Patent Publication Number: US-10774320-B2

Title: Methods and microorganisms for the production of glycolic acid and/or glyoxylic acid

Description:
FIELD OF THE INVENTION 
     The present invention relates to methods and recombinant microorganisms useful for the production of glycolic acid and/or glyoxylic acid. The microorganisms of the invention are modified in a way that the glycolic acid and/or glyoxylic acid yield on carbon source is increased by overexpressing phosphoketolase activity. The methods of the invention involve either only one step of fermentation of the modified microorganisms of the invention or one step of fermentation of the modified microorganisms of the invention and one step of either biological or chemical conversion of the glycolic acid or of the glyoxylic acid. 
     BACKGROUND OF THE INVENTION 
     Carboxylic acids are organic compounds that contain at least one carboxyl group. Carboxylic acids occur widely and include the amino acids (which make up proteins) and acetic acid (which is part of vinegar and occurs in metabolism) for instance. Carboxylic acids are used in the production of polymers, pharmaceuticals, solvents, and food additives. Industrially important carboxylic acids include acetic acid (component of vinegar, precursor to solvents and coatings), acrylic and methacrylic acids (precursors to polymers, adhesives), adipic acid (polymers), citric acid (beverages), ethylenediaminetetraacetic acid (EDTA) (chelating agent), fatty acids (coatings), maleic acid (polymers), propionic acid (food preservative), terephthalic acid (polymers), butyric acid (food additive), succinic acid (food additive, polymers). 
     Glycolic acid (HOCH 2 COOH, CAS Number: 79-14-1), or glycolate for its conjugate base, is the simplest member of the alpha-hydroxy acid family of carboxylic acids. Glycolic acid has dual functionality with both alcohol and moderately strong acid functional groups on a very small molecule. Its properties make it ideal for a broad spectrum of consumer and industrial applications, including use in water well rehabilitation, leather industry, oil and gas industry, laundry and textile industry, cleaning products, and as a component in personal care products. It can also be used to produce a variety of polymeric materials, including thermoplastic resins comprising polyglycolic acid which have excellent gas barrier properties, and thus may be used to make packaging materials having the same properties (e.g., beverage containers, etc.). The polyester polymers gradually hydrolyze in aqueous environments at controllable rates. This property makes them useful in biomedical applications such as dissolvable sutures and in applications where a controlled release of acid is needed to reduce pH. Currently, more than 50,000 tons of glycolic acid are consumed annually worldwide. 
     Although glycolic acid occurs naturally as a trace component in sugarcane, beets, grapes and fruits, it is mainly synthetically produced. Other technologies to produce glycolic acid are described in the literature or in patent applications. For instance, EP 2025759 and EP 2025760 patent applications describe a method for producing the said hydroxycarboxylic acid from aliphatic polyhydric alcohol having a hydroxyl group at the end by using a microorganism. This method is a bioconversion, as is the one described by Michihiko Kataoka in the paper on the production of glycolic acid using ethylene glycol-oxidizing microorganisms (Kataoka et al., 2001). Glycolic acid is also produced by bioconversion from glycolonitrile using mutant nitrilases with improved nitrilase activity and that technique was disclosed in patent applications WO 2006/069110, WO 2009/059104, and WO 2009/059096, or by bioconversion from ethylene glycol, glycolaldehyde or glyoxal as disclosed in patent applications JP 2007/228927 or WO 2005/106005. Methods and microorganisms for producing glycolic acid by fermentation from renewable resources wherein carbohydrates are converted to glycolic acid by one direct step of fermentation were disclosed in patent applications WO 2007/141316, WO 2010/108909, WO 2011/036213, WO 2011/157728, WO 2012/025780, CN105647844A, CN106011185A, and WO 2016/079440 for methods using  Escherichia coli  strains, and in WO 2013/050659, WO 2014/162063, and WO 2016/193540 using  Saccharomyces cerevisiae  or  Kluyveromyces lactis  strains. 
     Glyoxylic acid or oxoacetic acid (HCOCOOH, CAS Number: 298-12-4) or glyoxylate for its conjugate base is a C2 carboxylic acid. Glyoxylic acid is an intermediate of the glyoxylate cycle, which enables organisms, such as bacteria, fungi and plants to convert fatty acids into carbohydrates. Glyoxylate is the byproduct of the amidation process in biosynthesis of several amidated peptides. It is a colourless solid that occurs naturally and is useful industrially. It is used as a cleaning agent for a variety of industrial applications, as a specialty chemical and biodegradable copolymer feedstock and as an ingredient in cosmetics. It is a useful compound for agricultural and pharmaceutical chemicals. Indeed, glyoxylic acid can be used in pharmaceutical industry since its condensation with phenols gives 4-hydroxymandelic acid which reacts with ammonia to give hydroxyphenylglycine, a precursor to the drug amoxicillin or which can be reduced to give 4-hydroxyphenylacetic acid, a precursor to the drug atenolol. Moreover acid-catalysed reaction of glyoxylic acid with urea leads to the production of allantoin used in cosmetics, ointments and in the treatment of some cancers (Cativiela et al., 2003). Finally, condensation with guaiacol in place of phenol provides a route to vanillin, used as a flavoring agent in foods, beverages, and pharmaceuticals. 
     Although glyoxylic acid occurs naturally as a trace component in unripe fruit and young green leaves, it is mainly synthetically produced. Other technologies to produce glyoxylic acid are described in the literature or in patent applications. For instance, glyoxylic acid may be chemically produced by heating dibromoacetic acid with some water or by electrolytic reduction of oxalic acid or by nitric oxidation of glyoxal. Some patent applications describe processes of production of glyoxylic acid by bioconversion, such as patent applications WO 1993/14214, U.S. Pat. No. 5,439,813, and WO 1994/28155 disclosing the bioconversion from glycolic acid using glycolate oxidase produced by a microorganism, as well as Isobe &amp; Nishise (1999). Patent application US 2007/0026510 discloses the bioconversion from glyoxal using an aldehyde oxidase. 
     The industrial interest of glycolic acid and glyoxylic acid coupled with environmental concerns due to chemical by-products formed during chemical productions render microbial production of such carboxylic acids an attractive prospect. 
     The inventors have identified new methods for the production of glycolic acid and/or glyoxylic acid from carbohydrates as sole carbon source involving at least one fermentative step and a modified microorganism in which activity of phosphoketolase is enhanced. 
     Phosphoketolase activity and genes encoding enzymes with such activities are known in the art (Papini et al., 2012). Two different phosphoketolase activities have been reported in bacteria. Xylulose 5-phosphate phosphoketolase catalyses the phosphate consuming conversion of xylulose 5-phosphate to glyceraldehyde 3-phosphate and acetylphosphate with release of water. Xylulose 5-phosphate phosphoketolase is, for instance, encoded by the xpkA gene from  Lactobacillus pentosus  (Posthuma et al., 2002). Fructose 6-phosphate phosphoketolase catalyses the phosphate consuming conversion of fructose 6-phosphate to erythrose 4-phosphate and acetylphosphate with release of water. Few genes encode phosphoketolases having both xylulose 5-phosphate phosphoketolase and fructose 6-phosphate phosphoketolase activities, as, for instance, is the case for the protein encoded by the gene xfp from  Bifidobacterium lactis  (Meile et al., 2001) or  Bifidobacterium animalis  (WO 2006/016705 and WO 2016/044713). 
     The use of phosphoketolase for the production of metabolites of interest is already known and has been disclosed in patent applications WO 2006/016705 and in WO 2016/044713. The metabolites of interest are glutamic acid, glutamine, proline, arginine, leucine, cysteine, succinate, polyhydrobutyrate and 1,4-butanediol. The use of phosphoketolase for the production of glycolic acid or glyoxylic acid has never been disclosed. 
     The methods and the microorganisms of the invention are new over the prior art since the use of phosphoketolase for the production of glycolic acid and/or glyoxylic acid has never been previously disclosed. The inventors have surprisingly found that the overproduction of phosphoketolase in the microorganisms of the invention improves the production of glycolic acid and/or glyoxylic acid. 
     The methods identified by the inventors involve either only one step of fermentation of the modified microorganisms of the invention or one step of fermentation of the modified microorganisms of the invention and one step of either biological or chemical conversion of the glycolic acid or of the glyoxylic acid. 
     SUMMARY OF THE INVENTION 
     The invention relates to recombinant microorganisms and methods for improving the production of glycolic acid and/or glyoxylic acid from carbohydrates as sole carbon source and using at least one step of fermentation and a modified microorganism wherein expression of at least one gene chosen among aceB, glcB, gcl and eda is attenuated and expression of a gene encoding xylulose 5-phosphate phosphoketolase and/or fructose 6-phosphate phosphoketolase is enhanced. Preferably the gene encoding the phosphoketolase is chosen among the xpkA gene from  Lactobacillus pentosus , the xfp gene from  Bifidobacterium animalis , or the xfp gene from  Bifidobacterium lactis , or their homologous genes. 
     The methods identified by the inventors involve either only one step of fermentation of the modified microorganism of the invention for the production of glycolic acid or glyoxylic acid or one step of fermentation of the modified microorganism of the invention for the production of the corresponding intermediates glycolic acid or glyoxylic acid and one step of biological or chemical conversion of the intermediates glycolic acid or glyoxylic acid into glyoxylic acid or glycolic acid, respectively. 
     Another method of the invention relates to the production of glyoxylic acid from glycine using a glycine oxidase and optionally a catalase. 
     The microorganisms of the invention are chosen among bacteria such as Enterobacteriaceae, Clostridiaceae, Corynebacteriaceae, Bacillaceae, Bifidobacteriaceae, Lactobacillaceae or yeast. More preferably, the microorganisms of the invention are from the  Escherichia coli  species. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Before describing the present invention in detail, it is to be understood that this invention is not limited to particularly exemplified methods and may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments of the invention only, and is not intended to be limiting, which will be limited only by the appended claims. 
     All publications, patents and patent applications cited herein, whether supra or infra, are hereby incorporated by reference in their entirety. 
     Furthermore, the practice of the present invention employs, unless otherwise indicated, conventional microbiological and molecular biological techniques within the skill of the art. Such techniques are well known to the skilled worker, and are explained fully in the literature. 
     It must be noted that as used herein and in the appended claims, the singular forms “a,” “an,” and “the” include plural reference unless the context clearly dictates otherwise. Thus, for example, a reference to “a microorganism” includes a plurality of such microorganisms, and a reference to “an endogenous gene” is a reference to one or more endogenous genes, and so forth. Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any materials and methods similar or equivalent to those described herein can be used to practice or test the present invention, the preferred materials and methods are now described. 
     In the claims that follow and in the consecutive description of the invention, except where the context requires otherwise due to express language or necessary implication, the word “comprise,” “contain,” “involve,” or “include” or variations such as “comprises,” “comprising,” “containing,” “involved,” “includes,” “including” are used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention. 
     The invention relates to recombinant microorganisms and methods for improving the production of glycolic acid and/or glyoxylic acid. The methods identified by the inventors involve either only one step of fermentation of the modified microorganism of the invention for the production of glycolic acid or glyoxylic acid or one step of fermentation of the modified microorganism of the invention for the production of the corresponding intermediates glycolic acid or glyoxylic acid and one step of bioconversion of the intermediates into, respectively, glyoxylic acid or glycolic acid. 
     The term “glycolic acid” designates the carboxylic acid with chemical formula HOCH2COOH, and CAS Number 79-14-1. 
     The term “glyoxylic acid” designates the carboxylic acid with chemical formula HCOCOOH, and CAS Number: 298-12-4. 
     The term “microorganism,” as used herein, refers to a bacterium, yeast or fungus which is not modified artificially. Preferentially, the microorganism is selected among Enterobacteriaceae, Clostridiaceae, Corynebacteriaceae, Bacillaceae, Bifidobacteriaceae, Lactobacillaceae or yeast. More preferentially, the microorganism is a species of  Escherichia, Klebsiella, Lactobacillus, Bifidobacterium, Corynebacterium, Kluyveromyces  or  Saccharomyces . Even more preferentially, the microorganism of the invention is from  Escherichia coli  species. 
     The term “recombinant microorganism” or “genetically modified microorganism,” as used herein, refers to a bacterium, yeast or fungus that is not found in nature and is genetically different from its equivalent found in nature. It means it is modified either by introduction or by deletion or by modification of genetic elements. It can also be transformed by forcing the development and evolution of new metabolic pathways by combining directed mutagenesis and evolution under specific selection pressure (see, for example, WO 2004/076659 or WO 2007/011939). 
     A microorganism may be modified to express exogenous genes if these genes are introduced into the microorganism with all of the elements allowing their expression in the host microorganism. The modification or “transformation” of microorganisms with exogenous DNA is a routine task for those skilled in the art. 
     A microorganism may be modified to modulate the expression level of an endogenous gene. 
     The term “endogenous gene” means that the gene was present in the microorganism before any genetic modification. Endogenous genes may be overexpressed by introducing heterologous sequences in addition to, or to replace, endogenous regulatory elements, or by introducing one or more supplementary copies of the gene into the chromosome or a plasmid. Endogenous genes may also be modified to modulate their expression and activity of the corresponding encoded protein. For example, mutations may be introduced into the coding sequence to modify the gene product or heterologous sequences may be introduced in addition to or to replace endogenous regulatory elements. Modulation of an endogenous gene may result in the up-regulation and/or enhancement of the activity of the gene product, or alternatively, down regulate and/or lower the activity of the endogenous gene product. 
     Another way to modulate their expression is to exchange the endogenous promoter of a gene (e.g., wild type promoter) with a stronger or weaker promoter to up or down regulate expression of the endogenous gene. These promoters may be homologous or heterologous. It is well within the ability of the person skilled in the art to select appropriate promoters. 
     Contrariwise, “exogenous gene” means that the gene was introduced into a microorganism, by means well-known to the man skilled in the art whereas this gene is not naturally occurring in the microorganism. Exogenous genes may be integrated into the host chromosome, or be expressed extra-chromosomally by plasmids or vectors. A variety of plasmids, which differ with respect to their origin of replication and their copy number in the cell, are well-known in the art. These genes may be homologous. 
     In the context of the invention, the term “homologous gene” is not limited to designate genes having a theoretical common genetic ancestor, but includes genes which may be genetically unrelated that have, none the less, evolved to encode protein which perform similar functions and/or have similar structure. Therefore, the term ‘functional homologue” for the purpose of the present invention relates to the fact that a certain enzymatic activity may not only be provided by a specific protein of defined amino acid sequence, but also by proteins of similar sequence from other (un)related microorganisms. 
     Using the references given in Genbank for known genes, those skilled in the art are able to determine the equivalent genes in other organisms, bacterial strains, yeast, fungi, mammals, plants, etc. This routine work is advantageously done using consensus sequences that can be determined by carrying out sequence alignments with genes derived from other microorganisms and designing degenerate probes to clone the corresponding gene in another organism. These routine methods of molecular biology are well known to those skilled in the art. 
     The terms “improved glycolic acid and/or glyoxylic acid production,” “improve glycolic acid and/or glyoxylic acid production” and grammatical equivalents thereof, as used herein, refer to an increased glycolic acid and/or glyoxylic acid/carbon source yield (ratio of gram/mol glycolic acid and/or glyoxylic acid produced per gram/mol carbon source consumed that it can be expressed in percent). Methods for determining the amount of carbon source consumed and of glycolic acid and/or glyoxylic acid produced are well-known to those in the art. The yield is higher in the recombinant microorganism compared to the corresponding unmodified microorganism. 
     The terms “microorganism optimised for the fermentative production of glycolic acid and/or glyoxylic acid” refers to microorganisms evolved and/or genetically modified to present an improved glycolic acid and/or glyoxylic acid production in comparison with the endogenous production of the corresponding wild-type microorganism. Such microorganisms “optimised” for glycolic acid and/or glyoxylic acid production are well-known in the art, and have been disclosed in particular in patent applications WO 2007/141316, WO 2010/108909, WO 2011/036213, WO 2011/157728, and WO 2012/025780. 
     According to the invention the terms “fermentative production,” “culture,” “fermentation step,” or “fermentation” are used to denote the growth of bacteria. This growth is generally conducted in fermenters with an appropriate culture medium adapted to the microorganism being used and containing at least one simple carbon source, and if necessary co-substrates. 
     An “appropriate culture medium” designates a medium (e.g., a sterile, liquid media) comprising nutrients essential or beneficial to the maintenance and/or growth of the cell such as carbon sources or carbon substrates, nitrogen sources, for example, peptone, yeast extracts, meat extracts, malt extracts, urea, ammonium sulfate, ammonium chloride, ammonium nitrate and ammonium phosphate; phosphorus sources, for example, monopotassium phosphate or dipotassium phosphate; trace elements (e.g., metal salts), for example magnesium salts, cobalt salts and/or manganese salts; as well as growth factors such as amino acids and vitamins. 
     The term “carbon source” or “carbon substrate” or “source of carbon” according to the present invention denotes any source of carbon that can be used by those skilled in the art to support the normal growth of a microorganism. The carbon source is chosen among carbohydrates which designate monosaccharides (such as glucose, galactose, xylose, fructose or lactose), oligosaccharides, disaccharides (such as sucrose, cellobiose or maltose), molasses, starch or its derivatives, hemicelluloses and combinations thereof. An especially preferred simple carbon source is glucose. Another preferred simple carbon source is sucrose. The carbon source can be derived from renewable feed-stock. Renewable feed-stock is defined as raw material required for certain industrial processes that can be regenerated within a brief delay and in sufficient amount to permit its transformation into the desired product. Vegetal biomass treated or not, is an interesting renewable carbon source. 
     According to the invention the terms “bioconversion step” or “bioconversion” or “biotransformation” or “bio catalysis” or “biological conversion” refer to the conversion of organic materials into desired products by a specific enzyme produced by an enzyme producer microorganism (different from the modified microorganisms of the invention). The reaction of bioconversion can be performed differently according to the enzyme, its mechanism and the constraint of the process and they are known by the man in the art: 
     1. The organic material may be contacted with the enzyme by adding to organic material the purified enzyme or, 
     2. The organic material may be contacted with the enzyme by adding to organic material the fermentation broth of the enzyme producer microorganism containing the enzyme cleared from the bacteria or, 
     3. The organic material may be contacted with the enzyme by adding to organic material the extract of lysed cells of the enzyme producer microorganism or, 
     4. The organic material may be contacted with the enzyme by adding to organic material the living cells of the enzyme producer microorganism previously treated to permit both the enzymatic reaction and the viability of said enzyme producer microorganism required for the reaction regeneration (specific cofactor availability). This system is called the whole-cell biocatalyst system. 
     In a preferred embodiment of the invention, the organic material originates from the first step of fermentation of the method of the invention that is to say glycolic acid or glyoxylic acid produced by the modified microorganisms of the invention. The organic material originating from the first step of fermentation can be more or less purified from the culture broth. In another embodiment of the invention, the organic material is the glycine which is chemically or biologically provided. 
     In a first aspect of the invention, the invention relates to a method for the production of glycolic acid and/or glyoxylic acid from carbohydrate as sole carbon source using at least one step of fermentation and a modified microorganism wherein the modified microorganism of the invention comprises attenuation of expression of at least one gene chosen among aceB, glcB, gcl and eda, and overexpression of at least one gene encoding xylulose 5-phosphate phosphoketolase and/or fructose 6-phosphate phosphoketolase. 
     As disclosed in patent application WO 2007/141316, deletion of at least one gene chosen among aceB encoding malate synthase, glcB encoding malate synthase, gcl encoding glyoxylate carboligase and eda encoding 2-keto-3-deoxygluconate 6-phosphate aldolase leads to a decrease of the conversion of glyoxylate allowing such accumulation of glyoxylate which could be recovered from the culture medium or could be further converted into glycolic acid. Preferably, in the microorganism of the invention expression of the genes aceB and glcB and gcl are attenuated. 
     More preferably, the modified microorganisms of the invention may be further modified as disclosed in patent applications WO 2010/108909, WO 2011/036213, WO 2011/157728, WO 2012/025780 with:
         attenuation of the genes glcDEFG encoding glycolate oxidase and/or aldA encoding glycolaldehyde dehydrogenase leading to the inability to substantially metabolize glycolate   increase of the glyoxylate pathway flux, obtained in particular by the attenuation of the genes icd encoding isocitrate dehydrogenase, aceK encoding lcd kinase-phosphatase, pta encoding phospho-transacetylase, ackA encoding acetate kinase, poxB encoding pyruvate oxidase, iclR or fadR encoding glyoxylate pathway repressors, and/or by the overexpression of the gene aceA encoding isocitrate lyase,   Decreasing the level of isocitrate dehydrogenase can be accomplished by introducing artificial promoters that drive the expression of the icd gene, coding for the isocitrate dehydrogenase, or by introducing mutations into the icd gene that reduce the enzymatic activity of the protein.   Since the activity of the protein lcd is reduced by phosphorylation, it may also be controlled by introducing mutant aceK genes that have increased kinase activity or reduced phosphatase activity compared to the wild type AceK enzyme.   increase of the availability of NADPH, obtained in particular by the attenuation of the genes pgi, udhA, edd.       

     The terms “attenuation” or “expression attenuated” mean in this context that the expression of a gene and/or the production of an enzyme is decreased or suppressed compared to the non-modified microorganism leading to a decrease in the intracellular concentration of a ribonucleic acid, a protein or an enzyme compared to the non-modified microorganism. The man skilled in the art knows different means and methods to measure ribonucleic acid concentration or protein concentration in the cell including, for instance, use of Reverse Transcription Polymerase Chain Reaction (RT-PCR) to determine ribonucleic acid concentration and use of specific antibody to determine concentration of specific protein. 
     Decrease or suppression of the production of an enzyme is obtained by the attenuation of the expression of gene encoding said enzyme. 
     Attenuation of genes may be achieved by means and methods known to the man skilled in the art. Generally, attenuation of gene expression may be achieved by:
         Mutating the coding region or the promoter region or,   Deleting all or a part of the promoter region necessary for gene expression or,   Deleting all or a part of the coding region of the gene by homologous recombination or,   Inserting an external element into the coding region or into the promoter region or,   Expressing the gene under the control of a weak promoter or an inducible promoter.       

     The man skilled in the art knows a variety of promoters exhibiting different strengths and which promoter to use for a weak or an inducible genetic expression. 
     The term “activity” of an enzyme is used interchangeably with the term “function” and designates, in the context of the invention, the reaction that is catalysed by the enzyme. The man skilled in the art knows how to measure the enzymatic activity of said enzyme. 
     The terms “attenuated activity” or “reduced activity” of an enzyme mean either a reduced specific catalytic activity of the protein obtained by mutation in the amino acids sequence and/or decreased concentrations of the protein in the cell obtained by mutation of the nucleotide sequence or by deletion of the coding region of the gene. 
     The terms “enhanced activity” or “increased activity” of an enzyme designate either an increased specific catalytic activity of the enzyme, and/or an increased quantity/availability of the enzyme in the cell, obtained for example by overexpressing the gene encoding the enzyme. 
     The terms “increased expression,” “enhanced expression,” or “overexpression” and grammatical equivalents thereof, are used interchangeably in the text and have a similar meaning. These terms mean that the expression of a gene or the production of an enzyme is increased compared to the non-modified microorganism leading to an increase in the intracellular concentration of a ribonucleic acid, a protein or an enzyme compared to the non-modified microorganism. The man skilled in the art knows different means and methods to measure ribonucleic acid concentration or protein concentration in the cell including, for instance, use of Reverse Transcription Polymerase Chain Reaction (RT-PCR) to determine ribonucleic acid concentration and use of specific antibody to determine concentration of specific protein. 
     Increased production of an enzyme is obtained by increasing expression of the gene encoding said enzyme. 
     To increase the expression of a gene, the man skilled in the art knows different techniques such as:
         Increasing the number of copies of the gene in the microorganism. The gene is encoded chromosomally or extrachromosomally. When the gene is located on the chromosome, several copies of the gene can be introduced on the chromosome by methods of recombination, known by the expert in the field (including gene replacement). When the gene is located extra-chromosomally, it may be carried by different types of plasmids that differ with respect to their origin of replication and thus their copy number in the cell. These plasmids are present in the microorganism in 1 to 5 copies, or about 20 copies, or up to 500 copies, depending on the nature of the plasmid: low copy number plasmids with tight replication (pSC101, RK2), low copy number plasmids (pACYC, pRSF1010) or high copy number plasmids (pSK bluescript II).   Using a promoter leading to a high level of expression of the gene. The man skilled in the art knows which promoters are the most convenient. For example, promoters Ptrc, Ptac, Plac, or the lambda promoters PR and PL are widely used. These promoters can be “inducible” by a particular compound or by a specific external condition like temperature or light. These promoters may be homologous or heterologous.   Attenuating the activity or the expression of a transcription repressor, specific or non-specific of the gene.   Using elements stabilizing the corresponding messenger RNA (Carrier and Keasling, 1999) or elements stabilizing the protein (e.g., GST tags, GE Healthcare).       

     The terms “encoding” or “coding” refer to the process by which a polynucleotide, through the mechanisms of transcription and translation, produces an amino acid sequence. The gene(s) encoding the enzyme(s) can be exogenous or endogenous. 
     The term “phosphoketolase” typically refers to enzymes with xylulose 5-phosphate phosphoketolase activity (EC 4.1.2.9) and/or fructose 6-phosphate phosphoketolase activity (EC 4.1.2.22). Xylulose 5-phosphate phosphoketolase activity means the activity of phosphate-consuming conversion of xylulose 5-phosphate to glyceraldehyde 3-phosphate and acetylphosphate with release of water. Fructose 6-phosphate phosphoketolase activity means the activity of phosphate-consuming conversion of fructose 6-phosphate to erythrose 4-phosphate and acetylphosphate with release of water. These two activities may be measured with the method described by Meile et al (2001). D-xylulose 5-phosphate phosphoketolase enzyme or its encoding gene may originate from bacteria having an activity of D-xylulose 5-phosphate phosphoketolase, including lactic acid bacterium, methanol-assimilating bacterium, methane-assimilating bacterium,  Streptococcus  bacterium, and more specifically bacteria belonging to the genera  Acetobacter, Bifidobacterium, Lactobacillus, Thiobacillus, Streptococcus, Methylococus, Butyrivibrio, Fibrobacter , and/or yeast belonging to the genera  Candida, Rhodotorula, Rhodosporidium, Pichia, Yarrowia Hansenula, Kluyveromyces Saccharomyces, Trichosporon, Wingea  or the like. Fructose 6-phosphate phosphoketolase enzyme or its encoding gene may originate from bacteria having an activity of fructose 6-phosphate phosphoketolase that belong to the genera  Acetobacter, Bifidobacterium, Chlorobium, Brucella, Methylococus, Gardnerella , and/or yeast belonging to  Rhodotorula, Candida, Saccharomyces  or the like. Moreover, it has been reported that some enzymes catalyse both activities of xylulose 5-phosphate phosphoketolase and fructose 6-phosphate phosphoketolase, such as Xfp from  Bifidobacterium animalis  (WO 2006/016705 and WO 2016/044713) or from  Bifidobacterium lactis  (Meile et al., 2001). 
     In the context of the invention, the modified microorganisms overexpress phosphoketolase encoding gene so as to enhance the production of glycolic acid and/or glyoxylic acid. If the microorganisms of the invention naturally express phosphoketolase, the gene(s) encoding phosphoketolase(s) are overexpressed using overexpression methods detailed above. On the contrary, if the microorganism of the invention does not naturally express phosphoketolase, exogenous phosphoketolase encoding gene is introduced into the microorganism. In this case the introduction of the gene leads to an overexpression of the phosphoketolase encoding gene. 
     Phosphoketolase encoding genes are chosen among: xfp gene from  Bifidobacterium animalis  (WO 2006/016705 and WO 2016/044713), xfp gene from  Bifidobacterium lactis  (Meile et al., 2001), xpkA from  Lactobacillus pentosus  (Posthuma et al., 2002) or their homologous genes: xpk1 gene from  Lactobacillus plantarum , xpk2 gene from  Lactobacillus plantarum , xpk gene from  Streptococcus agalactiae  NEM316, ptk gene from  Lactococcus lactis  subsp.  lactis , xpk gene from  Lactobacillus johnsonii , xpk gene from  Lactobacillus acidophilus , xfp gene from  Bifidobacterium longum , xfp gene from  Chlorobium tepidum , xfp gene from  Brucella suis , xfp gene from  Brucella abortus . Preferentially, the microorganism overexpresses xfp gene from  Bifidobacterium lactis  (Meile et al., 2001), xfp gene from  Bifidobacterium animalis  (WO 2006/016705 and WO 2016/044713), or xpkA from  Lactobacillus pentosus  (Posthuma et al., 2002). Even more preferably, the microorganism overexpresses the xfp gene from  Bifidobacterium animalis  (WO 2006/016705 and WO 2016/044713) or the xpkA gene from  Lactobacillus pentosus  (Posthuma et al., 2002). 
     In a preferred embodiment of the invention, the modified microorganism further overexpresses ycdW or its homologous genes in order to increase the conversion of glyoxylic acid into glycolic acid as disclosed in patent application WO 2007/141316. It is an object of the invention to provide a method for the production of glycolic acid using a modified microorganism in which the expression of at least one gene chosen among aceB, glcB, gcl and eda is attenuated, the expression of xfp gene from  Bifidobacterium lactis , xfp gene from  Bifidobacterium animalis , or xpkA from  Lactobacillus pentosus  and the expression of ycdW gene and/or yiaE gene are overexpressed. Preferably, in the method for the production of glycolic acid using a modified microorganism, the expression of at least one gene chosen among aceB, glcB, gcl and eda is attenuated, the expression of the xfp gene from  Bifidobacterium animalis  or the xpkA gene from  Lactobacillus pentosus  is overexpressed, and the expression of the ycdW gene and/or the yiaE gene is overexpressed. 
     This method for production of glycolic acid allows the production of glyoxylic acid by either biological or chemical conversion from glycolic acid. This second method of the invention comprises the step of optional isolation of glycolic acid from fermentation broth, conversion of glycolic acid into glyoxylic acid either by bioconversion using a glycolate oxidase encoded by the gene gldDEFG from  Escherichia coli  and a catalase encoded by the genes katE or katG from  Escherichia coli  (Loewen &amp; Switala, 1986) or by chemical conversion for example using a nitroxyl radical catalyst such as AZADO (Furukawa et al., 2016) and recovering glyoxylic acid from conversion medium. 
     The optional isolation of glycolic acid may lie at least in the withdrawal of the microbial cells from the fermentation broth. Glycolic acid may be further purified from the other organic species from the fermentation broth as disclosed in patent applications WO2012/153041, WO2012/153042 and WO2012/153043 by successive distillations. Alternatively, glycolic acid may be purified by iterative crystallization steps as disclosed in U.S. Pat. No. 7,439,391 or by liquid-liquid extraction using adequate solvent. Usable solvents are well-known by the man in the art, whom is able to choose the most convenient solvent. Another way to purify glycolic acid from the fermentation broth is the use of fermentative extraction process also known as reactive extraction process or extractive fermentation process. Extractive fermentation could be considered as an integrated process, in which a reaction process i.e. fermentation, is combined with a purification operation, i.e. liquid extraction as disclosed in patent applications WO 2009/042950 or WO 1999/54856. The process presents the advantage of allowing the removal of glycolic acid as soon as it is produced, reducing the inhibition of cell growth due to the toxic effect of glycolic acid, and thus to produce and recover glycolic acid in one continuous step, thereby reducing the downstream processing and the recovery costs. The solvent is chosen among carbon-bonded oxygen-bearing solvents or phosphorus-bonded oxygen-bearing solvents or high-molecular weight aliphatic amines. Preferred solvents are tri-n-octyl phosphine oxide, tri-n-butyl phosphate, lauryl-trialkylmethylamine, tri-n-octylamine, tri-iso-octylamine, tri-n-(octyl-decyl)-amine, quaternary alkylammonium salt, polyethylene glycols, polyethyleneimine and polypropyleneimine. More preferentially, the solvent used is tri-n-octyl phosphine oxide. Advantageously, extractive fermentation may be completed by a subsequent step of crystallization or distillation. 
     The step of bioconversion of glycolic acid into glyoxylic acid is mediated by a glycolate oxidase also known as (S)-2-hydroxy-acid oxidase (EC number: 1.1.3.15) which catalyses the reaction:
 
glycolate+O 2   glyoxylate+H 2 O 2  
 
     This enzyme belongs to the family of oxidoreductases, specifically those acting on the CH—OH group of the donor with oxygen as the acceptor. 
     The glycolate oxidase used in this invention may correspond to any naturally occurring form of the enzyme or any variant of these naturally enzymes which exhibit better stability or catalytic efficiency. The naturally occurring glycolate oxidase used may be extracted and purified from spinach or beet leaves or, alternatively, the gene encoding these enzymes may be inserted in a producer microorganism as disclosed in patent applications WO 1994/20631 and WO 1995/01444. Alternatively, the gldDEFG genes from  Escherichia coli  encoding glycolate oxidase are overexpressed in a producer organism. The producer organism is chosen among  Pichia pastoris, Hansenula polymorpha, Aspergillus nidulans  or  Escherichia coli . Most preferably,  Escherichia coli  is used. As glycolate oxidase generates H 2 O 2 , it is advantageous to use in combination with the glycolate oxidase a catalase (EC Number: 1.11.1.6) which is endogenously expressed by  Pichia pastoris, Hansenula polymorpha, Aspergillus nidulans  or  Escherichia coli . In  Escherichia coli , catalase is encoded by katE or katG genes. Preferably, glycolate oxidase and catalase are overexpressed in the same microorganism. Use of glycolate oxidase and catalase is disclosed in patent applications WO 1996/00793, WO 1994/20631 or in WO 1995/01444. 
     For the bioconversion, the glycolate oxidase and/or catalase are contacted with the glycolic acid by adding directly the purified enzymes to the solution of glycolic acid (partially purified or not) or adding the fermentation broth of the glycolate oxidase and/or catalase producer microorganism or an extract of lysed cell of the glycolate oxidase and/or catalase producer microorganism. To improve efficiency of bioconversion, immobilized enzymes may be used after their purification as disclosed in U.S. Pat. No. 5,439,813. 
     Glyoxylic acid formed is then purified using means well known by the man skilled in the art as for instance crystallization or precipitation with calcium hydroxide or liquid-liquid extraction. 
     In another preferred embodiment of the invention, the modified microorganism is further engineered so as to attenuate expression of at least ycdW genes in order to avoid the conversion of glyoxylic acid into glycolic acid. It is also an object of the invention to provide a method for the production of glyoxylic acid using a modified microorganism in which the expression of at least one gene chosen among aceB, glcB, gcl and eda is attenuated, the expression of xfp gene from  Bifidobacterium lactis , the expression of xfp gene from  Bifidobacterium animalis , or xpkA gene from  Lactobacillus pentosus  are overexpressed and the expression of ycdW gene and yiaE gene are attenuated or completely abolished. Preferably, in the method for the production of glyoxylic acid using a modified microorganism, the expression of at least one gene chosen among aceB, glcB, gcl and eda is attenuated, and the expression of the xfp gene from  Bifidobacterium animalis  or the xpkA gene from  Lactobacillus pentosus  is overexpressed, and the expression of the ycdW gene and the yiaE gene are attenuated or completely abolished. 
     This method for production of glyoxylic acid allows the production of glycolic acid by either biological or chemical conversion from glyoxylic acid. This fourth method of the invention comprises the step of optional isolation of glyoxylic acid from fermentation broth, conversion of glyoxylic acid into glycolic acid either by bioconversion using a glyoxylate reductase or by chemical conversion using, for example, sodium borohydride and recovering glycolic acid from conversion medium. Said glyoxylate reductase can be encoded by the gene ycdW from  Escherichia coli  or by the gene grxA from  Rhizobium etli.    
     The optional isolation of glyoxylic acid may lie at least in the withdrawal of the microbial cells from the fermentation broth. Glyoxylic acid may be further purified from the other organic species from the fermentation broth by ion exchange or by precipitation/crystallization methods or by liquid-liquid extraction. Another way to purify glyoxylic acid from the fermentation broth is the use of extractive fermentation process. Extractive fermentation could be considered as an integrated process, in which a reaction process i.e. fermentation, is combined with a purification operation, i.e. liquid extraction as disclosed in patent applications WO 2009/042950 or WO 1999/54856. The process presents the advantage of allowing the removal of glyoxylic acid as soon as it is produced, reducing the inhibition of cell growth due to the toxic effect of glyoxylic acid, and thus to produce and recover glyoxylic acid in one continuous step, thereby reducing the downstream processing and the recovery costs. The solvent is chosen among carbon-bonded oxygen-bearing solvents or phosphorus-bonded oxygen-bearing solvents or high-molecular weight aliphatic amines. Preferred solvents are tri-n-octyl phosphine oxide, tri-n-butyl phosphate, lauryl-trialkylmethylamine, tri-n-octylamine, tri-iso-octylamine, tri-n-(octyl-decyl)-amine, quaternary alkylammonium salt, polyethylene glycols, polyethyleneimine and polypropyleneimine. More preferentially, the solvent used is tri-n-octyl phosphine oxide. Advantageously, extractive fermentation may be completed by a subsequent step of crystallization or distillation. Alternatively, to the extractive fermentation, to decrease toxicity of glyoxylic acid produced during fermentation, it is possible to add in the fermentation broth a molecule known to form a complex with aldehydes such as semicarbazide, or carbohydrazide or 2,4-dinitrophenylhydrazine so as to complex with glyoxylic acid and thus reduce toxicity of the aldehyde for the cells. This could allow for an improved productivity of glyoxylic acid (Sardari et al., 2014). 
     The step of bioconversion of glyoxylic acid into glycolic acid is mediated by a glyoxylate reductase. Glyoxylate reductase was first isolated from spinach leaves and is an enzyme that catalyses the reduction of glyoxylic acid to glycolic acid, using the cofactor NADH or NADPH. Glycolate reductase may be NADH dependent (EC Number: 1.1.1.26) or NADPH dependent (EC Number: 1.1.1.79). Examples of NADH dependent glyoxylate reductases which could be used are: GxrA encoded by gxrA gene from  Rhizobium etli , GOR1 encoded by GOR1 gene from  Saccharomyces cerevisiae , HprA encoded by hprA gene from  Methylobacterium extorquens  or GyaR encoded by gyaR gene from  Pyrococcus furiosus . Examples of NADPH dependent glyoxylate reductases are: YcdW encoded by ycdW gene from  Escherichia coli , YiaE encoded by yiaE gene from  Escherichia coli , YjbG encoded by yjgB gene from  Escherichia coli , YafB encoded by yafB gene from  Escherichia coli , YqhD encoded by yqhD gene from  Escherichia coli  or GLYR1 and GLYR2 encoded by GLYR1 and GLYR2 genes from  Arabidopsis thaliana  . . . . Advantageously, the glyoxylate reductase producer microorganism is  Escherichia coli.    
     For the bioconversion of glyoxylic acid into glycolic acid, the glyoxylate reductase enzymes require either NADH or NADPH cofactors. In such a case, the bioconversion needs to be done in presence of the specific enzyme and living cells able to produce and regenerate the cofactors during the reaction. This is a so called whole-cell biocatalyst system. A solution of glyoxylic acid partially purified or not is therefore contacted with the fermentation broth of the glyoxylate reductase producer microorganism having overproduced the glyoxylate reductase enzyme and being still viable in order to produce and regenerate the cofactors. To improve efficiency of bioconversion, immobilized enzymes may be used but still in presence of a living microorganism pre-treated in order to help the reaction. 
     Glycolic acid formed is then purified using means well-known by the man skilled in the art such as distillation, crystallization, precipitation with calcium hydroxide or liquid-liquid extraction. 
     In another embodiment of the invention, glyoxylic acid may be produced by bioconversion of glycine (CAS Number: 56-40-6) by using a glycine oxidase. Glycine oxidase (EC Number: 1.4.3.19) catalyses the reaction:
 
Glycine+H 2 O+O 2   glyoxylate+NH 3 +H 2 O 2  
 
     The glycine oxidase used in this invention may correspond to any naturally occurring form of the enzyme or any variant of these naturally enzymes which exhibit better stability or catalytic efficiency as disclosed in patent application US 2016/0002610. Glycine oxidase has been identified in  Bacillus subtilis  (Nishiya &amp; Imanaka, 1998; Job et al., 2002). This enzyme may be extracted and purified from  Bacillus subtilis  or, alternatively, the gene encoding this enzyme may be inserted in a producer microorganism. The producer organism is preferably  Escherichia coli . As glycine oxidase generates H 2 O 2 , it is advantageous to use, in combination with the glycine oxidase, a catalase (EC Number: 1.11.1.6) which is endogenously expressed by  Escherichia coli  by katE and katG genes. Preferably, glycine oxidase and catalase are overexpressed in the same microorganism. 
     For the bioconversion, the glycine oxidase and/or catalase are contacted with glycine by adding the purified enzymes directly to the solution of glycine or by adding the fermentation broth of the glycine oxidase and/or catalase producer microorganism or an extract of lysed cell(s) of the glycine oxidase and/or catalase producer microorganism. To improve efficiency of bioconversion, immobilized enzymes may be used after their purification as disclosed in U.S. Pat. No. 5,439,813. 
     Finally, the invention relates to a microorganism modified for the production of glycolic acid and/or glyoxylic acid wherein expression of at least one gene chosen among aceB, glcB, gcl and eda is attenuated and expression of at least one gene encoding phosphoketolase is enhanced. The phosphoketolase is encoded by xfp gene from  Bifidobacterium animalis , xfp gene from  Bifidobacterium lactis , or xpkA gene from  Lactobacillus pentosus.    
     For the production of glycolic acid, the microorganism of the invention is further modified to overexpress ycdW gene from  Escherichia coli  or at least one of its homologous genes. 
     For the production of glyoxylic acid, the microorganism of the invention is further modified to attenuate or completely abolish the expression of at least the gene ycdW. 
     The microorganism of the invention is chosen among Enterobacteriaceae, Clostridiaceae, Corynebacteriaceae, Bacillaceae, Bifidobacteriaceae, Lactobacillaceae, or yeast. More preferentially, the microorganism of the invention is from the  Escherichia coli  species. 
     EXAMPLES 
     The following experiments demonstrate how to produce glycolic acid or glyoxylic acid, using a modified glycolic acid producer  E. coli  recombinant strain as background. 
     In the examples given below, methods well-known in the art were used to construct  E. coli  strains containing replicating vectors and/or various chromosomal insertions, deletions, and substitutions using homologous recombination, as is well-described by Datsenko &amp; Wanner, (2000). 
     Protocols 
     Several protocols have been used to construct the glyoxylic acid producing strains described in the following examples. 
     Protocol 1 (Chromosomal modifications by homologous recombination, selection of recombinants and antibiotic cassette excision flanked by FRT sequences) and protocol 2 (Transduction of phage P1) used in this invention have been fully described in patent application WO 2013/001055. 
     Protocol 3: Antibiotic cassette excision flanked by LoxP sequences 
     The resistance genes flanked by LoxP sequences were removed by using plasmid pJW168 (Palmeros et al., 2000) carrying gene coding for the Cre recombinase. Briefly, the clones harbouring the pJW168 plasmid were cultivated at 37° C. or 42° C. on LB and then tested for loss of antibiotic resistance at 30° C. Antibiotic sensitive clones were then verified by PCR using adequate primers. 
     Example 1: Suppression of the Overexpression of ycdW Gene in an  E. coli  Glycolic Acid Overproducer Recombinant Strain—Description of Strain 1 and Construction of Strains 2 to 3 
     Description of Strain 1 
     The strain described in Example 2, part 2 of patent application WO 2011/157728, and which corresponds to the parental strain of AG1413, i.e. the AG1413 strain without the pME101-ycdW-TT07-PaceA-aceA-TT01 plasmid, will be named strain 1 in the current patent application. 
     Construction of Strain 2 
     Before using strain 1, the antibiotic resistance cassettes were removed from loci icd (SEQ ID No 01, coding the protein having the sequence of SEQ ID No 02) and aceK (SEQ ID No 03, coding the protein having the sequence of SEQ ID No 04), using the Flp recombinase (according to Protocol 1) and the Cre recombinase (according to Protocol 3), respectively. The kanamycin and chloramphenicol sensible transformants were then selected and the absence of the antibiotic markers was verified by PCR analysis with appropriate oligonucleotides. The strain retained was named strain 2. 
     Construction of Strain 3 
     The ycdW gene (SEQ ID No 05, coding the protein having the sequence of SEQ ID No 06), coding for the glyoxylate/hydroxypyruvate reductase, and carried by the pME101-ycdW-TT07-PaceA-aceA-TT01 plasmid described in patent application WO 2010/108909, was removed from this plasmid, together with its promoter and lacI gene (SEQ ID No 07, coding the protein having the sequence of SEQ ID No 08), by restriction enzymes and ligase, giving the pAG0094 plasmid. 
     The plasmid pAG0094 was introduced into strain 2, giving rise to strain 3. 
     Example 2: Improving Glyoxylic Acid Production by a Complete Removal of ycdW Gene in an  E. coli  Glycolic Acid Overproducer Recombinant Strain—Construction of Strains 4 to 6 
     The endogenous copy of ycdW gene was deleted in  E. coli  recombinant glycolic acid producer strain 2. 
     To achieve the deletion of ycdW gene, the homologous recombination strategy described by Datsenko &amp; Wanner, 2000 (according to Protocol 1) was used. 
     For ycdW deletion, a fragment carrying a resistance marker flanked by DNA sequences homologous to upstream and downstream regions of the ycdW gene was PCR amplified by the overlapping PCR technique (overlapping oligonucleotides). The sequences for recombination into upstream and downstream regions of ycdW gene are referred to as SEQ ID No 09 and 10. The PCR product obtained “ΔycdW::Km” was then introduced by electroporation into the strain MG1655 (pKD46). The antibiotic resistant transformants were selected and the deletion of ycdW gene with the associated resistance cassette was verified by a PCR analysis with appropriate oligonucleotides. The strain retained was designated MG1655 ΔycdW::Km. Finally, the ΔycdW::Km deletion was transferred by P1 phage transduction (according to Protocol 2) from MG1655 ΔycdW::Km into strain 2. Kanamycin resistant transductants were selected and the presence of LycdW::Km deletion was verified by a PCR analysis with appropriate oligonucleotides. The strain retained was named strain 4. 
     The endogenous copy of aceA gene (SEQ ID No 11, coding the protein having the sequence of SEQ ID No 12) was then deleted in strain 4. 
     To achieve the deletion of aceA gene, the homologous recombination strategy described by Datsenko &amp; Wanner, 2000 (according to Protocol 1) was used. As aceB (SEQ ID No 13, coding the protein having the sequence of SEQ ID No 14) and aceK (SEQ ID No 03, coding the protein having the sequence of SEQ ID No 04) genes have been previously deleted in strain 4, the homologous recombination strategy is equivalent to the one used to delete aceBAK operon. 
     For aceBAK deletion, a fragment carrying an antibiotic resistance marker flanked by DNA sequences homologous to upstream and downstream regions of the aceBAK operon was PCR amplified by the overlapping PCR technique (overlapping oligonucleotides). The sequences for recombination into upstream and downstream regions of aceBAK operon are referred as SEQ ID No 15 and 16. The PCR product obtained “ΔaceBAK::Cm” was then introduced by electroporation into the strain MG1655 (pKD46). The antibiotic resistant transformants were selected and the deletion of aceBAK operon with the associated resistance cassette was verified by a PCR analysis with appropriate oligonucleotides. The strain retained was designated MG1655 ΔaceBAK::Cm. Finally, the ΔaceBAK::Cm deletion was transferred by P1 phage transduction (according to Protocol 2) from MG1655 ΔaceBAK::Cm into strain 4. Chloramphenicol resistant transductants were selected and the presence of ΔaceBAK::Cm deletion was verified by a PCR analysis with appropriate oligonucleotides. The strain retained was named strain 5. 
     The ycdW gene (SEQ ID No 05 and No 06), coding for the glyoxylate/hydroxypyruvate reductase, and carried by the pME101-ycdW-TT07-PaceA-aceA-TT01 plasmid, was removed from this plasmid by restriction enzymes and ligase, without deleting its promoter and the lacI gene (unlike pAG0094 described in Example 1), giving the pAG0303 plasmid. Therefore, on this plasmid, a part of aceA gene expression is driven via its natural promoter, but it is also possible to increase the level of aceA expression by adding IPTG via the remaining promoter of the ycdW and lacI genes. 
     The plasmid pAG0303 was introduced into strain 5, giving rise to strain 6. 
     Example 3: Glycolic Acid and Glyoxylic Acid Production by Fermentation with Strains 3 and 6 
     Production strains were evaluated in Erlenmeyer baffled flasks. 
     A 5 mL preculture was grown at 37° C. for 16 hours in a mixed medium (10% LB medium (Sigma 25%) with 2.5 g·L −1  glucose and 90% minimal medium M1). It was used to inoculate a 50 mL culture to an OD 600  of 0.2 in medium M1. The composition of medium M1 is described in Table 1. 
     When necessary, antibiotics were added in the medium (spectinomycin at a final concentration of 50 mg·L −1 ) and IPTG at a final concentration of 100 μM. The temperature of the cultures was 30° C. 
     When the culture had reached an OD 600  up to 5, extracellular metabolites were analysed using HPLC with refractometric detection (organic acids and glucose). 
     For each strain, several repetitions were performed. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Composition of M1 medium. 
               
            
           
           
               
               
               
            
               
                   
                 Compound 
                 Concentration (g · L −1 ) 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                   
                 Citric acid•H 2 O 
                 6.00 
               
               
                   
                 MgSO 4 •7H 2 O 
                 1.00 
               
               
                   
                 CaCl 2 •2H 2 O 
                 0.04 
               
               
                   
                 CoCl 2 •6H 2 O 
                 0.0080 
               
               
                   
                 MnSO 4 •H 2 O 
                 0.0200 
               
               
                   
                 CuCl 2 •2H 2 O 
                 0.0020 
               
               
                   
                 H 3 BO 3   
                 0.0010 
               
               
                   
                 Na 2 MoO 4 •2H 2 O 
                 0.0004 
               
               
                   
                 ZnSO 4 •7H 2 O 
                 0.0040 
               
               
                   
                 Na 2 HPO 4   
                 2.00 
               
               
                   
                 KH 2 PO 4   
                 10.48 
               
               
                   
                 K 2 HPO 4 •3H 2 O 
                 10.48 
               
               
                   
                 (NH 4 ) 2 HPO 4   
                 8.00 
               
               
                   
                 (NH 4 ) 2 SO 4   
                 5.00 
               
               
                   
                 NH 4 Cl 
                 0.13 
               
               
                   
                 FeSO 4 •7H 2 O 
                 0.04 
               
               
                   
                 Thiamin 
                 0.0100 
               
               
                   
                 MOPS 
                 40 
               
               
                   
                 Glucose 
                 10 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Glycolic Acid (GA) and Glyoxylic Acid (GxA) titers and 
               
               
                 yields (Y) in g of GA or GxA per gram of sugar consumed 
               
               
                 for the producing strains in shake flasks. For the precise 
               
               
                 definition of GA or GxA/glucose yields see below. 
               
            
           
           
               
               
               
            
               
                   
                 Strain 3 
                 Strain 6 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                   
                 [GA] (g · L −1 ) 
                 1.1 
                 0.4 
               
               
                   
                 [GxA] (g · L −1 ) 
                 0.2 
                 0.8 
               
               
                   
                 Y GA (g · g −1 ) 
                 0.11 
                 0.03 
               
               
                   
                 Y GxA (g · g −1 ) 
                 0.02 
                 0.10 
               
               
                   
                   
               
            
           
         
       
     
     As can be seen in Table 2 above, upon the deletion of ycdW copies and the controlled induction of the aceA gene, strain 6 produced much higher amounts of Glyoxylic Acid than strain 3. As expected, this production of glyoxylic acid is linked to the drop of Glycolic Acid synthesis in strain 6. 
     The GA and GxA yields were expressed as follows: 
     
       
         
           
             
               Y 
               GA 
             
             = 
             
               
                 GA 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   ( 
                   g 
                   ) 
                 
               
               
                 consumed 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 glucose 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   ( 
                   g 
                   ) 
                 
               
             
           
         
       
       
         
           
             
               Y 
               GxA 
             
             = 
             
               
                 GxA 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   ( 
                   g 
                   ) 
                 
               
               
                 consumed 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 glucose 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   ( 
                   g 
                   ) 
                 
               
             
           
         
       
     
     Example 4: Improving Glycolic Acid Production by Overexpression of Heterologous Gene Coding for Phosphoketolase Enzyme in an  E. coli  Glycolic Acid Overproducer Recombinant Strain—Construction of Strains 7 to 12 
     Construction of Strains 7 to 9: Reconstruction of  E. coli  Pta Gene Coding for Phosphate Acetyltransferase in an  E. coli  Glycolic Acid Overproducer Recombinant Strain 
     In strain AG1413 and its parental strain, strain 1, both ackA (SEQ ID No 17, coding the protein having the sequence of SEQ ID No 18) and pta (SEQ ID No 19, coding the protein having the sequence of SEQ ID No 20) genes, which are organized in operons, have been previously deleted. In order to again convert acetyl-phosphate (which is generated by phosphoketolase in the following strains) into acetyl-CoA, the pta gene has been reconstructed into a strain 1 background, while at the same time conserving the deletion of ackA gene. 
     To achieve the reconstruction of the pta gene, the ΔackA::Km mutant from the Keio collection, deleted only for the ackA gene and carrying a wildtype pta gene, was used. The ΔockA::Km deletion associated with the pta wildtype gene was transferred by P1 phage transduction (according to Protocol 2) from the ΔackA::Km Keio mutant to strain 2. Kanamycin resistant transductants were selected and the presence of both the ΔackA::Km deletion and pta wildtype gene were verified by a PCR analysis with appropriate oligonucleotides. The strain retained was named strain 7. 
     Then the antibiotic cassette was removed from the ΔackA deletion using the Flp recombinase (according to Protocol 1) in strain 7. The kanamycin sensible transformants were selected and the absence of the antibiotic marker was verified by PCR analysis with appropriate oligonucleotides. The strain retained was named strain 8. 
     For the Phosphate acetyltransferase activity assay, the phosphate-dependent release of CoA from acetyl-CoA was monitored with Ellman&#39;s thiol reagent, 5,5′-dithiobis(2-nitrobenzoic acid) (DTNB), by measuring at 30° C. the formation of the thiophenolate anion at 412 nm (ε 412 =13.6 mM −1  cm −1 ). The assay mixture (1 ml) contained 100 mM KH 2 PO 4  (pH 8), 0.5 mM DTNB, and 1 mM acetyl-CoA. The activity value determined without acetyl-CoA in the assay was subtracted. Specific activity is expressed as milli units (mUI) per milligram of protein. 
     The phosphate acetyltransferase activity of the strain 7 is 10 times higher than that of strain 8 (1100 mUI/mg for strain 7 and 110 mUI/mg for strain 8). 
     Then the plasmid pME101-ycdW-TT07-PaceA-aceA-TT01, described in patent application WO2010/108909, was introduced into strain 8, giving rise to strain 9. 
     Construction of Strains 10 to 12: Overexpression of Heterologous Genes Coding for Phosphoketolase Enzyme in an  E. coli  Glycolic Acid Overproducer Recombinant Strain 
     In order to increase the acetyl-CoA pool, and thereby glycolic acid production, different phosphoketolases were overproduced in an  E. coli  glycolic acid overproducer strain. 
     Three different genes coding for phosphoketolase enzymes were overexpressed individually in strain 9:
         xfp (SEQ ID No 21, coding the protein having the sequence of SEQ ID No 22) from  Bifidobacterium animalis,      fxpk (SEQ ID No 23 coding the protein having the sequence of SEQ ID No 24) from  Bifidobacterium adolescentis,      xpk (SEQ ID No 25 coding the protein having the sequence of SEQ ID No 26) from  Lactobacillus pentosus.          

     For each gene (identified by the “O1ec” suffix), a codon-harmonized version optimized for production in  E. coli  was synthetically synthesised by GeneArt Gene Synthesis Service from Thermo Fisher Scientific. Each gene was subcloned on a pBBR1MCS5 vector (Kovach et al, 1995), together with the artificial Ptrc promoter (the artificial promoter is the one described for the overexpression of cysPUWAM operon in patent application WO 2009/043803; Brosius et al, 1985), using the appropriate oligonucleotides, giving respectively the plasmids detailed in Table 3, below. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Plasmids for the overexpression of phosphoketolase genes 
               
            
           
           
               
               
               
               
            
               
                   
                 Name of codon- 
                   
                   
               
               
                   
                 harmonized 
               
               
                   
                 version of gene 
                   
                 Name of plasmids for 
               
               
                   
                 coding for the 
                 SEQ ID 
                 overexpression of 
               
               
                 Origin organism 
                 phosphoketolase 
                 No 
                 phosphoketolase genes 
               
               
                   
               
               
                 
                   Bifidobacterium 
                 
                 xfpO1ec 
                 27 
                 pBBR1MCS5-Ptrc01- 
               
               
                 
                   animalis 
                 
                   
                   
                 xfpO1ec 
               
               
                 
                   Bifidobacterium 
                 
                 fxpkO1ec 
                 28 
                 pBBR1MCS5-Ptrc01- 
               
               
                 
                   adolescentis 
                 
                   
                   
                 fxpkO1ec 
               
               
                 
                   Lactobacillus 
                 
                 xpkO1ec 
                 29 
                 pBBR1MCS5-Ptrc01- 
               
               
                 
                   pentosus 
                 
                   
                   
                 xpkO1ec 
               
               
                   
               
            
           
         
       
     
     Each plasmid was individually introduced into strain 9, giving rise to the strains described in Table 4, below. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Strains comprising plasmids for the overexpression 
               
               
                 of phosphoketolase genes: 
               
            
           
           
               
               
            
               
                   
                 Name of resulting strains 
               
               
                 Name of plasmids for overexpression of 
                 corresponding to strain 
               
               
                 phosphoketolase genes 
                 9 carrying the plasmids 
               
               
                   
               
               
                 pBBR1MCS5-Ptrc01-xfpO1ec 
                 Strain 10 
               
               
                 pBBR1MCS5-Ptrc01-fxpkO1ec 
                 Strain 11 
               
               
                 pBBR1MCS5-Ptrc01-xpkO1ec 
                 Strain 12 
               
               
                   
               
            
           
         
       
     
     The pBBR1 plasmid stably replicates at moderate copy number. 
     In order to have several levels of overexpression of phosphoketolase genes, the genes were also cloned into the following plasmids with a known different number of copies into the cell:
         the low copy number pCL1920 vector (Lerner &amp; Inouye, 1990) pME101-ycdW-TT07-PaceA-aceA-TT01 plasmid,   and the one copy number bacterial artificial chromosome pBAC plasmid (Epicentre®).       

     Phosphoketolase overexpression plasmids, together with pME101-ycdW-TT07-PaceA-aceA-TT01 plasmid, were also introduced in strain 7 and 8. 
     Performances in Shake Flasks and Reactors 
     First, strains were evaluated in shake flasks as described in Example 3. 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Glycolic Acid (GA) yields in g of GA per gram of sugar consumed 
               
               
                 for the producing strains in shake flasks. For the precise 
               
               
                 definition of GA/glucose yields see Example 3. 
               
            
           
           
               
               
            
               
                   
                 GA Yield (g · g −1 ) 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                   
                 Strain 9 
                 0.286 
               
               
                   
                 Strain 10 
                 0.395 
               
               
                   
                 Strain 11 
                 0.337 
               
               
                   
                 Strain 12 
                 0.389 
               
               
                   
                   
               
            
           
         
       
     
     As shown in Table 5, upon overexpression of all the tested phosphoketolase genes, the Glycolic Acid production yield increased. The yield was higher with the overexpression of the phosphoketolase genes from  Bifidobacterium animalis  and from  Lactobacillus pentosus.    
     These strains were evaluated in 2 L fermenters (Pierre Guerin) using a fedbatch strategy. 
     A first preculture in tubes was carried out at 37° C. for 10 hours in a mixed medium (10% LB medium (Sigma 25%) with 2.5 g·L −1  glucose and 90% minimal medium M1). It was used to inoculate a 50 mL second seed to an OD 600  of 0.2 in medium M1. This step of preculture was carried out at 37° C. in 500 mL Erlenmeyer flask filled with 50 mL of synthetic medium (M1) supplemented with 40 g·L −1  of MOPS and 10 g·L −1  of glucose. This second preculture was used for the inoculation of the fermentor after getting an OD 600 nm  close to 9. 
     The reactor filled with 700 mL of synthetic medium (M2) supplemented with 20 g·L −1  of glucose, was inoculated at an initial optical density of about 0.5. The composition of M2 medium is described in Table 6. The culture was carried out at 30° C. with agitation and the dissolved oxygen concentration was maintained at values between 20 and 40%, preferentially 30% saturation, by increasing the agitation and aeration. The pH was adjusted to 6.8 by automatic addition of NH 4 OH/NaOH solution (15/5 w/w). 
     The culture was conducted in a batch mode until exhaustion of glucose. At that time, a solution of 700 g·L −1  glucose supplemented with magnesium sulfate, oligo-elements and thiamine was added to restore a concentration of 20 g·L −1  of glucose in the reactor. Further additions were performed each time that glucose was once again exhausted. After the fifth pulse, the pH was increased to the value of 7.4 thanks to a ramp of three hours. 
     The cultures were stopped after 40 to 45 hours. Extracellular metabolites were analysed using HPLC with refractometric detection (organic acids and glucose). 
     For each strain, several repetitions were performed. The final performances are presented in Table 7. 
     
       
         
           
               
             
               
                 TABLE 6 
               
             
            
               
                   
               
               
                 Composition of M2 medium. 
               
            
           
           
               
               
               
            
               
                   
                 Compound 
                 Concentration (g · L −1 ) 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                   
                 Citric acid•H 2 O 
                 3.00 
               
               
                   
                 MgSO 4 •7H 2 O 
                 1.00 
               
               
                   
                 CaCl 2 •2H 2 O 
                 0.04 
               
               
                   
                 CoCl 2 •6H 2 O 
                 0.0080 
               
               
                   
                 MnSO 4 •H 2 O 
                 0.0200 
               
               
                   
                 CuCl 2 •2H 2 O 
                 0.0020 
               
               
                   
                 H 3 BO 3   
                 0.0010 
               
               
                   
                 Na 2 MoO 4 •2H 2 O 
                 0.0004 
               
               
                   
                 ZnSO 4 •7H 2 O 
                 0.0040 
               
               
                   
                 KH 2 PO 4   
                 0.70 
               
               
                   
                 K 2 HPO 4 •3H 2 O 
                 1.17 
               
               
                   
                 NH 4 H 2 PO 4   
                 2.99 
               
               
                   
                 (NH 4 ) 2 HPO 4   
                 3.45 
               
               
                   
                 (NH 4 ) 2 SO 4   
                 8.75 
               
               
                   
                 NH 4 Cl 
                 0.13 
               
               
                   
                 FeSO 4 •7H 2 O 
                 0.04 
               
               
                   
                 Thiamin 
                 0.0100 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 7 
               
             
            
               
                   
               
               
                 Final performances of strains 9, 10 and 12 in 2 L reactors. 
               
            
           
           
               
               
               
               
            
               
                 Strain 
                 GA final Titer 
                 GA final productivity 
                 GA final Yield 
               
               
                   
               
               
                 Strain 9 
                 Reference 
                 Reference 
                 Reference 
               
               
                 Strain 10 
                 ~ 
                 ~ 
                 ++ 
               
               
                 Strain 12 
                 ~ 
                 ~ 
                 + 
               
               
                   
               
               
                 The symbol ~ indicates that the variation of the parameter is between −5% and 5% compared to the reference strain. The symbol + indicates an increase between 5 and 10% and the symbol ++ indicates an increase between 10 and 20%. 
               
            
           
         
       
     
     Upon overexpression of phosphoketolase genes, the production yield of strains 10 and 12 shows an increase between 5 to 20% according to the nature of the heterologous gene tested. We did not see any effect on the titer or on the productivity. With these results, the benefit of the phosphoketolase activity on GA production was confirmed. 
     Levels of expression of the phosphoketolase genes did not change the tendency of the results. Glycolic acid production was improved with different levels of overexpression of the 3 genes; xfpO1ec, fxpkO1ec, xpkO1e, carried on pCL1920 or on pBAC (data not shown). 
     Yields are Calculated as Follows: 
     The fermentor volume was calculated by adding to the initial volume the amount of solutions added to regulate the pH and to feed the culture and by subtracting the volume used for sampling and lost by evaporation. 
     The fedbatch volume was followed continuously by weighing the feeding stock. The amount of injected glucose was then calculated on the basis of the injected weight, the density of the solution and the glucose concentration controlled by the method of Brix ([Glucose]). The GA yield was expressed as follows: 
     
       
         
           
             
               Y 
               GA 
             
             = 
             
               
                 
                   
                     GA 
                     f 
                   
                   * 
                   
                     V 
                     f 
                   
                 
                 - 
                 
                   
                     GA 
                     0 
                   
                   * 
                   
                     V 
                     0 
                   
                 
               
               
                 Consumed 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 glucose 
               
             
           
         
       
     
     GA 0  and GA f  correspond respectively to the initial and final GA concentrations, and V 0  and V f  to the initial and the final volumes. The consumed glucose is expressed in g. 
     D-Xylulose 5-Phosphate and Fructose 6-Phosphate Phosphoketolase Assays on Crude Extract (X5PPK and F6PPK) 
     Phosphoketolase activity was measured spectrophotometrically as ferric acetyl hydroxamate produced from the enzymatically generated acetyl phosphate according to Racker et al, 1962 and Meile et al, 2001. The standard reaction mixture of 0.075 ml consisted of 33.3 mM potassium phosphate (pH 6.5), L-cysteine hydrochloride (1.9 mM), sodium fluoride (23 mM), sodium iodoacetate (8 mM), either D-fructose 6-phosphate (F6P) (100 mM) or D-xylulose 5-phosphate (X5P) (27 mM) as a substrate and the crude extract to initiate the reaction. After incubating at 37° C. for 10 or 30 min, the enzymatic reaction was stopped by the addition of 0.075 ml of hydroxylamine hydrochloride (2 M, pH 6.5). After ten minutes at room temperature, 0.05 ml of 15% (wt/vol) trichloroacetic acid, 0.05 ml of 4 M HCl, and 0.05 ml of FeCl 3 ×6 H 2 O (5% [wt/vol] in 0.1 M HCl) were added for the final color development of the ferric hydroxamate. After incubation at 25° C. under agitation for 5 min, the mix was centrifuged for 5 min at 2250×g, 200 μl of supernatant was transferred to a new micro plate to measure the absorbance. The formation of ferric hydroxamate was then spectrophotometrically quantified at 505 nm by comparing to a series of acetyl phosphate standards between 1.5 mM and 150 mM. One unit of phosphoketolase activity is defined as the amount of extract forming 1 mmol of acetyl phosphate per min from either F6P or X5P. The activity value determined without substrate in the assay was subtracted. Specific activity is expressed as milli units per milligram of protein. 
     The D-Xylulose 5-Phosphate and Fructose 6-Phosphate phosphoketolase activities of strains 10 to 12 are provided in Table 8. 
     
       
         
           
               
             
               
                 TABLE 8 
               
             
            
               
                   
               
               
                 D-Xylulose 5-Phosphate and 6-Phosphate phosphoketolase 
               
               
                 activities measured on crude extract 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 X5PPK activity 
                 F6PPK activity 
               
               
                   
                 Strain 
                 mUI/mg 
                 mUI/mg 
               
               
                   
                   
               
               
                   
                 Strain 9 
                 ND 
                 ND 
               
               
                   
                 Strain 10 
                 690 
                 250 
               
               
                   
                 Strain 11 
                 115 
                  30 
               
               
                   
                 Strain 12 
                 650 
                 ND 
               
               
                   
                   
               
               
                   
                 ND: Not detectable 
               
            
           
         
       
     
     The phosphoketolase enzymes encoded by xfp from  Bifidobacterium animalis  and fxpk from  Bifidobacterium adolescentis  expressed in  E. coli  catalyse both activities on Xylulose 5-phosphate and fructose 6-phopshate substrates. Nevertheless, these enzymes are more active on the substrate Xylulose 5-phosphate than on the fructose 6-phopshate as can be seen in Table 8. In contrast, the phosphoketolase enzyme encoded by xpk from  Lactobacillus pentosus  expressed in  E. coli  was exclusively active on the substrate Xylulose 5-phosphate. 
     Results of this example, showing Glycolic Acid production of strains 10, 11 and 12 that carry different phosphoketolase (PK) enzymes as compared to a referent strain 9 without any PK, demonstrate that the phosphoketolase activity improves the yield of Glycolic Acid per glucose consumed regardless of the substrate specificity of the enzyme (X5PPK or F6PPK). 
     Example 5: Improving Glyoxylic Acid Production by Both Suppression of ycdW Expression and Overexpression of Heterologous Genes Coding for Phosphoketolase Enzyme in an  E. coli  Glycolic Acid Overproducer Recombinant Strain—Construction of Strains 13 to 17 
     Construction of Strains 13 and 14: Suppression of Expression of  E. coli  ycdW Gene Coding for Glyoxylate/Hydroxypyruvate Reductase in ΔackA+pta AG1413 Strain 
     The strain AG1413 possesses two copies of the ycdW gene, one on the chromosome and one on the pME101-ycdW-TT07-PaceA-aceA-TT01 plasmid. 
     To delete the chromosomal copy of ycdW in a strain AG1413 background, the ΔycdW::Km deletion, described in Example 2, was transferred by P1 phage transduction (according to Protocol 2) from MG1655 ΔycdW::Km into strain 2. Kanamycin resistant transductants were selected and the presence of ΔycdW::Km deletion was verified by a PCR analysis with appropriate oligonucleotides. The strain retained was named strain 13. 
     Then, the plasmid pAG0303 described in Example 2, overexpressing aceA gene without the ycdW gene, was introduced into strain 13, giving rise to strain 14. 
     Construction of Strains 15 to 16: Suppression of Expression of the  E. coli  ycdW Gene Coding for Glyoxylate/Hydroxypyruvate Reductase in Pta Wild-Type Strain 9 
     The strain 9 possesses two copies of the ycdW gene, one on the chromosome and one on the pME101-ycdW-TT07-PaceA-aceA-TT01 plasmid. 
     To delete the chromosomal copy of ycdW in a strain 9 background, the ΔycdW::Km deletion, described in Example 2, was transferred by P1 phage transduction (according to Protocol 2) from MG1655 ΔycdW::Km into strain 8. Kanamycin resistant transductants were selected and the presence of ΔycdW::Km deletion was verified by a PCR analysis with appropriate oligonucleotides. The strain retained was named strain 15. 
     Then the plasmid pAG0303 described in Example 2, overexpressing aceA gene without the ycdW gene, was introduced into strain 15, giving rise to strain 16. 
     Construction of Strain 17: Overexpression of  Bifidobacterium animalis  Gene Coding for Phosphoketolase Enzyme in an  E. coli  Glyoxylic Acid Overproducer Recombinant Strain 
     The codon-harmonized version of  Bifidobacterium animalis  xfpO1ec gene coding for phosphoketolase enzyme was overexpressed into strain 16. 
     To overexpress this gene into strain 16, the plasmid pBBR1MCS5-Ptrc01-xfpO1ec described in Example 4, was introduced into strain 16, giving rise to strain 17. 
     In order to test different levels of overexpression of the phosphoketolase gene, the xfpO1ec gene was also cloned into a plasmid with a smaller copy number than pBBR1: the low copy number pCL1920 vector (Lerner &amp; Inouye, 1990), pAG303 described in Example 2. 
     All modifications described above were also made in a strain 7 background. 
     The same work was also performed for both of the other phosphoketolase genes, fxpkO1ec and xpkO1ec. 
     Performances in Shake Flasks 
     Strains were evaluated in shake flasks as described in Example 3. 
     
       
         
           
               
             
               
                 TABLE 9 
               
             
            
               
                   
               
               
                 Glyoxylic Acid (GxA) yields in g of GxA per gram of sugar 
               
               
                 consumed for the producing strains in shake flasks. For the 
               
               
                 precise definition of GxA/glucose yields see Example 3. 
               
            
           
           
               
               
               
            
               
                   
                 Strain 
                 GxA Yield (g · g −1 ) 
               
               
                   
                   
               
               
                   
                 Strain 14 
                 0.02 
               
               
                   
                 Strain 16 
                 0.02 
               
               
                   
                 Strain 17 
                 0.03 
               
               
                   
                   
               
            
           
         
       
     
     As can be seen in Table 9 above, the overexpression of phosphoketolase gene from  Bifidobacterium animalis  slightly increased the production of Glyoxylic Acid (strain 17 versus strain 14). 
     The same results, an increase in glyoxylic acid production, were also obtained with the other phosphoketolase enzymes tested (data not shown). 
     D-Xylulose 5-Phosphate and Fructose 6-Phosphate Phosphoketolase Assays on Crude Extract (X5PPK and F6PPK) 
     Phosphoketolase activity was measured according to the protocol described above in Example 4. The D-Xylulose 5-Phosphate and Fructose 6-Phosphate phosphoketolase activities of strains 16 and 17 are provided in Table 10. 
     
       
         
           
               
             
               
                 TABLE 10 
               
             
            
               
                   
               
               
                 D-Xylulose 5-Phosphate and 6-Phosphate phosphoketolase 
               
               
                 activities measured on crude extract 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 X5PPK activity 
                 F6PPK activity 
               
               
                   
                 Strain 
                 mUI/mg 
                 mUI/mg 
               
               
                   
                   
               
               
                   
                 Strain 16 
                 ND 
                 ND 
               
               
                   
                 Strain 17 
                 710 
                 300 
               
               
                   
                   
               
               
                   
                 ND: Not detectable 
               
            
           
         
       
     
     The phosphoketolase enzyme encoded by xfp from  Bifidobacterium animalis  and expressed in  E. coli  catalyses both activities on Xylulose 5-phosphate and fructose 6-phopshate substrates. The enzyme was more active on the substrate Xylulose 5-phosphate. 
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