Metabolic engineering of Lactobacillus helveticus CNRZ32 for production of pure L-(+)-lactic acid

Genomic Modifications of Lactic Acid Bacteria and Their Applications in Dairy Fermentation

Lactic Acid Bacteria (LAB) have a long history of safe use in milk fermentation and are generally recognized as health-promoting microorganisms when present in fermented foods. LAB are also important components of the human intestinal microbiota and are widely used as probiotics. Considering their safe and health-beneficial properties, LAB are considered appropriate vehicles that can be genetically modified for food, industrial and pharmaceutical applications. Here, this review describes (1) the potential opportunities for application of genetically modified LAB strains in dairy fermentation and (2) the various genomic modification tools for LAB strains, such as random mutagenesis, adaptive laboratory evolution, conjugation, homologous recombination, recombineering, and CRISPR (clustered regularly interspaced short palindromic repeat)- Cas (CRISPR-associated protein) based genome engineering. Lastly, this review also discusses the potential future developments of these genomic modification technologies and their applications in dairy fermentations.

Homofermentative Lactobacilli isolated from organic sources exhibit potential ability of lactic acid production

There has been an increasing interest in recent years in lactic acid bacteria that are derived from organic sources for lactic acid production. This research article presents the isolation and identification of homofermentative lactic acid bacteria from various novel organic sources, followed by qualitative and quantitative analyses of lactic acid produced. A total of 32 isolates were identified initially from various sources, such as curd (C1, C2), probiotics (P1, P2, and P3), silage (Si1 and Si2), soil samples (S1, S2, and S3), vermicompost (V1 and V2), and Farmyard manure. Biochemical tests such as Gram's staining, catalase test, and oxidase test were conducted for preliminary identification of lactic acid bacteria using De Man, Rogosa, and Sharpe agar (MRS) media. Through selection and identification, based on colony morphology and biochemical characteristics, 18 isolates were identified as lactic acid bacteria. The subsequent analysis included a tube test, screening for organic acid production, and homofermentative screening using homofermentative-heterofermentative differential (HHD) medium for qualitative analysis of lactic acid. The results revealed that 9 out of 18 selected strains were homofermentative and had promising potential for the production of lactic acid. Furthermore, six isolates (P1-1, S1-3, C2-1, V2-3, P2-1, and C1-1) from all of the nine positive strains were subjected to pH testing (0, 24, 48, and 72 h) and titrimetric assay for estimation of % crude lactic acid present. The presence of lactic acid was confirmed using thin-layer chromatography (TLC). L (+)-Lactic acid was quantified using a K-LATE enzymatic assay kit, for the best three isolates (P1-1, S1-3, C2-1). Finally, the strains were subjected to 16SrRNA sequencing and were identified as Lactobacilli. Based on the findings of the study, it could be concluded that homofermentative lactic acid bacteria with significant LA-producing ability can be obtained from different organic sources and may prove to be useful in the successful production of lactic acid for biotechnological applications.

Dual Role of Probiotic Lactic Acid Bacteria Cultures for Fermentation and Control Pathogenic Bacteria in Fruit-Enriched Fermented Milk

The food industry has been developing new products with health benefits, extended shelf life, and without chemical preservation. Bacteriocin-producing lactic acid bacteria (LAB) strains have been evaluated for food fermentation to prevent contamination and increase shelf life. In this study, potentially probiotic LAB strains, Lactiplantibacillus (Lb.) plantarum ST8Sh, Lacticaseibacillus (Lb.) casei SJRP38, and commercial starter Streptococcus (St.) thermophilus ST080, were evaluated for their production of antimicrobial compounds, lactic acid and enzyme production, carbohydrate assimilation, and susceptibility to antibiotics. The characterization of antimicrobial compounds, the proteolytic activity, and its inhibitory property against Listeria (List.) monocytogenes and Staphylococcus (Staph.) spp. was evaluated in buriti and passion fruit-supplemented fermented milk formulations (FMF) produced with LAB strains. Lb. plantarum ST8Sh was found to inhibit List. monocytogenes through bacteriocin production and produced both L(+) and D(-) lactic acid isomers, while Lb. casei SJRP38 mainly produced L(+) lactic acid. The carbohydrate assimilation profiles were compatible with those usually found in LAB. The potentially probiotic strains were susceptible to streptomycin and tobramycin, while Lb. plantarum ST8Sh was also susceptible to ciprofloxacin. All FMF produced high amounts of L(+) lactic acid and the viability of total lactobacilli remained higher than 8.5 log CFU/mL during monitored storage period. Staph. aureus ATCC 43300 in fermented milk with passion fruit pulp (FMFP) and fermented milk with buriti pulp (FMB), and Staph. epidermidis KACC 13234 in all formulations were completely inhibited after 14 days of storage. The combination of Lb. plantarum ST8Sh and Lb. casei SJRP38 and fruit pulps can provide increased safety and shelf-life for fermented products, and natural food preservation meets the trends of the food market.

Metabolic engineering of Zymomonas mobilis for co-production of D-lactic acid and ethanol using waste feedstocks of molasses and corncob residue hydrolysate

Lactate is the precursor for polylactide. In this study, a lactate producer of Z. mobilis was constructed by replacing ZMO0038 with LmldhA gene driven by a strong promoter PadhB, replacing ZMO1650 with native pdc gene driven by Ptet, and replacing native pdc with another copy of LmldhA driven by PadhB to divert carbon from ethanol to D-lactate. The resultant strain ZML-pdc-ldh produced 13.8 ± 0.2 g/L lactate and 16.9 ± 0.3 g/L ethanol using 48 g/L glucose. Lactate production of ZML-pdc-ldh was further investigated after fermentation optimization in pH-controlled fermenters. ZML-pdc-ldh produced 24.2 ± 0.6 g/L lactate and 12.9 ± 0.8 g/L ethanol as well as 36.2 ± 1.0 g/L lactate and 40.3 ± 0.3 g/L ethanol, resulting in total carbon conversion rate of 98.3% ± 2.5% and 96.2% ± 0.1% with final product productivity of 1.9 ± 0.0 g/L/h and 2.2 ± 0.0 g/L/h in RMG5 and RMG12, respectively. Moreover, ZML-pdc-ldh produced 32.9 ± 0.1 g/L D-lactate and 27.7 ± 0.2 g/L ethanol as well as 42.8 ± 0.0 g/L D-lactate and 53.1 ± 0.7 g/L ethanol with 97.1% ± 0.0% and 99.1% ± 0.8% carbon conversion rate using 20% molasses or corncob residue hydrolysate, respectively. Our study thus demonstrated that it is effective for lactate production by fermentation condition optimization and metabolic engineering to strengthen heterologous ldh expression while reducing the native ethanol production pathway. The capability of recombinant lactate-producer of Z. mobilis for efficient waste feedstock conversion makes it a promising biorefinery platform for carbon-neutral biochemical production.

Lactobacillus in Food Animal Production—A Forerunner for Clean Label Prospects in Animal-Derived Products

Lactobacillus, the largest genus within the lactic acid bacteria group, has served diverse roles in improving the quality of foods for centuries. The heterogeneity within this genus has resulted in the industry's continued use of their well-known functions and exploration of novel applications. Moreover, the perceived health benefits in many applications have also made them fond favorites of consumers and researchers alike. Their familiarity lends to their utility in the growing “clean label” movement, of which consumers prefer fewer additions to the food label and opt for recognizable and naturally-derived substances. Our review primarily focuses on the historical use of lactobacilli for their antimicrobial functionality in improving preharvest safety, a critical step to validate their role as biocontrol agents and antibiotic alternatives in food animal production. We also explore their potential as candidates catering to the consumer-driven demand for more authentic, transparent, and socially responsible labeling of animal products.

Microbial cell factories for the production of three-carbon backbone organic acids from agro-industrial wastes

At present, mass production of basic and valuable commodities is dependent on linear petroleum-based industries, which ultimately makes the depletion of finite natural reserves and accumulation of non-biodegradable and hazardous wastes. Therefore, an ecofriendly and sustainable solution should be established for a circular economy where infinite resources, such as agro-industrial wastes, are fully utilized as substrates in the production of target value-added chemicals. Hereby, recent advances in metabolic engineering strategies and techniques used in the development of microbial cell factories for enhanced production of three-carbon platform chemicals such as lactic acid, propionic acid, and 3-hydroxypropionic acid are discussed. Further developments and future perspectives in the production of these organic acids from agro-industrial wastes from the dairy, sugar, and biodiesel industries are also highlighted to demonstrate the importance of waste-based biorefineries for organic acid production.

In silico, in vitro and in vivo safety evaluation of Limosilactobacillus reuteri strains ATCC PTA-126787 & ATCC PTA-126788 for potential probiotic applications

The last two decades have witnessed a tremendous growth in probiotics and in the numbers of publications on their potential health benefits. Owing to their distinguishing beneficial effects and long history of safe use, species belonging to the Lactobacillus genus are among the most widely used probiotic species in human food and dietary supplements and are finding increased use in animal feed. Here, we isolated, identified, and evaluated the safety of two novel Limosilactobacillus reuteri (L. reuteri) isolates, ATCC PTA-126787 & ATCC PTA-126788. More specifically, we sequenced the genomes of these two L. reuteri strains using the PacBio sequencing platform. Using a combination of biochemical and genetic methods, we identified the two strains as belonging to L. reuteri species. Detailed in silico analyses showed that the two strains do not encode for any known genetic sequences of concern for human or animal health. In vitro assays confirmed that the strains are susceptible to clinically relevant antibiotics and do not produce potentially harmful by-products such as biogenic amines. In vitro bile and acid tolerance studies demonstrated that the two strains have similar survival profiles as the commercial L. reuteri probiotic strain DSM 17938. Most importantly, daily administration of the two probiotic strains to broiler chickens in drinking water for 26 days did not induce any adverse effect, clinical disease, or histopathological lesions, supporting the safety of the strains in an in vivo avian model. All together, these data provide in silico, in vitro and in vivo evidence of the safety of the two novel candidates for potential probiotic applications in humans as well as animals.

Use of glycerol waste in lactic acid bacteria metabolism for the production of lactic acid: State of the art in Poland

Lactic acid is a naturally existing organic acid, which may be used in many different branches of industrial application. It can be made in the sugar fermentation process from renewable raw lactic acid, which is an indispensable raw material, including in the agricultural, food, and pharmaceutical industries. It is an ecological product that has enjoyed great popularity in recent years. In 2010, the US Department of Energy published a report about lactic acid to be a potential building element for future technology, whose demand grows year by year. The lactic acid molecule naturally exists in plants, microorganisms, and animals and can also be produced by carbohydrate fermentation or chemical synthesis from coal, petroleum products, and natural gas. In industry, lactic acid can be produced by chemical synthesis or fermentation. Although racemic lactic acid is always produced chemically from petrochemical sources, the optically pure L(+) – or D(−) – lactic acid forms can be obtained by microbial fermentation of renewable resources when an appropriate microorganism is selected. Depending on the application, one form of optically pure LA is preferred over the other. Additionally, microbial fermentation offers benefits including cheap renewable substrates, low production temperatures, and low energy consumption. Due to these advantages, the most commonly used biotechnological production process with the use of biocatalysts, i.e., lactic acid bacteria. The cost of raw materials is one of the major factors in the economic production of lactic acid. As substrate costs cannot be reduced by scaling up the process, extensive research is currently underway to find new substrates for the production of LA. These searches include starch raw materials, lignocellulosic biomass, as well as waste from the food and refining industries. Here, the greatest attention is still drawn to molasses and whey as the largest sources of lactose, vitamins, and carbohydrates, as well as glycerol – a by-product of the biodiesel component production process. Focusing on the importance of lactic acid and its subsequent use as a product, but also a valuable raw material for polymerization (exactly to PLA), this review summarizes information about the properties and applications of lactic acid, as well as about its production and purification processes. An industrial installation for the production of lactic acid is only planned to be launched in Poland. As of today, there is no commercial-scale production of this bio-raw material. Thus, there is great potential for the application of the lactic acid production technology and research should be carried out on its development.

Recent Advances in Lactic Acid Production by Lactic Acid Bacteria

Lactic acid can synthesize high value-added chemicals such as poly lactic acid. In order to further minimize the cost of lactic acid production, some effective strategies (e.g., effective mutagenesis and metabolic engineering) have been applied to increase productive capacity of lactic acid bacteria. In addition, low-cost cheap raw materials (e.g., cheap carbon source and cheap nitrogen source) are also used to reduce the cost of lactic acid production. In this review, we summarized the recent developments in lactic acid production, including efficient strain modification technology (high-efficiency mutagenesis means, adaptive laboratory evolution, and metabolic engineering), the use of low-cost cheap raw materials, and also discussed the future prospects of this field, which could promote the development of lactic acid industry.

Advancing mechanistic understanding and bioengineering of probiotic lactobacilli and bifidobacteria by genome editing

Typical traditional probiotics lactobacilli and bifidobacteria are gaining great interest to be developed as living diagnostics and therapeutics for improving human health. However, the mechanistic basis underlying their inherent health beneficial property remain incompletely understood which can slow down the translational pipeline in the functional food and pharmaceutical field. Efficient genome editing will advance the understanding of the molecular mechanism of the probiotics' physiological properties and their interaction with the host and the host microbiota, thereby further promote the development of next-generation designer probiotics with improved robustness and tailored functionalities. With the expansion of genome editing strategies such as CRISPR-Cas-based tools and IPSD assisted genome engineering as well as other synthetic biology technologies, the research and application of these health-promoting bacteria for the food and pharmaceutical industry will be further enhanced.

Role of lactic acid bacteria in flavor development in traditional Chinese fermented foods: A review

Traditional Chinese fermented foods are favored by consumers due to their unique flavor, texture and nutritional values. A large number of microorganisms participate in the process of fermentation, especially lactic acid bacteria (LAB), which are present in almost all fermented foods and contribute to flavor development. The formation process of flavor is complex and involves the biochemical conversion of various food components. It is very important to fully understand the conversion process to direct the flavor formation in foods. A comprehensive link between the LAB community and the flavor formation in traditional Chinese fermented foods is reviewed. The main mechanisms involved in the flavor formation dominated by LAB are carbohydrate metabolism, proteolysis and amino acid catabolism, and lipolysis and fatty acid metabolism. This review highlights some useful novel approaches for flavor enhancement, including the application of functional starter cultures and metabolic engineering, which may provide significant advances toward improving the flavor of fermented foods for a promising market.

Lactic acid production - producing microorganisms and substrates sources-state of art

Lactic acid is an organic compound produced via fermentation by different microorganisms that are able to use different carbohydrate sources. Lactic acid bacteria are the main bacteria used to produce lactic acid and among these, Lactobacillus spp. have been showing interesting fermentation capacities. The use of Bacillus spp. revealed good possibilities to reduce the fermentative costs. Interestingly, lactic acid high productivity was achieved by Corynebacterium glutamicum and E. coli, mainly after engineering genetic modification. Fungi, like Rhizopus spp. can metabolize different renewable carbon resources, with advantageously amylolytic properties to produce lactic acid. Additionally, yeasts can tolerate environmental restrictions (for example acidic conditions), being the wild-type low lactic acid producers that have been improved by genetic manipulation. Microalgae and cyanobacteria, as photosynthetic microorganisms can be an alternative lactic acid producer without carbohydrate feed costs. For lactic acid production, it is necessary to have substrates in the fermentation medium. Different carbohydrate sources can be used, from plant waste as molasses, starchy, lignocellulosic materials as agricultural and forestry residues. Dairy waste also can be used by the addition of supplementary components with a nitrogen source.

Metabolically engineered Lactobacillus gasseri JCM 1131 as a novel producer of optically pure L- and D-lactate

High-quality environmentally-friendly bioplastics can be produced by mixing poly-L-lactate with poly-D-lactate. On an industrial scale, this process simultaneously consumes large amounts of both optically pure lactate stereoisomers. However, because optimal growth conditions of L-lactate producers often differ from those of D-lactate producers, each stereoisomer is produced in a specialised facility, which raises cost and lowers sustainability. To address this challenge, we metabolically engineered Lactobacillus gasseri JCM 1131^T, a bioprocess-friendly and genetically malleable strain of homofermentative lactic acid bacterium, to efficiently produce either pure L- or pure D-lactate under the same bioprocess conditions. Transformation of L. gasseri with plasmids carrying additional genes for L- or D-lactate dehydrogenases failed to affect the ratio of produced stereoisomers, but inactivation of the endogenous genes created strains which yielded 0.96 g of either L- or D-lactate per gram of glucose. In this study, the plasmid pHBintE, routinely used for gene disruption in Bacillus megaterium, was used for the first time to inactivate genes in lactobacilli. Strains with inactivated genes for endogenous lactate dehydrogenases efficiently fermented sugars released by enzymatic hydrolysis of alkali pre-treated wheat straw, an abundant lignocellulose-containing raw material, producing 0.37-0.42 g of lactate per gram of solid part of alkali-treated wheat straw. Thus, the constructed strains are primed to serve as producers of both optically pure L-lactate and D-lactate in the next-generation biorefineries.

Safety Aspects of Genetically Modified Lactic Acid Bacteria

Lactic acid bacteria (LAB) have a long history of use in the food industry. Some species are part of the normal human microbiota and have beneficial properties for human health. Their long-standing use and considerable biotechnological potential have led to the development of various systems for their engineering. Together with novel approaches such as CRISPR-Cas, the established systems for engineering now allow significant improvements to LAB strains. Nevertheless, genetically modified LAB (GM-LAB) still encounter disapproval and are under extensive regulatory requirements. This review presents data on the prospects for LAB to obtain 'generally recognized as safe' (GRAS) status. Genetic modification of LAB is discussed, together with problems that can arise from their engineering, including their dissemination into the environment and the spread of antibiotic resistance markers. Possible solutions that would allow the use of GM-LAB are described, such as biocontainment, alternative selection markers, and use of homologous DNA. The use of GM-LAB as cell factories in closed systems that prevent their environmental release is the least problematic aspect, and this is also discussed.

A novel D(-)-lactic acid-inducible promoter regulated by the GntR-family protein D-LldR of Pseudomonas fluorescens

Lactic acid has two stereoisomers of D(-)- and L(+)-forms, both of which are important monomers of biodegradable plastic, poly-lactic acid. In this study, a novel d-lactate inducible system was identified in Pseudomonas fluorescens A506, partially characterized and tested as biosensor. The d-lactate catabolic operon (lldP-dld-II) was negatively regulated through the inversely transcribed D-lldR (encoding a GntR-type regulator), where the repression is relieved by addition of d-lactate. The derepression was specific to d-lactate and marginally affected by l-lactate. The D-LldR-responsive operator, showing dyad symmetry and separated by one base, was located between +11 and + 27 from the transcription start site of the lldP-dld-II operon. By site-directed mutagenesis, a motif with a dyad symmetry (AATTGGTAtTACCAATT), present in the upstream region of lldP, was identified as essential for the binding of LldR. d-lactate biosensors were developed by connecting the upregulation by d-lactate to a green fluorescent readout. About ~6.0-fold induction by 100 mM d-lactate was observed compared to l-lactate.

Cloning of D-lactate dehydrogenase genes of Lactobacillus delbrueckii subsp. bulgaricus and their roles in D-lactic acid production

Lactobacillus delbrueckii subsp. bulgaricus is a heterogenous lactic acid bacterium that converts pyruvate mainly to D-lactic acid using D-lactate dehydrogenases (D-LDHs), whose functional properties remain poorly characterized. Here, the D-LDHs genes (ldb0101, ldb0813, ldb1010, ldb1147 and ldb2021) were cloned and overexpressed in Escherichia coli JM109 from an inducible pUC18 vector, respectively, and the resulting strains were compared in terms of D-lactic acid production. The strain expressing ldb0101 and ldb1010 gene individually produced more D-lactate than other three strains. Further study revealed that Ldb0101 activity was down-regulated by the oxygen and, therefore, achieved a highest titer of D-lactate (1.94 g/L) under anaerobic condition, and introduction of ldb1010 gene enhanced D-lactate formation (0.94 and 0.85 g/L, respectively) both in aerobic and anaerobic conditions due to a relatively stable q _d-lactate. Our results suggested that the enzyme Ldb0101 and Ldb1010 played a role of more importance in D-lactate formation. To the best of our knowledge, we demonstrate for the first time the roles of different D-LDH homologs from L. bulgaricus in D-lactic acid production.

Lactobacilli and pediococci as versatile cell factories - Evaluation of strain properties and genetic tools

This review discusses opportunities and bottlenecks for cell factory development of Lactic Acid Bacteria (LAB), with an emphasis on lactobacilli and pediococci, their metabolism and genetic tools. In order to enable economically feasible bio-based production of chemicals and fuels in a biorefinery, the choice of product, substrate and production organism is important. Currently, the most frequently used production hosts include Escherichia coli and Saccharomyces cerevisiae, but promising examples are available of alternative hosts such as LAB. Particularly lactobacilli and pediococci can offer benefits such as thermotolerance, an extended substrate range and increased tolerance to stresses such as low pH or high alcohol concentrations. This review will evaluate the properties and metabolism of these organisms, and provide an overview of their current biotechnological applications and metabolic engineering. We substantiate the review by including experimental results from screening various lactobacilli and pediococci for transformability, growth temperature range and ability to grow under biotechnologically relevant stress conditions. Since availability of efficient genetic engineering tools is a crucial prerequisite for industrial strain development, genetic tool development is extensively discussed. A range of genetic tools exist for Lactococcus lactis, but for other species of LAB like lactobacilli and pediococci such tools are less well developed. Whereas lactobacilli and pediococci have a long history of use in food and beverage fermentation, their use as platform organisms for production purposes is rather new. By harnessing their properties such as thermotolerance and stress resistance, and by using emerging high-throughput genetic tools, these organisms are very promising as versatile cell factories for biorefinery applications.

The Effect of a Newly Developed Oat-Banana Fermented Beverage with a Beta-glucan Additive on ldhL Gene Expression in Streptococcus thermophilus TKM3 KKP 2030p

The number of people who cannot consume dairy products due to intolerance or allergies to food components is increasing. Consumers are increasingly searching for alternative, nondairy beverages which have an advantageous effect on the body and which stimulate gut microflora. Previous studies have shown that Streptococcus thermophilus TKM3 KKP 2030p can be an efficient starter culture for the fermentation of plant material and can also ensure an acceptable sensory profile and a high number of lactic acid bacteria during 4 week cold storage. The aim of this study was to determine the relative gene expression of the L-lactate dehydrogenase gene (ldhL) in selected strain using the RT qPCR technique at particular time points. The gene expression experiments were conducted in laboratory broth (LABm) and fruit-cereal (OBPromOat) matrix with a beta-glucan additive. Levels of gene relative expression in the selected strain in these two media were correlated with the amount of L-lactate produced. The results showed that the plant matrix stimulated stronger ldhL gene expression in the first 4 h of the experiment (although the synthesis of L-lactate was less than in laboratory broth). This study broadens existing knowledge of the transcriptional mechanisms which arise from the adaptation of the analyzed strain to different habitats during L-lactate synthesis. This could contribute to the development of new alternative food products for people who show intolerance to milk proteins or lactose.

Biosurfactant/s from Lactobacilli species: Properties, challenges and potential biomedical applications

Lactic acid bacteria are generally believed to have positive roles in maintaining good health and immune system in humans. A number of Lactobacilli spp. are known to produce important metabolites, among which biosurfactants in particular have shown antimicrobial activity against several pathogens in the intestinal tract and female urogenital tract partly through interfering with biofilm formation and adhesion to the epithelial cells surfaces. Around 46 reports are documented on biosurfactant production from Lactobacillus spp. of which six can be broadly classified as cell free biosurfactant and 40 as cell associated biosurfactants and only approximately 50% of those have reported on the structural composition which, in order of occurrence were mainly proteinaceous, glycolipidic, glycoproteins, or glycolipopeptides in nature. Due to the proteinaceous nature, most biosurfactant produced by strains of Lactobacillus are generally believed to be surlactin type with high potential toward impeding pathogens adherence. Researchers have recently focused on the anti-adhesive and antibiofilm properties of Lactobacilli-derived biosurfactants. This review briefly discusses the significance of Lactobacilli-derived biosurfactants and their potential applications in various fields. In addition, we highlight the exceptional prospects and challenges in fermentation economics of Lactobacillus spp.-derived biosurfactants' production processes.

Mn(2+) and Mg(2+) synergistically enhanced lactic acid production by Lactobacillus rhamnosus FTDC 8313 via affecting different stages of the hexose monophosphate pathway

AIMS

The study aimed to evaluate the effects of Mn(2+) and Mg(2+) on lactic acid production using response surface methodology and to further study their effects on interactions between the enzymes and substrates along the hexose monophosphate pathway using a molecular modelling approach.

METHODS AND RESULTS

A rotatable central composite design matrix for lactic acid production was generated with two independent factors namely, manganese sulfate and magnesium sulfate. The second-order regression model indicated that the quadratic model was significant (P < 0·05), suggesting that the model accurately represented the data in the experimental region. Three-dimensional response surface showed that lactic acid production was high along the region where the ratio of MnSO4 to MgSO4 was almost 1 : 1, justifying the need for both Mg(2+) and Mn(2+) to be present simultaneously in stimulating the production of lactic acid. Molecular docking simulation was performed on a total of 13 essential enzymes involved in the hexose monophosphate pathway for the production of lactic acid with four different conditions namely in the presence of Mg(2+) , Mn(2+) , both Mg(2+) and Mn(2+) and in the absence of metal ions. Results showed that the presence of both Mg(2+) and Mn(2+) within the binding site improved the binding affinity for substrates in five enzymes namely, glucose-6-phosphate dehydrogenase, phosphogluconate dehydrogenase, glyceraldehyde-3-phosphate dehydrogenase, phosphopyruvate hydratase and pyruvate kinase.

CONCLUSIONS

Using response surface methodology and molecular modelling approach, we illustrated that Mg(2+) and Mn(2+) synergistically enhanced lactic acid production by Lactobacillus rhamnosus FTDC 8313 via affecting different stages of the hexose monophosphate pathway.

SIGNIFICANCE AND IMPACTS OF THE STUDY

Mg(2+) and Mn(2+) synergistically improved lactic acid production of Lact. rhamnosus via improved binding affinity of the enzyme-substrate along the hexose monophosphate pathway, instead of purely affecting growth as previously understood.

Metabolic engineering of lactic acid bacteria for the production of industrially important compounds

Lactic acid bacteria (LAB) are receiving increased attention for use as cell factories for the production of metabolites with wide use by the food and pharmaceutical industries. The availability of efficient tools for genetic modification of LAB during the past decade permitted the application of metabolic engineering strategies at the levels of both the primary and the more complex secondary metabolism. The recent developments in the area with a focus on the production of industrially important metabolites will be discussed in this review.

Relative catalytic efficiency of ldhL- and ldhD-encoded products is crucial for optical purity of lactic acid produced by lactobacillus strains

NAD-dependent l- and d-lactate dehydrogenases coexist in Lactobacillus genomes and may convert pyruvic acid into l-lactic acid and d-lactic acid, respectively. Our findings suggest that the relative catalytic efficiencies of ldhL- and ldhD-encoded products are crucial for the optical purity of lactic acid produced by Lactobacillus strains.

Non-sterilized fermentative production of polymer-grade L-lactic acid by a newly isolated thermophilic strain Bacillus sp. 2-6

Background

The demand for lactic acid has been increasing considerably because of its use as a monomer for the synthesis of polylactic acid (PLA), which is a promising and environment-friendly alternative to plastics derived from petrochemicals. Optically pure l-lactic acid is essential for polymerization of PLA. The high fermentation cost of l-lactic acid is another limitation for PLA polymers to compete with conventional plastics.

Methodology/Principal Findings

A Bacillus sp. strain 2–6 for production of l-lactic acid was isolated at 55°C from soil samples. Its thermophilic characteristic made it a good lactic acid producer because optically pure l-lactic acid could be produced by this strain under open condition without sterilization. In 5-liter batch fermentation of Bacillus sp. 2–6, 118.0 g/liter of l-lactic acid with an optical purity of 99.4% was obtained from 121.3 g/liter of glucose. The yield was 97.3% and the average productivity was 4.37 g/liter/h. The maximum l-lactic acid concentration of 182.0 g/liter was obtained from 30-liter fed-batch fermentation with an average productivity of 3.03 g/liter/h and product optical purity of 99.4%.

Conclusions/Significance

With the newly isolated Bacillus sp. strain 2–6, high concentration of optically pure l-lactic acid could be produced efficiently in open fermentation without sterilization, which would lead to a new cost-effective method for polymer-grade l-lactic acid production from renewable resources.

Microbial conversion of sugars from plant biomass to lactic acid or ethanol

Concerns for our environment and unease with our dependence on foreign oil have renewed interest in converting plant biomass into fuels and 'green' chemicals. The volume of plant matter available makes lignocellulose conversion desirable, although no single isolated organism has been shown to depolymerize lignocellulose and efficiently metabolize the resulting sugars into a specific product. This work reviews selected chemicals and fuels that can be produced from microbial fermentation of plant-derived cell-wall sugars and directed engineering for improvement of microbial biocatalysts. Lactic acid and ethanol production are highlighted, with a focus on engineered Escherichia coli.

Metabolic engineering of Lactobacillus plantarum for production of L-ribulose

L-Ribulose is a rare and expensive sugar that can be used as a precursor for the production of other rare sugars of high market value such as L-ribose. In this work we describe a production process for L-ribulose using L-arabinose, a common component of polymers of lignocellulosic materials, as the starting material. A ribulokinase-deficient mutant of the heterofermentative lactic acid bacterium Lactobacillus plantarum NCIMB8826 was constructed. Expression of araA, which encodes the critical enzyme L-arabinose isomerase, was repressed by high glucose concentrations in batch cultivations. A fed-batch cultivation strategy was therefore used to maximize L-arabinose isomerase production during growth. Resting cells of the ribulokinase-deficient mutant were used for the production of L-ribulose. The isomerization of L-arabinose to L-ribulose was very unfavorable for L-ribulose formation. However, high L-ribulose yields were obtained by complexing the produced L-ribulose with borate. The process for L-ribulose production in borate buffer by resting cells was optimized using central composite designs. The experiment design suggested that the process has an optimal operation point around an L-arabinose concentration of 100 g liter(-1), a borate concentration of 500 mM, and a temperature of 48 degrees C, where the statistical software predicted an initial L-ribulose production rate of 29.1 g liter(-1) h(-1), a best-achievable process productivity of 14.8 g liter(-1) h(-1), and a conversion of L-arabinose to L-ribulose of 0.70 mol mol(-1).