Patent Application
20150056673
The invention relates to a recombinant microorganism for the production of hydrocarbon monomers for use in the synthesis of polymers. More precisely, the invention relates to a microorganism which has been genetically modified so as to produce monomers of this type from a carbon source, but also through depolymerization of polymers present in the culture medium. The invention also relates to a method for producing monomers using recombinant microorganisms of this type, as well as to a method for synthesizing polymers from the monomers produced.
Polymers are used in a large number of technical fields, in particular in the form of plastic material, from food packaging to the medical field, via clothing, the automobile industry, etc. As an example, polyamides, and more particularly nylons, are used both for the manufacture of soles for shoes and in the form of microfilaments for surgical sutures. Similarly, certain polyesters (for example polyethylene terephthalate—PET, polylactic acid—PLA, etc.) are used in the manufacture of clothes and packaging, but also in the form of a thermoset resin for the manufacture of automobile or other parts.
Until recently, the polymers used in the majority of industries were obtained from non-renewable fossil energy sources such as oil. However, the difficulties associated with recycling such polymeric materials and the ever-increasing dearth of non-renewable fossil energy renders using them ever more questionable.
Thence, for a number of years, the manufacture of polymers from renewable raw materials or from biomass has been under development. Polylactic acid (PLA), for example, is a biodegradable polymer produced from lactic acid which has mechanical properties which are comparable to those of polymers obtained from the petrochemicals industry. Microorganisms have been isolated and/or developed for their capacity to produce hydrocarbon monomers and/or polymers from a suitable renewable carbon source.
As an example, various microorganisms have been developed to produce lactic acid from carbon sources such as starch, pentoses, etc. In particular, recombinant yeasts expressing a lactate dehydrogenase have been produced in order to synthesize lactic acid in an appropriate culture medium (WO03102152).
Similarly, yeasts have been genetically modified in order to express a fumarate reductase and to produce succinic acid from fumaric acid (WO2009065778).
Usually, such microorganisms, recombinant or otherwise, have to be cultured in a suitable culture medium comprising expensive substrates (glucose, saccharose, etc.). In order to improve the production performance of such microorganisms, it is also necessary to optimize the culture conditions (temperature, stirring, nature of the carbon source etc.), which tends to further increase the production costs.
In order to reduce the production costs, attempts have been made to produce these metabolites directly from biomass which cannot be upgraded directly, such as sugar beet juice (WO2008000699), date juice (Boudjelal et al.—Rev. Energ. Ren: Production et Valorisation-Biomasse [Biomass—Production and Upgrading] (2001) 41-46), etc. However, the pre-treatment of biomass to promote the growth of microorganisms therein is complicated and lengthy, which means that the production costs cannot be brought down in a satisfactory manner.
Further, the quantity of hydrocarbon monomers produced by the microorganisms only exceeds 30 g/litre with difficulty, even under optimal culture conditions.
In addition, the quantity of hydrocarbon monomers and/or associated polymers produced by the microorganisms is still relatively low and the associated costs are too high for it to be a real alternative to polyethylene (PE) and other petrochemical polymers. Although the idea of producing polymers from renewable sources is seductive, there is currently no system for production by fermentation which overcomes the problems of cost and/or yield and which can satisfactorily respond to demand.
The aim of the invention is to at least partially overcome at least one of the problems discussed above by proposing a recombinant microorganism which is capable of producing large quantities of hydrocarbon monomers for use in the synthesis of polymers.
To this end, the invention proposes a microorganism, in particular a bacterium, a yeast or a fungus, which is genetically modified so as to be capable of synthesizing one or more monomer(s) by fermentation of a carbon source and of depolymerizing, simultaneously or otherwise, a polymer present in the culture medium so as to liberate the monomers of which it is composed therefrom. The monomers produced thereby by that same microorganism derive from two different and independent sources. Furthermore, the use of a microorganism of the invention of this type means that the polymers can be recycled, and thus problems with the accumulation of plastic waste can be overcome. Indeed, the polymers which are capable of being depolymerized by the microorganism of the invention can both be integrated into a product manufactured from plastic material and also be directly available in the culture medium, in solution or otherwise. Advantageously, the microorganism of the invention is capable of depolymerizing high molecular weight polymers, not only into oligomers with a lower molecular weight, but right down to the building block or monomer.
Thus, the invention concerns a microorganism which is genetically modified so as to
i) synthesize a hydrocarbon monomer by fermentation of a carbon source, and
ii) depolymerize a polymer constituted at least by hydrocarbon monomer which it is capable of synthesizing.
The invention also concerns a method for producing a hydrocarbon monomer, comprising the steps of
The invention also envisages a coculture of microorganisms comprising at least a first microorganism in accordance with the invention and at least a second microorganism, optionally genetically modified, which is capable of synthesizing a polymer constituted at least by monomers produced by the first microorganism.
The invention also concerns a method for synthesizing polymers using a coculture of microorganisms in accordance with the invention, comprising the steps of
1.25 g/L,
2.5 g/L, ▴3.75 g/L,
5 g/L).
) with 20 g/L of PLA and 3.75 g/L of proteinase K (▪) or with 20 g/L of PLA (
).
) with 20 g/L of PLA and 3.75 g/L of proteinase K (▪) or with 20 g/L of PLA (
).
) with 20 g/L of PLA and 3.5 g/L of proteinase K (▪) or with 20 g/L of PLA (
) and calculation of lactate differential (B) between the condition without PLA and the conditions with PLA.
) with 20 g/L of PLA and 3.75 g/L of proteinase K (▪) or with 20 g/L of PLA (
).
In the context of the invention, the term “microorganism” means any unicellular eukaryotic organism such as yeasts, microalgae and fungi, or prokaryotic organism such as bacteria.
Preferably, the microorganism is a microorganism from a genus selected from Aspergillus, Cupriavidus, Clostridium, Corynebacterium, Escherichia, Pseudomonas, Yarrowia, Aeromonas, Candida, Burkholderia, Thermobifida, Fusarium, Pichia, Saccharomyces and Bacillus. Preferably, the microorganism is selected from Aspergillus Niger, Cupriavidus necator, Clostridium acetobutylicum, Corynebacterium glutamicum, Escherichia coli, Pseudomonas aeruginosa, Pseudomonas putida, Pseudomonas entomophila, Pseudomonas oleovorans, Yarrowia lipolytica, Aeromonas hydrophile, Candida tropicalis, Candida antartica, Burkholderia xenovorans, Burkholderia mallei, Burkholderia pseudomallei, Thermobifida fusca, Fusarium solani, Pichia pastoris, Saccharomyces cerevisiae, Bacillus subtilis and Bacillus megaterium.
The term “genetically modified” microorganism means that the genome of the microorganism has been modified in a manner such that said microorganism exhibits two pathways, of synthesis and of depolymerization. As an example, the microorganism may be a microorganism which is naturally capable of synthesizing a monomer of interest from a carbon source and which is genetically modified to express the enzyme or enzymes necessary for the desired depolymerization, or vice versa. The microorganism may also not naturally exhibit any of the desired synthesis and depolymerization pathways, and be genetically modified for these two pathways. The microorganism may also naturally have one and/or the other of these pathways and be genetically modified to promote and/or enhance the expression of the genes involved. In accordance with the invention, the genome of the microorganism is modified so as to integrate at least one nucleic sequence coding for at least one enzyme involved in the pathway for biosynthesis of a hydrocarbon monomer and/or in the depolymerization of a polymer, or coding for a biologically active fragment thereof. Said nucleic sequence may have been introduced into the genome of said microorganism or one of its ancestors, using any suitable molecular cloning method. In the context of the invention, the “genome of a microorganism” means all of the genetic material contained in said microorganism, including the extrachromosomal genetic material contained, for example, in the plasmids, episomes, synthetic chromosomes, etc. The nucleic sequence introduced may be a heterologous sequence, i.e. it does not exist in the natural state in said microorganism, or a homologous sequence. Advantageously, a transcriptional unit is introduced into the genome of the microorganism which comprises the nucleic sequence of interest, placed under the control of one or more promoter(s).
The carbon source used by the microorganism of the invention preferably comprises refined sugars such as glucose, galactose, xylose, saccharose, etc., but also any organic material of vegetable or animal origin, or biomass, which is capable of being degraded by the microorganism to produce fermentable sugars. Thus, the carbon source may comprise hemicellulosic, cellulosic, lignocellulosic etc. biomass, for example molasses and/or sugar cane bagasse, sugar refinery waste water or spent liquor, sugar crystallization mother liquors, etc. The carbon source may also comprise glycerol or oils (triglycerides). In all cases, the carbon source does not include the monomer produced by depolymerization of the polymer of the invention.
The simultaneous fermentation and depolymerization characteristic of the present invention is defined by the fact that the microorganism is brought into contact with the carbon source and with the polymer at the same time (or within a very short time period, for example of the order of 15 minutes maximum), under the same physico-chemical conditions (for example temperature, pH and/or culture medium). Thus, the hydrocarbon monomer produced by the two pathways (fermentation and depolymerization) may be recovered simultaneously in the culture medium.
The term “hydrocarbon monomer” means an aliphatic or aromatic monomer essentially composed of carbon atoms and hydrogen. It generally comprises 1 to 8 carbon atoms, preferably 2 to 6 carbon atoms. The hydrocarbon monomer may also comprise at least one heteroatom such as nitrogen or oxygen, generally in the form of an acid, alcohol, amine function etc.
A hydrocarbon monomer of the invention may in particular be a dicarboxylic acid or one of its derivatives such as an acid dianhydride, a diol, a hydroxylated acid such as α-hydroxylated acids, a diester, a lactone, a cyclic diester (glycolide, lactide, etc.), a diamine or a hydroxylamine.
A non-limiting list of these monomers is constituted by lactic acid, glycolic acid, 3-hydroxybutyric acid, 4-hydroxyvaleric acid, butanediol, propanediol, ethylene glycol, succinic acid, glutaric acid, adipic acid, terephthalic acid, furandicarboxylic acid, caprolactone, caprolactam, putrescine, cadaverine, hexamethylene diamine, esters, esteramides, etc.
The term “oligomer” means a succession of monomers in accordance with the invention comprising more than one monomer, and fewer monomers than the polymer supplied to the microorganism.
Polymers which are capable of being depolymerized by the microorganism of the invention are polymers the principal chain of which is essentially constituted by carbon atoms, and in particular polyesters and/or polyamides. The polymers of the invention may be homopolymers or copolymers, which may be branched. The polyesters may be aliphatic and/or aromatic. Examples of aliphatic polyesters which may be cited are polylactic acid (PLA), polyhydroxyalkanoates (PHAs), poly(butylene succinate) (PBS), poly(butylene succinate-co-adipate) (PBSA) or poly-β-caprolactone (pCAPA). Aliphatic polyesters which may in particular be cited are poly(ethylene terephtalate) (PET), poly(trimethylene terephtalate) (PTT), poly(butylene terephtalate) (PBT) or poly(butylene-adipate-terephtalate) (PBAT). Polyamides which may in particular be cited include poly(caprolactam) or nylon-6; poly(undecanamide) or nylon-11; poly(hexamethylenediamine-adipate) or nylon-6,6.
The invention in particular envisages the depolymerization of aliphatic polyesters and the synthesis of all or a part of the esterification intermediates, acid and/or alcohol, of said polyester, such as, for example, the depolymerization of polylactic acid (PLA) and the synthesis of lactic acid; the depolymerization of poly(3-hydroxybutyrate) (PHB) and the synthesis of 3-hydroxybutyric acid; the depolymerization of poly(3-hydroxybutyrate-co-4-hydroxyvalerate) (PHBV) and the synthesis of 3-hydroxybutyric acid and/or 4-hydroxyvaleric acid; the depolymerization of poly(butylene-succinate) (PBS) and the synthesis of 1,4-butanediol and/or succinic acid; the depolymerization of poly(butylene-succinate-co-adipate) (PBSA) and the synthesis of 1,4-butanediol and/or succinic acid and/or adipic acid.
The invention also envisages the depolymerization of aromatic polyesters and the synthesis of all or part of the associated monomers, such as, for example, the depolymerization of poly(ethylene-terephthalate) (PET) and the synthesis of ethylene glycol and/or terephthalic acid; the depolymerization of poly(trimethylene-terephthalate) (PTT) and the synthesis of 1,3-propanediol and/or of terephthalic acid; the depolymerization of poly(butylene-terephthalate) (PBT) and the synthesis of 1,4-butanediol and/or terephthalic acid; the depolymerization of poly(butylene-adipate-terephthalate (PBAT) and the synthesis of 1,4-butanediol and/or adipic acid and/or terephthalic acid.
The invention also envisages the depolymerization of polyamides and the synthesis of all or a part of the associated monomers, such as, for example, the depolymerization of poly(caprolactam) or nylon-6 and the synthesis of caprolactam, or of 6-amino-hexanoic acid; the depolymerization of poly(undecanamide) or nylon-11 and the synthesis of undecanamide; the depolymerization of poly(hexamethylenediamine-adipate) or nylon-6,6 and the synthesis of hexamethylenediamine and/or adipic acid.
In accordance with the invention, the same microorganism is capable of synthesizing at least one hydrocarbon monomer and of depolymerizing a polymer constituted at least by that same monomer. In accordance with the invention, the microorganism is capable of depolymerizing said polymer in a manner so as to recover the monomer of interest from it. In the case in which the polymer comprises other monomers, said polymer can be only partially depolymerized so as to liberate the monomer of interest, the other monomers possibly being in the final depolymerization form in the form of oligomers.
In a particular embodiment of the invention, the microorganism is also genetically modified to attenuate at least one pathway for the degradation of a hydrocarbon monomer which it is capable of synthesizing.
In fact, in some cases, the microorganisms of the invention may be naturally capable of using the monomer which it synthesizes as a carbon source for growth thereof. However, these monomers may be of particular industrial interest, in particular for the subsequent synthesis of polymers, and it may be advantageous to prevent their degradation. To this end, in accordance with the invention it is possible to delete all or part of the genes coding for the enzyme or enzymes involved in the degradation of the monomer of interest, using any appropriate recombination/deletion method. It is also possible to attenuate the expression of all or a portion of the genes involved in the degradation of the monomer of interest. To this end, it is, for example, possible to introduce one or more nucleic sequences coding for regulators, such as transcription factors, interfering RNAs, siRNA, etc., which are capable of attenuating the expression of these genes. Similarly, it is possible to modify the promoter sequence of at least one of the genes involved in the degradation of the monomer of interest, so as to attenuate/prevent transcription therein.
In order to prevent the degradation of the synthesized monomers, it is also possible, instead of or as a complement to the above genetic modifications, to regularly enrich the culture medium for the microorganism with substrates which can easily be assimilated by the microorganism, so that it preferentially uses those substrates rather than the synthesized monomer.
The microorganism of the invention may also be genetically modified so as to be able to synthesize a polymer constituted entirely or in part by a monomer which it is capable of synthesizing. As an example, the microorganism may be genetically modified so as to integrate and express a nucleic sequence coding for a particular polymer synthase. Thus, the same microorganism may, in accordance with the invention, produce a monomer with the aid of a fermentation and a targeted depolymerization of a polymer to be recycled/degraded, and to produce a new polymer using the monomers produced. In the case in which the polymer of interest is a heteropolymer, it is possible to enrich the culture medium with the other constituent monomer or monomers of said polymer.
In particular, the invention envisages the depolymerization of aliphatic polyesters such as polylactic acid (PLA), polyhydroxyalkanoates (PHAs), etc., and the synthesis of all or a portion of the associated monomers.
Thus, in a first embodiment, the microorganism of the invention is genetically modified to depolymerize PLA and thus to obtain lactic acid, and concomitantly to synthesize lactic acid from a carbon source.
The enzyme to depolymerize PLA is advantageously selected from a serine protease, a lipase and a PLA depolymerase. Preferably, the enzyme for depolymerizing PLA is selected from the proteinase K of Tritirachium album, the lipase A from Aspergillus niger, the lipase AY from Candida rugosa, the lipase F from Rhizopus oryzae, the lipase PS from Burkholderia sp. or the lipase cutinase-like enzyme from the S-2 strain of Cryptococcus sp. Similarly, the enzyme for the synthesis of lactic acid is advantageously a lactate dehydrogenase. In a particular example, the microorganism is a lactic microorganism, for example a lactic bacterium, naturally excreting a lactate dehydrogenase, i.e. in the wild state.
The invention also provides a method for producing a hydrocarbon monomer, comprising the steps of
The hydrocarbon monomer produced both by fermentation and by depolymerization is advantageously present in the culture medium. It is then possible to treat the culture medium which is enriched in these monomers using any known technique, so as to isolate them and/or to purify them.
In accordance with the invention, it is possible to produce all sorts of monomers, simply by adapting the microorganism used and the genetic modifications introduced. The choice of microorganism as well as the genetic modifications to be provided are within the purview of the skilled person in the light of the teaching of the present application.
The conditions for bringing the microorganism into contact with the carbon source and the polymer, such as the temperature, pH, etc., may be adapted by the skilled person to optimize/favour both production of the monomer by depolymerization of polymer and production of monomer by fermentation starting from the carbon source.
The invention also proposes a coculture, for example in a fermenter, of at least two microorganisms, one of which is a genetically modified microorganism of the invention, the second microorganism itself being capable of synthesizing a polymer constituted by at least the monomer produced by the microorganism of the invention.
Thus, the monomer produced by the microorganism of the invention and excreted in the culture medium is used by the second microorganism to produce a polymer of interest which could then be exploited in any type of industry. The method of the invention can thus be used to completely recycle polymers, from their depolymerization into monomers which can be used again, to their re-use in the form of new polymers which can be used in the form of plastics for new applications.
Thus, for example, the microorganism of the invention is a genetically modified lactic bacterium which expresses at least one lactate dehydrogenase and at least one enzyme which is capable of depolymerizing PLA selected from PLA depolymerases, proteases and lipases. This lactic bacterium may be co-cultivated with another microorganism, which may be genetically modified, which can synthesize PLA. Advantageously, the two cocultured microorganisms are selected so that they do not compete for the carbon source, preferably sugars, present in the culture medium. In certain embodiments of the present invention, the term “comprise” can signify “essentially consist of” or “consist of”.
The invention will be better understood from the examples below, given by way of entirely non-limiting illustration of the invention.
Method and Apparatus
The cloning techniques used to prepare the recombinant plasmid are those conforming to the techniques described in the publication by Sambrook et al., 1989 (“Molecular cloning: a Laboratory Manual. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.”).
The pld gene from Amycolatopsis sp. K104-1 with the sequence SEQ ID No 1 (Nakamura et al. 2001—Appl. Environ. Microbiol. 67:345-353) is introduced into the plasmid pNZ8048 by homologous recombination (Kuipers et al. 1998—J. Biotechnol. 64:15-21). The recombinant plasmid integrating the pld gene is termed “pNZ-pld”.
The wild type strain Lactococcus lactis MG1363 is transformed by the plasmid pNZ-pld introduced into the cell by electroporation using the method described by Ho et al. in 1995 (“Transformation of Lactococcus by electroporation” Methods Mol. Biol. 47:195-199). The recombinant bacterium integrating the pNZ-pld plasmid is denoted “MG1363-pNZ-pld”. The negative control “MG1363-pNZ8048” corresponds to the wild type strain Lactococcus lactis MG1363 transformed by the empty plasmid pNZ8048.
PLA Depolymerase Activity
The MG1363-pNZ-pld and MG1363-pNZ8048 strains are cultivated in parallel in 2 L fermenters with CDM (“Chemically Defined Medium”) at 30° C., under anaerobic conditions. Glucose is added to the culture medium to reach a final concentration of 1% (weight/volume), and the pH is kept at 6.5 by automatic addition of NaOH.
The cultures are each divided into two sub-cultures, each batch of two sub-cultures (MG1363-pNZ-pld-1/MG1363-pNZ8048-1 and MG1363-pNZ-pld-2/MG1363-pNZ8048-2) being kept under identical conditions.
Only the second batch (MG1363-pNZ-pld-2/MG1363-pNZ8048-2) receives PLA with a molecular mass of 220,000 Da (Shimadzu Co.—Kyoto, Japan), emulsified into the culture medium to 0.1% (weight/volume).
The PLA depolymerase activity is measured after 2 days of culture, by applying the method described in Nakamura et al.—2001 (“Purification and characterization of an extracellular poly(L-lactic-acid) depolymerase from a soil isolate Amycolatopsis sp. Strain K104-1” Appl. Environ. Microbiol. 67:345-353).
Culture supernatant from the first batch is removed after two days of culture and analysed by mass spectrometry (Varian Inova 500) in order to evaluate the lactic acid content. For the two samples (MG1363-pNZ-pld-1 and MG1363-pNZ8048-1), a single peak at 90 Da is observed, corresponding to the lactic acid produced by the bacteria by fermentation. Thus, the recombinant strain MG1363-pNZ-pld is well adapted to producing lactic acid by fermentation of glucose present in the culture medium.
At the same time, culture supernatant from the second batch is removed and analysed by mass spectrometry (Varian Inova 500) in order to evaluate the quantity of lactic acid oligomers (essentially dimers and trimers) present.
For the two samples, (MG1363-pNZ-pld-2 and MG1363-pNZ8048-2), a peak is observed at 220,000 Da, corresponding to PLA present in the culture medium, and a peak at 90 Da corresponding to lactic acid. Further, for the sample corresponding to the culture medium of MG1363-pNZ-pld-2, the presence of several peaks with an intermediate molecular weight is observed, these peaks being absent for MG1363-pNZ8048-1. These intermediate peaks corresponds to lactic acid oligomers of various sizes, which confirms that the PLA of the culture medium has indeed been degraded by the recombinant strain MG1363-pNZ-pld-2.
The recombinant strain MG1363-pNZ-pld, expressing a PLA depolymerase, is indeed capable of producing lactic acid by fermentation and of depolymerizing high molecular weight PLA present in the culture medium.
The culture supernatants of Example 1, corresponding to batches MG1363-pNZ-pld-1, MG1363-pNZ-pld-2, MG1363-pNZ8048-1 and MG1363-pNZ8048-2, are analysed by HPLC in order to determine the quantity of lactic acid liberated. To this end, an Aminex HPX-87H column, thermostatted at 50° C. and eluted at 0.5 mL/min with a 5 mM sulphuric acid solution, is used for the quantification.
This analysis could demonstrate that the quantity of lactic acid in the supernatant from batch MG1363-pNZ-pld-2 is higher than the quantity of lactic acid of the 3 other conditions (MG1363-pNZ-pld-1, MG1363-pNZ8048-1 and MG1363-pNZ8048-2). Similarly, the presence of a lactic acid dimer is observed only in the culture supernatant from batch MG1363-pNZ-pld-2.
A complementary analysis on a C18 column is carried out using the method described by Vu et al., 2005 (“Oligomer distribution in concentrated lactic acid solutions”, Fluid Phase Equilibria, 236, 125-135). The analysis of the culture supernatant from batch MG1363-pNZ-pld-2 confirms the presence of lactic acid oligomers up to a degree of polymerization of 6.
The example below deals with the combination of the Lactococcus lactis IL 1403 strain and proteinase K for the production of lactic acid. The proteinase K is proteinase K from Tritirachium album, available from EUROMEDEX under reference EU0090.
3.1. Selection of the Strain and Culture Medium
The strain used is the Lactococcus lactis IL 1403 strain. This strain was cultured on chemically defined medium (CDM medium) supplemented with 5 g/L of glucose and buffered to a pH of 8. A rich medium (containing sources of peptones, proteins or oligopeptides) was not used so as not to include in the culture medium substrates which would be preferred to proteinase K which could compete with the PLA which is to be degraded.
3.2. Selection of Culture Conditions
The aim was to determine the growth rate of L. lactis IL 1403 for 4 pH values: 6.6, 7.0, 7.5 and 8.0. The experiments were carried out in tubes containing 15 mL of chemically defined medium with 5 g/L of glucose at 30° C. These experiments were carried out under anaerobic conditions and under aerobic conditions.
Growth was monitored by measuring the optical density at 580 nm for 24 h, and the mean growth rate could be determined.
While the condition at pH 6.6 corresponds to the reference condition for L. lactis IL 1403, the maximum growth rate decreases slightly at pH 7.5, but slightly more at a pH of 8 (25-30%). The aeration conditions had little influence.
These results show that L. lactis IL 1403 can be cultured at a pH of 8 with little influence on its growth rate.
3.3. Selection of PLA Depolymerization Conditions
A first study was carried out in order to determine the efficiency of hydrolysis of the proteinase K at different concentrations (1.25, 2.5, 3.75 and 5 g/L) in the chemically defined medium (growth medium for L. lactis IL 1403) at 30° C. (growth temperature for lactic bacteria) and at a pH of 8 (optimal pH for proteinase K). The concentration of lactic acid liberated after 24 h of hydrolysis was measured by HPLC for 5 initial concentrations of PLA: 2.5, 5, 10, 20 and 40 g/L, and 4 concentrations of proteinase K: 1.25, 2.5, 3.75 and 5 g/L (
This study shows that after 24 hours, between 27 and 54 mM of lactic acid could be obtained by hydrolysis of PLA by proteinase K in CDM at an initial pH of 8.0. The best hydrolysis yield was obtained for concentrations of PLA and proteinase K of 20 g/L and 3.75 g/L respectively.
3.4. Combination of Lactic Bacteria and PLA Depolymerase
The experiments were carried out in 2 L fermenters (Biostat B plus, Sartorius; 1 L working volume) controlled by a computer. Three culture conditions were tested, starting from a chemically defined medium containing only 2.5 g/L of glucose. The pH was adjusted to a value of 8.0 by adding 10N KOH. The temperature was 30° C. The three conditions are described below:
A. CDM+IL 1403
B. CDM+IL 1403+PLA 20 g/L+Proteinase K (3.75 g/L)
C. CDM+IL 1403+PLA 20 g/L
The PLA was autoclaved dry directly in the fermenters. The medium and the proteinase K were added under sterile conditions.
The cultures were monitored kinetically: sampling every two hours, measuring the optical densities at 580 nm, freezing the culture supernatants after centrifuging for subsequent analysis of the substrate and co-products by HPLC. The correlation between the OD and the dry weight for the L. lactis IL 1403 strain is known: 1 g/L=0.3 ODU.
3.4.1. Growth
The profiles for the growth rate of L. lactis IL 1403 cultured in the absence and in the presence of PLA are similar. The μmax values are 0.65 h−1 (no PLA) and 0.70 h−1 (with PLA). In contrast, the growth rate profile when proteinase K is added to the medium as a supplement to PLA is very different. The μmax is 0.85 h−1 (
Irrespective of the medium conditions, growth stops after 4 h30 of culture and corresponds to complete consumption of the glucose.
3.4.2. Catabolic Balance
The various culture conditions for L. lactis IL 1403 were monitored dynamically over 9 hours, then two final samples were taken at 15 hours and at 24 hours.
The change in the concentrations of glucose in the culture medium is strictly similar under the three culture conditions. The glucose is completely consumed after 4 h of culture (
Regarding the specific glucose consumption rates during the exponential growth phase (1 h-3 h), they are very close in the medium without PLA and with PLA. The maximum values for the specific glucose consumption rates (qglucose) are respectively 27 and 24 mmole/g/h. When proteinase K is added to the medium, this qglucose is only 17 mmole/g/h. This difference in qglucose is in agreement with a higher biomass in the fermenter containing proteinase K.
The lactate is the major fermentation product. Adding PLA and proteinase K does not modify the homolactic nature of the strain in any way. More than 90% of the pyruvate stream leads to the production of lactic acid, while less than 7% is directed towards the production of acetic acid. The catabolic carbon balances are equilibrated between 89% and 95%, taking into account the CO2 theoretically produced by the metabolism. The overall stoichiometric balances in mM of carbon, calculated after 24 h of culture, are as follows:
IL 1403
IL 1403 with PLA
IL 1403 with PLA and proteinase K
Overall, the change in the lactate concentrations followed the same profile in the three culture conditions, however with slight differences in the onset of culture when PLA is present with or without proteinase K (
3.5. Conclusion
In the tests carried out in a regulated pH fermenter, the degradation of PLA to lactic acid by proteinase K could not be detected. The production of lactate could be considered to derive exclusively from the consumption of glucose: 1.66 to 1.74 mmole of lactate produced per mmole of glucose, the value normally encountered with this strain.
This demonstrates that simply adding PLA-depolymerase to a culture of lactic bacteria cannot increase the lactic acid produced in the method.
4.1. Construction of an Escherichia coli Strain Expressing a PBS-Depolymerase
A strain of Escherichia coli, SBS 550 MG-PHL413, expressing a PBS-depolymerase (Cutinase-like enzyme (CLE) from Cryptococcus sp. S-2) is constructed. The gene sequence for the CLE is presented in SEQ ID NO: 2; the corresponding amino acid sequence is presented in SEQ ID NO: 3.
To this end, the gene for the CLE is amplified by PCR starting from genomic DNA of the strain Cryptococcus sp. S-2 (Masaki et al., 2005: Kazuo Masaki, Numbi Ramudu Kamini, Hiroko Ikeda, and Haruyuki Iefuji, “Cutinase-Like Enzyme from the Yeast Cryptococcus sp. Strain S-2 Hydrolyzes Polylactic Acid and Other Biodegradable Plastics”, AEM, 71, 7548-7550).
The following primers are used:
At the same time, the vector pET26b(+) (Novagen) is linearized by digestion with the restriction enzymes BamHI and XhoI.
The insert and the linearized vector are purified on gel using the QIAquick Gel Extraction kit (Qiagen). A ligation is carried out with ligase T4 (T4 DNA ligase, New England Biolabs). The ligation product is transformed in E. coli DH5a then spread onto LB-Agar medium containing 40 μg/mL of kanamycin. After leaving overnight at 37° C., one colony is removed and cultured in 5 mL of LB medium containing 40 μg/mL of kanamycin at 37° C. overnight (16 to 18 h). The amplified vector is purified using the QIAprep Spin Miniprep Kit (Qiagen) and verified by sequencing.
The vector obtained thereby is denoted pET26b-CLE.
The vector pET26b-CLE is transformed in the E. coli SBS 550 MG-PHL413 strain (Sanchez et al., 2005: Ailen M. Sanchez, George N. Bennett, Ka-Yiu San, “Novel pathway engineering design of the anaerobic central metabolic pathway in Escherichia coli to increase succinate yield and productivity, Metabolic Engineering 7 (2005) 229-239). A negative control is constructed by transformation of the empty vector pET26b in E. coli SBS 550 MG-PHL413. The two strains obtained thereby are respectively denoted E. coli SBS 550 MG-CLE and E. coli SBS 550 MG pET26b.
4.2. Production of Succinic Acid by Culture of the Strain E. coli SBS 550 MG-CLE
The strains E. coli SBS 550 MG-CLE and E. coli SBS 550 MG pET26b are cultured in a 2 L fermenter in a mineral medium supplemented with 20 g/L of glucose and 20 g/L of PBS (Bionolle 1001, Showa Highpolymer Corp. Ltd. (Tokyo, Japan) micronised to 500 μm). The temperature is maintained at 37° C., and the pH at 6.5 by adding KOH. A first culture phase under aerobic conditions produced biomass. When the optical density at 600 nm reaches 0.6, a solution of IPTG is added to each fermenter in order to obtain a final concentration of 1 mM and to induce the production of PBS-depolymerase in the fermenter, corresponding to the culture of the strain E. coli SBS 550 MG-CLE. Culture is continued under anaerobic conditions by continuously injecting 0.4 vvm of CO2 until all of the glucose has been consumed.
Samples are taken from the culture medium at regular intervals in order to measure the bacterial growth and to analyse the composition of the supernatant. The glucose concentration and the succinic acid production are measured by HPLC on a Bio-rad HPX 87H column. Detection is carried out by refractometry.
During fermentation, glucose is indeed consumed by each of the strains and succinic acid is produced. The fermenter corresponding to culture of the strain E. coli SBS 550 MG-CLE exhibites a higher production of succinic acid, corresponding to the depolymerization of PBS by the CLE (PBS-depolymerase) expressed by the strain E. coli SBS 550 MG-CLE.
5.1. Construction of a Strain of Clostridium acetobutylicum Expressing a PTT-Depolymerase.
The gene Bta1 corresponding to cutinase TfH is amplified from genomic DNA obtained from the strain Thermobifida fusca DSM 43793. The sequence of the Bta1 gene of the cutinase TfH is presented in SEQ ID NO: 4; the corresponding amino acid sequence is presented in SEQ ID NO: 5. At the same time, the signal peptide PS-Cel48A of the cellulase from Clostridium acetobutylicum ATCC 824 (Cel48A, Sabathé, F., A. Belaich, and P. Soucaille. 2002. Characterization of the cellulolytic complex (cellulosome) of Clostridium acetobutylicum. FEMS Microbiol. Lett. 217:15-22) was amplified by PCR (Polymerase Chain Reaction).
The expression vector pSOS952 (Perret, S., Casalot, L., Fierobe, H. P., Tardif, C., Sabathe, F., Belaich, J. P., and Belaich, A., 2004. “Production of heterologous and chimeric scaffoldins by Clostridium acetobutylicum ATCC 824”. J. Bacteriol. 186, 253-257) is linearized by digestion with the restriction enzymes BamHI and Nar I.
Finally, the above gene bta1 of the signal peptide PS-Cel48A is cloned into the linearized vector using the In-Fusion cloning kit (In-Fusion® HD Cloning Kit, Clontech). The vector obtained thereby is denoted pSOS952-bta1.
The vector pSOS952-bta1 is transformed in E. coli ER2275 (pAN1) so that it could be methylated by the methyltransferase φ3T I using the method described by Mermelstein and Papoutsakis, 1993 and Green et al., 1996. After purification, the plasmid is transformed by electroporation in the strain Clostridium acetobutylicum DG1 (pSPD5) (Gonzalez et al., 2005). The recombinant strain obtained is denoted C. acetobutylicum DG1-pSOS952-bta1.
A control strain (negative control) is constructed by transformation of the vector p505952 in Clostridium acetobutylicum DG1 (pSPD5).
5.2. Production of 1,3-propanediol by Culture of the Strain C. acetobutylicum DG1-pSOS952-bta1
A pre-culture of the two strains obtained thereby is carried out at 35° C. on a synthetic medium containing 10 g/L of glycerol as the carbon source and under anaerobic conditions (sealed flask and in a nitrogen Gonzalez et al., 2005). Each pre-culture is used to seed a 250 mL fermenter so as to have an absorbance at 600 nm equal to 0.05 at t0. The culture medium is constituted by the synthetic medium described by Gonzalez et al., 2005, supplemented with 50 g/L glycerol and PTT (polytrimethylene terephthalate, Sorona micronised to 500 μm, DuPont) and kept under anaerobic conditions under a nitrogen atmosphere. The pH of the fermenter is kept at 6.5 by adding NH4OH.
Samples of the culture medium are taken at regular intervals in order to measure the bacterial growth and to analyse the composition of the supernatant. The concentration of glycerol and the production of 1,3-propanediol are measured by means of HPLC on a Bio-rad HPX 97H column. Refractometry is used for detection.
During the fermentation, the glycerol is indeed consumed by each of the strains and 1,3-propanediol and acetate are produced. The fermenter corresponding to culture of the strain C. acetobutylicum DG1-pSOS952-bta1 exhibits a greater production of 1,3 propanediol as well as the production of terephthalic acid, which corresponds to depolymerization of PTT by PTT-depolymerase secreted into the culture medium.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1252733 | Mar 2012 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2013/056583 | 3/27/2013 | WO | 00 |
