- Original article
- Open Access
Direct production of organic acids from starch by cell surface-engineered Corynebacterium glutamicum in anaerobic conditions
© Tsuge et al.; licensee Springer. 2013
Received: 7 October 2013
Accepted: 12 December 2013
Published: 17 December 2013
We produced organic acids, including lactate and succinate, directly from soluble starch under anaerobic conditions using high cell-density cultures of Corynebacterium glutamicum displaying α-amylase (AmyA) from Streptococcus bovis 148 on the cell surface. Notably, reactions performed under anaerobic conditions at 35 and 40°C, which are higher than the optimal growth temperature of 30°C, showed 32% and 19%, respectively, higher productivity of the organic acids lactate, succinate, and acetate compared to that at 30°C. However, α-amylase was not stably anchored and released into the medium from the cell surface during reactions at these higher temperatures, as demonstrated by the 61% and 85% decreases in activity, respectively, from baseline, compared to the only 8% decrease at 30°C. The AmyA-displaying C. glutamicum cells retained their starch-degrading capacity during five 10 h reaction cycles at 30°C, producing 107.8 g/l of total organic acids, including 88.9 g/l lactate and 14.0 g/l succinate. The applicability of cell surface-engineering technology for the production of organic acids from biomass by high cell-density cultures of C. glutamicum under anaerobic conditions was demonstrated.
In the new era of green chemistry, the utilization of renewable resources, such as plant biomass, and environmentally friendly chemical products are eagerly desired. In particular, the organic acids lactic and succinic acids are recognized as ideal candidates for the building blocks of next-generation plastics. Both compounds were selected within the top 30 value-added chemicals manufactured from biomass by the U.S. Department of Energy (Werpy et al. 2004).
Corynebacterium glutamicum is a Gram-positive bacterium with a high G + C content and has been widely used in industry for the large-scale production of amino acids, including glutamate and lysine (Nakayama et al. 1961; Kinoshita 1985). Recently, C. glutamicum was shown to have increased metabolic flow through the glycolytic pathway and reductive branch of the tricarboxylic acid (TCA) cycle under oxygen-deprived conditions, resulting in the exclusive production of the organic acids lactate, succinate, and acetate (Inui et al. 2004). High cell-density cultivation promoted the consumption of oxygen and obviated the need for the removal of oxygen. Using this system, in which resting cell of C. glutamicum were cultured at high cell-density under anaerobic conditions, several organic acids, including lactate and succinate, were produced at high levels (Okino et al. 2005 Okino et al. 2008a Okino et al. 2008b Litsanov et al. 2012 Tsuge et al. 2013).
The cell-surface display of heterologous proteins has several potential biotechnological applications, including adsorption or degradation of environmental pollutants, recovery of rare metal ions, biosensors, and recombinant protein production (Kuroda and Ueda 2010, 2011). In addition to these applications, yeast and bacteria displaying fusion proteins composed of a cell surface-anchoring protein and a biomass-degrading protein have been effectively used for the fermentation of biomass resources, such as starch, xylan, cellobiose, and cellulose (Katahira et al. 2004; Matano et al. 2012; Murai et al. 1997; Narita et al. 2006; Yamada et al. 2013). The main advantage of cell surface-engineering technology is that cells displaying useful proteins can be reutilized multiple times, similar to biocatalysts (Matano et al. 2013; Yamakawa et al. 2012). We previously developed a cell surface-display system in C. glutamicum using the Bacillus subtilis PgsA and C. glutamicum PorC proteins as anchors for displaying α-amylase from Streptococcus bovis 148 and β–glucosidase from Saccharophagus degradans 2–40 for lysine production from starch and cellobiose, respectively, under growing conditions (Tateno et al. 2007a; Adachi et al. 2013). Although it was anticipated that the co-utilization of cell surface-engineering technology zand high cell-density systems would improve organic acid production from biomass, the efficacy of this combined approach in C. glutamicum has not been demonstrated.
In the present study, we demonstrated the application of cell surface-engineering technology for the production of organic acids by C. glutamicum cells cultured at high-density under anaerobic conditions in a minimal salts medium. Using this approach, organic acids, including lactate and succinate, were produced directly from starch by C. glutamicum displaying α-amylase on the cell surface. Moreover, we reutilized the C. glutamicum cells for the repeated production of organic acids to evaluate the stability and durability of the cell-surface displayed protein in C. glutamicum.
Materials and methods
Bacterial strains, media, growth conditions, and plasmids
Strains, plasmids, and primers used in this study
Strain, plasmid or primer
Relevant genotype or description
Reference or source
endA1 hsdR17 (rK12 – mK12 +) supE44 thi-1 recA1 gyrA96 relA1 lac F′[proA + B + lacI q Z ΔM15::Tn10] (Tetr)
dam, dcm supE44, hsdR17, thi leu, rpsL1, lacY, galK, galT, ara, tonA, thr, tsx, Δ(lac-proAB)/F’ [traD36, proAB + , lacIq , lacZ Δ M15]
Kmr; E. coli-C. glutamicum shuttle vector containing cspB promoter
Tateno et al. 2007b
Kmr; pCC carrying pgsA
Kmr; pCC-pgsA carrying amyA gene fused with 3’ terminal of pgsA
ACGCGTCGACTTACTTGTCATCGTCATCCTTGTAGTCGAGCTC TTTAGATTTTAGTT TGTCACTATGATC
All polymerase chain reactions (PCR) were performed using KOD-Plus-DNA polymerase (Toyobo Co., Ltd., Osaka, Japan). The pgsA gene was amplified by PCR using the plasmid pHLA (Narita et al. 2006) as template with primers BamHI-pgsA_F and pgsA-SacI_R (Table 1). The amplified fragment was digested with Bam HI and Sac I, and then ligated into Bam HI/Sac I digested plasmid pCC (Tateno et al. 2007b). The resulting plasmid was designated pCC-pgsA. The α-amylase gene (amyA) from Streptococcus bovis 148 was amplified by PCR using pCCS-amyA (Tateno et al. 2007b) as template and primers SacI-amyA_F and amyA-XhoI_R (Table 1). The amplified fragment was digested with Sac I and Xho I, and was then ligated into Sac I/Xho I digested pCC-pgsA. The resulting plasmid was designated pCC-pgsA-amyA.
Transformation of E. coli was performed by the CaCl2 method. Transformation of C. glutamicum was carried out as described previously (Tateno et al. 2007a).
Bioprocess conditions for organic acid production
For organic acid production, cells were harvested from 500 ml cultures in A+ medium by centrifugation (5000 × g, 4°C for 10 min) after 16 h cultivation at 30°C with starting at an optical density at 600 nm (OD600) of 0.05. The cell pellet was resuspended in 40 ml K10 medium, which consisted of 168 mg NH4H2PO4, 151 mg (NH4)2HPO4, 250 mg KCl, 400 mg MgSO4 · 7H2O, 16 mg FeSO4 · 7H2O, 16 mg MnSO4 · 7H2O, 0.16 mg biotin, and 0.16 mg thiamine (per liter), to give a final OD600 of 60. 40 ml of 24 g/l soluble starch, which was sterilized by autoclaving, was added to Bio Jr.8 fermentor (ABLE Biott, Japan), and the temperature was set at 30, 35, or 40°C. After the temperature had stabilized, 40 ml of the cell suspension was added to the bioreactor, resulting in a 80 ml culture with an OD600 of 30 and a final concentration of 12 g/l soluble starch. For repeated reactions, a 45 g/l soluble starch solution was used. After each reaction, cells were harvested by centrifugation (5000 × g, 4°C for 10 min) and then washed once with K10 medium. The cell pellet was resuspended in 40 ml of K10 medium, to which 40 ml of 90 g/l soluble starch solution was added, giving 80 ml cell suspensions containing 45 g/l soluble starch. Cell suspensions were agitated at 120 rpm without aeration. Anaerobic conditions were achieved by the rapid consumption of residual oxygen in the medium by cells. The pH of the medium was maintained at 7.0 throughout the reaction using a 5.0 N ammonia solution.
OD600 was measured using a spectrophotometer (UVmini-1240; Shimadzu, Japan). Organic acids (lactate, succinate, and acetate) were quantified in centrifuged samples (15,000 rpm, 4°C, 10 min) using a high-performance liquid chromatograph (Shimadzu) equipped with a UV/VIS detector (SPD-20A) and a BioRad Aminex 87H column (Bio-Rad Laboratories, USA) operating at 50°C with a 5 mM H2SO4 mobile phase at a flow rate of 0.6 ml/min. The starch concentration was measured using an EnzyChrom Starch Assay Kit (Funakoshi) according to the manufacturer’s instructions.
α-Amylase activity measurement
α-Amylase activity was measured with an α-amylase measurement kit (Kikkoman, Tokyo, Japan) as described previously (Tateno et al. 2007a). An OD600 of 1.0 corresponded to 0.39 mg dry weight cells ml-1.
PgsA-anchored α-amylase sufficiently fermented soluble starch in high-cell density cultures of C. glutamicum cells
A C. glutamicum strain displaying PgsA-anchored α-amylase was previously shown to efficiently produce lysine from starch by direct simultaneous saccharification and fermentation under aerobic growing conditions (Tateno et al. 2007a). Here, we investigated if this system could be applied for the production of organic acids using high cell-density cultures of C. glutamicum cells under anaerobic conditions. We considered that the cell-surface display of α-amylase, as opposed to secretion, was ideal for the system because cells could be reutilized multiple times with high-cell density conditions. We fused the α-amylase-encoding amyA gene from Streptococcus bovis 148 with the C-terminal region of pgsA from Bacillus subtilis in pCC vector downstream of the cspB promoter, and then introduced the constructed vector (pCC-pgsA-amyA) into a wild-type C. glutamicum ATCC13032 strain. Wild-type C. glutamicum produces mainly lactate (1.79 mol/mol glucose) and smaller amounts of succinate (0.09 mol/mol glucose) and acetate (0.01 mol/mol glucose) under anaerobic conditions (Okino et al. 2005).
Summary of organic acid production from starch at various temperatures
Starch consumed (g/l)
α-Amylase activity (U/g dry weight cell)
Total organic acids
0.2 ± 0.2
0.1 ± 0.06
0.3 ± 0.05
0.2 ± 0.2
0.3 ± 0.1
0.5 ± 0.1
1.1 ± 0.1
9.2 ± 0.2
10.4 ± 1.0
9.5 ± 0.5
6.0 ± 0.3
1.5 ± 0.2
0.7 ± 0.1
8.2 ± 0.2
0.88 ± 0.01
12.8 ± 2.3
9.9 ± 1.0
3.9 ± 1.9
7.7 ± 0.5
2.1 ± 0.2
1.0 ± 0.1
10.8 ± 0.2
0.86 ± 0.13
12.0 ± 2.1
9.3 ± 0.1
1.4 ± 0.7
7.3 ± 1.2
1.5 ± 0.3
0.9 ± 0.1
9.7 ± 0.7
0.83 ± 0.13
The AmyA-displaying strain mainly produced lactate (73% of total organic acids after 10 h), whereas the control strain predominantly generated acetate (45% of total organic acids after 10 h). Glucose remained below the limit of detection (data not shown) in the culture medium of the AmyA-displaying strain after 2 h, indicating that the glucose formed from starch by α-amylase was immediately transported into cells through the phosphoenolpyruvate:carbohydrate phosphotransferase system (Tanaka et al. 2008). The α-amylase activity in the AmyA-displaying strain was 10.4 ± 1.0 (U/g dry weight cell) at 0 h, but was only 0.1 ± 0.06 (U/g dry weight cell) in the control strain (Table 2). After 10 h of culture, the AmyA-displaying strain had retained its α-amylase activity, which was 9.5 ± 0.5 (U/g dry weight cell).
Effects of temperature on starch degradation and organic acid production
The above-described bioprocess experiment was conducted at 30°C, which is the optimal growth temperature of C. glutamicum. However, because the optimum temperature for S. bovis AmyA activity is 50°C in vitro (Satoh et al. 1993), we also examined the effect of high temperature (35 and 40°C) on organic acid production by the recombinant strain from starch. In addition, performing fermentations at high temperature would reduce production costs associated with cooling the reactor, as the metabolic activities of microorganisms typically generate heat (Abdel-Banat et al. 2010).
High-density cell suspensions, which had an OD600 of 30 and a total volume of 80 ml, were cultured at 35 and 40°C under anaerobic conditions. Under these conditions, starch was efficiently degraded by the AmyA-displaying strain and organic acids were produced. Starch consumption was highest (12.8 g/l) at 35°C after 10 h, representing a 39% increase compared to that consumed at 30°C (Table 2). Interestingly, a similar amount of starch was degraded at 40°C, even though the cells barely grew at this temperature. Glucose was not detected at either of the higher temperatures, indicating it was immediately imported into cells and consumed after the degradation of starch. The total amount of produced organic acids was 32% and 18% higher at 35 and 40°C, respectively, than at 30°C (Table 2). In addition, the ratio of produced organic acids did not markedly change depending on the reaction temperature. Notably, however, α-amylase activity on the cell surface decreased by 61% after 10 h at 35°C, whereas a decrease of only 8% was observed at 30°C (Table 2). AmyA-displaying cells cultured at 40°C exhibited an even higher decrease (85%) of α-amylase activity. These results indicated that AmyA proteins were stably anchored on the cell surface at 30°C, whereas most anchored AmyA proteins were released into the medium at 35 and 40°C. The results also showed that although AmyA proteins were not anchored on the cell surface at 35 or 40°C, the metabolic activity of C. glutamicum cells was maintained.
Repeated production of organic acids from starch
Cell-surface engineering is an ideal technology for the repeated use of cells, but only if the displayed protein maintained activity after several culture cycles and associated centrifugation steps, and medium exchange. Here, the ability of cell-surface engineered C. glutamicum cells to produce organic acids in repeated culture cycles was therefore investigated.
Production of organic acids from starch by C. glutamicum cells in repeated cycles
Start OD 600
Consumed starch (g/l)
α-Amylase activity (U/g dry weight cell)
Total organic acids
In the present study, we demonstrated the applicability of cell surface-engineering technology in C. glutamicum for production of the organic acids lactate and succinate from high-density cultures under anaerobic conditions. Although the Streptococcus bovis 148 AmyA protein used in this study has optimal in-vitro activity at 50°C, we demonstrated that the in vivo production of organic acids from starch could be achieved at 35 or 40°C, which is markedly higher than the optimal growth temperature of 30°C for C. glutamicum. At these temperatures, the consumption of starch and production of organic acids were accelerated with increasing reaction temperature. In a previous study examining lysine production from starch by C. glutamicum cells expressing PgsA-anchored α-amylase, cultivation at 40°C resulted in severe growth defects that were accompanied by dramatic decreases in lysine production and starch consumption compared to those observed at 30, 34, and 37°C (Tateno et al. 2007a). This difference was apparently because lysine production was performed under growing conditions. In contrast, the resting-cell reactions performed here with high cell-density cultures of C. glutamicum maintained their metabolic capacity despite the apparent lack of cell growth at higher temperature, and showed even higher starch consumption and organic acid production. Therefore, the optimal temperatures for cell growth and the central metabolic pathway that converts glucose to lactic acid, succinic acid, and acetic acid are likely different. However, because the ratio of produced organic acids did not alter dramatically at different temperatures, the optimal temperature for enzymes in the central metabolic pathway of C. glutamicum is likely to be similar. In resting cells of Brevundimonas diminuta, the production of ascorbic acid-2-phosphate was enhanced at 40°C compared to the optimal growth temperature of 30°C (Shin et al. 2007). Production of chemicals and fuels at higher temperature is advantageous for industrial processes, because it would reduce the risk of contamination during fermentation. More directly, higher temperatures would reduce the costs for cooling of the reactor. For example, a 5°C increase in the fermentation temperature would reportedly save approximately $30,000 per year for a 30 m3 scale plant (Abdel-Banat et al. 2010).
The AmyA protein fused with the C-terminus of cell-surface protein PgsA remained stable anchoring to the cell surface after several reaction cycles at 30°C, which included cell washing and medium exchange events. This finding contrasted that of the previous study on lysine production showing that cell-surface expressed AmyA protein was released into the medium at 30°C. There are two possible explanations for this difference. First, a certain proportion of fusion α-amylase proteins may be firmly anchored on the cell surface, whereas other fusion proteins failed to anchor after being transported to the cell surface. The second possibility is that stable anchoring of the fusion protein might be dependent on cell division, as we observed that organic acid production occurred in the absence of apparent cell growth. In either case, screen of more stable anchoring proteins would be desired to display α-amylase, or other protein of interest, on the cell surface of C. glutamicum cells. More stable anchoring proteins could lead to enhance process productivity at high temperature with repeated use of C. glutamicum cells, that could not be obtained by protein secretion.
In conclusion, we demonstrated that cell surface-engineering technology was advantageous for the repeated use of C. glutamicum cells cultured at high cell-density for organic acid production. This approach could be applied for displaying other biomass-degrading enzymes for the direct production of organic acids from renewable biomass.
This work was supported in part by the commission for Development of Artificial Gene Synthesis Technology for Creating Innovative Biomaterial from the Ministry of Economy, Trade and Industry (METI), Japan. This work was also supported in part by Special Coordination Funds for promoting Science and Technology, Creation of Innovation Centers for Advanced Interdisciplinary Research Areas (Innovative Bioproduction, Kobe), Japan.
- Abdel-Banat BM, Hoshida H, Ano A, Nonklang S, Akada R: High-temperature fermentation: how can processes for ethanol production at high temperatures become superior to the traditional process using mesophilic yeast? Appl Microbiol Biotechnol 2010, 85: 861–867. 10.1007/s00253-009-2248-5PubMedView ArticleGoogle Scholar
- Adachi N, Takahashi C, Ono-Murota N, Yamaguchi R, Tanaka T, Kondo A: Direct L-lysine production from cellobiose by Corynebacterium glutamicum displaying beta-glucosidase on its cell surface. Appl Microbiol Biotechnol 2013, 97: 7165–7172. 10.1007/s00253-013-5009-4PubMedView ArticleGoogle Scholar
- Inui M, Murakami S, Okino S, Kawaguchi H, Vertès AA, Yukawa H: Metabolic analysis of Corynebacterium glutamicum during lactate and succinate productions under oxygen deprivation conditions. J Mol Microbiol Biotechnol 2004, 7: 182–196. 10.1159/000079827PubMedView ArticleGoogle Scholar
- Katahira S, Fujita Y, Mizuike A, Fukuda H, Kondo A: Construction of a xylan-fermenting yeast strain through codisplay of xylanolytic enzymes on the surface of xylose-utilizing Saccharomyces cerevisiae cells. Appl Environ Microbiol 2004, 70: 5407–5414. 10.1128/AEM.70.9.5407-5414.2004PubMed CentralPubMedView ArticleGoogle Scholar
- Kinoshita S: Glutamic acid bacteria. In Biology of industrial microorganisms. Edited by: Demain AL, Solomon NA. London: Cummings; 1985:115–146.Google Scholar
- Kuroda K, Ueda M: Engineering of microorganisms towards recovery of rare metal ions. Appl Microbiol Biotechnol 2010, 87: 53–60. 10.1007/s00253-010-2581-8PubMedView ArticleGoogle Scholar
- Kuroda K, Ueda M: Cell surface engineering of yeast for applications in white biotechnology. Biotechnol Lett 2011, 33: 1–9. 10.1007/s10529-010-0403-9PubMedView ArticleGoogle Scholar
- Litsanov B, Brocker M, Bott M: Toward homosuccinate fermentation: metabolic engineering of Corynebacterium glutamicum for anaerobic production of succinate from glucose and formate. Appl Environ Microbiol 2012, 78: 3325–3337. 10.1128/AEM.07790-11PubMed CentralPubMedView ArticleGoogle Scholar
- Matano Y, Hasunuma T, Kondo A: Display of cellulases on the cell surface of Saccharomyces cerevisiae for high yield ethanol production from high-solid lignocellulosic biomass. Bioresource Technol 2012, 108: 128–133.View ArticleGoogle Scholar
- Matano Y, Hasunuma T, Kondo A: Cell recycle batch fermentation of high-solid lignocellulose using a recombinant cellulase-displaying yeast strain for high yield ethanol production in consolidated bioprocessing. Bioresource Technol 2013, 135: 403–409.View ArticleGoogle Scholar
- Murai T, Ueda M, Yamamura M, Atomi H, Shibasaki Y, Kamasawa N, Osumi M, Amachi T, Tanaka A: Construction of a starch-utilizing yeast by cell surface engineering. Appl Environ Microbiol 1997, 63: 1362–1366.PubMed CentralPubMedGoogle Scholar
- Nakayama K, Kitada S, Kinoshita S: Studies on lysine fermentation I. The control mechanism on lysine accumulation by homoserine and threonine. J Gen Appl Microbiol 1961, 7: 145–154. 10.2323/jgam.7.145View ArticleGoogle Scholar
- Narita J, Okano K, Kitao T, Ishida S, Sewaki T, Sung MH, Fukuda H, Kondo A: Display of alpha-amylase on the surface of Lactobacillus casei cells by use of the PgsA anchor protein, and production of lactic acid from starch. Appl Environ Microbiol 2006, 72: 269–275. 10.1128/AEM.72.1.269-275.2006PubMed CentralPubMedView ArticleGoogle Scholar
- Okino S, Inui M, Yukawa H: Production of organic acids by Corynebacterium glutamicum under oxygen deprivation. Appl Microbiol Biotechnol 2005, 68: 475–480. 10.1007/s00253-005-1900-yPubMedView ArticleGoogle Scholar
- Okino S, Noburyu R, Suda M, Jojima T, Inui M, Yukawa H: An efficient succinic acid production process in a metabolically engineered Corynebacterium glutamicum strain. Appl Microbiol Biotechnol 2008, 81: 459–464. 10.1007/s00253-008-1668-yPubMedView ArticleGoogle Scholar
- Okino S, Suda M, Fujikura K, Inui M, Yukawa H: Production of D-lactic acid by Corynebacterium glutamicum under oxygen deprivation. Appl Microbiol Biotechnol 2008, 78: 449–454. 10.1007/s00253-007-1336-7PubMedView ArticleGoogle Scholar
- Sambrook J, Fritsh E, Maniatis T: Molecular cloning: a laboratory manual. Cold Spring Harbor, NY: Cold Spring Harbor Laboratory Press; 1989.Google Scholar
- Satoh E, Niimura Y, Uchimura T, Kozaki M, Komagata K: Molecular cloning and expression of two α-amylase genes from Streptococcus bovis 148 in Escherichia coli . Appl Environ Microbiol 1993, 63: 4941–4944.Google Scholar
- Shin WJ, Kim BY, Bang WG: Optimization of ascorbic acid-2-phosphate production from ascorbic acid using resting cell of Brevundimonas diminuta . J Microbiol Biotechnol 2007, 17: 769–773.PubMedGoogle Scholar
- Tanaka Y, Okai N, Teramoto H, Inui M, Yukawa H: Regulation of the expression of phosphoenolpyruvate: carbohydrate phosphotransferase system (PTS) genes in Corynebacterium glutamicum R. Microbiology 2008, 154: 264–274. 10.1099/mic.0.2007/008862-0PubMedView ArticleGoogle Scholar
- Tateno T, Fukuda H, Kondo K: Production of L-Lysine from starch by Corynebacterium glutamicum displaying α-amylase on its cell surface. Appl Microbiol Biotechnol 2007, 74: 1213–1220. 10.1007/s00253-006-0766-yPubMedView ArticleGoogle Scholar
- Tateno T, Fukuda H, Kondo A: Direct production of L-lysine from raw corn starch by Corynebacterium glutamicum secreting Streptococcus bovis alpha-amylase using cspB promoter and signal sequence. Appl Microbiol Biotechnol 2007, 77: 533–541. 10.1007/s00253-007-1191-6PubMedView ArticleGoogle Scholar
- Tsuge Y, Yamamoto S, Kato N, Inui M, Yukawa H: Reactions upstream of glycerate-1,3-bisphosphate drive Corynebacterium glutamicum D-lactate productivity under oxygen deprivation. Appl Microbiol Biotechnol 2013, 97: 6693–6703. 10.1007/s00253-013-4986-7PubMedView ArticleGoogle Scholar
- Werpy T, Petersen G: Top value added chemicals from biomass. Volume 1: results of screening for potential candidates from sugars and synthesis gas. Oak Ridge, TN, USA: U.S. Department of Energy; 2004:76.Google Scholar
- Yamada R, Nakatani Y, Ogino C, Kondo A: Efficient direct ethanol production from cellulose by cellulase- and cellodextrin transporter-co-expressing Saccharomyces cerevisiae . AMB Express 2013,24(3):34.View ArticleGoogle Scholar
- Yamakawa S, Yamada R, Tanaka T, Ogino C, Kondo A: Repeated fermentation from raw starch using Saccharomyces cerevisiae displaying both glucoamylase and α-amylase. Enzyme Microb Technol 2012, 50: 343–347. 10.1016/j.enzmictec.2012.03.005PubMedView ArticleGoogle 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.