Patent Publication Number: US-11655486-B2

Title: Microorganism and method for improved 1,3-propanediol production by fermentation on a culture medium with high glycerine content

Description:
The present invention relates to a new method and microorganism for the production of 1,3-propanediol from a culture medium with high glycerol content, preferably wherein said glycerol is industrial glycerine. More particularly, the microorganism overexpresses the hcpR and/or frdX genes. 
     BACKGROUND OF THE INVENTION 
     1,3-Propanediol (PDO), also called trimethylene glycol or propylene glycol, is one of the oldest known fermentation products. It was originally identified as early as 1881 by August Freund in a glycerol fermented culture containing  Clostridium pasteurianum . PDO is a typical product of glycerol fermentation, though it has been found in anaerobic conversions of other organic substrates. Very few organisms, all of them bacteria, are able to form PDO. These include enterobacteria of the genera  Klebsiella  ( K. pneumoniae ),  Enterobacter  ( E. agglomerans ) and  Citrobacter  ( C. freundii ),  Lactobacilli  ( L. brevis  and  L. buchneri ) and  Clostridia  ( C. butyricum, C. pasteurianum ). Of these,  C. butyricum  is considered to be the best “natural producer” of PDO in terms of both yield and titer. 
     PDO, as a bi-functional organic compound, can potentially be used in many different synthesis reactions, including as a monomer for polycondensations to produce polyesters, polyethers and polyurethanes, and in particular, polytrimethylene terephtalate (PTT). Given the structure and reactivity of PDO, it may also be used as a constituent in solvents, adhesives, detergents, cosmetics, textiles (e.g. clothing fibers or flooring) and plastics (e.g. car industry, in packing, or as a coating). 
     While various chemical methods can be used to produce PDO, they generate waste streams containing extremely polluting substances, thereby preventing chemically produced PDO from being cost competitive with petrochemically available diols, such as 1,2-ethanediol, 1,2-propanediol, and 1,4-butanediol. Although a more environmentally friendly method for the biological conversion of D-glucose to PDO using  Escherichia coli  has been described, this method has several major disadvantages. Notably, culture is discontinuous, due to instability of the producing strain, and further requires the addition of the expensive co-factor vitamin B 12 . Indeed, while PDO can be produced in  E. coli  via a vitamin B 12 -dependent pathway,  E. coli  itself does not produce this co-factor. 
     Due to the availability of large amounts of industrial glycerine, which comprises glycerol, produced by the bio-diesel industry, a continuous, vitamin-B 12 -free process with a higher carbon yield using industrial glycerine as a substrate would be advantageous. 
     While pure glycerol has a wide variety of applications (e.g. as a food, pharmaceutical, or cosmetic additive), the industrial glycerine produced during biodiesel synthesis generally contains 80-85% of glycerol mixed with salts and water, and therefore requires additional purification steps before it can be used as an additive. As a result, industrial glycerine is treated as a waste product, rather than a valuable commodity, and therefore represents an abundant and inexpensive fermentation substrate for PDO, when compared to other carbon sources, such as glucose or pure glycerol. 
       Clostridia  represent very promising strains for the production of PDO. Indeed,  C. butyricum  is able to use pure, but also industrial glycerine, as the sole carbon source for the production of PDO via a B 12 -independent pathway in batch and two-stage continuous fermentation (Papanikolaou et al., 2000). However, at the highest glycerol concentration, the maximal PDO titre obtained was 48.1 g/L at a dilution rate of 0.02 h −1 , corresponding to a productivity of 0.96 g/L/h. The cultures were conducted with a maximum glycerol concentration in the fed medium of 90 g/L and in the presence of yeast extract, a costly compound containing organic nitrogen that is known to increase bacterial biomass. 
     WO 2006/128381 discloses the use of industrial glycerine for the production of PDO with batch and fed-batch cultures using “natural producers” of PDO, such as  Klebsiella pneumoniae, C. butyricum  or  C. pasteuricum . However, the medium used in WO 2006/128381 also contains yeast extract. The maximum productivity reached is also similar to that found by Papanikolaou et al., 2000, comprised between 0.8 and 1.1 g/L/h. 
     The performance of a recombinant  C. acetobutylicum  strain containing the vitamin B 12 -independent glycerol-dehydratase and the PDO-dehydrogenase from  C. butyricum , called  C. acetobutylicum  DG1 pSPD5 has been described in Gonzalez-Pajuelo et al., 2005. This strain originally grows and produces PDO in a fed medium containing up to 120 g/L of pure glycerol. In addition, analyses with a fed medium containing 60 g/L of pure or industrial glycerine did not show any differences. These results were also obtained in presence of yeast extract. However, industrial glycerine comprising glycerol concentrations higher than 60 g/L were not tested. 
     More recently, WO 2010/128070 has disclosed a  C. acetobutylicum  DG1 pSPD5 strain that has been further adapted to grow on a high concentration of industrial glycerine and in the absence of yeast extract. The resulting population of  C. acetobutylicum  DG1 pSPD5 adapted strains was able to produce PDO in culture media containing a relatively high-quality industrial glycerine sourced from Novance (Compiegne, France) with a glycerol concentration of up to 120 g/L with a PDO titer of up to 53.5 g/L, a yield of up to 0.53 g/g and productivity of up to 2.86 g/L/h. 
     In patent application WO 2012/062832, the inventors described the isolation of clone “c08” from a population of  C. acetobutylicum  DG1 pSPD5 adapted strains obtained by the same process as that described in WO 2010/128070. This clone was able to produce PDO in a culture medium comprising a relatively high-quality industrial glycerine sourced from Novance (Compiègne, France), having a glycerol concentration of approximately 105 g/L. A PDO titer of up to 50.45 g/L, a yield of up to 0.53 g/g and productivity of up to 3.18 g/L/h was observed for the initial population, while the isolated clone c08 showed increased PDO production under the same conditions, with a PDO titer of up to 51.30 g/L, a yield of up to 0.50 g/g and productivity of up to 3.05 g/L/h. 
     Despite these improvements, there remains a need for increased PDO production (e.g. yield, titer, and/or productivity) from glycerol, in particular from industrial glycerine. There also exists a need for methods and microorganisms producing PDO from industrial glycerine having higher amounts of impurities, which may inhibit PDO production, and/or from industrial glycerine obtained from different origins. Indeed, the composition of industrial glycerine can vary from one manufacturer to another and even between batches. Furthermore, industrial glycerine is increasingly contaminated, having increased levels of impurities called Matter Organic Non-Glycerol (MONG), including fatty acids (e.g. oleic acid, linoleic acid), alcohols, salts and metals, which can inhibit growth and/or PDO production. Finally, there exists a need for methods and microorganisms having a reduced level of residual glycerol. Indeed, reducing residual glycerol levels facilitates downstream PDO purification. 
     The present invention provides a method and microorganism for the improved production of PDO, in particular from industrial glycerine substrates. Indeed, the inventors have surprisingly found that the overexpression of the hcpR and/or frdX genes, further improves PDO production, as a higher titer of PDO and better yield are observed when compared to the performance of unmodified strains in which the hcpR and/or frdX genes are not overexpressed. In addition, the inventors have surprisingly found that less residual glycerol is present during continuous cultivation. The inventors have also found that strains overexpressing the hcpR and/or frdX genes are able to produce PDO at this improved level in the presence of high concentrations of glycerol contained in industrial glycerine (e.g. up to about 105 g/L), from increasingly impure industrial glycerine, and from industrial glycerine from a variety of origins. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The present invention concerns a method for the fermentative production of PDO, comprising culturing a recombinant microorganism converting glycerol into PDO and overexpressing a nitric oxide-responsive transcriptional regulator and/or a ferredoxin-3 like protein, on a medium comprising glycerol. Preferably, the genes coding said proteins are the hcpR and frdX genes, respectively. Preferably, the hcpR and/or frdX gene(s) are overexpressed. 
     In a preferred embodiment of the method of the invention, the hcpR and/or frdX gene(s) are overexpressed by genetic modification, such as, without any limitation, by mutating the promoter regulating the expression of the hcpR and/or frdX genes, by mutating the intergenic region between the hcpR and frdX genes, by gene duplication, or by overexpressing the hcpR and/or frdX genes from a plasmid. 
     In a preferred embodiment of the method of the invention, the hcpR and/or frdX genes are overexpressed in the recombinant microorganism by mutating the intergenic region between the two genes, preferably via an insertion, more preferably via an insertion comprising as least one nucleotide, wherein said at least one nucleotide is preferably an ‘A’ nucleotide. In a particularly preferred embodiment, the intergenic region is mutated by a single base insertion, preferably an ‘A’ nucleotide. According to a preferred embodiment, said insertion occurs in a region comprising repeated ‘A’ nucleotides, preferably comprising at least 7 ‘A’ nucleotides. Preferably, the at least one ‘A’ nucleotide insertion is incorporated between positions 1014234 and 1014240 of the  C. acetobutylicum  ATCC 824 genome (NCBI reference sequence: NC_003030.1). 
     In a particular embodiment of the method of the invention, the recombinant microorganism is adapted to grow in the presence of a high concentration of glycerol, specifically of industrial glycerine. Preferably, the glycerol concentration in the industrial glycerine is comprised between 90 and 120 g/L, preferably about 105 g/L. Preferably, the industrial glycerine comprises at least 5% fatty acids. The industrial glycerine is preferably a by-product of biodiesel production. Ultimately, PDO is preferably purified. 
     In a further embodiment of the method of the invention, the recombinant microorganism is a bacterium, preferably selected from species of the genus  Clostridium  or  Klebsiella , more preferably selected from  Clostridium acetobutylicum, Clostridium butyricum, Clostridium pasteurianum , and  Klebsiella pneumoniae.    
     According to a particular embodiment of the method, the recombinant microorganism is co-cultured with at least one other microorganism in a microbial consortium, preferably with at least one other microorganism of the  Clostridium  genus, more preferably with  Clostridium sporogenes  or  Clostridium sphenoides , even more preferably with both  Clostridium sporogenes  and  Clostridium sphenoides.    
     The present invention also concerns a recombinant microorganism for the production of PDO from glycerol wherein said microorganism converts glycerol into PDO and overexpresses a nitric oxide-responsive transcriptional regulator and/or a ferredoxin-3 like protein. Preferably, in said recombinant microorganism, the genes coding said proteins are the hcpR and frdX genes, respectively. Preferably, the hcpR and/or frdX gene(s) are overexpressed. Preferably, the hcpR and/or frdX gene(s) are overexpressed by genetic modification, such as, without any limitation, by mutating the promoter regulating the expression of the hcpR and/or frdX genes, by mutating the intergenic region between the hcpR and frdX genes, by gene duplication, or by overexpressing the hcpR and/or frdX genes from a plasmid. 
     In a preferred embodiment, the hcpR and/or frdX genes are overexpressed in the recombinant microorganism by mutating the intergenic region between the two genes, preferably via an insertion, more preferably via an insertion comprising at least one nucleotide, wherein said at least one nucleotide is preferably an ‘A’ nucleotide. In a particularly preferred embodiment, the intergenic region is mutated by a single base insertion, preferably an ‘A’ nucleotide. According to a preferred embodiment, said insertion occurs in a region comprising repeated ‘A’ nucleotides, preferably comprising at least 7 ‘A’ nucleotides. Preferably, the at least one ‘A’ nucleotide insertion is incorporated between positions 1014234 and 1014240 of the  C. acetobutylicum  ATCC824 genome (NCBI reference sequence: NC_003030.1). 
     Particularly, the present invention concerns a recombinant microorganism adapted to grow on a culture medium having a high concentration of industrial glycerine, preferably wherein the glycerol concentration in the industrial glycerine is comprised between 90 and 120 g/L. The industrial glycerine may comprise impurities. In one embodiment, the industrial glycerine comprises at least 5% fatty acids. 
     In a further embodiment, the recombinant microorganism of the invention is a bacterium, preferably selected from species of the genus  Clostridium  or  Klebsiella , more preferably selected from  Clostridium acetobutylicum, Clostridium butyricum, Clostridium pasteurianum , and  Klebsiella pneumoniae . In a particular embodiment, the microorganism of the invention is co-cultured with at least one other microorganism in a microbial consortium, preferably with at least one other microorganism of the  Clostridium  genus, more preferably with  Clostridium sporogenes  or  Clostridium sphenoides , even more preferably with both  Clostridium sporogenes  and  Clostridium sphenoides.    
     The present invention further relates to the recombinant microorganism according to the method described herein. 
     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. However, publications mentioned herein are cited for the purpose of describing and disclosing the protocols, reagents and vectors that are reported in the publications and that might be used in connection with the invention. Nothing herein is to be construed as an admission that the invention is not entitled to antedate such disclosure by virtue of prior invention. 
     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. See, for example, Prescott et al. (1999) and Sambrook et al. (1989) (2001). 
     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 material and methods similar or equivalent to those described herein can be used to practice or test the present invention, the preferred material and methods are now described. As used herein, the following terms may be used for interpretation of the claims and specification. 
     In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word “comprise” or variations such as “comprises” or “comprising” is 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 present invention relates to a new method and microorganism for the fermentative production of PDO. 
     The term “microorganism” as used herein refers to all types of unicellular organisms, including prokaryotic organisms like bacteria, and eukaryotic organisms like yeasts and fungi, that can generally be found in nature. In the context of the present invention, the microorganism is preferably a bacterium, more preferably selected among the group consisting of Enterobacteriaceae, Bacillaceae, Clostridiaceae, Streptomycetaceae and Corynebacteriaceae. The terms “ Escherichia,” “Klebsiella,” “Bacillus,” “Clostridium,” “Clostridia ,” and “ Corynebacterium ” refer to all bacterial species belonging to these families or genera. As a non-limiting example, the bacterial species may be selected among the group consisting of  Escherichia  sp. (preferably  Escherichia coli ),  Klebsiella  sp. (preferably  Klebsiella pneumoniae ),  Bacillus  sp. (preferably  Bacillus subtilis ),  Clostridium  sp. (preferably  Clostridium acetobutylicum, Clostridium butyricum , and  Clostridium pasteurianum ) and  Corynebacterium  sp. (preferably  Corynebacterium glutamicum ). 
     The term “recombinant microorganism” or “genetically modified microorganism” as used herein refers to a microorganism or a strain of microorganism that has been genetically modified or genetically engineered, for example by adaptation. This means, according to the usual meaning of these terms, that the microorganism of the invention is not found in nature and is genetically modified when compared to a “parental” microorganism, from which it is derived. The “parental” microorganism may occur in nature (i.e. a wild type microorganism) or may have been previously modified, but does not express or over-express the one or more proteins of the present invention (i.e. HcdR and/or FrdX). Accordingly, the recombinant microorganisms of the invention have been modified to express or over-express at least the HcdR and/or FrdX proteins that were not expressed or over-expressed in the parental microorganism. 
     Preferably, the parental microorganism is selected from the microorganisms listed herein. In a particular embodiment, the parental microorganism is selected from the  Clostridium  species  C. acetobutylicum, C. butyricum, Clostridium pasteurianum , and related isolates, or from  Klebsiella  species, such as  K. pneumoniae  and related isolates. More preferably, the parental microorganism is selected from  C. acetobutylicum  strains described in described in Gonzalez-Pajuelo et al., 2005 or in PCT patent application no. WO 2010/128070 or WO 2012/062832. Even more preferably, the parental microorganism is selected from  C. acetobutylicum  DG1 pSPD5 strains, such as the DG1 pSPD5 PD0001VE05 strain. 
     A variety of genetic modifications may be made to the recombinant microorganism of the invention. As a non-limiting example, endogenous genes can be attenuated, deleted, or over-expressed in the recombinant microorganism, while exogenous genes can be introduced, carried by a plasmid, or integrated into the genome of the strain, for expression within the cell. Such modifications can be performed, for example, by genetic engineering, by adaptation, wherein a microorganism is cultured in that apply a specific stress on the microorganism and induce mutagenesis, or by forcing the development and evolution of new metabolic pathways by combining directed mutagenesis and evolution under specific selection pressure. 
     In the context of the present invention, the method for the fermentative production of PDO comprises culturing a microorganism converting glycerol into PDO, and overexpressing a nitric oxide-responsive transcriptional regulator and/or a ferredoxin-3 like protein, on a medium comprising glycerol. 
     The nitric oxide-responsive transcriptional regulator described herein, otherwise known as HcpR, is described in database regprecise.lbl.gov/RegPrecise/ as a transcriptional regulator of the Crp family. It comprises a cAMP binding domain and regulatory subunit of cAMP-dependent protein kinase. It is preferably encoded by the gene CA_C0884 of  C. acetobutylicum . Exemplary gene and amino acid sequences are shown in SEQ ID NOs: 1 and 2, respectively. 
     The ferredoxin 3-like protein described herein, otherwise known as FrdX, notably comprises a 4Fe-4S ferredoxin iron-sulfur binding domain. It is preferably encoded by the gene CA_C0885 of  C. acetobutylicum . Exemplary gene and amino acid sequences are shown in SEQ ID NOs: 3 and 4, respectively. 
     The nucleotide sequences of the above-mentioned genes, or the amino acid sequences encoded by said genes, are described in Table 1 below, according to their accession number and version in a database and/or according to their sequence identification. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 hcpR and frdX genes and proteins of the invention 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                 Gene 
                   
                 Protein 
               
               
                   
                   
                   
                 NCBI reference 
                   
                 SEQ ID 
                   
                 SEQ ID 
               
               
                 Microorganism 
                 Product name 
                 Strain sequence 
                 sequence and version 
                 Locus tag name 
                 NO 
                 Protein ID 
                 NO 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 
                   Clostridium 
                 
                 cAMP- 
                 ATCC 824 
                 NC_003030.1 
                 CA_C0884 
                 1 
                 NP_347520.1 
                 2 
               
               
                 
                   acetobutylicum 
                 
                 binding domain- 
                 chromosome, 
                   
                   
                   
                   
                   
               
               
                   
                 containing  
                 complete genome 
                   
                   
                   
                   
                   
               
               
                   
                 protein nitric 
                   
                   
                   
                   
                   
                   
               
               
                   
                 oxide-responsive 
                   
                   
                   
                   
                   
                   
               
               
                   
                 transcriptional 
                   
                   
                   
                   
                   
                   
               
               
                   
                 regulator 
                   
                   
                   
                   
                   
                   
               
               
                 
                   Clostridium 
                 
                 Crp/Fnr 
                 GL11 
                 NZ_MCGV0100000.1 
                 BEE63_RS05095 
                 5 
                 WP_066020350 
                 6 
               
               
                 
                   pasteurianum 
                 
                 family 
                 contig. 1, whole 
                   
                   
                   
                   
                   
               
               
                   
                 transcriptional 
                 genome shotgun 
                   
                   
                   
                   
                   
               
               
                   
                 regulator 
                 sequence 
                   
                   
                   
                   
                   
               
               
                 
                   Clostridium 
                 
                 Crp/Fnr family 
                 5521 
                 NZ_ABDT01000093.2 
                 CBY_RS14495 
                 7 
                 WP_003410673.1 
                 8 
               
               
                 
                   butyricum 
                 
                 transcriptional 
                 gcontig_1106103650362, 
                   
                   
                   
                   
                   
               
               
                   
                 regulator 
                 whole genome shotgun 
                   
                   
                   
                   
                   
               
               
                   
                   
                 sequence 
                   
                   
                   
                   
                   
               
               
                 
                   Clostridium 
                 
                 Crp/Fnr family 
                 DSM 10702 
                 NZ_AQQF01000197.1 
                 K670_RS19640 
                 9 
                 WP_035765453.1 
                 10 
               
               
                 
                   butyricum 
                 
                 regulator 
                 contig000197,  
                   
                   
                   
                   
                   
               
               
                   
                 transcriptional 
                 whole genome shotgun 
                   
                   
                   
                   
                   
               
               
                   
                 regulator 
                 sequence 
                   
                   
                   
                   
                   
               
               
                 
                   Clostridium 
                 
                 Crp/Fnr family 
                 NEC8, whole genome 
                 NC_CBYK010000011.1 
                 AT697_RS15885 
                 11 
                 WP_046057470.1 
                 12 
               
               
                 
                   butyricum 
                 
                 transcriptional 
                 shotgun sequence 
                   
                   
                   
                   
                   
               
               
                   
                 regulator 
                   
                   
                   
                   
                   
                   
               
               
                 
                   Clostridium 
                 
                 ferredoxin 3-like 
                 ATCC 824 chromosome, 
                 NC_003030.1 
                 CA_C0885 
                 3 
                 NP_347521.1 
                 4 
               
               
                 
                   acetobutylicum 
                 
                 protein 
                 complete genome 
                   
                   
                   
                   
                   
               
               
                 
                   Clostridium 
                 
                 Fe—S- 
                 GL11 contig. 1, whole 
                 NZ_MCGV01000001.1 
                 BEE63_RS05090 
                 13 
                 WP_066020349.1 
                 14 
               
               
                 
                   pasteurianum 
                 
                 binding protein 
                 genome shotgun sequence 
                   
                   
                   
                   
                   
               
               
                   
               
            
           
         
       
     
     The terms “overexpress,” “overexpression,” and like, as used herein should be taken broadly to include any increase in expression in the corresponding protein, as compared to the expression level protein in an unmodified or “parental” microorganism under the same conditions. It should not be taken to mean that the protein is expressed at any particular level. The term “expression level,” as used herein, refers to the amount (e.g. relative amount, concentration) of a protein of interest (or of the gene or mRNA encoding said protein) expressed in a microorganism, which is measurable by methods well-known in the art, such as by qRT-PCR, Western Blot-Immunoblot, Enzyme-linked immunosorbent assay (e.g. ELISA), quantitative proteomic approaches and the like. 
     A person skilled in the art can easily induce the overexpression of a protein of interest in a microorganism. 
     As a non-limiting example, one or more endogenous genes, such as the hcpR and/or frdX genes, may be overexpressed by introducing heterologous sequences which favour upregulation in addition to endogenous regulatory elements, or by substituting those endogenous regulatory elements with such heterologous sequences, or by introducing one or more supplementary copies of the endogenous gene chromosomally (i.e. into the chromosome) or extra-chromosomally (e.g. into a plasmid or vector) within the microorganism. In this regard, several copies of a gene can be introduced on a chromosome by methods well-known in the art, such as by genetic recombination. In particular, standard techniques of chromosomal modification may be used in  C. acetobutylicum , for example according to the method of homologous recombination described in patent application WO 2008/040387 for  Clostridium . In a preferred embodiment of the invention, a second copy of the gene is introduced in the chromosome (i.e. gene duplication). Alternatively, or in addition, a gene may be introduced into the microorganism on a plasmid or vector, and expressed extra-chromosomally. As a non-limiting example, it may be carried by different types of plasmid that may differ with respect to their origin of replication, depending on the microorganism in which they can replicate, and by their copy number in the cell. For example, a microorganism transformed by a plasmid can contain 1 to 5 copies of the plasmid, about 20 copies, or even up to 500 copies, depending on the nature of the origin of replication of the selected plasmid. A variety of plasmids, which differ in respect of their origin of replication and of their copy number in a cell, are well known in the art and can be easily selected by the skilled practitioner for such purpose. Examples of plasmids which can replicate in  C. acetobutylicum  include, without limitation, the pSOS plasmids (Tummala et al. 1999), plasmids of pSYL series (Lee, 1992), and plasmids of the pMTL series (Chambers et al. 1988), etc. 
     Another way to overexpress an endogenous gene is to exchange its promoter (i.e. wild-type promoter) with a stronger promoter. Promoters suitable for such purposes may be homologous (originating from the same species) or heterologous (originating from a different species), and are well-known in the art. Indeed, the skilled person can easily select an appropriate promoter for inducing the expression of an endogenous gene, such as the hcpR and/or frdX genes. Promoters that are the most convenient for increasing gene expression level are well-known to the skilled person in the art: these include, among others, promoters Ptrc, Ptac, Plac, and the lambda promoter P R  and P L . These promoters are “inducible” by a particular compound or by specific external conditions such as temperature or light, and/or may be homologous or heterologous. Specific examples of  C. acetobutylicum  promoters leading to high overexpression of genes include the thl, adc, ptb promoters (Tummala et al., 1999). 
     Endogenous gene expression levels can also be increased by introducing mutations into the coding sequence of a gene or into non-coding sequences. These mutations may be synonymous, when no modification in the corresponding amino acid occurs, or non-synonymous, when the corresponding amino acid is altered. Synonymous mutations do not have any impact on the function of translated proteins, but may have an impact on the regulation of the corresponding genes or even of other genes, if the mutated sequence is located in a binding site for a regulator factor. Non-synonymous mutations may have an impact on the function of the translated protein as well as on regulation depending the nature of the mutated sequence. 
     In particular, mutations in non-coding sequences may be located upstream of the coding sequence (i.e. in the promoter region, in an enhancer, silencer, or insulator region, in a specific transcription factor binding site) or downstream of the coding sequence. Mutations introduced in the promoter region may be in the core promoter, proximal promoter or distal promoter. Mutations may be introduced by site-directed mutagenesis using, for example, Polymerase Chain Reaction (PCR), by random mutagenesis techniques for example via mutagenic agents (Ultra-Violet rays or chemical agents like nitrosoguanidine (NTG) or ethylmethanesulfonate (EMS)) or DNA shuffling or error-prone PCR or using culture conditions that apply a specific stress on the microorganism and induce mutagenesis. The insertion of one or more supplementary nucleotide in the region located upstream of a gene can notably modulate gene expression. As a non-limiting example, one or more mutations may be introduced into the intergenic region located between the hcpR and/or frdX genes (the full sequence comprising the hcpR and frdX genes and the parental intergenic region is shown in the sequence of SEQ ID NO: 15, while the parental sequence of the intergenic region alone is shown in the sequence of SEQ ID NO: 16). As a non-limiting example, the insertion of an “A” nucleotide may be introduced in the intergenic region. An example of such an insertion is shown in the sequence of SEQ ID NO: 17. 
     In the context of the present invention, the recombinant microorganism preferably overexpresses the hcpR and/or frdX gene(s). Preferably, the hcpR and/or frdX gene(s) are overexpressed by at least 1.5-fold, more preferably by at least about 2-fold, even more preferably by at least 3-fold or by about 4-fold. 
     According to a first preferred embodiment, the hcpR and/or frdX gene(s) are overexpressed by mutating the intergenic region between the two genes, preferably via an insertion. Preferably, said intergenic mutation modifies the promoter region regulating the expression of the hcpR and/or frdX gene(s), inducing overexpression. Preferably, said intergenic mutation induces the overexpression of both the hcpR and frdX genes. Indeed, the hcpR and frdX genes are arranged as a bi-directional gene pair in  C. acetobutylicum . However, according to the type of intergenic mutation, only one of the hcpR and frdX genes may be overexpressed. Preferably, said intergenic mutation is comprised between positions 1014117 and 1014239 of the  C. acetobutylicum  ATCC 824 genome. 
     Preferably, the intergenic region is mutated by the insertion of at least one nucleotide selected from the ‘A,’ ‘C,’ ‘T,’ and ‘G’ nucleotides. Preferably, the intergenic region is mutated by the insertion of at least one ‘A’ nucleotide, more preferably one ‘A’ nucleotide. Preferably, the at least one nucleotide is inserted into a region wherein a same nucleotide is repeated at least two, three, four, five, six, seven or more times (e.g. insertion of an ‘A’ nucleotide in a nucleotide stretch comprising ‘AAAAAA’). When said insertion is an ‘A’ nucleotide, said insertion may be further located at position 1014240 of the  C. acetobutylicum  ATCC 824 genome. Preferably, said at least one ‘A’ is incorporated between positions 1014234 and 1014240 of the  C. acetobutylicum  ATCC 824 genome. 
     According to a second preferred embodiment, the hcpR and/or frdX gene(s) is/are overexpressed by introduction of said gene(s) into the microorganism on an exogenous vector or plasmid, more preferably under the control of an inducible promoter. 
     According to a third preferred embodiment, the hcpR and/or frdX gene(s) is/are overexpressed by introduction of at least one additional copy of said gene in the chromosome (i.e. gene duplication). 
     In view of the gene and amino acid sequences provided herein, and using the information available in databases such as UniProt (for proteins), GenBank (for genes), or NCBI (for proteins or genes), the skilled practitioner can easily determine the sequence of a specific protein and/or gene of a microorganism, and identify equivalent proteins or genes, or homologs thereof, in other microorganisms. This routine work can be performed for example by alignment of a specific gene (or protein) sequence of a microorganism with gene (or protein) sequences or the genome (or proteome) of other microorganisms, which can be found in the above-mentioned databases. Such sequence alignment can advantageously be performed using the BLAST algorithm developed by Altschul et al. (1990). Once sequence homology has been established between sequences, a consensus sequence can be derived and used to design degenerate probes in order to clone the corresponding homolog gene (and hence homolog protein) of the related microorganism. These routine methods of molecular biology are well-known to the person skilled in the art. 
     It shall be further understood that, in the context of the present invention, should an exogenous gene encoding a protein of interest be expressed in a specific microorganism, a synthetic version of this gene is preferably constructed by replacing non-preferred codons or less preferred codons with preferred codons of said microorganism which encode the same amino acid. It is indeed well-known in the art that codon usage varies between microorganism species, which may impact the recombinant expression level of the protein of interest. To overcome this issue, codon optimization methods have been developed, and are extensively described by Graf et al. (2000), Deml et al. (2001) and Davis &amp; Olsen (2011). Several software programs have notably been developed for codon optimization determination such as the GeneOptimizer® software (Lifetechnologies) or the OptimumGene™ software of (GenScript). In other words, the exogenous gene encoding a protein of interest is preferably codon-optimized for expression in a specific microorganism. 
     In the context of the method and microorganism of the invention, the recombinant microorganism may comprise additional modifications with regards to the parent strain. As a non-limiting example, the recombinant microorganism may have been previously adapted for increased PDO production from a culture medium comprising glycerol. Methods for directing the glycerol metabolism towards production of PDO are known in the art (see for instance WO 2006/128381, González-Pajuelo &amp; al. 2006). 
     In a preferred embodiment, if the recombinant microorganism is  C. acetobutylicum , it has preferably been previously adapted, more preferably by an anaerobic continuous process, for growth and production of PDO from a culture medium with high glycerol content, presenting an increased flux of PDO production. The adaptation of the strain  C. acetobutylicum  is preferably carried out by an anaerobic continuous process, which is a technique well known by the skilled person. Among the particulars of this process known by the one skilled in the art, it may be for example mentioned that fed medium is added to the fermenter continuously and an equivalent amount of converted nutrient solution with microorganisms is simultaneously removed from the system. The rate of nutrient exchange is expressed as the dilution rate. Hence the dilution rate is applied to the culture medium, takes into consideration maximum growth rate of the microorganism and impacts the rate of intake and withdrawal of the medium. 
     The  C. acetobutylicum  strain may be adapted by introducing extra copies of the PDO operon from  C. butyricum , encoding enzymes involved in the vitamin B 12 -independent PDO pathway. In particular, the PDO operon from  C. butyricum  may be overexpressed by either a plasmid or integrated into the chromosome of the strain  C. acetobutylicum  to be adapted. For example, the pSPD5 plasmid can be used for overexpression of the PDO operon in  C. acetobutylicum  (Gonzalez-Pajuelo et al., 2006). 
     Alternatively or in addition, said recombinant microorganism may be adapted to grow in a culture medium having a high glycerol content, specifically a high concentration of glycerol originating from industrial glycerine, by a selection pressure culturing process as disclosed in WO 2010/128070 (see in particular page 7, line 10 to page 8, line 23 and Examples 2, 3, and 4). 
     Strains of  C. acetobutylicum  that have been genetically modified for the production of PDO from glycerol as a sole source of carbon are known in the art and disclosed, particularly, in applications WO 2001/04324 and WO 2010/128070. Finally, the  C. acetobutylicum  strain may be previously adapted by the process described in WO 2012/062832. 
     As a non-limiting example, the strain  C. acetobutylicum  to be adapted may be cultivated in continuous culture using a fed medium containing between 90 and 120 g/L of glycerol and preferably of about 105 g/L of raw glycerol. Said “adaptation” of the producing microorganism is obtained by culturing the microorganism on a culture medium comprising high industrial glycerine content at a low dilution rate, and selecting the adapted microorganism able to grow on the culture medium having high concentration of glycerol originating from industrial glycerine. A dilution rate between 0.005 and 0.02 h −1  corresponds to a “low dilution rate” while a dilution rate between 0.02 and 0.1 h −1  corresponds to common dilution rates. As a non-limiting example, the strain  C. acetobutylicum  to be adapted may be cultivated at a low dilution rate over a period ranging from 24 hours to 10 days, preferably more than 2 days, more preferably about 8 days. The dilution rate is generally comprised between 0.005 and 0.1 h −1 , preferably between 0.005 and 0.02 h −1 . The dilution rate can be changed during the adaptation method, eventually with a first step comprised between 0.005 and 0.02 h −1  and a second step where the dilution rate is increased up to 0.1 h −1 , more preferably up to 0.06 h −1 , even more preferably up to 0.07 h −1 . 
     In a specific embodiment of the invention, the strain  C. acetobutylicum  to be adapted is cultivated in continuous culture using a fed medium containing between 90 and 120 g/L of glycerol and preferably of about 105 g/L of raw glycerol, at a low dilution rate comprised between 0.005 and 0.02 h −1 , preferably 0.02 h −1 . Over a period of up to 10 days, preferably between 5 and 8 days, the strain  C. acetobutylicum  is adapted to the high glycerine concentration present in the fed medium, and the dilution rate can be increased up to 0.1 h −1 , preferably up to 0.07 h −1 . 
     The “culture medium” refers to a culture medium appropriate for the growth and the diol-production of the recombinant microorganism, wherein said culture medium comprises glycerol, which may or may not be the sole carbon source present in the culture medium. The culture medium may or may not comprise an organic nitrogen source. Nitrogen is a naturally occurring element that is essential for growth and reproduction in both plants and animals. It is found in amino acids and in many other organic and inorganic compounds. “Organic nitrogen” means, according to the invention, a nitrogen comprising organic compound obtained from living organisms. Usual sources of organic nitrogen for bacterial culture comprise yeast extract. 
     Glycerol may be the sole carbon source present in the culture medium. Alternatively, one or more additional carbon sources (e.g. glucose) may be present in the culture medium. The glycerol present in the culture medium may be pure or industrial glycerine. 
     Preferably, the culture medium does not comprise yeast extract. Even more preferably, the culture medium does not comprise any organic nitrogen source. Preferably, glycerol is the sole carbon source present in the culture medium. 
     The term “glycerol” may be used interchangeably herein with the term “glycerine,” and may comprise pure and/or industrial glycerine. Industrial glycerine may be obtained as a bioproduct from biodiesel production, (e.g. from the transesterification of glycerides with short-chain alcohols). Alternatively, industrial glycerine may be obtained from plant or animal oils and fats, for example by saponification, transesterification and/or hydrolysis reactions. 
     “Pure glycerol” refers to a glycerol product having a purity of greater than 95% by weight, of glycerol. In some cases, pure glycerol may have a purity of greater than 97%, even more of greater than 99% or 99.2%. The remainder of the glycerol product is preferably composed of water (e.g. about 3 to 5% by weight). Pure glycerol has very few remaining impurities. Pure glycerol may notably be obtained by purification using one or more processes, such as distillation. 
     “Industrial glycerine” refers to a glycerol product obtained from an industrial process without substantial purification. Industrial glycerine can also be designated as “raw glycerine,” “raw glycerol,” “crude glycerol,” or “industrial glycerol.” Industrial glycerine generally comprises from 5 to 95% by weight of glycerol, in particular from 40 to 90% by weight of glycerol, more particularly from 60 to 90% by weight of glycerol, even more particularly from about 65 to 85% by weight of glycerol or about 70% to 85% by weight of glycerol. The remaining fraction of industrial glycerine may comprise water also referred to as moisture, mineral salts, and Matter Organic Non-Glycerol (MONG), which comprises other non-glycerol organic compounds and glycerides. In particular, industrial glycerine further comprises impurities, such as alcohol (e.g. methanol), mineral salts, unreacted mono-, di-, and tri-acylglycerols, methyl esters and one or more fatty acids (e.g. palmitic acid, stearic acid, oleic acid, linolenic acid, linoleic acid and arachidic acid). 
     According to a preferred embodiment, industrial glycerine preferably contains more than about 60%, 70% or 80% by weight of glycerol and less than about 15% by weight of water and impurities. According to a preferred embodiment, the concentration of mineral salts is less than 10%, preferably less than 5%. According to a preferred embodiment, the concentration of fatty acids comprised in the MONG, in certificates of analysis, is less than 20%, preferably less than 10%, even more preferably less than 5%. According to an alternative preferred embodiment, the concentration of fatty acids or MONG is preferably at least 5%, more preferably at least 10%, even more preferably comprised between 5 and 10%. 
     Industrial processes from which industrial glycerine is obtained as a by-product are, inter alia, manufacturing methods where fats and oils, particularly fats and oils of plant or animal origin, are processed into industrial products such as detergent or lubricants. In such manufacturing methods, industrial glycerine is considered as a by-product. The exact composition of industrial glycerine will depend on the initial glycerol source (e.g. animal fat or plant oil, such as sunflower oil, canola oil, soybean oil, mustard seed, etc.), and the method and conditions used in glycerol extraction and downstream treatment, for example, in biodiesel production. 
     In a particular embodiment, the industrial glycerine is a by-product from biodiesel production. Preferably, the industrial glycerine comprises known impurities of glycerine obtained from biodiesel production, comprising about 80 to 85% of glycerol with salts, water and some other organic compounds (i.e. MONG), comprising compounds such as fatty acids, such as those as listed above. Industrial glycerine obtained from biodiesel production has not been subjected to further purification steps. 
     The terms “high glycerol content” or “high glycerol concentration” as used herein refers to glycerol concentrations equal or superior to 90 g/l of glycerol in the culture medium. In preferred embodiments, the culture medium comprises glycerol at a concentration comprised between 90 and 120 g/L, preferably comprised between 105 and 110 g/L, more preferably about 105 g/L or 109 g/L. 
     In some cases, PDO production may be further increased and/or residual glycerol levels further decreased by co-culturing the recombinant microorganism in the presence of at least one other microorganism, in a so-called microbial consortium. The terms “microbial consortium” or “co-culture” are used interchangeably to denote the use of two or more microbial species in the fermentation process. Preferably, the one or more additional strains used in co-culture do not ferment glycerol or produce PDO. 
     As a non-limiting example, the microbial consortium may comprise at least two  Clostridium  strains, such as one  C. acetobutylicum  strain and at least one strain chosen among strains of the  Clostridium  genus, such as strains of  C. sporogenes  and/or strains of  C. sphenoides . As a further example, the microbial consortium may comprise at least three  Clostridium  strains, such as at least one  C. acetobutylicum  strain, at least one  C. sporogenes  and at least one strain of  C. sphenoides . In either of the cases described above, the majority of the microbial consortium may belong to the  C. acetobutylicum  species. For example, the microbial consortium may comprise more than 85% of  C. acetobutylicum , from 0.001% to 0.2% of  C. sporogenes  and/or from 0.1% to 15% of  C. sphenoides , considering that the totality of the cells contained in the culture corresponds to 100%. In particular, the microbial consortium may comprise from 85% to 99.8% of  C. acetobutylicum , from 0.001% to 0.15% of  C. sporogenes  and/or from 0.2% to 15% of  C. sphenoides , or from 90% to 99.8% of  C. acetobutylicum , from 0.002% to 0.13% of  C. sporogenes  and/or from 0.2% to 10% of  C. sphenoides.    
     Thus, in a preferred embodiment, the microorganism of the present invention is co-cultured in a microbial consortium. Said microbial consortium preferably comprises the recombinant microorganism disclosed herein, or as described in the method of the invention disclosed herein, in co-culture with at least one, preferably two, other microorganisms. According to a particularly preferred embodiment, the recombinant microorganism is co-cultured with at least one strain of  C. sphenoides , more preferably with at least one strain of  C. sphenoides  and at least one strain of  C. sporogenes . Preferably, the recombinant microorganism of the invention is a  C. acetobutylicum  strain, even more preferably adapted for growth and production of PDO from a culture medium with high glycerol content and specifically with a high concentration of glycerol contained in industrial glycerine. 
     In a preferred embodiment, the microbial consortium of the invention comprises more than 85% of  C. acetobutylicum , from 0.001% to 0.2% of  C. sporogenes  and/or from 0.1% to 15% of  C. sphenoides , considering that the totality of the cells contained in the culture corresponds to 100%. In a more preferred embodiment, the microbial consortium comprises from 85% to 99.8% of  C. acetobutylicum , from 0.001% to 0.15% of  C. sporogenes  and/or from 0.2% to 15% of  C. sphenoides . In an even more preferred embodiment, the microbial consortium comprises from 90% to 99.8% of  C. acetobutylicum , from 0.002% to 0.13% of  C. sporogenes  and/or from 0.2% to 10% of  C. sphenoides.    
     In the method of the present invention, the production of PDO is preferably carried out by an anaerobic continuous fermentation by culturing the recombinant microorganism, or microbial consortium of the invention described above in a culture medium comprising glycerol as the sole source of carbon, said culture medium being a minimal medium without addition of organic nitrogen. 
     The term “minimal medium” means a culture medium strictly mineral comprising a chemically defined composition on which organisms are grown apart from the glycerine solution. Such culture media are disclosed in the art, particularly in WO 2010/128070 and WO 2011/042434, the contents of which are incorporated herein by reference in their entirety. 
     In a preferred embodiment, the PDO thus obtained from the method according to the invention is further purified. Methods for recovering and eventually purifying PDO from a fermentation medium are known to the skilled person. PDO may be isolated by distillation. In most embodiments, PDO is distilled from the fermentation medium with a by-product, such as acetate, and then further purified by known methods. A particularly preferred purification method is disclosed in applications WO 2009/068110 and WO 2010/037843, the content of which are incorporated herein by reference in their entirety. 
     Continuous fermentation processes are known to the person skilled in the art. The fermentation process is generally conducted in reactors with an inorganic culture medium of known defined composition adapted to the bacteria used, containing at least industrial glycerine, a by-product from biodiesel production, and if necessary a co-substrate for the production of the metabolite. 
     This method of the invention is preferably realized in a continuous process. The person skilled in the art is able to manage each of these experimental conditions, and to define the culture conditions for the microorganisms according to the invention according to his general knowledge. In particular clostridia are fermented at a temperature between 20° C. and 60° C., preferentially between 25° C. and 40° C. for  C. acetobutylicum.    
     In a specific embodiment of the invention, the strain  C. acetobutylicum  DG1 pSPD5 Type 130P or Type 008P is cultivated in continuous culture using a fed medium containing approximately 105 g/L or 109 g/L of raw glycerol, at a dilution rate comprised between 0.035 and 0.08 h −1 , preferably 0.07 h −1  (see examples 1 and 2). Said method, in its different embodiments, leads to production of PDO of at least 52 g/L, with a yield comprised between 0.4 and 0.6 g/g and a productivity superior to 2.9 g/L/h for a dilution rate of 0.7 h −1 . Preferably, the yield is comprised between 0.4 and 0.5 g/g and the productivity is superior to 3.6 g/L/h, even more preferably superior to 3.65 g/L/h. The method of the invention, in certain embodiments, further leads to a residual glycerol level of less than 3.7 g/L. Preferably, the residual glycerol level is approximately 3.6 g/L. 
    
    
     
       FIGURES 
         FIG.  1   : Obtention of  Clostridium acetobutylicum  DG1 pSPD5 Type 130P strain by adaptation of the Type 008P strain on raw glycerine. Dynamic of OD 620nm  (OD units or ODU, squares), residual glycerol (g/L; hatches), PDO concentration (g/L; triangles) and feed flow rate (mL/h; circles) of the continuous culture are shown as a function of culture duration (days; d). 
         FIG.  2   : Chromosomal organization of the hcpR and frdX genes in  C. acetobutylicum . The genes hcpR and frdX are organized as a bi-directional gene pair, with a 123 bp intergenic region located between the two genes. Nucleotide positions within the  C. acetobutylicum  ATCC 824 genome (NCBI reference sequence: NC_003030.1) are indicated. 
         FIG.  3   : Overexpression of the hcpR and frdX genes by quantitative PCR. Both hcpR and frdX were overexpressed in the 130P strain, as compared to the parent strain Type 008P, as determined by qRT-PCR. Black bars: Type 008P; Grey bars: Type 130P. 
     
    
    
     EXAMPLES 
     The present invention is further defined in the following examples. It should be understood that these example, while indicating preferred embodiments of the invention, are given by way of illustration only. From above disclosure and these examples, the person skilled in the art can make various changes of the invention to adapt it to various uses and conditions without modifying the essential means of the invention. 
     Example 1 
     Continuous Culture of  Clostridium acetobutylicum  DG1 (pSPD5) on Raw Industrial Glycerol and Obtention of the Microorganism Type 130P 
     Bacterial Strains: 
     
         
         
           
             Type 008P:  C. acetobutylicum  strain DG1 pSPD5 adapted on high concentrations of raw glycerine as described in patent application WO 2010/128070 
             Type 130P:  C. acetobutylicum  strain DG1 pSPD5 Type issued from a continuous culture form  C. acetobutylicum  strain DG1 pSPD5 Type 008P on high concentrations of raw industrial glycerine, and overexpressing the hcpR and frdX genes, as described herein
 
The synthetic media used for clostridia batch cultivations contained, per liter of tap water: glycerol, 30 g; KH 2 PO 4 , 0.5 g; K 2 HPO 4 , 0.5 g; MgSO 4 , 7H 2 O, 0.2 g; CoCl 2  6H 2 O, 0.01 g; H 2 SO 4 , 0.1 ml; NH 4 Cl, 1.5 g; biotin, 0.16 mg; p-amino benzoic acid, 32 mg; FeSO 4 , 7H 2 O, 0.028 g. The pH of the medium was adjusted to 6.3 with NH 4 OH 3N. Commercial glycerol purchased from SDS Carlo_Erba (purity 99%) was used for batch cultivation. The feed medium for continuous cultures contained, per liter of tap water: glycerol from raw glycerine, 105 g; KH 2 PO 4 , 0.50 g; K 2 HPO 4 , 0.50 g; MgSO 4 , 7H 2 O, 0.2 g; NH 4 Cl, 1.5 g; CoCl 2  6H 2 O, 0.026 g; biotin, 0.16 mg; p-amino benzoic acid, 32 mg; FeSO 4 , 7H 2 O, 0.04 g; anti-foam, 0.05 ml; ZnSO 4 , 7H 2 O, 8 mg; CuCl 2 , 2H 2 O, 4 mg; MnSO 4 , H 2 O, 0.04 g; H 3 BO 3 , 2 mg; Na 2 MoO 4 , 2H 2 O, 0.8 mg. Medium pH was adjusted between 3.5 and 4 with H 2 SO 4  96%.
 
           
         
       
    
     Raw glycerine, from the transesterification process for biodiesel, was provided by two different providers and had the following composition:
         from ADM (Rolle, Switzerland) (using vegetable oil; purity 80.9%; Moisture 12.6%; MONG 0.39%; Ash 6.2%),   from Greenergy (London, UK) (using cooking oil; purity 76.5%; Moisture 10.2%; MONG 7.1%; Ash 6.3%).       

     Optionally, these glycerines were pretreated by acidification. 
     The purity and MONG composition has an incidence on the toxicity of the glycerine on the microorganism. The Greenergy (London, UK) glycerine is both less pure and more dirty, therefore more toxic for the microorganism, than the ADM (Rolle, Switzerland) glycerine, because of the high concentration of MONG. Indeed, MONG concentration in the Greenergy (London, UK) glycerine is above 5%. 
     The following example shows the adaptation of the strain Type 008P on coarser glycerine (from ADM to Greenergy) to get a new strain named Type 130P, that is able to grow and produce PDO on less refined industrial glycerine. This adaptation is highly advantageous, as less refined industrial glycerine is a cheaper raw material for the fermentation process. 
     Experimental Set-Up: 
     Continuous cultures were performed in a 5 l Tryton bioreactor (Pierre Guerin, France) with a working volume of 2000 ml. The culture volume was kept constant at 2000 ml by automatic regulation of the culture level. Cultures were stirred at 200 RPM, the temperature was set to 35° C. and pH was maintained constant at 6.5 by automatic addition of NH 4 OH 5.5N. To create anaerobic conditions, the sterilized medium in the vessel was flushed with sterile O 2 -free nitrogen for one hour at 60° C. and flushed again until 35° C. was attained (flushing during 2 hours). The bioreactor gas outlet was protected from oxygen by a pyrogallol arrangement (Vasconcelos et al, 1994). After sterilization, the feed medium was also flushed with sterile O 2 -free nitrogen until room temperature was reached and kept under nitrogen to avoid O 2  entry. 
     Analytical Procedures: 
     Cell concentration was measured turbidimetrically at 620 nm (OD 620nm ) and correlated with cell dry weight, which was determined directly. Glycerol, PDO, ethanol, lactate, acetic and butyric acid concentrations were determined by HPLC analysis. Separation was performed on a Biorad Aminex HPX-87H column and detection was achieved by refractive index. Operating conditions were as follows: mobile phase sulphuric acid 0.5 mM; flow rate 0.5 ml/min, temperature, 25° C. 
     Batch and Continuous Cultures Process and Results: 
     A culture growing in a 100 ml flask on synthetic medium (the same as described above for batch culture but with the addition of acetic acid, 2.2 g/L and MOPS, 23.03 g/L) taken at the end of exponential growth phase was used as inoculum (5% v/v). 
     Cultures were first grown in batch mode. At the early exponential growth phase, we performed a pulse of glycerol with the feed medium (the same as described for feed culture). Glycerol from raw glycerine was added at a static flow rate during 3 hours (i.e. an addition of 18 g/L of glycerol). Then, the growth continued in batch mode and before the end of the exponential growth phase the continuous feeding started with a dilution rate of 0.035 h −1  of feed medium containing 105 g/L of glycerol from raw glycerine provided by ADM (Rolle, Switzerland) only. As can be seen in  FIG.  1   , after 3 days with a dilution rate of 0.035 h −1 , glycerol accumulation started and reached 46.6 g/L at 6.5 residence times (RT, calculated according to the formula shown below), corresponding to the first peak of residual glycerine. This accumulation was coupled with a decrease of PDO production (up to 31 g/L instead of 52 g/L) and biomass production (1.8 ODU instead of 5.6 ODU). This accumulation was followed by a quick re-consumption, after 9 RT at a dilution rate of D=0.035 h −1  residual glycerol was drop down at 2.9 g/L. At this time (12 days after the inoculation), the dilution rate was increased from 0.035 h −1  to 0.070 h −1  in five days. After 9 RT at a dilution rate of D=0.07 h −1 , performances stabilized at 5.5±1.1 g/L of glycerol and 51.6±0.7 g/L of PDO. 
     After this stabilization, (28 days after the inoculation (see  FIG.  1   ), raw glycerine of the feed was changed to a blend of raw glycerine provided by ADM (50%; Rolle, Switzerland) and by Greenergy (50%; London, UK), thereby increasing the level of MONG, and glycerine toxicity for microorganisms. 
     This modification of the feed composition induced cycles of glycerol accumulation (max at 35.1 g/L) and drops of PDO production (min at 37.4 g/L) during 13 days. The culture was monitored for stabilization via the key factors (OD, residual glycerol and PDO concentration), and the new adapted strain 130P was identified for storage at day 45 (see  FIG.  1   ). 
     At this step, the new strain was sequenced and compared to the sequence of 008P. We identified the intergenic mutation described in Example 3 below. 
     Performances of the resulting strain Type 130P are presented below in Table 2. 
     Formula for the Calculation of Residence Time from Dilution Rate 
     
       
         
           
             RT 
             = 
             
               1 
               DR 
             
           
         
       
       
         
           
             RT 
             ⁢ 
             
               : 
             
             ⁢ 
             
                 
             
             ⁢ 
             residence 
             ⁢ 
             
                 
             
             ⁢ 
             time 
             ⁢ 
             
                 
             
             ⁢ 
             
               ( 
               h 
               ) 
             
           
         
       
       
         
           
             DR 
             ⁢ 
             
               : 
             
             ⁢ 
             
                 
             
             ⁢ 
             dilution 
             ⁢ 
             
                 
             
             ⁢ 
             rate 
             ⁢ 
             
                 
             
             ⁢ 
             
               ( 
               
                 h 
                 
                   - 
                   1 
                 
               
               ) 
             
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Performances of the  C .  acetobutylicum  DG1  
               
               
                 pSPD5 strain type 130P. The feed medium  
               
               
                 contained 105 g/L of glycerol from raw  
               
               
                 glycerine provided by ADM  
               
               
                 (Rolle, Switzerland) and Greenergy  
               
               
                 (London, UK) at a dilution rate of 0.070 h −1 . 
               
            
           
           
               
               
               
            
               
                   
                   
                 PDO Production  
               
               
                   
                   
                 performances 
               
               
                   
                   
                 Type 130P strain 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                   
                 Feed glycerol  
                 109 
               
               
                   
                 (ADM/Greenergy) (g.l −1 ) 
                   
               
               
                   
                 PDO (g.l −1 ) 
                 52.0 
               
               
                   
                 YPDO (g.g −1 ) 
                 0.48 
               
               
                   
                 QPDO (g.l −1 .h −1 ) 
                 3.62 
               
               
                   
                 Dilution rate (h −1 ) 
                 0.070 
               
               
                   
                 Residual glycerol (g.l −1 ) 
                 7.50 
               
               
                   
                 Biomass (g.l −1 ) 
                 1.4 
               
               
                   
                 Acetic acid (g.l −1 ) 
                 5.1 
               
               
                   
                   
               
               
                   
                 YPDO: PDO yield (g/g of glycerol engaged) 
               
               
                   
                 QPDO: PDO volumetric productivity 
               
            
           
         
       
     
     Example 2 
     PDO Production Performances of  C. acetobutylicum  DG1 pSPD5 Strains Type 008P and 130P in a Chemostat by Continuous Culture with High Concentration of Raw Glycerine 
     The synthetic media used for clostridia batch cultivations contained per liter of tap water: glycerol, 30 g; KH 2 PO 4 , 0.5 g; K 2 HPO 4 , 0.5 g; MgSO 4 , 7H 2 O, 0.2 g; CoCl 2  6H 2 O, 0.01 g; H 2 SO 4 , 0.1 ml; NH 4 Cl, 1.5 g; biotin, 0.16 mg; p-amino benzoic acid, 32 mg and FeSO 4 , 7H 2 O, 0.028 g. The pH of the medium was adjusted to 6.3 with NH 4 OH 3N. Commercial glycerol purchased from SDS Carlo_Erba (purity 99%) was used for batch cultivation. The feed medium for continuous cultures contained per liter of tap water: glycerol from raw glycerine, 105 g; KH 2 PO 4 , 0.50 g; K 2 HPO 4 , 0.50 g; MgSO 4 , 7H 2 O, 0.2 g; NH 4 Cl, between 0 to 1.5 g; CoCl 2  6H 2 O, between 0.013 to 0.026 g; biotin, between 0.08 to 0.16 mg; p-amino benzoic acid, between 16 to 32 mg; FeSO 4 , 7H 2 O, 0.04 g; anti-foam, 0.05 ml; ZnSO 4 , 7H 2 O, 8 mg; CuCl 2 , 2H 2 O, 4 mg; MnSO 4 , H 2 O, 0.02 g to 0.04 g; H 3 BO 3  between 0 to 2 mg; Na 2 MoO 4 , 2H 2 O, between 0 to 0.8 mg. Medium pH was adjusted between 3.5 and 4 with H 2 SO 4  96%. 
     Raw glycerine, from the transesterification process for biodiesel, was obtained from several different sources and had the following composition:
         Novance (Compiegne, France) (using vegetable oil; purity between 82 to 85%; Moisture between 8 to 13%; MONG between 0.1 to 0.3%; Ash 1.4%)   ADM (Rolle, Switzerland) (using vegetable oil; purity 80.9%; Moisture 12.6%; MONG 0.39%; Ash 6.2%), used in a blend with Greenergy glycerine   Greenergy (London, UK) (using cooking oil; purity 76.5%; Moisture 10.2%; MONG 7.1%; Ash 6.3%), used in a blend with ADM glycerine       

     Optionally, these glycerine were pretreated by acidification. 
     As explained above in Example 1, the purity and MONG composition has an incidence on the toxicity of the glycerine on the microorganism. 
     Experimental set-up is as described in Example 1, above. 
     Batch and Continuous Cultures Process: 
     A culture growing in a 100 ml flask on synthetic medium (the same as described above for batch culture but with addition of acetic acid, 2.2 g/L and MOPS, 23.03 g/L) taken at the end of exponential growth phase was used as inoculum (5% v/v). 
     Cultures were first grown in batch mode. At the early exponential growth phase we performed a pulse of glycerol with the feed medium (the same as described for feed culture). Glycerol from raw glycerine was added at a static flow rate during 3 hours (i.e. an addition of 18 g/L of glycerol). Then, the growth continued in batch mode and before the end of the exponential growth phase the continuous feeding started with a dilution rate of 0.035 h −1 . Five to eight days after inoculation of the bioreactor, the dilution rate was increased from 0.035 h −1  to 0.070 h −1  in five days. After that, stabilization of the culture was followed by PDO production and glycerol consumption using the HPLC protocol described in example 1 in Analytical procedures. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Performances of the  C .  acetobutylium  Type 008P and of the Type  
               
               
                 130P strains in continous culture. The feed medium contained  
               
               
                 105 g/L of glycerol from raw glycerine at dilution of 0.070 h −1 . 
               
               
                 Mean data from respectively 8 and 17 chemostats. Providers of  
               
               
                 glycerine used in the cultures are indicated for each strain.  
               
               
                 Novance (Compiègne, France) corresponds to a relatively  
               
               
                 clean and pure glycerine while ADM (Rolle, Switzerland)  
               
               
                 and Greenergy (London, UK) provide glycerine that is less  
               
               
                 pure with more contaminants, and therefore more toxic to 
               
               
                 the microorganism. 
               
            
           
           
               
               
               
               
            
               
                   
                 Type 008P  
                   
                   
               
               
                   
                 strain 
                 Type 008P strain 
                 Type 130P strain  
               
               
                   
                 Raw  
                 Raw glycerine 
                 Raw glycerine  
               
               
                   
                 glycerine 
                 used: ADM/ 
                 used: Novance or 
               
               
                   
                 used:  
                 Greenergy  
                 ADM/ 
               
               
                   
                 Novance 
                 blend 
                 Greenergy blend* 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 Feed glycerol (g.l −1 ) 
                 105 
                 104 
                 106 
               
               
                 1,3-propanediol (g.l −1 ) 
                 49.8 
                 41.7 
                 52.3 
               
               
                 YPDO (g.g −1 ) 
                 0.47 
                 0.40 
                 0.49 
               
               
                 QPDO (g.l −1 .h −1 ) 
                 3.55 
                 2.90 
                 3.70 
               
               
                 Dilution rate (h −1 ) 
                 0.072 
                 0.070 
                 0.071 
               
               
                 Residual  
                 5.7 
                 18.6 
                 3.6 
               
               
                 glycerol (g.l −1 ) 
                   
                   
                   
               
               
                 Biomass (g.l −1 ) 
                 2.3 
                 1.7 
                 2.3 
               
               
                 Acetic acid (g.l −1 ) 
                 2.5 
                 2.8 
                 3.1 
               
               
                 Butyric acid (g.l −1 ) 
                 10.8 
                 8.2 
                 10.6 
               
               
                   
               
               
                 YPDO: PDO yield (g/g of glycerol engaged) 
               
               
                 QPDO: PDO volumetric productivity 
               
               
                 *Performances for strain Type 130P did not change significantly when different glycerine types were used (i.e. Novance or ADM/Greenergy blend) 
               
            
           
         
       
     
     These results show that the Type 130P strain bearing an intergenic mutation (in this case between the nucleotides at positions 1014234 to 1014240 on the chromosome according to  C. acetobutylicum  ATCC 824) that induces overexpression of hcpR and frdX genes surprisingly exhibits a better PDO production with higher titer and yield and a lesser residual glycerine than its parental strain, 008P. 
     These results also demonstrate the great advantage of the strain Type 130P which grew and produced much more PDO than the mother strain Type 008P which does not carry the genetic modification (Table 3). Indeed, all key industrial parameters (higher titer and yield of PDO and less residual glycerol) were improved for the 130P when compared to the 008P in culture conditions with industrial glycerine more toxic than usually used with Type 008P strain. 
     Thus, upon overexpression of the hcpR and frdX genes, the  C. acetobutylicum  DG1 pSPD5 strain produces more PDO and is more robust and therefore more suitable for an industrial process. 
     Example 3 
     Intergenic Mutation Description 
     Unexpectedly, a single nucleotide insertion in the intergenic region between CA_C0884 and CA_C0885 genes (illustrated in  FIG.  2   , SEQ ID NO: 15) has the effect of improving the production of PDO and the resistance to impurities MONG present in glycerin. 
     The nucleotide insertion was detected by nucleic acid sequencing of PCR fragment amplified on DNA of strain  Clostridium acetobutylicum  DG1 pSPD5 Type 008P compared to DNA of  Clostridium acetobutylicum  DG1 pSPD5 Type 130P using oligonucleotides Intergenic region forward primer (SEQ ID NO: 18) and Intergenic region reverse primer (SEQ ID NO: 19). The ‘A’ insertion mutation was identified in the intergenic region between CA_C0884 and CA_C0885 genes in a region of repeating ‘A’ nucleotide as mentioned in SEQ ID NO: 17 compared to the parental type sequence (SEQ ID NO: 16). The gene CA_C0884 hcpR (SEQ ID NO: 1) codes for a nitric oxide-responsive transcriptional regulator (SEQ ID NO: 2) and the gene CA_C0885 frdX (SEQ ID NO: 3) codes for a ferredoxin 3-like protein (SEQ ID NO: 4). 
     Example 4 
     CA_C0884 and CA_C0885 Gene Expression with or without the Intergenic Nucleotide Insertion 
     RNA Isolation 
     RNA was extracted from 2 ml of flask culture, transferred into a 6 ml mixture of phenol (5%)/ethanol (95%) and centrifuged at 3000 g at 4° C. for 5 minutes. Pellet was homogenized in 100 μL lysozyme 100 mg/mL, incubated 30 minutes at 37° C. and RNA was extracted using the Maxwell RSC Simply RNA Tissue kit (Promega) in a Maxwell RSC instrument (Promega). 
     Quantification of a Specific Ribonucleic Acid Sequence by Quantitative Reverse Transcription PCR (qRT-PCR) 
     As RNA cannot serve as a PCR template, the first step in gene expression profiling by qRT-PCR is the reverse transcription of the RNA template into cDNA, followed by its exponential amplification in a PCR reaction. 
     Reverse transcription was performed with 0.2 μg of total RNA and reverse transcribed into cDNA using SuperScript ViloIV (Invitrogen) in the presence of random primers and oligo dT primers. 
     The reverse transcriptase reaction was done in a total volume of 20 μl. After completion of the reaction, the mixture was held at 85° C. 
     Relative quantification in samples was determined by quantitative PCR using the SsoAdvanced Universal SYBR Green Supermix (Bio-rad Mitry Mory, France). Quantitative PCR was performed on a Bio-Rad C1000™ Thermal Cycler equipped with a CFX96™ Real-Time System (Bio-Rad). 
     PCR reactions mixtures consisted of 1×Sso Advanced Universal SYBR Green Supermix (Bio-Rad), 6 μL of a mix of forward (F) and reverse (R) primers (1 μM), 2 μL of diluted sample and nuclease free water to reach a final volume of 20 μL. Amplification was achieved according to the following thermal cycling program: initial melting at 98° C. for 2 min (1 cycle) followed by 40 cycles of melting at 98° C. for 10 sec, annealing of primers and elongation at 60° C. for 30 sec. (Melt Curve 65 to 95° C., increment 0.5° C. every 5 sec). For each experiment, threshold levels (Ct) were set during the exponential phase of the qPCR reaction using CFX Manager™ 3.1 software (Bio-rad). 
     The expression level of each gene was determined by quantitative reverse transcription PCR (qRT-PCR). The CA_C0884 gene based primers used were CA_C0884 gene based forward primer (SEQ ID NO:20) and CA_C0884 gene based reverse primer (SEQ ID NO:21) and the CA_C0885 gene based primers used were CA_C0885 gene based forward primer (SEQ ID NO:22) and CA_C0885 gene based reverse primer (SEQ ID NO:23). The amount of each target gene relative to the housekeeping gene DNA gyrase subunit A (gyrA: CA_C1628, primers used CA_C1628 gene based forward primer (SEQ ID NO:24) and CA_C1628 gene based reverse primer (SEQ ID NO:25) was determined for each sample using the comparative threshold cycle (Ct) method, with serial dilutions of ATCC824 genomic DNA at known concentrations used as the calibrator for each experiment. Approximately equal efficiencies of the primers were confirmed using serial dilutions of ATCC824 genomic DNA templates in order to use the comparative Ct method. 
     The relative expression level of both genes CA_C0884 and CA_C0885 was significantly higher in strain Type 130P carrying the intergenic mutation compared to the strain Type 008P with the parental type intergenic region ( FIG.  3   ). 
     These data demonstrate that the nucleotide insertion occurring in the intergenic region between CA_C0884 and CA_C0885 of recombinant  Clostridium acetobutylicum  DG1 psPD5 strain producing PDO, allows the overexpression of the two said genes hcpR and frdX. In the presence of this mutation, PDO production performance features are improved and the strain is much more resistant to dirty, high content MONG compounds present in industrial glycerine. 
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