Production of 1,3-PDO and butanol by a mutant strain of Clostridium pasteurianum with increased tolerance towards crude glycerol
© Jensen et al.; licensee Springer. 2012
Received: 27 July 2012
Accepted: 30 July 2012
Published: 17 August 2012
The production of biodiesel results in a concomitant production of crude glycerol (10% w/w). Clostridium pasteurianum can utilize glycerol as sole carbon source and converts it into 1,3-propanediol, ethanol, butanol, and CO2. Reduced growth and productivities on crude glycerol as compared to technical grade glycerol have previously been observed. In this study, we applied random mutagenesis mediated by ethane methyl sulfonate (EMS) to develop a mutant strain of C. pasteurianum tolerating high concentrations of crude glycerol. At an initial crude glycerol concentration of 25 g/l the amount of dry cell mass produced by the mutant strain was six times higher than the amount produced by the wild type. Growth of the mutant strain was even detected at an initial crude glycerol concentration of 105 g/l. A pH controlled reactor with in situ removal of butanol by gas-stripping was used to evaluate the performance of the mutant strain. Utilizing stored crude glycerol, the mutant strain showed significantly increased rates compared to the wild type. A maximum glycerol utilization rate of 7.59 g/l/h was observed along with productivities of 1.80 g/l/h and 1.21 g/l/h of butanol and 1,3-PDO, respectively. These rates are higher than what previously has been published for C. pasteurianum growing on technical grade glycerol in fed batch reactors. In addition, high yields of the main products (butanol and 1,3-PDO) were detected and these two products were efficiently separated in two steams using gas-stripping.
Limited availability of fossil resources and increasing global impact from the release of fossil fuel-derived CO2 has increased the development of biological production of renewable alternatives. Biodiesel production from plant lipids is considered a renewable alternative to mineral oil-derived diesel ([Demirbas 2007]). The increasing market for biodiesel has substantially altered the cost and availability of glycerol released from transesterification of fatty acids from lipids. Without purification, this glycerol (crude glycerol), is considered a waste ([Johnson and Taconi 2007]; Dobson et al. ) and it is, therefore, important that new and sustainable solutions for utilization of the crude glycerol are developed (Pyle et al. ; [Yazdani and Gonzalez 2007]; [Willke and Vorlop 2008]).
The major microbial conversion route of glycerol leads to the production of 1,3-propanediol (1,3-PDO) (Pagliaro et al. ). Currently, 1,3-PDO is used as a diol component in the plastic poly-trimethylene terephthalate (PTT), which is comparable to nylon ([Willke and Vorlop 2008]). PTT is mainly used for carpet and textile fiber production (DuPont™ Sorona® ). The organisms best known for producing 1,3-PDO are Clostridium butyricum and Klebsiella pneumoniae, which both can achieve high yields and productivities (Biebl et al. ; Zeng et al. ).
Since the 1,3-PDO pathway does not lead to ATP production, other pathways are needed for the generation of energy. The formation of acetate and/or butyrate leads to ATP production but is also associated with the generation of reducing equivalents, which are regenerated by the 1,3-PDO pathway (Rossi et al. ; [Zeng 1996]).
The most commonly used strategy for 1,3-PDO production is fed-batch fermentation, because it combines high product concentrations with a low excess of glycerol in the fermentation broth, which both are critical for the downstream processing ([Zeng and Biebl 2002]). However, accumulation of by-products such as acetate, butyrate, or ethanol can cause inhibition of the organism ([Biebl 1991]).
Butanol is an important bulk chemical for the synthesis of a variety of chemical products and an efficient biofuel with properties clearly superior to ethanol such as lower enthalpy of vaporization, lower solubility of water, less corrosiveness, and a much higher energy density (Dürre ).
The production of butanol and 1,3-PDO from biodiesel derived glycerol will not only constitute a sustainable utilization of waste glycerol for fuel production, but also a means to produce two chemicals, which can be used in the chemical industry.
The most studied organism for biological production of butanol is C. acetobutylicum. However, C. acetobutylicum cannot grow solely on glycerol as it cannot re-oxidize the excess NADH generated in the cellular glycerol catabolism. (Girbal et al. ; Gonzalez-Pajuelo et al. )
In contrast to C. butyricum which produces 1,3-PDO along with the formation of butyrate and acetate, C. pasteurianum produces butanol, 1,3-PDO, and ethanol as main products. Fermentation of glycerol by C. pasteurianum was firstly described by Nakas et al. () in an attempt to obtain a marketable product from photosynthetically produced glycerol from halophilic algae.
Only few publications have dealt with optimization of glycerol fermentation by C. pasteurianum ([Biebl 2001]; Dabrock et al. ). Recently, a study demonstrating the possibility of utilizing crude glycerol in fermentative butanol production has been published (Taconi et al. ). However, growth and productivity were significantly lower compared to results obtained from fermentation of pure glycerol. In order to achieve a high productivity, upstream purification has been successfully applied (Venkataramanan et al. ; Jensen et al. ). Another approach is to develop strains capable of coping with the toxicity of the crude glycerol. To construct/develop such an organism a directed or a random approach can be used. Even though directed strain improvement by systems metabolic engineering may be necessary as reported by (Lee et al. ) and Lee et al. () the lack of a complete genome sequence and adequate tools limits the application on C. pasteurianum.
In this study we have, therefore, chosen to develop a strain of C. pasteurianum with elevated tolerance towards to crude glycerol derived from the biodiesel production by use of chemical mutagenesis and selection.
Materials and methods
The strain C. pasteurianum (DMSZ 525) was purchased from the German Collection of Microorganism and Cell Cultures (DSMZ), Göttingen, Germany.
Medium and conditions
The minimal medium used for both reactor and 10 ml batch fermentations was described by Jensen et al. (). When solid media were used, 15 g/l agar was added to the media prior to autoclavation. All cultures were incubated anaerobically at 37°C at pH 6.0 under a gas phase of N2/CO2 (80:20). The carbon source was either purified (technical grade) glycerol (Sigma-Aldrich, St. Louis, Missouri, USA) or biodiesel derived crude glycerol (Meroco, Leopoldov, Slovakia). The crude glycerol was derived from 100% rape seed oil and was specified to contain 800–850 g/l glycerol, less than 1 g/l methanol, 55 g/l NaCl and 2.5% MONG (matter organic non glycerol) by the manufacturer. All amounts of crude glycerol presented are based on measured concentrations.
Chemical mutagenesis and evolutionary adaption
A fresh C. pasteurianum culture (50 or 100μL) was plated on petri dishes containing solid minimal medium supplemented with 50 g/l and 70 g/l crude glycerol. A drop (approximately 7 μl) of ethane methyl sulfonate (EMS) was placed in the middle of the plates. The plates were incubated at 37°C for three days in an anaerobic jar together with a humid anaerocult® A (Merck, Darmstadt, Germany). An inoculated plate (without EMS) was incubated as a control in the jar.
After incubation, single colonies were picked from the edge of the clearing zones caused by EMS. The selected colonies were streaked on plates with similar or higher glycerol concentration without EMS. When growth was detected, the colonies were repeatedly streaked on plates with similar or higher glycerol concentration to increase the tolerance towards the crude glycerol. The crude glycerol concentration in the medium was; 50, 60, 70, 80, 90, 100, and 110 g/l.
Colonies able to grow on 110 g/l crude glycerol plates were inoculated in liquid minimal medium supplemented with 25 g/l crude glycerol. The liquid culture was used as inoculums for the toxicity test.
Treatment of the crude glycerol
Glycerol was stored at room temperature for 10 months as previously described (Jensen et al. . Approximately 0.05 g of activated stone carbon 0.4 - 0.85 mm per 10 ml of medium (Gert Strand, Malmö, Sweden) was added prior to autoclavation (Jensen et al. ).
To assess the inhibition of the glycerol, toxicity tests were performed in batch experiments. A series of vials were prepared with different concentrations of glycerol ranging from 10 to 200 g/l. The vials were inoculated with 0.2 ml of an overnight culture. After 20 hours of incubation, growth was evaluated based upon the amount of dry cell mass. The dry cell mass was determined spectrophotometrically after establishing a linear correlation between the dry cell mass and cell suspension absorbance at 595 nm. All tests were conducted in at least duplicates.
The reactor fermentation experiments were carried out as described by Jensen et al. () in 500 ml glass reactors at 37°C. The active volume was 400 ml and the inoculation volume was 10%. pH was maintained at 6.0 by addition of 1 M KOH. Mixing was performed by magnetic stirring. For in situ removal of solvent, gas-stripping was applied, circulating the produced gas by means of a peristaltic pump at a flow rate of approximately 600 ml/min.
For stoichiometric calculations the composition of cell biomass was assumed to be CH1.65 N0.23O0.45 (Hild et al. ).
Liquid samples were analyzed for glycerol, lactate, acetate, 1,3-PDO, butyrate, ethanol, acetone, and butanol using high-pressure liquid chromatography. The HPLC system was equipped with a Rezex ROA-Organic Acid column (Phenomenex, Torrance, California, USA) and a RI 101 refractive index detector (Shodex, Kawasaki, Japan). The mobile phase was 4.5 mM H2SO4, pumped at a flow-rate of 0.6 mL/min. Prior to the analysis the samples were centrifuged at 12,000 x g for 10 minutes. The supernatant was separated and diluted to a suitable concentration range before it was loaded on the HPLC.
The two main compounds 1,3-PDO and butanol were produced in almost equimolar amounts with a 18% surplus of butanol after the mass-balance had been adjusted.
Random mutagenesis is a generally an accepted and widely used approach for increasing tolerance of a bacterial strain towards an unknown inhibitor. EMS and N-methyl-N-nitro-N-nitrosoguanidine (MNNG), which both act by direct mutagenesis inducing base substitutions or deletions, have been used with success in the closely related C. acetobutylicum ([Annous and Blaschek 1991]; Elkanouni et al. ). Recently, Malaviya et al. () succeeded in developing an effective butanol producing mutant strain of C. pasteurianum by the use of MNNG. However, both [Lemmel (1985]) and Syed et al. () obtained C. acetobutylicum mutants with higher tolerance and productivity using EMS and reported that MNNG was a less effective mutagenic compound. Our success with increased tolerance towards crude glycerol and increased butanol productivity by EMS mutagenesis confirmed the efficiency of EMS for development of C. pasteurianum mutants. As the mutant strain was developed by chemical mutagenesis it will not be classified as a Gene Modified Organism (GMO) reducing the operating costs and safety precautions necessary to run a GMO-based production.
Comparison of fermentation data obtained in this study with a related study
Technical grade glycerol(Jensen et al.)
Stored crude glycerol + activated stone carbon (Jensen et al.)
Stored crude glycerol + activated stone carbon
Maximum Glycerol rate
Maximum 1,3-PDO rate
Maximum Butanol rate
The maximum glycerol utilization rate attained by MNO6 was 7.59 g/l/h whereas the wild type strain reached rates of 4.08 g/l/h and 4.94 g/l/h utilizing stored crude glycerol and technical grade glycerol, respectively. This corresponds to an increased rate of 86% and 55% compared to the wild type on crude and technical grade glycerol. The production rates were similarly increased by 33% for 1,3-PDO and 46% for butanol compared to the wild type grown on technical grade glycerol. When the rates achieved with MNO6 were compared to rates of the wild type grown on similar glycerol, maximum 1,3-PDO productivity was increased by 89% and maximum butanol productivity was increased by 49%. Malaviya et al. () demonstrated significantly increased production rates in a high cell density continuous bioreactor using a so-called hyper producing C. pasteurianum mutant strain. Under batch conditions the mutant strain had a 14% higher butanol productivity compared to the wild type. This is lower than the increase in productivity by MNO6 demonstrated in this study.
In a glucose fed batch bioreactor with gas-stripping, Ezeji et al. () achieved a maximum butanol productivity of 1.81 g/l/h (65% of total solvent productivity) utilizing the well known ABE producing strain C. beijerinckii BA101. This is similar to the maximum butanol productivity achieved by MNO6. However, Ezeji et al. reached the productivity by utilizing pure glucose where MNO6 utilized stored crude glycerol.
In the mass balance the carbon recovery was 87% (Table 1). A similar carbon recovery was observed in the fermentation by the wild type utilizing stored crude glycerol supplemented with activated stone carbon (Table 1). We assume that the resulting carbon was non-condensed butanol in the gas-phase. Also, activated stone carbon has been considered as an adsorbent for butanol in downstream processing (Qureshi et al. ; [Groot and Luyben 1986]). When fermentations were carried out without activated stone carbon (on technical grade glycerol) a higher carbon recovery was observed (Table 1).
After adjusting the mass balance, the yield of butanol from stored crude glycerol by MNO6 was 0.296 mol/mol compared to 0.264 mol/mol by the wild type utilizing technical grade glycerol. Venkataramanan et al. () reported a butanol yield of 0.347 mol/mol by C. pasteurianum utilizing purified crude glycerol and 0.322 mol/mol when technical grade glycerol was used. Both yields are higher than those achieved by MNO6 and the wild type. Low initial glycerol concentration, low utilization of glycerol as well as other dissimilarities in growth conditions in the two studies, could cause this difference.
Strain MNO6 had a 1,3-PDO yield of 0.249 mol/mol, which is 47% higher than the yield by the wild type (0.169 mol/mol) grown on stored crude glycerol. Even though the 1,3-PDO yield was lower than the theoretical maximum of 0.720 mol/mol ([Zeng 1996]) MNO6 has an interesting potential, as 1,3-PDO is produced simultaneously with butanol. Productivity of 1,3-PDO in fed batch has been reported to 0.9-3.0 g/l/h by the dedicated 1,3-PDO producer C. butyricum ([Willke and Vorlop 2008]; Chatzifragkou et al. ; Wilkens et al. ; [Reimann and Biebl 1996]). The achieved maximum 1,3-PDO production rate of 1.21 g/l/h by MNO6 is comparable to C. butyricum.
MNO6 produced ethanol in very small amounts (0.018 mol/mol). Compared to the fermentation by the wild type, the ethanol yield was reduced by 68% and 89% from stored crude glycerol and technical grade glycerol, respectively. The biomass yield of MNO6 was also significantly reduced, constituting only 90% of the wild type. The reduced amount of biomass diminishes the need for ATP. By extrapolating the ATP yield from the mass balances (both in this study and from Jensen et al.  (data not shown)) it is clear that also the ATP yield is reduced by 10% in MNO6. The reduced ATP requirement/production may be causing the shift in product pattern, as observed in this study.
In a fed-batch fermentation with MNO6 and in situ removal of butanol, 1,3-PDO would accumulate in the reactor, reaching high titers critical for downstream processing. Recently, the discovery of a bacterial strain producing both 1,3-PDO and ethanol from crude glycerol has been published (Rossi et al. ). In order to reach high 1,3-PDO titers, the concentration of the second product, ethanol will also increase and possibly inhibit the organism. The fermentation by MNO6 only leads to small amounts of ethanol but high concentrations of butanol, which is even more toxic than ethanol. However, as butanol can be removed simultaneously by gas-stripping, it is possible to achieve high 1,3-PDO titers.
In this study, we have demonstrated that our mutant strain of C. pasteurianum can tolerate high concentrations of crude glycerol, has a high glycerol utilization rate, and high productivity of butanol and 1,3-PDO. Based on these results and on the results on non-treated crude glycerol, we consider MNO6 a more robust and more efficient strain than the wild type and, therefore, also better suited for industrial applications.
We acknowledge the financial support of the European Union Seventh Framework Program FP7 (Glyfinery).
- Annous BA, Blaschek HP: Isolation and characterization of Clostridium acetobutylicum with enhanced amylolytic activity. Appl Environ Microbiol 1991,57(9):2544–2548.PubMed CentralPubMedGoogle Scholar
- Biebl H: Glycerol fermentation of 1,3-propanediol by Clostridium butyricum -Measurement of product inhibition by use of a pH-autostat. Appl Microbiol Biotechnol 1991,35(6):701–705.Google Scholar
- Biebl H: Fermentation of glycerol by Clostridium pasteurianum - batch and continuous culture studies. J Ind Microbiol & Biotechnol 2001,27(1):18–26. 10.1038/sj.jim.7000155Google Scholar
- Biebl H, Menzel K, Zeng AP, Deckwer WD: Microbial production of 1,3-propanediol. Appl Microbiol Biotechnol 1999,52(3):289–297. 10.1007/s002530051523PubMedGoogle Scholar
- Borden JR, Papoutsakis ET: Dynamics of genomic-library enrichment and identification of solvent tolerance genes for Clostridium acetobutylicum. Appl Environ Microbiol 2007,73(9):3061–3068. 10.1128/AEM.02296-06PubMed CentralPubMedGoogle Scholar
- Chatzifragkou A, Papanikolaou S, Dietz D, Doulgeraki AI, Nychas GJE, Zeng AP: Production of 1,3-propanediol by Clostridium butyricum growing on biodiesel-derived crude glycerol through a non-sterilized fermentation process. Appl Microbiol Biotechnol 2011,91(1):101–112. 10.1007/s00253-011-3247-xPubMedGoogle Scholar
- Dabrock B, Bahl H, Gottschalk G: Parameters affecting solvent production by Clostridium pasteurianum. Appl Environ Microbiol 1992,58(4):1233–1239.PubMed CentralPubMedGoogle Scholar
- Demirbas A: Importance of biodiesel as transportation fuel. Energy Policy 2007,35(9):4661–4670. 10.1016/j.enpol.2007.04.003Google Scholar
- Dobson R, Gray V, Rumbold K: Microbial utilization of crude glycerol for the production of value-added products. J Ind Microbiol Biotechnol 2012,39(2):217–226. 10.1007/s10295-011-1038-0PubMedGoogle Scholar
- DuPont™ Sorona® (2011)[http://www2.dupont.com/Sorona_Consumer/en_US/index.html] Accessed 12-Dec-2011
- Dürre P: Fermentative butanol production - Bulk chemical and biofuel. Ann N Y Acad Sci 2008, 1125: 353–362. 10.1196/annals.1419.009PubMedGoogle Scholar
- Elkanouni A, Junelles AM, Janatiidrissi R, Petitdemange H, Gay R: Clostridium acetobutylicum mutants isolated for resistance to the pyruvate halogen analogs. Curr Microbiol 1989,18(3):139–144. 10.1007/BF01569561Google Scholar
- Ezeji TC, Qureshi N, Blaschek HP: Acetone butanol ethanol (ABE) production from concentrated substrate: reduction in substrate inhibition by fed-batch technique and product inhibition by gas stripping. Appl Microbiol Biotechnol 2004,63(6):653–658. 10.1007/s00253-003-1400-xPubMedGoogle Scholar
- Girbal L, Croux C, Vasconcelos I, Soucaille P: Regulation of metabolic shifts in Clostridium acetobutylicum ATCC-824. FEMS Microbiol Rev 1995,17(3):287–297. 10.1111/j.1574-6976.1995.tb00212.xGoogle Scholar
- Gonzalez-Pajuelo M, Meynial-Salles I, Mendes F, Andrade JC, Vasconcelos I, Soucaille P: Metabolic engineering of Clostridium acetobutylicum for the industrial production of 1,3-propanediol from glycerol. Metab Eng 2005,7(5–6):329–336.PubMedGoogle Scholar
- Groot WJ, Luyben K: In situ product recovery by adsorption in the butanol/isopropanol batch fermentation. Appl Microbiol Biotechnol 1986,25(1):29–31.Google Scholar
- Hild HM, Stuckey DC, Leak DJ: Effect of nutrient limitation on product formation during continuous fermentation of xylose with Thermoanaerobacter ethanolicus JW200 Fe(7). Appl Microbiol Biotechnol 2003,60(6):679–686.PubMedGoogle Scholar
- Jensen TØ, Kvist T, Mikkelsen M, Christensen P, Westermann P: Fermentation of crude glycerol from biodiesel production by Clostridium pasteurianum. J Ind Microbiol Biotechnol 2012,39(5):709–717. 10.1007/s10295-011-1077-6PubMedGoogle Scholar
- Johnson DT, Taconi KA: The glycerin glut: Options for the value-added conversion of crude glycerol resulting from biodiesel production. Environ Prog 2007,26(4):338–348. 10.1002/ep.10225Google Scholar
- Lee SY, Lee DY, Kim TY: Systems biotechnology for strain improvement. Trends Biotechnol 2005,23(7):349–358. 10.1016/j.tibtech.2005.05.003PubMedGoogle Scholar
- Lee SY, Park JH, Jang SH, Nielsen LK, Kim J, Jung KS: Fermentative butanol production by clostridia. Biotechnol Bioeng 2008,101(2):209–228. 10.1002/bit.22003PubMedGoogle Scholar
- Lemmel SA: Mutagenesis in Clostridium acetobutylicum. Biotechnol Lett 1985,7(10):711–716. 10.1007/BF01032281Google Scholar
- Malaviya A, Jang Y-S, Lee S: Continuous butanol production with reduced byproducts formation from glycerol by a hyper producing mutant of Clostridium pasteurianum. Appl Microbiol Biotechnol 2012,93(4):1485–1494. 10.1007/s00253-011-3629-0PubMedGoogle Scholar
- Nakas JP, Schaedle M, Parkinson CM, Coonley CE, Tanenbaum SW: System-development for linked-fermentation production of solvents from algal biomass. Appl Environ Microbiol 1983,46(5):1017–1023.PubMed CentralPubMedGoogle Scholar
- Pagliaro M, Ciriminna R, Kimura H, Rossi M, Della Pina C: From glycerol to value-added products. Angew Chem Int Ed 2007,46(24):4434–4440. 10.1002/anie.200604694Google Scholar
- Pyle DJ, Garcia RA, Wen ZY: Producing docosahexaenoic acid (DHA)-rich algae from biodiesel-derived crude glycerol: Effects of impurities on DHA production and algal biomass composition. J Agric Food Chem 2008,56(11):3933–3939. 10.1021/jf800602sPubMedGoogle Scholar
- Qureshi N, Hughes S, Maddox IS, Cotta MA: Energy-efficient recovery of butanol from model solutions and fermentation broth by adsorption. Bioprocess Biosystems Eng 2005,27(4):215–222. 10.1007/s00449-005-0402-8Google Scholar
- Reimann A, Biebl H: Production of 1,3-propanediol by Clostridium butyricum DSM 5431 and product tolerant mutants in fedbatch culture: Feeding strategy for glycerol and ammonium. Biotechnol Lett 1996,18(7):827–832. 10.1007/BF00127897Google Scholar
- Rossi DM, da Costa JB, de Souza EA, Peralba MDR, Ayub MAZ: Bioconversion of residual glycerol from biodiesel synthesis into 1,3-propanediol and ethanol by isolated bacteria from environmental consortia. Renew Energ 2012,39(1):223–227. 10.1016/j.renene.2011.08.005Google Scholar
- Syed QUA, Nadeem M, Nelofer R: Enhanced butanol production by mutant strains of Clostridium acetobutylicum in molasses medium. Turk J Biochem 2008,33(1):25–30.Google Scholar
- Taconi KA, Venkataramanan KP, Johnson DT: Growth and solvent production by Clostridium pasteurianum ATCC (R) 6013 (TM) utilizing biodiesel-derived crude glycerol as the sole carbon source. Environ Prog Sustainable Energy 2009,28(1):100–110. 10.1002/ep.10350Google Scholar
- Venkataramanan KP, Boatman JJ, Kurniawan Y, Taconi KA, Bothun GD, Scholz C: Impact of impurities in biodiesel-derived crude glycerol on the fermentation by Clostridium pasteurianum ATCC 6013. Appl Microbiol Biotechnol 2012,93(3):1325–1335. 10.1007/s00253-011-3766-5PubMedGoogle Scholar
- Wilkens E, Ringel AK, Hortig D, Willke T, Vorlop KD: High-level production of 1,3-propanediol from crude glycerol by Clostridium butyricum AKR102a. Appl Microbiol Biotechnol 2012,93(3):1057–1063. 10.1007/s00253-011-3595-6PubMedGoogle Scholar
- Willke T, Vorlop K: Biotransformation of glycerol into 1,3-propanediol. Eur J Lipid Sci Technol 2008,110(9):831–840. 10.1002/ejlt.200800057Google Scholar
- Yazdani SS, Gonzalez R: Anaerobic fermentation of glycerol: a path to economic viability for the biofuels industry. Curr Opin Biotechnol 2007,18(3):213–219. 10.1016/j.copbio.2007.05.002PubMedGoogle Scholar
- Zeng A-P: Pathway and kinetic analysis of 1,3-propanediol production from glycerol fermentation by Clostridium butyricum. Bioprocess Eng 1996,14(4):169–175. 10.1007/BF01464731Google Scholar
- Zeng A-P, Biebl H: Bulk Chemicals from Biotechnology: The Case of 1,3-Propanediol Production and the New Trends. Adv Biochem Eng Biotechnol 2002, 74: 239–259.PubMedGoogle Scholar
- Zeng A-P, Ross A, Biebl H, Tag C, Gunzel B, Deckwer WD: Multiple product inhibition and growth modeling of Clostridium butyricum and Klebsiella pneumoniae in glycerol fermentation. Biotechnol Bioeng 1994,44(8):902–911. 10.1002/bit.260440806PubMedGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.