Isolation and characterisation of lactic acid bacterium for effective fermentation of cellobiose into optically pure homo l-(+)-lactic acid

Efficient conversion of hemicellulose sugars from spent sulfite liquor into optically pure L-lactic acid by Enterococcus mundtii

Spent sulfite liquor (SSL), a waste stream from wood pulp production, has great potential as carbon source for future industrial fermentations. In the present study, SSL was separated into a hemicellulose derived sugar syrup (HDSS) and a lignosulfonic fraction by simulated moving bed chromatography. The recovery of SSL sugars in the HDSS was 89% and the fermentation inhibitors furfural, 5-hydroxymethylfurfural and acetic acid were removed by 98.7%, 60.5% and 75.5%, respectively. The obtained sugars have been converted to L-lactic acid, a building block for bioplastics, by fermentation with the lactic acid bacterium Enterococcus mundtii DSM4838. Batch fermentations on HDSS produced up to 56.3 g/L L-lactic acid. Simultaneous conversion of pentose and hexose sugars during fed-batch fermentation of wildtype E. mundtii led to 87.9 g/L optically pure (>99%) L-lactic acid, with maximum productivities of 3.25 g/L.h and yields approaching 1.00 g/g during feeding phase from HDSS as carbon source.

Isolation of cellulase-producing Microbulbifer sp. from marine teleost blackfish (Girella melanichthys) intestine and the enzyme characterization

Most animals cannot digest cellulose but have symbiotic microbes that degrade the matrix polysaccharides of plant matter. Herbivorous and omnivorous marine fish are similarly expected to rely on symbiotic microbes, but reports to date on cellulase-producing bacteria in fish intestines are limited. Here, we report the isolation of new cellulase-producing bacteria from the marine omnivorous teleost, blackfish (Girella melanichthys), and the characterization of cellulase activity. Three strains of cellulase-producing bacteria sp. were isolated from the hindgut of wild G. melanichthys. The strains of cellulase-producing bacteria grew in medium with artificial seawater but not in NaCl alone. Growth was optimum at 20-35°C, but there was no growth at 40°C, suggesting adaptation in a marine environment at a low temperature. Isolates were identified to Microbulbifer sp., among which GL-2 strain produced a high enzyme activity. The GL-2 strain was further used for enzyme characterization with carboxymethyl cellulose (CMC) as the substrate. Maximum activity of the cellulase was observed at 60°C, and activity was more than 30% at 20°C, while commercial cellulase Enthiron showed an optimum activity at 50°C and 17% activity at 20°C. Hydrolytic products by GL-2 cellulase were cellobiose but not glucose, suggesting a deficiency of β-glucosidase activity. Active gel electrophoresis containing CMC showed five bands, suggesting several cellulolytic enzymes. The GL-2 strain and its enzyme are potential probiotics for aquaculture fish and the industrial production of cellobiose.

Transcriptome profile of carbon catabolite repression in an efficient l-(+)-lactic acid-producing bacterium Enterococcus mundtii QU25 grown in media with combinations of cellobiose, xylose, and glucose

Enterococcus mundtii QU25, a non-dairy lactic acid bacterium of the phylum Firmicutes, is capable of simultaneously fermenting cellobiose and xylose, and is described as a promising strain for the industrial production of optically pure l-lactic acid (≥ 99.9%) via homo-fermentation of lignocellulosic hydrolysates. Generally, Firmicutes bacteria show preferential consumption of sugar (usually glucose), termed carbon catabolite repression (CCR), while hampering the catabolism of other sugars. In our previous study, QU25 exhibited apparent CCR in a glucose-xylose mixture phenotypically, and transcriptional repression of the xylose operon encoding initial xylose metabolism genes, likely occurred in a CcpA-dependent manner. QU25 did not exhibit CCR phenotypically in a cellobiose-xylose mixture. The aim of the current study is to elucidate the transcriptional change associated with the simultaneous utilization of cellobiose and xylose. To this end, we performed RNA-seq analysis in the exponential growth phase of E. mundtii QU25 cells grown in glucose, cellobiose, and/or xylose as either sole or co-carbon sources. Our transcriptomic data showed that the xylose operon was weakly repressed in cells grown in a cellobiose-xylose mixture compared with that in cells grown in a glucose-xylose mixture. Furthermore, the gene expression of talC, the sole gene encoding transaldolase, is expected to be repressed by CcpA-mediated CCR. QU25 metabolized xylose without using transaldolase, which is necessary for homolactic fermentation from pentoses using the pentose-phosphate pathway. Hence, the metabolism of xylose in the presence of cellobiose by QU25 may have been due to 1) sufficient amounts of proteins encoded by the xylose operon genes for xylose metabolism despite of the slight repression of the operon, and 2) bypassing of the pentose-phosphate pathway without the TalC activity. Accordingly, we have determined the targets of genetic modification in QU25 to metabolize cellobiose, xylose and glucose simultaneously for application of the lactic fermentation from lignocellulosic hydrolysates.

Gentiobiose and cellobiose content in fresh and fermenting cucumbers and utilization of such disaccharides by lactic acid bacteria in fermented cucumber juice medium

The content of cellobiose and gentiobiose, cellulose-derived dissacharides, in fresh and fermented cucumber was evaluated along with the ability of Lactobacillus plantarum, Lactobacillus pentosus, Lactobacillus buchneri and Lactobacillus brevis to utilize them during and after fermentation. The disaccharide content in fresh and fermenting cucumbers was below the detection level (10 µM) using HPLC for analysis. Utilization of cellobiose and gentiobiose by lactic acid bacteria (LAB) was tested in fermented cucumber juice medium (FCJM), a model system for the bioconversion and postfermentation lacking glucose and fructose. Changes in the fermentation metabolites were followed using HPLC and pH measurements as a function of time. The disaccharides were utilized by L. plantarum, L. pentosus, and L. buchneri in FCJM at pH 4.7 ± 0.1, representative of the active fermentation period, and converted to lactic acid. The disaccharides were not utilized in FCJM at pH 3.7 ± 0.1, representative of the end of fermentation. While L. brevis was unable to utilize cellobiose efficiently in FCJM, they were able to remove gentiobiose at pH 4.7 ± 0.1. Some strain level differences in cellobiose utilization were observed. It is concluded that the disaccharides are absent in the fresh cucumber and the typical fermentation. The LAB prevalent in the bioconversion utilizes cellobiose and gentiobiose, if available, at pH 4.7 ± 0.1. The LAB would not remove the disaccharides, which could become available from cellulose degradation by the acid resistant indigenous microbiota, after the pH is reduced to 3.7 ± 0.1.

Dynamic simulation of continuous mixed sugar fermentation with increasing cell retention time for lactic acid production using Enterococcus mundtii QU 25

BACKGROUND

The simultaneous and effective conversion of both pentose and hexose in fermentation is a critical and challenging task toward the lignocellulosic economy. This study aims to investigate the feasibility of an innovative co-fermentation process featuring with a cell recycling unit (CF/CR) for mixed sugar utilization. A l-lactic acid-producing strain Enterococcus mundtii QU 25 was applied in the continuous fermentation process, and the mixed sugars were utilized at different productivities after the flowing conditions were changed. A mathematical model was constructed with the experiments to optimize the biological process and clarify the cell metabolism through kinetics analysis. The structured model, kinetic parameters, and achievement of the fermentation strategy shall provide new insights toward whole sugar fermentation via real-time monitoring for process control and optimization.

RESULTS

Significant carbon catabolite repression in co-fermentation using a glucose/xylose mixture was overcome by replacing glucose with cellobiose, and the ratio of consumed pentose to consumed hexose increased significantly from 0.096 to 0.461 by mass. An outstanding product concentration of 65.2 g L^-1 and productivity of 13.03 g L^-1 h^-1 were achieved with 50 g L^-1 cellobiose and 30 g L^-1 xylose at an optimized dilution rate of 0.2 h^-1, and the cell retention time gradually increased. Among the total lactic acid production, xylose contributed to more than 34% of the mixed sugars, which was close to the related contents in agricultural residuals. The model successfully simulated the transition of sugar consumption, cell growth, and lactic acid production among the batch, continuous process, and CF/CR systems.

CONCLUSION

Cell retention time played a critical role in balancing pentose and hexose consumption, cell decay, and lactic acid production in the CF/CR process. With increasing cell concentration, consumption of mixed sugars increased with the productivity of the final product; hence, the impact of substrate inhibition was reduced. With the validated parameters, the model showed the highest accuracy simulating the CF/CR process, and significantly longer cell retention times compared to hydraulic retention time were tested.

Non-carbon loss long-term continuous lactic acid production from mixed sugars using thermophilic Enterococcus faecium QU 50

In this study, a non-sterile (open) continuous fermentation (OCF) process with no-carbon loss was developed to improve lactic acid (LA) productivity and operational stability from the co-utilization of lignocellulose-derived sugars by thermophilic Enterococcus faecium QU 50. The effects of different sugar mixtures on LA production were firstly investigated in conventional OCF at 50°C, pH 6.5 and a dilution rate of 0.20 hr^-1 . The xylose consumption ratio was greatly lower than that of glucose in fermentations with glucose/xylose mixtures, indicating apparent carbon catabolite repression (CCR). However, CCR could be efficiently eliminated by feeding solutions containing the cellobiose/xylose mixture. In OCF at a dilution rate ca. 0.10 hr^-1 , strain QU 50 produced 42.6 g L^-1 of l-LA with a yield of 0.912 g g^-1 -consumed sugars, LA yield of 0.655 g g^-1 based on mixed sugar-loaded, and a productivity of 4.31 g L^-1  hr^-1 from simulated energy cane hydrolyzate. In OCF with high cell density by cell recycling, simultaneous and complete co-utilization of sugars was achieved with stable LA production at 60.1 ± 3.25 g L^-1 with LA yield of 0.944 g g^-1 -consumed sugar and LA productivity of 6.49 ± 0.357 g L^-1  hr^-1 . Besides this, a dramatic increase in LA yield of 0.927 g g^-1 based on mixed sugar-loaded with prolonged operational stability for at least 500 hr (>20 days) was established. This robust system demonstrates an initial green step with a no-carbon loss under energy-saving toward the feasibility of sustainable LA production from lignocellulosic sugars.

Constitutive expression of phosphoketolase, a key enzyme for metabolic shift from homo- to heterolactic fermentation in Enterococcus mundtii QU 25

Phosphoketolase (PK) is responsible for heterolactic fermentation; however, the PK gene of Enterococcus mundtii QU 25, xfpA, is transcribed constitutively, even under homolactic fermentation conditions. In order to deduce the regulatory mechanisms of PK activity in QU 25, XfpA levels in QU 25 cells under hetero- and homolactic fermentation conditions were tested using western blotting. The results showed that the XfpA protein expression was similar under both conditions and that the expression products formed complexes, most likely homodimers, indicating that the regulation of PK activity is downstream of translation.

Modular Engineering of Biomass Degradation Pathways

Production of fuels and chemicals from renewable lignocellulosic feedstocks is a promising alternative to petroleum-derived compounds. Due to the complexity of lignocellulosic feedstocks, microbial conversion of all potential substrates will require substantial metabolic engineering. Non-model microbes offer desirable physiological traits, but also increase the difficulty of heterologous pathway engineering and optimization. The development of modular design principles that allow metabolic pathways to be used in a variety of novel microbes with minimal strain-specific optimization will enable the rapid construction of microbes for commercial production of biofuels and bioproducts. In this review, we discuss variability of lignocellulosic feedstocks, pathways for catabolism of lignocellulose-derived compounds, challenges to heterologous engineering of catabolic pathways, and opportunities to apply modular pathway design. Implementation of these approaches will simplify the process of modifying non-model microbes to convert diverse lignocellulosic feedstocks.

High Improvement in Lactic Acid Productivity by New Alkaliphilic Bacterium Using Repeated Batch Fermentation Integrated with Increased Substrate Concentration

Optically pure lactic acid (LA) is an important chemical platform that has a wide range of industrial and biotechnological applications. Improved parameters for cost effective LA production are of great interest for industrial developments. In the present study, an alkaliphilic lactic acid bacterium, BoM 1-2, was selected among 369 newly obtained bacterial isolates. It was characterized using API 50 CHL kit and identified as Enterococcus hirae BoM 1-2 by 16S rRNA gene sequence analysis. Efficient polymer-grade L-lactic acid production was achieved at pH 9.0 and 40°C. In batch fermentation strategy using 20 g L⁻¹ glucose, 19.6 g L⁻¹ lactic acid was obtained with volumetric productivity of 2.18 g L⁻¹h⁻¹. While using 100 g L⁻¹ glucose, 96.0 g L⁻¹ lactic acid was obtained with volumetric productivity of 1.07 g L⁻¹h^−1. The highest lactic acid concentration of 180.6 g L⁻¹ was achieved in multipulse fed batch strategy with volumetric productivity of 0.65 g L⁻¹h⁻¹. To achieve higher productivity, repeated fermentation processes were applied using the two different strategies. In the first strategy, the lactic acid productivity was increased from 1.97 g L⁻¹h⁻¹ to 4.48 g L⁻¹h⁻¹ when the total of 10 repeated runs were carried out using 60 g L⁻¹ glucose, but lactic acid productivity decreased to 2.95 g L⁻¹h⁻¹ using 100 g L⁻¹ glucose. In second strategy, repeated fermentation coupled with gradual increase in glucose concentration from 40 to 100 g L⁻¹ was conducted for 24 runs. A dramatic increase in LA productivity up to 39.9 g L⁻¹h⁻¹ (18-fold compared to first run) was achieved using 40 g L⁻¹ glucose while volumetric productivity ranging between 24.8 and 29.9 g L⁻¹h⁻¹ was achieved using 60–100 g L⁻¹ glucose.

Greener L-lactic acid production through in situ extractive fermentation by an acid-tolerant Lactobacillus strain

Lactic acid (LA) fermentation requires a neutralizer for a physiologically acceptable range. However, a neutralizer generates a large amount of gypsum, an environmental pollutant. Furthermore, the downstream processing is complicated and expensive, comprising 50-70% of the total cost. We previously developed a Lactobacillus delbrueckii FM1, which can produce undissociated LA without neutralizer. Here, we improved FM1 by adaptive evolution at pH 4.5, which generated Adp FM1 showing an ~ 1.80-fold increase in LA production compared to FM1. The LA production via fed-batch fermentation yielded 36.2 g/L of LA, with a productivity of 0.500 g/L/h. However, cell viability was reduced due to the acidic pH and/or end-product inhibition. Therefore, an in situ LA recovery process using an extractive solvent was employed to maintain cell viability. Adp FM1 produced 49.2 g/L of LA via in situ LA-extractive fed-batch fermentation, which was ~ 1.4-fold higher than that without LA extraction. Adp FM1 provided a total LA productivity of 0.512 g/L/h in 96 h. Among the tested strains, Adp FM1 exhibited the highest H⁺-ATPase activity and a 415-fold increase in H⁺-ATPase gene expression compared to the parent strain. These results suggest that the in situ LA extractive fermentation process will ease downstream processing and prove to be a more economical and environmentally friendly option compared to the present fermentation. To our knowledge, this is the first report on the production of undissociated L-LA by Lactobacillus using an in situ recovery process, with high LA production levels and productivity.

Carboxylic acid production from ensiled crops in anaerobic solid-state fermentation - trace elements as pH controlling agents support microbial chain elongation with lactic acid

BACKGROUND

For the production of carboxylic acid platform chemicals like medium-chain fatty acids (MCFA) by anaerobic fermentation, pH control is required. However, adding buffer solutions is ineffective in leach-bed reactors.

AIM

In order to increase the MCFA production by maize silage fermentation and to engineer the process we investigated the effect of solid alkaline iron and manganese additives on the process performance and microbial community dynamics.

RESULTS

Without additives, the pH dropped to 3.9 and lactic acid bacteria were favored. Total product yields of 207 ± 5.4 g organic acids (C₂-C₆) and alcohols per kg volatile solids were reached. The addition of trace elements increased the pH value and the product spectrum and yields changed. With a commercial iron additive, the product yields were higher (293 ± 15.2 g/kg_{volatile solids}) and supposedly clostridia used lactic acid for microbial chain elongation of acetic acid producing n-butyric acid. With the addition of pure Fe(OH)₃ or Mn(OH)₂, the total product yields were lower than in the other reactors. However, increased production of MCFA and the occurrence of distinct bacterial taxa (Lachnospiraceae, Ruminococcaceae and Megasphaera) related to this metabolic function were observed.

CONCLUSIONS

The application of alkaline trace metal additives as pH stabilizing agents can mitigate spatial metabolic heterogeneities when trace metal deficient substrates like specific crops or residues thereof are applied.

Co-fermentation of the main sugar types from a beechwood organosolv hydrolysate by several strains of Bacillus coagulans results in effective lactic acid production

Bacillus coagulans is an interesting facultative anaerobic microorganism for biotechnological production of lactic acid that arouses interest. To determine the efficiency of biotechnological production of lactic acid from lignocellulosic feedstock hydrolysates, five Bacillus coagulans strains were grown in lignocellulose organosolv hydrolysate from ethanol/water-pulped beechwood. Parameter estimation based on a Monod-type model was used to derive the basic key parameters for a performance evaluation of the batch process. Three of the Bacillus coagulans strains, including DSM No. 2314, were able to produce lactate, primarily via uptake of glucose and xylose. Two other strains were identified as having the ability of utilizing cellobiose to a high degree, but they also had a lower affinity to xylose. The lactate yield concentration varied from 79.4 ± 2.1 g/L to 93.7 ± 1.4 g/L (85.4 ± 4.7 % of consumed carbohydrates) from the diluted organosolv hydrolysate.

Genome Sequence of Enterococcus mundtii EM01, Isolated from Bombyx mori Midgut and Responsible for Flacherie Disease in Silkworms Reared on an Artificial Diet

The whole genome sequence of Enterococcus mundtii strain EM01 is reported here. The isolate proved to be the cause of flacherie in Bombyx mori. To date, the genomes of 11 other E. mundtii strains have been sequenced. EM01 is the only strain that displayed active pathological effects on its associated animal species.

Stimulation of d- and l-lactate dehydrogenases transcriptional levels in presence of diammonium hydrogen phosphate resulting to enhanced lactic acid production by Lactobacillus strain

The present study revealed the effect of nitrogen sources on lactic acid production and stimulation of d- and l-lactate dehydrogenases (LDH) of parent Lactobacillus lactis NCIM 2368 and its mutant RM2-24 generated after UV mutagenesis. Both the parent and mutant strains were evaluated for d-lactic acid production in control and modified media. The modified media did not show remarkable effect on lactic acid production in case of parent whereas mutant exhibited significant enhancement in d-lactic acid production along with the appearance of l-lactic acid in the broth. Both LDH activities and specific activities were found to be higher in mutant than the parent strain. These results suggested that the diammonium hydrogen phosphate in modified media triggered the expression of LDH genes leading to enhanced lactic acid production. This observation has been proved by studying the expression levels of d- and l-LDH genes of parent and mutant in control and modified media using quantitative RT-PCR technique. In case of mutant, the transcriptional levels of d-LDH and l-LDH increased ∼17 fold and ∼1.38 fold respectively in modified medium compared to the values obtained with control medium. In case of parent, no significant change in transcriptional levels of d- and l-LDH was found when the cells were grown in either control medium or modified medium. This study suggested that the mutant, RM2-24 has l-LDH gene which is expressed in presence of (NH₄)₂HPO₄ resulting in l-lactic acid production. Co-production of l-lactic acid in d-lactic acid fermentation may be detrimental in the PLA production.

Evaluation of marine psychrophile, Psychrobacter namhaensis SO89, as a probiotic in Nile tilapia (Oreochromis niloticus) diets

Marine environment represents a promising source of new, unconventional bioactive compounds with health-promoting abilities, which can be used as food supplements. The present study was carried out to evaluate the efficacy of marine Psychrobacter namhaensis SO89 on growth performance and immune response of Nile tilapia (Oreochromis niloticus). P. namhaensis were isolated from marine environments and phylogenetically identified by 16S rRNA gene sequences. The bacterial isolate was incorporated in Nile tilapia diets (30% crude protein) at three concentrations (0.0, 0.5 and 1.0%; w/w) (designated as T₀, T_0.5 and T₁, respectively), which were equivalent to 0.0, 2.8 × 10⁷ and 5.6 × 10⁷ CFU g^-1 diet, respectively. The diets were fed to Nile tilapia fingerlings (4.58 ± 0.14 g average weight) at a daily rate of 3% of their live body weights (BW), 3 times a day for 50 days. The best growth rates and feed utilization efficiency were obtained at 0.5% P. namhaensis SO89 concentration. Hematocrit (Ht%), hemoglobin (Hb%), erythrocytes (RBC) and total leukocyte (WBCs) values were significantly higher in P. namhaensis SO89- fed groups than in the control group. Similarly, immunoglobulin M (IgM), alternative complement hemolysis (ACH50), phagocytic and lysozyme activities significantly increased following dietary P. namhaensis SO89 supplementation at 0.5% concentration compared to the control group. The expression of IL-4 and IL-12 genes was also significantly up-regulated in P. namhaensis SO89-treated groups up to 0.5% concentration. Increasing bacterial concentration to 1% resulted in a significant decrease in fish performance and immune response. The present results suggest that marine psychrotolerant (Psychrobacter namhaensis) can be considered as a novel feed additive in Nile tilapia feeds.

Cost-effective simultaneous saccharification and fermentation of l-lactic acid from bagasse sulfite pulp by Bacillus coagulans CC17

The main barriers to cost-effective lactic acid production from lignocellulose are the high cost of enzymes and the ineffective utilization of the xylose within the hydrolysate. In the present study, the thermophilic Bacillus coagulans strain CC17 was used for the simultaneous saccharification and fermentation (SSF) of bagasse sulfite pulp (BSP) to produce l-lactic acid. Unexpectedly, SSF by CC17 required approximately 33.33% less fungal cellulase than did separate hydrolysis and fermentation (SHF). More interestingly, CC17 can co-ferment cellobiose and xylose without any exogenous β-glucosidase in SSF. Moreover, adding xylanase could increase the concentration of lactic acid produced via SSF. Up to 110g/L of l-lactic acid was obtained using fed-batch SSF, resulting in a lactic acid yield of 0.72g/g cellulose. These results suggest that SSF using CC17 has a remarkable advantage over SHF and that a potentially low-cost and highly-efficient fermentation process can be established using this protocol.

Novel pH control strategy for efficient production of optically active l-lactic acid from kitchen refuse using a mixed culture system

Uninvestigated control factors of meta-fermentation, the fermentative production of pure chemicals and fuels in a mixed culture system, were examined for production of optically pure l-lactic acid (LA) from food waste. In meta-fermentations by pH swing control, l-LA production with 100% optical purity (OPl-LA) was achieved even using unsterilized model kitchen refuse medium with preferential proliferation of l-LA-producing Bacillus coagulans, a minor member in the seed, whereas agitation decreased OPl-LA drastically. pH constant control shortened the fermentation time but decreased OPl-LA and LA selectivity (SLA) by stimulating growth of heterofermentative Bacillus thermoamylovorans. Deliberately switching from pH swing control to constant control exhibited the best performance for l-LA production: maximum accumulation, 39.2gL(-1); OPl-LA, 100%; SLA, 96.6%; productivity, 1.09gL(-1)h(-1). These results present a novel pH control strategy for efficient l-LA production in meta-fermentation based on a concept different from that of pure culture systems.

Engineered biosynthesis of biodegradable polymers

Advances in science and technology have resulted in the rapid development of biobased plastics and the major drivers for this expansion are rising environmental concerns of plastic pollution and the depletion of fossil-fuels. This paper presents a broad view on the recent developments of three promising biobased plastics, polylactic acid (PLA), polyhydroxyalkanoate (PHA) and polybutylene succinate (PBS), well known for their biodegradability. The article discusses the natural and recombinant host organisms used for fermentative production of monomers, alternative carbon feedstocks that have been used to lower production cost, different metabolic engineering strategies used to improve product titers, various fermentation technologies employed to increase productivities and finally, the different downstream processes used for recovery and purification of the monomers and polymers.

Probiotics production and alternative encapsulation methodologies to improve their viabilities under adverse environmental conditions

Probiotic products are dietary supplements containing live microorganisms producing beneficial health effects on the host by improving intestinal balance and nutrient absorption. Among probiotic microorganisms, those classified as lactic acid bacteria are of major importance to the food and feed industries. Probiotic cells can be produced using alternative carbon and nitrogen sources, such as agroindustrial residues, at the same time contributing to reduce process costs. On the other hand, the survival of probiotic cells in formulated food products, as well as in the host gut, is an essential nutritional aspect concerning health benefits. Therefore, several cell microencapsulation techniques have been investigated as a way to improve cell viability and survival under adverse environmental conditions, such as the gastrointestinal milieu of hosts. In this review, different aspects of probiotic cells and technologies of their related products are discussed, including formulation of culture media, and aspects of cell microencapsulation techniques required to improve their survival in the host.

Highly efficient l-lactic acid production from xylose in cell recycle continuous fermentation using Enterococcus mundtii QU 25

We report an effective cell recycling continuous fermentation of xylose to l-lactic acid with high concentration, productivity, and yield using strain QU 25. pH was found to affect the yield and corn steep liquor as feeding medium enhanced the yield.

Transcriptional regulation of xylose utilization in Enterococcus mundtii QU 25

In the xylose and/or glucose utilization by QU 25,the transcriptional regulation of related genes is involved in the catabolite repression,not in the metabolic shift between homo- and hetero-lactic fermentations.

An alternative allosteric regulation mechanism of an acidophilic l-lactate dehydrogenase from Enterococcus mundtii 15-1A

A plant-derived Enterococcus mundtii 15-1A, that has been previously isolated from Brassica rapa L. subsp. nipposinica (L.H. Bailey) Hanelt var. linearifolia by our group, possesses two kinds of l-lactate dehydrogenase (l-LDH): LDH-1 and LDH-2. LDH-1 was activated under low concentration of fluctose-1,6-bisphosphate (FBP) at both pH 5.5 and 7.5. Although LDH-2 was also activated under the low concentration of FBP at pH 5.5, a high concentration of FBP is necessary to activate it at pH 7.5. The present study shows the crystal structures of the acidophilic LDH-2 in a complex with and without FBP and NADH. Although the tertiary structure of the ligands-bound LDH-2 is similar to that of the active form of other bacterial l-LDHs, the structure without the ligands is different from that of any other previously determined l-LDHs. Major structural alterations between the two structures of LDH-2 were observed at two regions in one subunit. At the N-terminal parts of the two regions, the ligands-bound form takes an α-helical structure, while the form without ligands displays more disordered and extended structures. A vacuum-ultraviolet circular dichroism analysis showed that the α-helix content of LDH-2 in solution is approximately 30% at pH 7.5, which is close to that in the crystal structure of the form without ligands. A D241N mutant of LDH-2, which was created by us to easily form an α-helix at one of the two parts, exhibited catalytic activity even in the absence of FBP at both pH 5.5 and 7.5.

Fermentative production of lactic acid from renewable materials: recent achievements, prospects, and limits

The development and implementation of renewable materials for the production of versatile chemical resources have gained considerable attention recently, as this offers an alternative to the environmental problems caused by the petroleum industry and the limited supply of fossil resources. Therefore, the concept of utilizing biomass or wastes from agricultural and industrial residues to produce useful chemical products has been widely accepted. Lactic acid plays an important role due to its versatile application in the food, medical, and cosmetics industries and as a potential raw material for the manufacture of biodegradable plastics. Currently, the fermentative production of optically pure lactic acid has increased because of the prospects of environmental friendliness and cost-effectiveness. In order to produce lactic acid with high yield and optical purity, many studies focus on wild microorganisms and metabolically engineered strains. This article reviews the most recent advances in the biotechnological production of lactic acid mainly by lactic acid bacteria, and discusses the feasibility and potential of various processes.

Complete genome sequence of Enterococcus mundtii QU 25, an efficient L-(+)-lactic acid-producing bacterium

Enterococcus mundtii QU 25, a non-dairy bacterial strain of ovine faecal origin, can ferment both cellobiose and xylose to produce l-lactic acid. The use of this strain is highly desirable for economical l-lactate production from renewable biomass substrates. Genome sequence determination is necessary for the genetic improvement of this strain. We report the complete genome sequence of strain QU 25, primarily determined using Pacific Biosciences sequencing technology. The E. mundtii QU 25 genome comprises a 3 022 186-bp single circular chromosome (GC content, 38.6%) and five circular plasmids: pQY182, pQY082, pQY039, pQY024, and pQY003. In all, 2900 protein-coding sequences, 63 tRNA genes, and 6 rRNA operons were predicted in the QU 25 chromosome. Plasmid pQY024 harbours genes for mundticin production. We found that strain QU 25 produces a bacteriocin, suggesting that mundticin-encoded genes on plasmid pQY024 were functional. For lactic acid fermentation, two gene clusters were identified—one involved in the initial metabolism of xylose and uptake of pentose and the second containing genes for the pentose phosphate pathway and uptake of related sugars. This is the first complete genome sequence of an E. mundtii strain. The data provide insights into lactate production in this bacterium and its evolution among enterococci.

l-(+)-Lactic acid production by co-fermentation of cellobiose and xylose without carbon catabolite repression using Enterococcus mundtii QU 25

We established an effective highl-lactic acid production system based on fed-batch bacterial cultures utilising lignocellulosic biomass-derived mixed sugars without carbon catabolite repression.

Bacillus sp. strain P38: an efficient producer of L-lactate from cellulosic hydrolysate, with high tolerance for 2-furfural

In this study, efficient polymer-grade L-lactic acid production was achieved with the strain Bacillus sp. P38 by using cellulosic hydrolysate as the sole carbon source. In fed-batch fermentation, 180 g L(-1)L-lactic acid was obtained with a volumetric productivity of 2.4 g L(-1)h(-1) and a yield of 0.96 g g(-1) total reducing sugars. No D-isomer of lactic acid was detected in the broth. Strain P38 tolerated up to 10 g L(-1) 2-furfural, and lactate production was sharply inhibited only when the 2-furfural concentration was higher than 6 g L(-1). Moreover, strain P38 also tolerated high concentrations (>6 g L(-1)) of other fermentation inhibitors in cellulosic hydrolysate, such as vanillin and acetic acid, although it was slightly sensitive to formic acid. The efficient L-lactic acid production, combined with high inhibitor tolerance and efficient pentose utilization, indicate that Bacillus sp. P38 is a promising producer of polymer-grade L-lactic acid from cellulosic biomass.

GH1-family 6-P-β-glucosidases from human microbiome lactic acid bacteria

In lactic acid bacteria and other bacteria, carbohydrate uptake is mostly governed by phosphoenolpyruvate-dependent phosphotransferase systems (PTSs). PTS-dependent translocation through the cell membrane is coupled with phosphorylation of the incoming sugar. After translocation through the bacterial membrane, the β-glycosidic bond in 6'-P-β-glucoside is cleaved, releasing 6-P-β-glucose and the respective aglycon. This reaction is catalyzed by 6-P-β-glucosidases, which belong to two glycoside hydrolase (GH) families: GH1 and GH4. Here, the high-resolution crystal structures of GH1 6-P-β-glucosidases from Lactobacillus plantarum (LpPbg1) and Streptococcus mutans (SmBgl) and their complexes with ligands are reported. Both enzymes show hydrolytic activity towards 6'-P-β-glucosides. The LpPbg1 structure has been determined in an apo form as well as in a complex with phosphate and a glucose molecule corresponding to the aglycon molecule. The S. mutans homolog contains a sulfate ion in the phosphate-dedicated subcavity. SmBgl was also crystallized in the presence of the reaction product 6-P-β-glucose. For a mutated variant of the S. mutans enzyme (E375Q), the structure of a 6'-P-salicin complex has also been determined. The presence of natural ligands enabled the definition of the structural elements that are responsible for substrate recognition during catalysis.

Efficient production of polymer-grade L-lactic acid from corn stover hydrolyzate by thermophilic Bacillus sp. strain XZL4

Lactic acid has been identified as one of the top 30 potential building-block chemicals from biomass. Therefore, the search for cheap raw materials is an objective to reduce the production costs. Efficient polymer-grade L-lactic acid production was achieved in this report by a thermophilic strain Bacillus sp. XZL4 using corn stover hydrolyzate as sole carbon source. High L-lactic acid concentration (81.0 g L^-1) was obtained from 162.5 g L^-1 concentrated corn stover hydrolyzate (total reducing sugar of 83.0 g L^-1) with a volumetric productivity of 1.86 g L^-1 h^-1 (0–36 h) and a product yield of 0.98 g g^-1 total reducing sugars. This is the highest L-lactic acid concentration and yield reported from corn stover hydrolyzate. And the high optical purity of L-lactic acid obtained in this study also indicated that Bacillus sp. XZL4 is a promising polymer-grade L-lactic-acid producer from cellulosic biomass.

Lactic acid production from lignocellulose-derived sugars using lactic acid bacteria: overview and limits

Lactic acid is an industrially important product with a large and rapidly expanding market due to its attractive and valuable multi-function properties. The economics of lactic acid production by fermentation is dependent on many factors, of which the cost of the raw materials is very significant. It is very expensive when sugars, e.g., glucose, sucrose, starch, etc., are used as the feedstock for lactic acid production. Therefore, lignocellulosic biomass is a promising feedstock for lactic acid production considering its great availability, sustainability, and low cost compared to refined sugars. Despite these advantages, the commercial use of lignocellulose for lactic acid production is still problematic. This review describes the "conventional" processes for producing lactic acid from lignocellulosic materials with lactic acid bacteria. These processes include: pretreatment of the biomass, enzyme hydrolysis to obtain fermentable sugars, fermentation technologies, and separation and purification of lactic acid. In addition, the difficulties associated with using this biomass for lactic acid production are especially introduced and several key properties that should be targeted for low-cost and advanced fermentation processes are pointed out. We also discuss the metabolism of lignocellulose-derived sugars by lactic acid bacteria.

Efficient homofermentative L-(+)-lactic acid production from xylose by a novel lactic acid bacterium, Enterococcus mundtii QU 25

Enterococcus mundtii QU 25, a newly isolated lactic acid bacterium, efficiently metabolized xylose into l-lactate. In batch fermentations, the strain produced 964 mM l-(+)-lactate from 691 mM xylose, with a yield of 1.41 mol/mol xylose consumed and an extremely high optical purity of ≥99.9% without acetate production.