cellular cultures. The term “as a suspension” are also used to distinguish the cells which are not cultured in a liquid medium, such as the cells cultured by adhering to a Petri dish. On the other hand, the term “suspension” comprises both freely dispersed cells and agglomerated cells, regardless of whether the agglomeration can occur spontaneously or not. 
     Alternatively, the recombinant microorganisms can be cultured being attached to a solid phase such as microbeads, beads, capillaries, hollow fibres, these different elements being typically of a material compatible with the microorganism as, for example, dextran, gelatin, glass and cellulose. 
     Moreover, the culturing processes which can be used during step (a) of the process according to the present invention comprise, without being limited thereto, a discontinuous culture, a continuous culture or a “fed-batch” culture. 
     A “continuous (cellular) culture” is a cellular culture characterised both by a continuous supply of a nutrient liquid feed and a continuous liquid flow. Alternatively, a continuous culture can be a “perfusion culture”, in which case the liquid flow contains a culture medium which is substantially devoid of cells, or a cellular concentration substantially lower than that in the bioreactor. In a perfusion culture, the cells can be retained, for example, by filtration, centrifugation or settling. 
     A “fed-batch” culture is a discontinuous cell culture to which a substrate, in concentrated, solid or liquid form, is periodically or continuously added during analysis. As in a discontinuous culture, a “fed-batch” culture is initiated by inoculated cells to the medium, but, unlike a discontinuous culture, there is a subsequent nutrient inflow, as through a concentrated nutrient feed. Unlike a continuous culture, there is no systematic liquid removal from the culture or cells in a “fed-batch” culture. 
     Step (a) of the process according to the invention can be implemented in any container adapted to a cellular culture. By way of particular examples, an Erlenmeyer flask, a bioreactor or a biofermenter with different volumes can be mentioned. Additional information on these different containers can be found in [5]. 
     In the process according to the invention, steps (a) and (b) can occur one after the other or, otherwise, simultaneously. Advantageously, during step (b) of the process according to the invention, glycolic acid is recovered in the culture medium. 
     Further characteristics and advantages of the present invention will further appear to those skilled in the art upon reading examples given below by way of illustrating and no-limiting purposes, in reference to the appended figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  already mentioned shows the three key reactions for glycolic acid production, isolated from the  E. coli  central carbon metabolism. 
         FIG. 2  already mentioned shows the non-natural pathway for glycolic acid production from D-glucose, D-xylose and L-arabinose. 
         FIG. 3  is a schematic representation of the enzymatic test for checking arabinose-5P isomerase (KdsD), fructose-6P aldolase (FSA) and aldehyde dehydrogenase (aldA) activity. KdsD, FSA and aldA have been purified, triose phosphate isomerase (Tpi) and glycerol-3P dehydrogenase (G3PDH) have been ordered to Sigma. 
         FIG. 4  shows the system comprising 7 purified enzymes (AraA, AraB, AraD, RPE, KdsD, FSA, AldA) which catalyse the conversion of L-arabinose into glycolic acid ( FIG. 4A ). The glycolic acid production with this system in the presence of L-arabinose has been compared with that of a system not having AraA; the enzymatic reaction is based on the NADH assay at 340 nm ( FIG. 4B ). 
         FIG. 5  shows the system comprising 6 purified enzymes (XylA, XylB, RPE, KdsD, FSA, AldA) which are capable of converting D-xylose into glycolic acid ( FIG. 5A ). The glycolic acid production with this system in the presence of D-xylose has been compared with that with a system not having XylA; the enzymatic reaction is based on the NADH assay at 340 nm ( FIG. 5B ). 
         FIG. 6  shows the system comprising 7 purified enzymes (Hxk, Pgi, Tkt, RPE, KdsD, FSA, AldA) which are capable of converting D-glucose into glycolic acid ( FIG. 6A ). The glycolic acid production with this system in the presence of D-glucose has been compared with that with a system not having Hxk; the enzymatic assay is based on the NADH assay at 340 nm ( FIG. 6B ). 
         FIG. 7  shows the highlighting of the in vivo functionality of the non-natural pathway according to the invention in  E. coli  MG1655 ΔtktA ΔtktB ΔglcD, with in bold: the non-natural pathway according to the invention, in dotted lines, the deletions. Xu5P: xylulose-5-phosphate, Ru5P: ribulose-5-phosphate, R5P: ribose-5-phosphate, S7P: sedoheptulose-7-phosphate, F6P: fructose-6-phosphate, F16BP: Fructose-1,6-bisphosphate, G6P: glucose-6-phosphate, DHAP: dihydroxyacetone phosphate, Glyald: glycolaldehyde, E4P: erythrose-4-phosphate, G3P: glyceraldehyde-3-phosphate. 
         FIG. 8  shows the glycolic acid production of the  E. coli  strain MG1655 ΔtktA ΔtktB ΔglcD expressing the kdsD-fsa-aldA dependent non-natural pathway according to the invention from D-xylose or L-arabinose, at 37° C., 100h. 
         FIG. 9  shows the glycolic acid production of the strain of  E. coli  WC3G ΔgapA ΔglcD ΔarcA ΔmgsA ΔfucA Δpkf proD-galP expressing the kdsD-fsa-aldA dependent non-natural pathway according to the invention from glucose, xylose and arabinose, at 37° C., 50h. 
     
    
    
     DETAILED DISCLOSURE OF PARTICULAR EMBODIMENTS 
     I. Proof of In Vitro Feasibility. 
     I.1. Material and Methods. 
     A. Construction of Plasmids. 
     To construct a system in vitro, the inventors have cloned the ORFs (“Open Reading Frames”) of the xylose isomerase (XylA), xylulokinase (XylB), arabinose isomerase (AraA), ribulose kinase (AraB), L-ribulose-5-phosphate 4-epimerase (AraD), ribulose-5-phosphate-3-epimerase (RPE), D-arabinose-5-phosphate isomerase (KdsD), fructose-6-phosphate aldolase (FSA corresponding to FSAA such as previously defined) and aldehyde dehydrogenase (AdA) of  E. coli  K12 MG1655 with a polyhistidine tag for a facilitated purification. The oligonucleotides used for amplifying ORFS of the  E. coli  enzymes are listed in Table 3 below, the sequences SEQ ID NO: making reference to the appended sequence listing. The  E. coli  genomic DNA which served as on template is that of the strain K12 MG1655. 
     The plasmids for the expression of the C-terminal polyhistidine tagged proteins XylA, XylB, AraA, AraB, AraD, AraB, Rpe, KdsD, FSA, AdA have been constructed by HiFi Assembly® (NEB) with pET28a as a receiving vector. pET28a has been linearised beforehand with the restriction enzymes HindIII and BamHI (NEB) (Table 4). The  E. coli  strain NEB5®, derived from DH5 alpha, has been used for cloning and storing the different plasmids. 
     
       
         
           
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                 Gene 
                 Primer 
                 Primer sequence 
               
               
                   
               
             
            
               
                 xylA 
                 Sense 
                 ctggtgccgcgcggcagccatATGCAAGCCTATTTTGAC 
               
               
                   
                   
                 (SEQ ID NO: 26) 
               
               
                   
                 Antisense 
                 gtcgacggagctcgaattcgTTATTTGTCGAACAGATAATGG 
               
               
                   
                   
                 (SEQ ID NO: 27) 
               
               
                   
               
               
                 xylB 
                 Sense 
                 ctggtgccgcgcggcagccatATGTATATCGGGATAGATCTTG 
               
               
                   
                   
                 (SEQ ID NO: 28) 
               
               
                   
                 Antisense 
                 gtcgacggagctcgaattcgTTACGCCATTAATGGCAG 
               
               
                   
                   
                 (SEQ ID NO: 29) 
               
               
                   
               
               
                 araA 
                 Sense 
                 ctggtgccgcgcggcagccatATGACGATTTTTGATAATTATGAAG 
               
               
                   
                   
                 (SEQ ID NO: 30) 
               
               
                   
                 Antisense 
                 gtcgacggagctcgaattcgTTAGCGACGAAACCCGTAATAC 
               
               
                   
                   
                 (SEQ ID NO: 31) 
               
               
                   
               
               
                 araB 
                 Sense 
                 ctggtgccgcgcggcagccatATGGCGATTGCAATTGG 
               
               
                   
                   
                 (SEQ ID NO: 32) 
               
               
                   
                 Antisense 
                 gtcgacggagctcgaattcgTTATAGAGTCGCAACGGCC 
               
               
                   
                   
                 (SEQ ID NO: 33) 
               
               
                   
               
               
                 araD 
                 Sense 
                 ctggtgccgcgcggcagccatATGTTAGAAGATCTCAAACG 
               
               
                   
                   
                 (SEQ ID NO: 34) 
               
               
                   
                 Antisense 
                 gtcgacggagctcgaattcgTTACTGCCCGTAATATGC 
               
               
                   
                   
                 (SEQ ID NO: 35) 
               
               
                   
               
               
                 rpe 
                 Sense 
                 ctggtgccgcgcggcagccatATGAAACAGTATTTGATTGC 
               
               
                   
                   
                 (SEQ ID NO: 36) 
               
               
                   
                 Antisense 
                 gtcgacggagctcgaattcgTTATTCATGACTTACCTTTGC 
               
               
                   
                   
                 (SEQ ID NO: 37) 
               
               
                   
               
               
                 kdsD 
                 Sense 
                 gccgcgcggcagccatatATGTCGCACGTAGAGTTAC 
               
               
                   
                   
                 (SEQ ID NO: 38) 
               
               
                   
                 Antisense 
                 gtcgacggagctcgaattcgTTACACTACGCCTGCACG 
               
               
                   
                   
                 (SEQ ID NO: 39) 
               
               
                   
               
               
                 fsa 
                 Sense 
                 gccgcgcggcagccatatATGGAACTGTATCTGGATACTTC 
               
               
                   
                   
                 (SEQ ID NO: 40) 
               
               
                   
                 Antisense 
                 gtcgacggagctcgaattcgTTAAATCGACGTTCTGCC 
               
               
                   
                   
                 (SEQ ID NO: 41) 
               
               
                   
               
               
                 aldA 
                 Sense 
                 ctggtgccgcgcggcagccatATGTCAGTACCCGTTCAAC 
               
               
                   
                   
                 (SEQ ID NO: 42) 
               
               
                   
                 Antisense 
                 gtcgacggagctcgaattcgTTAAGACTGTAAATAAACCACC 
               
               
                   
                   
                 (SEQ ID NO: 43) 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 plasmids used for the proof of in vitro concept 
               
            
           
           
               
               
               
            
               
                 Name 
                 Description 
                 Reference 
               
               
                   
               
               
                 pET28 
                 Kan  R , ColE1 ori 
                 Novagen ® 
               
               
                 pVT-FSA 
                 fsa-bearing pET28 
                 This study 
               
               
                 pVT-KDSD 
                 kdsD-bearing pET28 
                 This study 
               
               
                 pVT-ALDA 
                 aldA-bearing pET28 
                 This study 
               
               
                 pVT-RPE 
                 rpe-bearing pET28 
                 This study 
               
               
                 pVT-XYLA 
                 xylA-bearing pET28 
                 This study 
               
               
                 pVT-XYLB 
                 xylB-bearing pET28 
                 This study 
               
               
                 pVT-ARAA 
                 araA-bearing pET28 
                 This study 
               
               
                 pVT-ARAB 
                 araB-bearing pET28 
                 This study 
               
               
                 pVT-ARAD 
                 araC-bearing pET28 
                 This study 
               
               
                   
               
            
           
         
       
     
     B. Construction of the Strains. 
       E. coli -competent cells BL21 (DE3) have been used for the expression of tagged proteins since these cells express RNA polymerase T7, and are thus compatible with the expression system T7 of the vector pET28a. The plasmids pET28a checked beforehand by sequencing have been transformed in the BL21(DE3) according to the NEB protocol. The strains obtained are stored in 50% glycerol at −80° C. (Table 5). 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                   Escherichia coli  strains used for the proof of in vitro concept 
               
            
           
           
               
               
               
            
               
                 Strain 
                 Genotype 
                 Reference 
               
               
                   
               
               
                 MG1655 
                 F-λ-ilvG-rfb-50 rph-1 
                 ATCC 47076 
               
               
                 BL21 (DE3) 
                 fhuA2 [Ion] ompT  
                 Invitrogen ® 
               
               
                   
                 gal (λ DE3) [dcm] ΔhsdS 
                   
               
               
                 prodFSA 
                 pVT-FSA-containing BL21 
                 This study 
               
               
                 prodKDSD 
                 pVT-KDSD-containing BL21 
                 This study 
               
               
                 prodALDA 
                 pVT-ALDA-containing BL21 
                 This study 
               
               
                 prodRPE 
                 pVT-RPE-containing BL21 
                 This study 
               
               
                 prodXYLA 
                 pVT-XYLA-containing BL21 
                 This study 
               
               
                 prodXYLB 
                 pVT-XYLB-containing BL21 
                 This study 
               
               
                 prodARA 
                 pVT-ARAA-containing BL21 
                 This study 
               
               
                 prodARAB 
                 pVT-ARAB-containing BL21 
                 This study 
               
               
                 prodARAC 
                 pVT-ARAC-containing BL21 
                 This study 
               
               
                   
               
            
           
         
       
     
     C. Expression and Purification of Polyhistidine-Tagged Proteins. 
     All the proteins expressed are soluble and have been produced from the expression vector pET28a transformed in an  E. coli  strain BL21 (DE3). A pre-culture in a LB (for “Luria-Bertani”) medium added with the antibiotic kanamycin is made overnight at 37° C. The pre-culture is used to seed a fresh culture of 200 mL of LB-Kanamycin at an optical density at 600 nm (OD 600 ) of 0.1 (37° C., 200 rpm). When the OD 600  is between 0.6 and 0.8, the expression of the protein of interest is induced by adding finally 1 mM IPTG. The proteins are expressed overnight at 16° C. The cells are collected as 50 ml fractions and centrifuged at 4 800 rpm for 15 min at 4° C. The cell pellets are preserved at −20° C. 
     The purification of proteins is made from the cell pellets obtained during the production step. All the steps are made while cold to avoid degradation of proteins by proteases. The cell pellets are re-suspended in 1.5 mL washing buffer (50 mM HEPES, pH 7.5; 0.3 M NaCl) and then sonicated on ice. A centrifugation step at 13 000 rpm, for 15 min at 4° C. enables the cell debris to be separated from the cytoplasmic liquid. The clarified lysates are deposited onto 600 ul cobalt resin (Clontech) previously balanced with the washing buffer. After 20 min at room temperature in contact with the resin, the tubes are centrifuged (700 rcf, 3 min, 4° C.). The supernatant is removed, the resin is contacted with 3 mL washing buffer for 10 min in order to remove non-specific interactions. After centrifugation (700 rcf, 3 min, 4° C.) and removing the supernatant, 3 mL of a solution of 15 mM imidazole are contacted with the resin for 5 min. The supernatant is separated from the resin by centrifugation, replaced by 500 μl of 200 mM imidazole. Imidazole causes elution of polyhistidine-tag bearing proteins. To promote protein stability at their optimum pH, the buffer has been modified. 
     The method used to measure the concentration of proteins in solution is based on the Bradford method. The Protein assay reagent sold by BioRad is diluted to %, the reaction mixture comprises 160 reagent and 40 diluted eluate (to 1/10 th  and to 1/20 th ). A standard range is made with BSA from 12.5 to 100 μg/ml. 
     D. Enzymatic Tests. 
     All the enzymes have been tested in 100 mM Tris, pH 7.5, 10 mM MgCl 2 , at 37° C. The co-factors and activators have been added if necessary. 
     The enzymes  Saccaromyces cerevisiae  hexokinase (Hxk),  Escherichia coli  transketolase (Tkt),  Escherichia coli  pyruvate kinase (PK) and  Escherichia coli  lactate dehydrogenase (LDH), triose-phosphate isomerase (Tpi) and glycerol-3-phosphate dehydrogenase (G3PDH) have been ordered to Sigma. The  Escherichia coli  phosphoglucose isomerase (Pgi) is from Megazyme. 
     All the enzymatic substrates have been bought to Sigma-Aldrich. The enzymatic tests are coupled to a redox reaction with as a co-factor, NADH which absorbs ultraviolet with a 340 nm peack with a coefficient of molar extinction of 6 220 M −1 ·cm −1 . The Bio Tek Epoch 2 spectrophotometer has been used for UV reaction monitoring. 
     Measurement of the aldehyde dehydrogenase (AldA) activity: this activity has been measured in the presence of 5 mM glycoaldehyde, 3 mM NAD +  and 3 mM ATP. For one molecule of glycoaldehyde oxidised into glycolate, one NAD +  molecule is reduced into NADH. 
     Measurement of the fructose-6P aldolase (FSA) activity: the enzyme FSA cleaves D-arabinose-5-phosphate into glycolaldehyde and glyceraldehyde-3-phosphate (GAP). The glycolaldehyde rate of appearance has been measured in the presence of AldA with 2 mM NAD + . To monitor GAP appearance, TPI and G3PDH have been added, in the presence of 0.4 mM NADH. 
     Measurement of the D-arabinose-5-phosphate isomerase (KdsD) activity: D-arabinose-5-phosphate isomerase catalyses the interconversion of D-ribulose-5-phosphate into D-arabinose-5-phosphate. The KdsD activity on D-ribulose-5-phosphate has been determined by adding FSA, aldA in excess in the presence of 3 mM NAD + . 
     In vitro conversion of D-xylose into glycolic acid: the conversion of D-xylose into glycolic acid requires XylA, XylB, Rpe, KdsD, FSA and aldA ( FIG. 5A ). The conversion of D-xylose into glycolic acid is demonstrated in a reaction mixture containing 100 mM Tris HCl pH 7.5, 10 mM MgCl 2 , 0.5 mM MnCl 2 , 3 mM ATP, 2 mM NAD + , 1-10 μg of each enzyme and 5 mM D-xylose. 
     In vitro conversion of L-arabinose into glycolic acid: the conversion of L-arabinose into glycolic acid requires AraA, AraB, AraD, Rpe, KdsD, FSA and aldA ( FIG. 4A ). The conversion of L-arabinose into glycolic acid is demonstrated in 100 mM Tris HCl, pH7.5, 10 mM MgCl 2 , 0.5 mM MnCl 2 , 3 mM ATP, 2 mM NAD + , 1-10 μg of each enzyme and 5 mM L-arabinose. 
     In vitro conversion of D-glucose into glycolic acid: The conversion of D-glucose into glycolic acid requires the enzymes Hxk, Pgi, Tkt, Rpe, KdsD, FSA and aldA ( FIG. 6A ). The proof of concept is made in 100 mM Tris HCl, pH 7.5, 10 mM MgCl 2 , 0.5 mM MnCl 2 , 1 mM TPP (“Thiamine Pyrophosphate”), 3 mM ATP, 2 mM NAD, 5 mM Glyceraldehyde-3P, 1-10 μg of each enzyme and 5 mM D-glucose. 
     I.2. Results. 
     A. In vitro conversion of D-ribulose-5P into glycolic acid. 
     Ribulose-5P is a metabolite common to the catabolism of arabinose, xylose and glucose in  Escherichia coli . The non-natural conversion pathway of D-ribulose-5P into glycolic acid being the object of the present invention consists of the enzymes KdsD (D-arabinose 5P isomerase), FSA (Fructose-6P aldolase) and AldA (glycolaldehyde dehydrogenase). The in vitro proof of concept consisted in reconstructing this pathway with purified enzymes in an adapted buffer. First, aldA has been characterised on glycolaldehyde, and then the FSA activity on D-arabinose-5P has been checked by coupling with aldA in order to check glycolaldehyde formation and by coupling with triose phosphate isomerase and glycerol-3P dehydrogenase to highlight glyceraldehyde-3P synthesis ( FIG. 3 ). The FSA activity measurements obtained by these 2 methods are identical. Finally, the KdsD activity on D-ribulose-5P has been measured by coupling with the excess enzymes FSA and aldA. This last enzymatic test enabled KdsD functionality to be checked and the entire glycolic acid synthesis pathway to be reconstituted. NADH is produced in an equimolar amount with glycolic acid. In order to check that NADH is really a glycolic acid production witness, the glycolic acid production has been measured by HPLC. 
     The catalytic constants of these 3 enzymes are shown in Table 6. Polyhistidine-tagged proteins KdsD, FSA and aldA are active. The reaction mixture containing KdsD, FSA and AdA in the presence of 5 mM of D-ribulose-5P has been analysed by HPLC, 1.5 mM of glycolic acid has been quantified. The glycolic acid production measured corresponds to the initial NAD +  amount in the reaction mixture, indicating that the reaction has been completed. 
     
       
         
           
               
             
               
                 TABLE 6 
               
             
            
               
                   
               
               
                 catalytic constants of polyhistidine-tagged arabinose-5P 
               
               
                 isomerase (KdsD), fructose-6P aldolase  
               
               
                 (FSA), aldehyde dehydrogenase (AldA) 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                   
                   
                 Vmax/Km  
               
               
                   
                   
                   
                 Vmax  
                 (.10 −6 ) 
               
               
                 Enzyme 
                 Substrate 
                 Km (mM) 
                 (U mg −1 ) 
                 (min −1  mg −1  ) 
               
               
                   
               
               
                 KdsD-His 
                 D-ribulose-5P 
                  1.34 ± 0.12 
                 1.14 ± 0.17 
                 850 
               
               
                 FSA-His 
                 D-arabinose-5P 
                 0.652 ± 0.15 
                 0.26 ± 0.09 
                 398 
               
               
                 aldA-His 
                 glycolaldehyde 
                  0.21 ± 0.05 
                 1.16 ± 0.31 
                 5 552 
               
               
                   
               
            
           
         
       
     
     B. Conversion of L-Arabinose or D-Xylose or D-Glucose into Glycolic Acid. 
     The enzymes catalyzing the conversion of L-arabinose, D-xylose and D-glucose into glycolic acid have been purified (AraA, AraB, AraD, XyA, XyB, Rpe, KdsD, Fsa, AldA) or ordered (Tkt, Glk, Pgi) in order to proof the concept of the glycolic acid production from these 3 substrates in vitro. 
     i. Conversion of L-Arabinose into Glycolic Acid. 
     The conversion of L-arabinose into glycolic acid and glyceraldehyde 3P is catalysed by AraA, araB, araD, Rpe, KdsD, FSA and aldA ( FIG. 4A ). The NADH synthesis observed during the enzymatic test in the presence of L-arabinose is the indirect witness of glycolic acid production (Table 7,  FIG. 4B ). 
     The conversion of L-arabinose into glycolic acid is demonstrated and this metabolic pathway is thus thermodynamically possible. 
     
       
         
           
               
             
               
                 TABLE 7 
               
             
            
               
                   
               
               
                 enzymatic activity measured for the conversion of L-arabinose 
               
               
                 into glycolic acid at 340 nm, 37° C.. An enzymatic  
               
               
                 activity unit (U) is defined as the conversion 
               
               
                 of one micromole of substrate per minute.  
               
               
                 The negative control is devoid of arabinose 
               
               
                 isomerase activity catalysed by AraA. 
               
            
           
           
               
               
               
            
               
                 Substrate (5 mM) 
                 Enzymes 
                 Activity (U) 
               
               
                   
               
            
           
           
               
               
               
            
               
                 L-arabinose 
                 AraA, AraB, AraD, Rpe, KdsD, FSA, 
                 11.59 
               
               
                   
                 AldA 
                   
               
               
                   
                 AraB, AraD, Rpe, KdsD, FSA, AldA 
                 0.00 
               
               
                   
               
            
           
         
       
     
     ii. Conversion of D-Xylose into Glycolic Acid. 
     The conversion of D-xylose into glycolic acid and glyceraldehyde 3P is catalysed by XylA, XylB, Rpe, KdsD, FSA and AldA ( FIG. 5A ). When these enzymes are contacted with D-xylose, D-xylose is converted into glycolic acid in vitro as evidenced by NADH production ( FIG. 5B , Table 8). 
     
       
         
           
               
             
               
                 TABLE 8 
               
             
            
               
                   
               
               
                 enzymatic activity measured for the conversion of D-xylose into 
               
               
                 glycolic acid at 340 nm, 37° C.. An enzymatic  
               
               
                 activity unit (U) is defined as the conversion of 
               
               
                 a micromole of substrate per minute. The negative  
               
               
                 control is devoid of xylulose isomerase 
               
               
                 activity catalysed by XylA. 
               
            
           
           
               
               
               
            
               
                 Substrate (5 mM) 
                 Enzyme(s) 
                 Activity (U) 
               
               
                   
               
            
           
           
               
               
               
            
               
                 D-xylose 
                 XylA, XylB, Rpe, KdsD, FSA, AldA 
                 15.07 
               
               
                   
                 XylB, Rpe, KdsD, FSA, AldA 
                 0.00 
               
               
                   
               
            
           
         
       
     
     The conversion pathway of D-xylose into glycolate is functional and thermodynamically possible. 
     iii. Conversion of D-Glucose into Glycolic Acid. 
     The conversion of D-glucose into glycolic acid has been demonstrated by contacting D-glucose and DL-glyceraldehyde-3P (GAP) with Hxk, Pgi, Tkt, Rpe, KdsD, FSA and AdA ( FIG. 6A ). The negative control shows a significant activity ( FIG. 6B , Table 9). GAP is a relatively instable molecule, it is thereby dephosphorylated spontaneously into glyceraldehyde. Baldoma et al. (1987) have demonstrated that  Escherichia coli  AldA was active on glycolaldehyde, lactaldehyde, methylglyoxal and L-glyceraldehyde [13]. NADH synthesis in the negative control is therefore not correlated with synthesis of glycolate but of glycerate from glyceraldehyde oxidation. This secondary activity is not relevant in vivo, especially as L-glyceraldehyde is not formed in  Escherichia coli . 
     
       
         
           
               
             
               
                 TABLE 9 
               
             
            
               
                   
               
               
                 enzymatic activity measured for the conversion of D-glucose into 
               
               
                 glycolic acid at 340 nm, 37° C.. An enzymatic  
               
               
                 activity unit is defined as the conversion of 
               
               
                 1 micromole of substrate per minute. The negative control  
               
               
                 is devoid of hexokinase (Hxk) activity. 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 Activity 
                 Corrected 
               
               
                 Substrates (5 mM) 
                 Enzymes 
                 (U) 
                 activity (U) 
               
               
                   
               
               
                 D-glucose, GAP 
                 Hxk, Pgi, Tkt, Rpe, 
                 23.5 
                 6.2 
               
               
                   
                 KdsD, Fsa, AldA 
                   
                   
               
               
                   
                 Pgi, Tkt, Rpe, KdsD, Fsa, AldA 
                 17.3 
               
               
                   
               
            
           
         
       
     
     From these results, the conversion pathway of D-glucose into glycolic acid is functional in vitro and is thus thermodynamically favourable. 
     The carbohydrates L-arabinose, D-xylose and D-glucose are naturally assimilated and converted into D-ribulose-5P in  Escherichia coli , the non-natural conversion of D-ribulose-5P into glycolic acid allowed by the overexpression of KdsD, FSA and aldA has been demonstrated. The implementation of the non-natural pathway KdsD-FSA-aldA is thermodynamically favourable, the complete assimilation and conversion pathways of the reconstructed pentoses and hexose enabled glycolic acid to be synthetised in vitro. 
     However, the in vitro demonstration is by definition isolated from the  E. coli  natural metabolism, these results do not take into account reactions involving intermediates, transmembrane transport efficiency of the substrates and glycolic acid excretion, co-factor availability, . . . . A complementary in vivo proof of concept has been made to enhance these preliminary results. 
     II. In Vivo Proof of Feasibility. 
     II.1. Material and Methods. 
     A. Construction of Plasmids. 
     i. Choice of the Vectors 
     The series of vectors pZ (Expressys) has the advantage of being modulable: it is easy to change the replication origin, resistance marker and vector promoter by restriction/ligation. 
     Vectors pZA23, pZA33, pZE23 and pZS23 have the promoter PA1/ac0-1 which is a promoter derived from the lactose operon promoter comprising the operator o. PA1lac0-1 is under the control of the repressor lacI: in its active form, the repressor lacI is linked to the operator o and inhibits the transcription whereas, when complexed with IPTG, it changes conformation and is no longer capable to be bound to the site o, whereby the transcription becomes possible. The promoter PA1lac0-1 is said to be IPTG-inducible. Even if  E. coli  naturally has a lacI gene copy in its genome upstream of the operon lac, most of the IPTG-inducible bacterial expression vectors bear the lac gene in order to ensure full inhibition of the transcription of genes which are under its dependence. The vectors pZ have the feature to have a lightened structure of the lacI gene which provides them with a small size (2 358 to 3 764 bp). 
     Modifications have been provided to the vector pZA33, the promoter PA1/ac0-1 has been replaced by the constitutive promoter proD and by the inducible promoter Ptac, generating the vectors pZA36 and pZA38 respectively. The promoter PA1/ac0-1 of pZS23 has been modified by the promoter proD generating the vectors pZS27. 
     ii. Cloning Method: HiFi Assembly 
     The HiFi assembly (NEB) method has been retained to construct the vectors used hereinafter. This method enables several fragments to be assembled. It has been validated for fragments with different sizes with variable overlapping regions (15-80 bp). In a single step, the fragments can be assembled, it is a method commonly used for its simplicity and flexibility. 
     The commercial mixture provided by New England Biolabs contains different enzymes: (a) an exonuclease which creates 3′ single strand ends, which facilitates assembly of the fragments which share a sequence complementarity; (b) a polymerase which fills the empty spaces after the fragments have been assembled; and (c) a ligase which links fragments together. 
     The genes of the glycolic acid production synthetic pathway according to the invention kdsD, fsa and aldA have been amplified by PCR from the genome DNA extracted from  E. coli  K12 MG1655 and inserted by HiFi Assembly® into the vectors pZ, linearised beforehand by PCR with primers hybridising on either side of the MCS. All the plasmids have been checked by sequencing before use. 
     iii. Expression Vectors for the Overexpression of kdsD, Fsa, aldA 
     The vectors used for the overexpression of kdsD,fsa and aldA are shown in Table 10 hereinafter. 
     
       
         
           
               
               
               
             
               
                 TABLE 10 
               
               
                   
               
               
                 Name 
                 Description 
                 Source 
               
               
                   
               
             
            
               
                 pZA23 
                 Kan  R , ori p15A, PA1lac0-1 
                 Expressys 
               
               
                 pZA33 
                 Chm R , ori p15A, PA1lac0-1 
                 Expressys 
               
               
                 pZS23 
                 Kan  R , ori pSC101, PA1lac0-1 
                 Expressys 
               
               
                 pZA36 
                 Chm  R , ori p15A, Ptac 
                 This study 
               
               
                 pZA37 
                 Chm  R , ori p15A, proC 
                 This study 
               
               
                 pZS28 
                 Kan  R , ori pSC101, proD 
                 This study 
               
               
                 pEXT20 
                 AmpR, ori ColE1, Ptac 
                 [33] 
               
               
                 pET28a 
                 Kan 11, ori ColE1, Ptac 
                 Novagen 
               
               
                 pKF3 
                 pZA36 kdsD fsa 
                 This study 
               
               
                 pA4 
                 pZS23 aldA 
                 This study 
               
               
                 pKF5 
                 pZA37 kdsD fsa 
                 This study 
               
               
                 pA8 
                 pZS28 aldA 
                 This study 
               
               
                   
               
            
           
         
       
     
     B. Construction of the Strains. 
     i. Deletions of the Genes 
     The deletions have been made by transduction, using the phage P1vir. The preparation of the lysates P1vir and the transduction procedures have been made as described previously with little modifications [34]. 
     Thus, the strains KEIO bearing a single deletion and a kanamycin antibiotic-resistance cassette (donor strain) have been infected with P1vir and high-titer lysates A have been obtained [35]. The donor strains (KEIO) have been cultured overnight in LB at 37° C. The day after, 5 ml of LB containing 0.2% glucose and 5 mM CaCl 2 ) have been inoculated with 200 of the donor strain and cultured for 30 min at 37° C. Then, 100 of P1vir lysate ( ˜ 5×10 8  phages/ml) have been added to each donor culture and incubated again at 37° C. for 2 to 3 hours until the culture was clear and the cells were completely lysed. The lysates have been filtered by using 25 mm sterile syringe filters with a 0.2 m support membrane (Pall) and preserved at 4° C. 
     The strain to be deleted (receiving strain) has been infected with P1vir containing a donor gene deletion cassette having a kanamycin resistance. For this, the receiving strain has been cultured in 5 ml LB medium at 37° C. The cells have been centrifuged at 1 500 g for 10 min and re-suspended in 1.5 ml of 10 mM MgSO 4  and 5 mM CaCl 2 . Lysate from the donor strain (0.1 ml) is added to the cellular suspension which is incubated for 30 min. Then, 0.1 ml of 1M sodium citrate is added to the cell and P1vir mixture. Then, 1 mL LB is added to the homogenised suspension before an incubation of 1h at 37° C., 200 rpm. 
     The cellular suspensions are spread on a solid LB medium with the appropriate antibiotic then the colonies are screened by PCR to highlight successful transduction events. 
     To remove the antibiotic cassette, the cells have been transformed with a plasmid pCP20 bearing the FLP recombinase. Each step has been checked by PCR. When the deleted strain is sensitive to kanamycin after removing the cassette, it can again be used as a receiving strain in order to add a new deletion from a new phage lysate. 
     ii. Preparation of Competent Bacteria and Transformation 
     The competent non-commercial strains are prepared according to the protocol of Chung et al, 1989, which is slightly modified [36]. A pre-culture is made in LB overnight to inoculate the day after a fresh LB culture at a OD 600  of 0.1. When the OD 600  reaches 0.3-0.5 (the bacteria can be made competent up to a OD 600  of 1), an amount of cellular culture equivalent to a OD 600  unit is sampled and centrifuged (8 000 rpm, 2 min). The supernatant is removed whereas the pellet is up taken in 300 TSS buffer (2.5% (wt/vol)  PEG 3350, 1 M MgCl 2 , 5% (vol/vol)  DMSO). The mixture is incubated for 10 min on ice. The plasmid can then be added to the competent cells. After 30 further minutes of ice incubation, a heat shock is made at 42° C. for 90 seconds. The cells transformed are put on ice for 10 min. 400 LB are added and the culture is incubated at 200 rpm for 1 h, at an adapted temperature (the temperature can not exceed 30° C. in the case of a transformation with a plasmid the replication origin of which is thermosensitive). The bacterial culture is centrifuged at 8 000 rpm for 2.5 min. 600 of supernatant are removed, the remaining volume is inoculated on a solid LB dish with the appropriate antibiotic. 
     iii. Strains for Glycolic Acid Production 
     The  E. coli  strains used in this study are listed in Table 11 hereinafter. 
     
       
         
           
               
               
               
             
               
                 TABLE 11 
               
               
                   
               
               
                 Strain 
                 Genotype 
                 Reference 
               
               
                   
               
             
            
               
                 MG1655 
                 F-λ-ilvG- rfb-50 rph-1 
                 ATCC 407076 
               
               
                 NEB5 
                 fhuA2 Δ(argF-lacZ)U169 phoA  
                 NEB 
               
               
                   
                 glnV44 ∅80 Δ(lacZ)M15 
                   
               
               
                   
                 gyrA96 recA1 relAl endA1 thi-1 hsdR17 
                   
               
               
                 BL21 (DE3) 
                 huA2 [Ion] ompT gal (λ DE3)  
                 NEB 
               
               
                   
                 [dcm] ΔhsdS) 
                   
               
               
                 Screen00 
                 MG1655 ΔtktA ΔtktB ΔglcD 
                 This study 
               
               
                 Screen09 
                 Screen00 containing pZA36  
                 This study 
               
               
                   
                 kdsD fsa (pKF3) and pZS23 
                   
               
               
                   
                 aldA (pA4) 
                   
               
               
                 Screen23 
                 Screen00 containing pZA37 kdsD  
                 This study 
               
               
                   
                 fsa (pKF5) and pZA28 
                   
               
               
                   
                 aldA (pA8) 
                   
               
               
                 WC3G 
                 W3CG F-, LAM-, gapA 10::Tn10,  
                 [37] 
               
               
                 gapA- 
                 IN(rrnD-rrnE), rph-1 
                   
               
               
                 BW25113 
                 F-, Δ(araD-araB)567,  
                 [26] 
               
               
                   
                 ΔlacZ4787(::rrnB-3), λ - , yrph-1, Δ(rhaD- 
                   
               
               
                   
                 rhaB)568, hsdR514 
                   
               
               
                 JW2771 
                 BW25113 ΔfucA 
                 [26] 
               
               
                 JW4364 
                 BW25113 ΔarcA 
                 [26] 
               
               
                 JW5129 
                 BW25113 ΔmgsA 
                 [26] 
               
               
                 JW2946 
                 BW25113 ΔgIcD 
                 [26] 
               
               
                 JW2771 
                 BW25113 ΔfucA 
                 [26] 
               
               
                 JW3887 
                 BW25113 ΔpfkA 
                 [26] 
               
               
                 GA00 
                 WC3G gapA- ΔglcD ΔarcA  
                 This study 
               
               
                   
                 ΔmgsA ΔfucA ΔpfkA proD galP 
                   
               
               
                 GA09 
                 GA00 containing pZA36 kdsD  
                 This study 
               
               
                   
                 fsa (pKF3) and pZS23 
                   
               
               
                   
                 aldA (pA4) 
                   
               
               
                 GA23 
                 GA00 containing pZA37 kdsD  
                 This study 
               
               
                   
                 fsa (pKF5) and pZA28 
                   
               
               
                   
                 aldA (pA8) 
               
               
                   
               
            
           
         
       
     
     C. Media and Culture Conditions. 
     i. Composition of the Media 
     The cells are cultured on the LB medium for the molecular biology (cloning, deletions, transformation) steps. This rich medium is comprised of 10 g/L trypton, 5 g/L yeast extracts and 5 g/L NaCl. 15 g/L agar are added for obtaining a solid medium. The LB, with or without agar, is sterilised by autoclaving for 20 min at 110° C. before use. 
     The cultures for glycolic acid production are made in a mineral medium M9 (Table 12) containing a carbon source (glucose, xylose or arabinose) at a 10 g/L or 20 g/L concentration and LB traces in order to reduce the latency phase (2 g/L trypton, 1 g/L yeast extracts and 1 g/L NaCl). 
     
       
         
           
               
             
               
                 TABLE 12 
               
             
            
               
                   
               
               
                 composition of the medium M9 
               
            
           
           
               
               
               
            
               
                   
                 Compound 
                 Final concentration (g/L) 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                   
                 Na 2 HPO 4  * 12 H 2 O 
                 18 
               
               
                   
                 KH 2 PO 4   
                 3 
               
               
                   
                 NaCl 
                 0.5 
               
               
                   
                 NH 4 Cl 
                 2 
               
               
                   
                 MgSO 4  * 7 H 2 O 
                 0.5 
               
               
                   
                 CaCl 2  * 2 H 2 O 
                 0.015 
               
               
                   
                 FeCl 3   
                 0.010 
               
               
                   
                 Thiamine HCl 
                 6.10 −3   
               
               
                   
                 NaEDTA 
                 0.49.10 −3   
               
               
                   
                 CoCl 2  * 6 H 2 O 
                 1.8.10 −3   
               
               
                   
                 ZnCl 2 SO 4  * 7 H 2 O 
                 1.8.10 −3   
               
               
                   
                 Na 2 MoO 4  * 2 H 2 O 
                 0.4 .10 −3   
               
               
                   
                 H 3 BO 3   
                 0.1.10 −3   
               
               
                   
                 MnSO 4  * H 2 O 
                 1.2.10 −3   
               
               
                   
                 1.2 mg/L CuCl 2  * 2 H 2 O 
                 1.2.10 −3   
               
               
                   
                   
               
            
           
         
       
     
     For the strain MG1655 ΔtktA ΔtktB ΔglcD, the medium M9 with LB traces is complemented with 500 M L-phenylalanine, 250 μM L-tyrosine, 200 μM L-tryptophane, 6 μM p-aminobenzoate, 6 μM p-hydroxydenzoate and 280 μM shikimate. For the strains the glyceraldehyde-3-phosphate dehydrogenase activity of which has been removed, the medium has been completed with 0.4 g/L malic acid adjusted at pH 7 with KOH beforehand. The medium is buffered by adding 20 g/L 3-(N-morpholino)propanesuphonic acid (MOPS) at pH 7 and then filtered through 0.2 μm membranes to obtain a sterile medium. If need be, the appropriate antibiotics have been added to the medium (100 g/mL ampicillin, 50 μg/mL kanamycin, 25 μg/mL chloramphenicol). For the strains containing an IPTG inducible vector, a 0.1 mM IPTG final concentration is used. All the products have been ordered to Sigma. The cultures are placed in a stirrer (Infors) at 200 rpm, 37° C. for the experiment time. The culture growth is monitored by measuring the 600 nm optical density (OD 600 ) with a spectrophotometer (Biochrom Libra S11). 
     ii. Fermentation Process in 250 mL Baffle Erlenmeyer Flasks 
     Strains Containing Constituted Promoters 
     Protocol 1 
     The strains are uptaken from of a glycerol stock preserved at −80° C. in 10 mL LB, at 37° C. overnight. The pre-cultures are centrifuged the day after at 4 000 rpm for 5 min and re-suspended in 20 mL of M9 medium with 1% xylose or arabinose in 100 mL Erlenmeyer flasks. An adaptation phase with a 24h duration enables the strains to adapt to pentose use. The cells are then centrifuged (4 000 rpm, 5 min) and then uptaken under the final culture conditions: at an initial OD 600  of 0.5 in 50 mL medium with a composition identical to that used during the adaptation phase (M9 with 1% xylose or arabinose). 250 mL baffle culture flasks are used for an optimum oxygenation. The cultures are stirred at 200 rpm, at 37° C. 
     Protocol 2 
     The strain GA23 is uptaken from of a glycerol stock preserved at −80° C. in 10 mL LB containing the following antibiotics: tetracyclin (10 μg/mL), kanamycin (50 μg/ml) and chloramphenicol (25 g/ml). The pre-culture is incubated at 37° C., 200 rpm overnight. The process is decoupled into two phases: a growth phase dedicated to biomass production and a production phase dedicated to the production of glycolic acid. The growth phase is achieved in 50 mL of medium M9 to pH 7 containing 1.5 g/L xylose, 5 g/L succinate, 73 mg/L L L-methionine, 73 mg/L L-tryptophan and 1 g/L casaminoacids inoculated with pre-culture at an OD 600  of 0.2 in a 250 ml erlenmeyer. After 24h of culture at 37° C., 200 rpm, the cells are centrifuged, washed and recovered in 50 mL M9 at pH 7 containing 10 g/L lignocellulosic sugar (glucose, xylose or arabinose), 73 mg/L L-methionine, 73 mg/L L-tryptophan and 1 g/L casaminoacids in a 250 ml erlenmeyer. The medium used for production is devoid of succinate and does not allow growth, the source of carbon (glucose, xylose or arabinose) is used for the production of glycolic acid. The production phase lasts 48h at 37° C., 200 rpm, samples are taken regularly to measure pH, optical density and track sugar consumption and GA production by HPLC. 
     Strains Containing Inducible Promoters 
     The LB pre-cultures of the strains containing a vector with an inducible promoter are cultured at an initial OD 600  of 0.1 in 30 mL of M9 with 1% glucose. When the OD 600  reaches 0.6, 0.1 mM IPTG is added to induce gene expression under the control of the IPTG inducible promoter (plac or ptac). 24h after, the cells are centrifuged, a sterile water washing enables glucose traces to be removed, and they are re-suspended in 20 mL of the medium chosen from the study (M9 with 1% xylose or arabinose, 0.1 mM IPTG) for the adaptation phase. The rest of the culturing protocol is identical to that for the strains with constitutive promoters. 
     iii. Fermentation Process in a 2 L BIOSTAT® B Startorius Stedim Biotech Bioreactor 
     Cultures of the previously described strain GA23 have been made in a 2 L bioreactor. The pre-cultures have been made in LB added with kanamycin, chloramphenicol and tetracyclin, cultured at 37° C., 200 rpm. A first 3 mL pre-culture has been made overnight, the same has been used for inoculating a second 15 mL pre-culture cultured for 24h. The same has been used for inoculating a third and last pre-culture in a 150 mL Erlenmeyer flask cultured for 8h. In the exponential phase, this last pre-culture has been used to inoculate the fermenter culture at a OD of 0.3. The M9 culture medium (Table 12) contains 10 g/L xylose or glucose, as well as LB (10%) and 0.4 g/L malic acid. The cultures have been made at 37° C., with stirring (300-1500 rpm) and venting maintaining dissolved oxygen above 20% of the air flow rate. The pH has been maintained at 7 with a KOH base solution. The culture has been conducted in a batch mode for 40h. 
     D. Measurements of the Concentrations of Extracellular Metabolites. 
     Sugar consumption and glycolic acid production are monitored by regularly sampling samples under sterile conditions which are centrifuged at 13 000 rpm for 5 min in a laboratory centrifuge (Eppendorf 5415D), filtered through a 0.2 μm filter and stored at −20° C. The measurement of extracellular metabolites is made by high performance liquid chromatography (HPLC) with an Ultimate 3000 chromatograph (Dionex, Sunnyvale, USA). The HPLC system is equipped with a cation exchange column (Aminex, HPX87H-300×7.8 mm, 9 μm, BioRad), an automatic injector (WPS-3000RS, Dionex), an IR detector (RID 10A, Shimadzu) and a UV detector (SPD-20A, Shimadzu). The mobile phase is a 1.25 mM sulfuric acid solution at a 0.5 mL/min flow rate. The samples are maintained at 4° C. and the injection of 20 is made in the column at 35° C. 
     II.2. Results Obtained with a Baffle Erlenmeyer Flask. 
     The inventors have demonstrated that the overexpression in  E. coli  of the 3 non-natural pathway enzymes according to the invention, i.e. arabinose-5-phosphate isomerase (KdsD), fructose-6-phosphate aldolase (FSA) and aldehyde dehydrogenase (AldA) is necessary and sufficient for synthesising glycolic acid from lignocellulosic monosaccharides. 
     A. With Pentoses as a Carbon Source. 
     To that end, a strain of  E. coli  K12 MG1655 deleted from genes tktA, tktB coding for transketolase has been constructed to highlight the functionality of the pathway according to the invention. Transketolase are major enzymes of the pentose phosphate pathway. In their absence, the growth on pentose is impossible, the intermediates pentose phosphate (D-ribose-5-phosphate and D-Xylulose-5-phosphate) are accumulated and can not be converted into glyceraldehyde-3phosphate for growth [38]. This strain needs the non-natural pathway according to the invention to grow from pentoses. Indeed, only the coupled activity of KdsD and FSA can generate glyceraldehyde-3-phosphate involved in glycolysis for the production of precursors necessary for growth [39]. Thus, a functional screen per growth test is contemplatable in which growth would be an indicator of the in vivo synthetic pathway efficiency ( FIG. 7 ). 
     The 3 genes have been cloned as an operon and expressed in a strain of  E. coli  MG1655 ΔtktA ΔtktB ΔglcD. To that end, a two-vector expression system has been used, the latter has been accepted by the strain without generating detrimental modifications for the expression of the synthetic pathway according to the invention. The expression system of the synthetic pathway according to the invention thus includes a vector having a medium copy number bearing kdsD-fsa co-expressed with a vector with a low copy number bearing aldA. 
     Two expression systems have been studied, the first one with vectors with IPTG inducible promoters (pZA36 kdsD fsa and pZS23 aldA, expression system n° 9) and the other one with vectors with constitutive promoters (pZA37 kdsD fsa and pZS28 aldA, expression system n° 23). 
     The cultures of the strains MG1655 ΔtktA ΔtktB ΔglcD expressing the systems 9 (Screen09) and 23 (Screen23) have been made at 37° C. and have shown a significant growth indicating that the overexpression of the kdsD and fsa genes is functional. HPLC analyses of the exometabolome of the bacterial cultures on L-arabinose and D-xylose have confirmed the presence of glycolic acid with both expression systems. This result demonstrates the feasibility of glycolic acid synthesis from pentoses by the in vivo synthetic pathway ( FIG. 8 ). Table 13 hereinafter shows the glycolic acid yield from xylose and arabinose of the strains Screen09 and Screen23. 
     
       
         
           
               
               
               
             
               
                 TABLE 13 
               
               
                   
               
             
            
               
                   
                 Glycolic 
                 Glycolic 
               
               
                   
                 acid/xylose 
                 acid/arabinose 
               
               
                 Strain 
                 (mol/mol) 
                 (mol/mol) 
               
               
                   
               
               
                 Screen00 
                 0.14 ± 0.03 
                 0.14 ± 0.03 
               
               
                 Screen09 
                 0.43 ± 0.02 
                 0.32 ± 0.01 
               
               
                 Screen23 
                 0.42 ± 0.02 
                 0.20 ± 0.03 
               
               
                   
               
               
                   
                 Glycolic  
                 Glycolic 
               
               
                   
                 acid/xylose 
                 acid/arabinose 
               
               
                 Strain 
                 (g/g) 
                 (g/g) 
               
               
                   
               
               
                 Screen00 
                 0.07 ± 0.03 
                 0.07 ± 0.03 
               
               
                 Screen09 
                 0.22 ± 0.02 
                 0.16 ± 0.01 
               
               
                 Screen23 
                 0.21 ± 0.02 
                 0.10 ± 0.03 
               
               
                   
               
            
           
         
       
     
     B. With Pentoses and Hexoses as a Carbon Source. 
     The expression systems 9 and 23 have been transformed in the strain of  E. coli  WC3G ΔgapA ΔglcD ΔarcA ΔmgsA ΔfucA Δpkf proD-galP (GA00) designed for the glycolic acid production from hexoses and pentoses with an optimum carbon preservation, generating strains GA09 and GA23, respectively. 
     The bacterial cultures have been made in an Erlenmeyer flask at 37° C. on D-glucose, L-arabinose and D-xylose for 46h. The overexpression of the kdsD,fsa and aldA genes is necessary for glycolic acid production. 
     With Protocol 1 Such as Previously Defined 
     The strain GA00 with the constitutive expression system n° 23 that is GA23 has shown a glycolic acid production from D-glucose (0.29 g/L), D-xylose (0.41 g/L) and L-arabinose (0.07 g/L). The glycolic acid production with the inducible expression system n° 9 that is GA09 is less significant on D-glucose (0.1 g/L), D-xylose (0.05 g/L) and L-arabinose (0.03 g/L) ( FIG. 9 ). The expression system including constitutive promoters is favourable. The yield of the strain GA23 in glycolic acid is 0.09 g/g (0.21 mol/mol) from glucose, 0.18 g/g (0.36 mol/mol) from xylose and 0.16 g/g (0.32 mol/mol) from arabinose. 
     With protocol 2 such as previously defined 
     The yield of the strain GA23 in glycolic acid is 0.096 g/g (0.19 mol/mol) from glucose, 0.3 g/g (0.60 mol/mol) from xylose and 0.34 g/g (0.68 mol/mol) from arabinose. 
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