High yields of 2,3-butanediol and mannitol in Lactococcus lactis through engineering of NAD⁺ cofactor recycling

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.

Expanding sugar alcohol industry: Microbial production of sugar alcohols and associated chemocatalytic derivatives

Sugar alcohols are polyols that are widely employed in the production of chemicals, pharmaceuticals, and food products. Chemical synthesis of polyols, however, is complex and necessitates the use of hazardous compounds. Therefore, the use of microbes to produce polyols has been proposed as an alternative to traditional synthesis strategies. Many biotechnological approaches have been described to enhancing sugar alcohols production and microbe-mediated sugar alcohol production has the potential to benefit from the availability of inexpensive substrate inputs. Among of them, microbe-mediated erythritol production has been implemented in an industrial scale, but microbial growth and substrate conversion rates are often limited by harsh environmental conditions. In this review, we focused on xylitol, mannitol, sorbitol, and erythritol, the four representative sugar alcohols. The main metabolic engineering strategies, such as regulation of key genes and cofactor balancing, for improving the production of these sugar alcohols were reviewed. The feasible strategies to enhance the stress tolerance of chassis cells, especially thermotolerance, were also summarized. Different low-cost substrates like glycerol, molasses, cellulose hydrolysate, and CO₂ employed for producing these sugar alcohols were presented. Given the value of polyols as precursor platform chemicals that can be leveraged to produce a diverse array of chemical products, we not only discuss the challenges encountered in the above parts, but also envisioned the development of their derivatives for broadening the application of sugar alcohols.

Mannitol Production by Heterofermentative Lactic Acid Bacteria: a Review

Obesity, diabetes, and other cardiovascular diseases are directly related to the high consumption of processed sugars with high caloric content. The current food industry has novel trends related to replacing highly caloric sugars with non-caloric or low-calorie sweeteners. Mannitol, a polyol, represents a suitable substitute because it has a low caloric content and does not induce a glycemic response, which is crucial for diabetic people. Consequently, this polyol has multiple applications in the food, pharmaceutical, and medicine industries. Mannitol can be produced by plant extraction, chemical or enzymatic synthesis, or microbial fermentation. Different in vitro processes have been developed regarding enzymatic synthesis to obtain mannitol from fructose, glucose, or starch-derived substrates. Various microorganisms such as yeast, fungi, and bacteria are applied for microbial fermentation. Among them, heterofermentative lactic acid bacteria (LAB) represent a reliable and feasible alternative due to their metabolic characteristics. In this regard, the yield and productivity of mannitol depend on the culture system, the growing conditions, and the culture medium composition. In situ mannitol production represents a novel approach to decrease the sugar content in food and beverages. Also, genetic engineering offers an interesting option to obtain mannitol-producing strains. This review presents and discusses the most significant advances that have been made in the mannitol production through fermentation by heterofermentative LAB, including the pertinent and critical analysis of culture conditions considering broth composition, reaction systems, and their effects on productivities and yields.

Diverse classes of constraints enable broader applicability of a linear programming-based dynamic metabolic modeling framework

Current metabolic modeling tools suffer from a variety of limitations, from scalability to simplifying assumptions, that preclude their use in many applications. We recently created a modeling framework, Linear Kinetics-Dynamic Flux Balance Analysis (LK-DFBA), that addresses a key gap: capturing metabolite dynamics and regulation while retaining a potentially scalable linear programming structure. Key to this framework's success are the linear kinetics and regulatory constraints imposed on the system. However, while the linearity of these constraints reduces computational complexity, it may not accurately capture the behavior of many biochemical systems. Here, we developed three new classes of LK-DFBA constraints to better model interactions between metabolites and the reactions they regulate. We tested these new approaches on several synthetic and biological systems, and also performed the first-ever comparison of LK-DFBA predictions to experimental data. We found that no single constraint approach was optimal across all systems examined, and systems with the same topological structure but different parameters were often best modeled by different types of constraints. However, we did find that when genetic perturbations were implemented in the systems, the optimal constraint approach typically remained the same as for the wild-type regardless of the model topology or parameterization, indicating that just a single wild-type dataset could allow identification of the ideal constraint to enable model predictivity for a given system. These results suggest that the availability of multiple constraint approaches will allow LK-DFBA to model a wider range of metabolic systems.

Deciphering the Regulation of the Mannitol Operon Paves the Way for Efficient Production of Mannitol in Lactococcus lactis

Lactococcus lactis has great potential for high-yield production of mannitol, which has not yet been fully realized. In this study, we characterize how the mannitol genes in L. lactis are organized and regulated and use this information to establish efficient mannitol production. Although the organization of the mannitol genes in L. lactis was similar to that in other Gram-positive bacteria, mtlF and mtlD, encoding the enzyme IIA component (EIIA^mtl) of the mannitol phosphotransferase system (PTS) and the mannitol-1-phosphate dehydrogenase, respectively, were separated by a transcriptional terminator, and the mannitol genes were found to be organized in two transcriptional units: an operon comprising mtlA, encoding the enzyme IIBC component (EIIBC^mtl) of the mannitol PTS, mtlR, encoding a transcriptional activator, and mtlF, as well as a separately expressed mtlD gene. The promoters driving expression of the two transcriptional units were somewhat similar, and both contained predicted catabolite responsive element (cre) genes. The presence of carbon catabolite repression was demonstrated and was shown to be relieved in stationary-phase cells. The transcriptional activator MtlR (mtlR), in some Gram-positive bacteria, is repressed by phosphorylation by EIIA^mtl, and when we knocked out mtlF, we indeed observed enhanced expression from the two promoters, which indicated that this mechanism was in place. Finally, by overexpressing the mtlD gene and using stationary-phase cells as biocatalysts, we attained 10.1 g/liter mannitol with a 55% yield, which, to the best of our knowledge, is the highest titer ever reported for L. lactis. Summing up, the results of our study should be useful for improving the mannitol-producing capacity of this important industrial organism. IMPORTANCE Lactococcus lactis is the most studied species of the lactic acid bacteria, and it is widely used in various food fermentations. To date, there have been several attempts to persuade L. lactis to produce mannitol, a sugar alcohol with important therapeutic and food applications. Until now, to achieve mannitol production in L. lactis with significant titer and yield, it has been necessary to introduce and express foreign genes, which precludes the use of such strains in foods, due to their recombinant status. In this study, we systematically characterize how the mannitol genes in L. lactis are regulated and demonstrate how this impacts mannitol production capability. We harnessed this information and managed to establish efficient mannitol production without introducing foreign genes.

Lifestyle, metabolism and environmental adaptation in Lactococcus lactis

Lactococcus lactis serves as a paradigm organism for the lactic acid bacteria (LAB). Extensive research into the molecular biology, metabolism and physiology of several model strains of this species has been fundamental for our understanding of the LAB. Genomic studies have provided new insights into the species L. lactis, including the resolution of the genetic basis of its subspecies division, as well as the control mechanisms involved in the fine-tuning of growth rate and energy metabolism. In addition, it has enabled novel approaches to study lactococcal lifestyle adaptations to the dairy application environment, including its adjustment to near-zero growth rates that are particularly relevant in the context of cheese ripening. This review highlights various insights in these areas and exemplifies the strength of combining experimental evolution with functional genomics and bacterial physiology research to expand our fundamental understanding of the L. lactis lifestyle under different environmental conditions.

Recent advances in microbial biosynthesis of C3 - C5 diols: Genetics and process engineering approaches

Diols derived from renewable feedstocks have significant commercial interest in polymer, pharmaceutical, cosmetics, flavors and fragrances, food and feed industries. In C3-C5 diols biological processes of 1,3-propanediol, 1,2-propanediol and 2,3-butanediol have been commercialized as other isomers are non-natural metabolites and lack natural biosynthetic pathways. However, the developments in the field of systems and synthetic biology paved a new path to learn, build, construct, and test for efficient chassis strains. The current review addresses the recent advancements in metabolic engineering, construction of novel pathways, process developments aimed at enhancing in production of C3-C5 diols. The requisites on developing an efficient and sustainable commercial bioprocess for C3-C5 diols were also discussed.

From Waste to Taste-Efficient Production of the Butter Aroma Compound Acetoin from Low-Value Dairy Side Streams Using a Natural (Nonengineered) Lactococcus lactis Dairy Isolate

Lactococcus lactis subsp. lactis biovar diacetylactis is widely used in dairy fermentations as it can form the butter aroma compounds acetoin and diacetyl from citrate in milk. Here, we explore the possibility of producing acetoin from the more abundant lactose. Starting from a dairy isolate of L. lactis biovar diacetylactis, we obtained a series of mutants with low lactate dehydrogenase (ldh) activity. One isolate, RD1M5, only had a single insertion mutation in the ldh gene compared to its parental strain as revealed by whole genome resequencing. We tested the ability of RD1M5 to produce acetoin in milk. With aeration, all the lactose could be consumed, and the only product was acetoin. In a simulated cheese fermentation, a 50% increase in acetoin concentration could be achieved. RD1M5 turned out to be an excellent cell factory for acetoin and was able to convert lactose in dairy waste into acetoin with high titer (41 g/L) and high yield (above 90% of the theoretical yield). Summing up, RD1M5 was found to be highly robust and to grow excellently in milk or dairy waste. Being natural in origin opens up for applications within dairies as well as for safe production of food-grade acetoin from low-cost substrates.

Harnessing Adaptive Evolution to Achieve Superior Mannitol Production by Lactococcus lactis Using Its Native Metabolism

Mannitol can be obtained as a by-product of certain heterolactic lactic acid bacteria, when grown on substrates containing fructose. Lactococcus lactis, a homolactic lactic acid bacterium, normally does not form mannitol but can be persuaded into doing so by expressing certain foreign enzyme activities. In this study, we find that L. lactis has an inherent capacity to form mannitol from glucose. By adaptively evolving L. lactis or derivatives blocked in NAD⁺ regenerating pathways, we manage to accelerate growth on mannitol. When cells of the adapted strains are resuspended in buffer containing glucose, 4-58% of the glucose metabolized is converted into mannitol, in contrast to nonadapted strains. The highest conversion was obtained for a strain lacking all major NAD⁺ regenerating pathways. Mannitol had an inhibitory effect on the conversion, which we speculated was due to the mannitol uptake system. After its inactivation, 60% of the glucose was converted into mannitol by cells suspended in glucose buffer. Using a two-stage setup, where biomass first was accumulated by aerated culturing, followed by a nonaerated phase (static conditions), it was possible to obtain 6.1 g/L mannitol, where 60% of the glucose had been converted into mannitol, which is the highest yield reported for L. lactis.

Engineering Lactococcus lactis for Increased Vitamin K2 Production

Cheese produced with Lactococcus lactis is the main source of vitamin K2 in the Western diet. Subclinical vitamin K2 deficiency is common, calling for foods with enhanced vitamin K2 content. In this study we describe analyses of vitamin K2 (menaquinone) production in the lactic acid bacterium L. lactis ssp. cremoris strain MG1363. By cloning and expression from strong promoters we have identified genes and bottlenecks in the biosynthetic pathways leading to the long-chained menaquinones, MK-8 and MK-9. Key genes of the biosynthetic menaquinone pathway were overexpressed, singly or combined, to examine how vitamin K2 production can be enhanced. We observed that the production of the long menaquinone polyprenyl side chain, rather than production of the napthoate ring (1,4-dihydroxy-2-naphtoic acid), limits total menaquinone synthesis. Overexpression of genes causing increased ring formation (menF and menA) led to overproduction of short chained MK-3, while overexpression of other key genes (mvk and llmg_0196) resulted in enhanced full-length MK-9 production. Of two putatively annotated prenyl diphosphate synthases we pinpoint llmg_0196 (preA) to be important for menaquinone production in L. lactis. The genes mvk, preA, menF, and menA were found to be important contributors to menaquinone levels as single overexpression of these genes double and more than triple the total menaquinone content in culture. Combined overexpression of mvk, preA, and menA increased menaquinone levels to a higher level than obtained individually. When the overproducing strains were applied for milk fermentations vitamin K2 content was effectively increased 3-fold compared to the wild type. The results provide a foundation for development of strains to ferment foods with increased functional value i.e., higher vitamin K2 content.

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.

Suppression of lactate production in fed-batch culture of some lactic acid bacteria with sucrose as the carbon source

We report a method for suppression of lactate production by lactic acid bacteria (LAB) in culture. LAB produce lactate to regenerate NAD⁺ that is consumed during glycolysis. Glucose suppresses NAD⁺ regeneration pathways other than lactate dehydrogenase and non-glycolytic ATP production pathways. Therefore, the carbon source was changed to sucrose, and fed-batch culture was performed to limit the glycolytic flux and thus suppress lactate production. As a result, lactate productivity (i.e., the amount of lactate produced per amount of grown cell) in the sucrose/fed-batch culture was decreased compared to that in glucose/batch culture, in all five LAB strains examined. The productivity level decreased to 24% and 46% in Lactobacillus reuteri JCM 1112 and Lactococcus lactis JCM 7638, respectively. Metabolic flux analysis of Lactobacillus reuteri JCM 1112 revealed increased contributions of the mannitol production pathway to NAD⁺ regeneration and the arginine deiminase pathway to ATP production in the sucrose/fed-batch culture.

A review of polyols - biotechnological production, food applications, regulation, labeling and health effects

Food research is constantly searching for new ways to replace sugar. This is due to the negative connotations of sugar consumption on health which has driven consumer demand for healthier products and is reflected on a national level by the taxation of sugary beverages. Sugar alcohols, a class of polyols, are present in varying levels in many fruits and vegetables and are also added to foods as low calorific sweeteners. The most commonly used polyols in food include sorbitol, mannitol, xylitol, erythritol, maltitol, lactitol and isomalt. Of these, microorganisms can produce sorbitol, mannitol, xylitol and erythritol either naturally or through genetic engineering. Production of polyols by microbes has been the focus of a lot of research for its potential as an alternative to current industrial scale production by chemical synthesis but can also be used for in situ production of natural sweeteners in fermented products using microbes approved for use in foods. This review on the generation of these natural sweetening compounds by microorganisms examines the current understanding and methods of microbial production of polyols that are applicable in the food industry. The review also considers the health benefits and effects of polyol usage and discusses regulations which are applicable to polyol use.

Co-culture of Lactobacillus delbrueckii and engineered Lactococcus lactis enhances stoichiometric yield of D-lactic acid from whey permeate

D-Lactic acid (D-LA) is an enantiomer of lactic acid, which has a niche application in synthesis of poly-lactic acid based (PLA) polymer owing to its contribution to the thermo-stability of stereo-complex PLA polymer. Utilization of renewable substrates such as whey permeate is pivotal to economically viable production of D-LA. In present work, we have demonstrated D-LA production from whey permeate by Lactobacillus delbrueckii and engineered Lactococcus lactis. We observed that lactose fermentation by a monoculture of L. delbrueckii yields D-LA and galactose as major products. The highest yield of D-LA obtained was 0.48 g g^-1 when initial lactose concentration was 30 g L^-1. Initial lactose concentration beyond 20 g L^-1 resulted in accumulation of glucose and galactose, and hence, reduced the stoichiometric yield of D-LA. L. lactis naturally produces L-lactic acid (L-LA), so a mutant strain of L. lactis (L. lactis Δldh ΔldhB ΔldhX) was used to prevent L-LA production and engineer it for D-LA production. Heterologous over-expression of D-lactate dehydrogenase (ldhA) in the recombinant strain L. lactis TSG1 resulted in 0.67 g g^-1 and 0.44 g g^-1 of D-LA yield from lactose and galactose, respectively. Co-expression of galactose permease (galP) and α-phosphoglucomutase (pgmA) with ldhA in the recombinant strain L. lactis TSG3 achieved a D-LA yield of 0.92 g g^-1 from galactose. A co-culture batch process of L. delbrueckii and L. lactis TSG3 achieved an enhanced stoichiometric yield of 0.90 g g^-1 and ~45 g L^-1D-LA from whey permeate (lactose). This is the highest reported yield of D-LA from lactose substrate, and the titres can be improved further by a suitably designed fed-batch co-culture process.

Systems Biology - A Guide for Understanding and Developing Improved Strains of Lactic Acid Bacteria

Lactic Acid Bacteria (LAB) are extensively employed in the production of various fermented foods, due to their safe status, ability to affect texture and flavor and finally due to the beneficial effect they have on shelf-life. More recently, LAB have also gained interest as production hosts for various useful compounds, particularly compounds with sensitive applications, such as food ingredients and therapeutics. As for all industrial microorganisms, it is important to have a good understanding of the physiology and metabolism of LAB in order to fully exploit their potential, and for this purpose, many systems biology approaches are available. Systems metabolic engineering, an approach that combines optimization of metabolic enzymes/pathways at the systems level, synthetic biology as well as in silico model simulation, has been used to build microbial cell factories for production of biofuels, food ingredients and biochemicals. When developing LAB for use in foods, genetic engineering is in general not an accepted approach. An alternative is to screen mutant libraries for candidates with desirable traits using high-throughput screening technologies or to use adaptive laboratory evolution to select for mutants with special properties. In both cases, by using omics data and data-driven technologies to scrutinize these, it is possible to find the underlying cause for the desired attributes of such mutants. This review aims to describe how systems biology tools can be used for obtaining both engineered as well as non-engineered LAB with novel and desired properties.

Efficient production of 2,3-butanediol by recombinant Saccharomyces cerevisiae through modulation of gene expression by cocktail δ-integration

In this study, the expression of 4 genes encoding α-acetolactate synthase, α-acetolactate decarboxylase, 2,3-butanediol dehydrogenase, and NADH oxidase was modulated using a previously developed cocktail δ-integration strategy. The resultant strain, YPH499/dPdAdG/BD6-10, was used in a fed-batch cultivation for the production of 2,3-butanediol. The concentration, production rate, and yield obtained were 80.0g/L, 4.00g/L/h, and 41.7%, respectively. The production rate and yield of the compound obtained are higher for this strain compared to reports published for Saccharomyces cerevisiae so far. The cocktail δ-integration strategy allows for modulation of multiple gene expression, without the exact knowledge of rate-limiting steps, and therefore, could be used as a promising strategy for the production of bio-based chemicals in recombinant S. cerevisiae.

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.

Synthesis of (3R)-acetoin and 2,3-butanediol isomers by metabolically engineered Lactococcus lactis

The potential that lies in harnessing the chemical synthesis capabilities inherent in living organisms is immense. Here we demonstrate how the biosynthetic machinery of Lactococcus lactis, can be diverted to make (3R)-acetoin and the derived 2,3-butanediol isomers meso-(2,3)-butanediol (m-BDO) and (2R,3R)-butanediol (R-BDO). Efficient production of (3R)-acetoin was accomplished using a strain where the competing lactate, acetate and ethanol forming pathways had been blocked. By introducing different alcohol dehydrogenases into this strain, either EcBDH from Enterobacter cloacae or SadB from Achromobacter xylosooxidans, it was possible to achieve high-yield production of m-BDO or R-BDO respectively. To achieve biosustainable production of these chemicals from dairy waste, we transformed the above strains with the lactose plasmid pLP712. This enabled efficient production of (3R)-acetoin, m-BDO and R-BDO from processed whey waste, with titers of 27, 51, and 32 g/L respectively. The corresponding yields obtained were 0.42, 0.47 and 0.40 g/g lactose, which is 82%, 89%, and 76% of maximum theoretical yield respectively. These results clearly demonstrate that L. lactis is an excellent choice as a cell factory for transforming lactose containing dairy waste into value added chemicals.

Engineering Corynebacterium glutamicum for the production of 2,3-butanediol

BACKGROUND

2,3-Butanediol is an important bulk chemical with a wide range of applications. In bacteria, this metabolite is synthesised from pyruvate via a three-step pathway involving α-acetolactate synthase, α-acetolactate decarboxylase and 2,3-butanediol dehydrogenase. Thus far, the best producers of 2,3-butanediol are pathogenic strains, hence, the development of more suitable organisms for industrial scale fermentation is needed. Herein, 2,3-butanediol production was engineered in the Generally Regarded As Safe (GRAS) organism Corynebacterium glutamicum. A two-stage fermentation process was implemented: first, cells were grown aerobically on acetate; in the subsequent production stage cells were used to convert glucose into 2,3-butanediol under non-growing and oxygen-limiting conditions.

RESULTS

A gene cluster, encoding the 2,3-butanediol biosynthetic pathway of Lactococcus lactis, was assembled and expressed in background strains, C. glutamicum ΔldhA, C. glutamicum ΔaceEΔpqoΔldhA and C. glutamicum ΔaceEΔpqoΔldhAΔmdh, tailored to minimize pyruvate-consuming reactions, i.e., to prevent carbon loss in lactic, acetic and succinic acids. Producer strains were characterized in terms of activity of the relevant enzymes in the 2,3-butanediol forming pathway, growth, and production of 2,3-butanediol under oxygen-limited conditions. Productivity was maximized by manipulating the aeration rate in the production phase. The final strain, C. glutamicum ΔaceEΔpqoΔldhAΔmdh(pEKEx2-als,aldB,Ptuf butA), under optimized conditions produced 2,3-butanediol with a 0.66 mol mol(-1) yield on glucose, an overall productivity of 0.2 g L(-1) h(-1) and a titer of 6.3 g L(-1).

CONCLUSIONS

We have successfully developed C. glutamicum into an efficient cell factory for 2,3-butanediol production. The use of the engineered strains as a basis for production of acetoin, a widespread food flavour, is proposed.

Adaptation of Lactococcus lactis to high growth temperature leads to a dramatic increase in acidification rate

Lactococcus lactis is essential for most cheese making, and this mesophilic bacterium has its growth optimum around 30 °C. We have, through adaptive evolution, isolated a mutant TM29 that grows well up to 39 °C, and continuous growth at 40 °C is possible if pre-incubated at a slightly lower temperature. At the maximal permissive temperature for the wild-type, 38 °C, TM29 grows 33% faster and has a 12% higher specific lactate production rate than its parent MG1363, which results in fast lactate accumulation. Genome sequencing was used to reveal the mutations accumulated, most of which were shown to affect thermal tolerance. Of the mutations with more pronounced effects, two affected expression of single proteins (chaperone; riboflavin transporter), two had pleiotropic effects (RNA polymerase) which changed the gene expression profile, and one resulted in a change in the coding sequence of CDP-diglyceride synthase. A large deletion containing 10 genes was also found to affect thermal tolerance significantly. With this study we demonstrate a simple approach to obtain non-GMO derivatives of the important L. lactis that possess properties desirable by the industry, e.g. thermal robustness and increased rate of acidification. The mutations we have identified provide a genetic basis for further investigation of thermal tolerance.

Enhanced production of 2,3-butanediol from sugarcane molasses

2,3-Butanediol has been known as a platform green chemical, and the production cost is the key problem for its large-scale production in which the carbon source occupies a major part. Sugarcane molasses is a by-product of sugar industry and considered as a cheap carbon source for biorefinery. In this paper, the fermentation of 2,3-butanediol with sugarcane molasses was studied by reducing the medium ingredients and operation steps. The fermentation medium was optimized by response surface methodology, and 2,3-butanediol production was explored under the deficiency of sterilization, molasses acidification, and organic nitrogen source. Based on these experiments, the fermentation medium with sugarcane molasses as carbon source was simplified to five ingredients, and the steps of molasses acidification and medium sterilization were reduced; thus, the cost was reduced and the production of 2,3-butanediol was enhanced. Under fed-batch fermentation, 99.5 g/L of 2,3-butanediol and acetoin was obtained at 60 h with a yield of 0.39 g/g sugar.

Engineering of cofactor regeneration enhances (2S,3S)-2,3-butanediol production from diacetyl

(2S,3S)-2,3-Butanediol ((2S,3S)-2,3-BD) is a potentially valuable liquid fuel and an excellent building block in asymmetric synthesis. In this study, cofactor engineering was applied to improve the efficiency of (2S,3S)-2,3-BD production and simplify the product purification. Two NADH regeneration enzymes, glucose dehydrogenase and formate dehydrogenase (FDH), were introduced into Escherichia coli with 2,3-BD dehydrogenase, respectively. Introduction of FDH resulted in higher (2S,3S)-2,3-BD concentration, productivity and yield from diacetyl, and large increase in the intracellular NADH concentration. In fed-batch bioconversion, the final titer, productivity and yield of (2S,3S)-2,3-BD on diacetyl reached 31.7 g/L, 2.3 g/(L·h) and 89.8%, the highest level of (2S,3S)-2,3-BD production thus far. Moreover, cosubstrate formate was almost totally converted to carbon dioxide and no organic acids were produced. The biocatalytic process presented should be a promising route for biotechnological production of NADH-dependent microbial metabolites.

A bacteriocin gene cluster able to enhance plasmid maintenance in Lactococcus lactis

BACKGROUND

Lactococcus lactis is widely used as a dairy starter and has been extensively studied. Based on the acquired knowledge on its physiology and metabolism, new applications have been envisaged and there is an increasing interest of using L. lactis as a cell factory. Plasmids constitute the main toolbox for L. lactis genetic engineering and most rely on antibiotic resistant markers for plasmid selection and maintenance. In this work, we have assessed the ability of the bacteriocin Lactococcin 972 (Lcn972) gene cluster to behave as a food-grade post-segregational killing system to stabilize recombinant plasmids in L. lactis in the absence of antibiotics. Lcn972 is a non-lantibiotic bacteriocin encoded by the 11-kbp plasmid pBL1 with a potent antimicrobial activity against Lactococcus.

RESULTS

Attempts to clone the full lcn972 operon with its own promoter (P972), the structural gene lcn972 and the immunity genes orf2-orf3 in the unstable plasmid pIL252 failed and only plasmids with a mutated promoter were recovered. Alternatively, cloning under other constitutive promoters was approached and achieved, but bacteriocin production levels were lower than those provided by pBL1. Segregational stability studies revealed that the recombinant plasmids that yielded high bacteriocin titers were maintained for at least 200 generations without antibiotic selection. In the case of expression vectors such as pTRL1, the Lcn972 gene cluster also contributed to plasmid maintenance without compromising the production of the fluorescent mCherry protein. Furthermore, unstable Lcn972 recombinant plasmids became integrated into the chromosome through the activity of insertion sequences, supporting the notion that Lcn972 does apply a strong selective pressure against susceptible cells. Despite of it, the Lcn972 gene cluster was not enough to avoid the use of antibiotics to select plasmid-bearing cells right after transformation.

CONCLUSIONS

Inserting the Lcn972 cluster into segregational unstable plasmids prevents their lost by segregation and probable could be applied as an alternative to the use of antibiotics to support safer and more sustainable biotechnological applications of genetically engineered L. lactis.

Effect of the inactivation of lactate dehydrogenase, ethanol dehydrogenase, and phosphotransacetylase on 2,3-butanediol production in Klebsiella pneumoniae strain

BACKGROUND

2,3-Butanediol (2,3-BD) is a high-value chemical usually produced petrochemically but which can also be synthesized by some bacteria. To date, Klebsiella pneumoniae is the most powerful 2,3-BD producer which can utilize a wide range of substrates. However, many by-products are also produced by K. pneumoniae, such as ethanol, lactate, and acetate, which negatively regulate the 2,3-BD yield and increase the costs of downstream separation and purification.

RESULTS

In this study, we constructed K. pneumoniae mutants with lactate dehydrogenase (LDH), acetaldehyde dehydrogenase (ADH), and phosphotransacetylase (PTA) deletion individually by suicide vector conjugation.These mutants showed different behavior of production formation. Knock out of ldhA had little influence on the yield of 2,3-BD, whereas knock out of adhE or pta significantly improved the formation of 2,3-BD. The accumulation of the intermediate of 2,3-BD biosynthesis, acetoin, was decreased in all the mutants. The mutants were then tested in five different carbon sources and increased 2,3-BD was observed. Also a double mutant strain with deletion of adhE and ldhA was constructed which resulted in accelerated fermentation and higher 2,3-BD production. In fed-batch culture this strain achieved more than 100 g/L 2,3-BD from glucose with a relatively high yield of 0.49 g/g.

CONCLUSION

2,3-BD production was dramatically improved with the inactivation of adhE and pta. The inactivation of ldhA could advance faster cell growth and shorter fermentation time. The double mutant strain with deletion of adhE and ldhA resulted in accelerated fermentation and higher 2,3-BD production. These results provide new insights for industrial production of 2,3-BD by K. pneumoniae.

The rebalanced pathway significantly enhances acetoin production by disruption of acetoin reductase gene and moderate-expression of a new water-forming NADH oxidase in Bacillus subtilis

Bacillus subtilis produces acetoin as a major extracellular product. However, the by-products of 2,3-butanediol, lactic acid and ethanol were accompanied in the NADH-dependent pathways. In this work, metabolic engineering strategies were proposed to redistribute the carbon flux to acetoin by manipulation the NADH levels. We first knocked out the acetoin reductase gene bdhA to block the main flux from acetoin to 2,3-butanediol. Then, among four putative candidates, we successfully screened an active water-forming NADH oxidase, YODC. Moderate-expression of YODC in the bdhA disrupted B. subtilis weakened the NADH-linked pathways to by-product pools of acetoin. Through these strategies, acetoin production was improved to 56.7g/l with an increase of 35.3%, while the production of 2,3-butanediol, lactic acid and ethanol were decreased by 92.3%, 70.1% and 75.0%, respectively, simultaneously the fermentation duration was decreased 1.7-fold. Acetoin productivity by B. subtilis was improved to 0.639g/(lh).

An extended dynamic model of Lactococcus lactis metabolism for mannitol and 2,3-butanediol production

Biomedical research and biotechnological production are greatly benefiting from the results provided by the development of dynamic models of microbial metabolism. Although several kinetic models of Lactococcus lactis (a Lactic Acid Bacterium (LAB) commonly used in the dairy industry) have been developed so far, most of them are simplified and focus only on specific metabolic pathways. Therefore, the application of mathematical models in the design of an engineering strategy for the production of industrially important products by L. lactis has been very limited. In this work, we extend the existing kinetic model of L. lactis central metabolism to include industrially relevant production pathways such as mannitol and 2,3-butanediol. In this way, we expect to study the dynamics of metabolite production and make predictive simulations in L. lactis. We used a system of ordinary differential equations (ODEs) with approximate Michaelis-Menten-like kinetics for each reaction, where the parameters were estimated from multivariate time-series metabolite concentrations obtained by our team through in vivo Nuclear Magnetic Resonance (NMR). The results show that the model captures observed transient dynamics when validated under a wide range of experimental conditions. Furthermore, we analyzed the model using global perturbations, which corroborate experimental evidence about metabolic responses upon enzymatic changes. These include that mannitol production is very sensitive to lactate dehydrogenase (LDH) in the wild type (W.T.) strain, and to mannitol phosphoenolpyruvate: a phosphotransferase system (PTS(Mtl)) in a LDH mutant strain. LDH reduction has also a positive control on 2,3-butanediol levels. Furthermore, it was found that overproduction of mannitol-1-phosphate dehydrogenase (MPD) in a LDH/PTS(Mtl) deficient strain can increase the mannitol levels. The results show that this model has prediction capability over new experimental conditions and offers promising possibilities to elucidate the effect of alterations in the main metabolism of L. lactis, with application in strain optimization.

Moderate expression of the transcriptional regulator ALsR enhances acetoin production by Bacillus subtilis

Acetoin, a major extracellular catabolic product of Bacillus subtilis cultured on glucose, is widely used to add flavor to food and also serves as a precursor for chemical synthesis. The biosynthesis of acetoin from pyruvate requires the enzymes α-acetolactate synthase (ALS) and α-acetolactate decarboxylase (ALDC), both of which are encoded by the alsSD operon. The transcriptional regulator ALsR is essential for the expression of alsSD. Here we focused on enhancing the production of acetoin by B. subtilis using different promoters to express ALsR. The expression of reporter genes was much higher under the control of the HpaII promoter than under control of the PbdhA promoter. Although the HpaII promoter highly enhanced transcription of the alsSD operon through overexpression of ALsR, the production of acetoin was not significantly increased. In contrast, moderate enhancement of ALsR expression using the PbdhA promoter significantly improved acetoin production. Compared with the wild-type, the enzyme activities of ALS and ALDC in B. subtilis harboring PbdhA were increased by approximately twofold, and the molar yield of acetoin from glucose was improved by 62.9 % in shake flask fermentation. In a 5-L fermentor, the engineered B. subtilis ultimately yielded 41.5 g/L of acetoin. Based on these results, we conclude that enhanced expression of ALDC and ALS by moderately elevated expression of the transcriptional regulator ALsR could increase acetoin production in recombinant B. subtilis.

NAD-dependent lactate dehydrogenase catalyses the first step in respiratory utilization of lactate by Lactococcus lactis

Lactococcus lactis can undergo respiration when hemin is added to an aerobic culture. The most distinctive feature of lactococcal respiration is that lactate could be consumed in the stationary phase concomitantly with the rapid accumulation of diacetyl and acetoin. However, the enzyme responsible for lactate utilization in this process has not yet been identified. As genes for fermentative NAD-dependent l-lactate dehydrogenase (l-nLDH) and potential electron transport chain (ETC)-related NAD-independent l-LDH (l-iLDH) exist in L. lactis, the activities of these enzymes were measured in this study using crude cell extracts prepared from respiratory and fermentation cultures. Further studies were conducted with purified preparations of recombinant LDH homologous proteins. The results showed that l-iLDH activity was hardly detected in both crude cell extracts and purified l-iLDH homologous protein while l-nLDH activity was very significant. This suggested that l-iLDHs were inactive in lactate utilization. The results of kinetic analyses and the effects of activator, inhibitor, substrate and product concentrations on the reaction equilibrium showed that l-nLDH was much more prone to catalyze the pyruvate reduction reaction but could reverse its role provided that the concentrations of NADH and pyruvate were extremely low while NAD and lactate were abundant. Metabolite analysis in respiratory culture revealed that the cellular status in the stationary phase was beneficial for l-nLDH to catalyze lactate oxidation. The factors accounting for the respiration- and stationary phase-dependent lactate utilization in L. lactis are discussed here.

•

LutABC proteins do not participate in lactate oxidation in Lactococcus lactis

•

Lactococcus lactis has very low NAD-independent lactate dehydrogenase activity

•

Fructose-1,6-bisphosphate-dependent lactate dehydrogenase can work in reverse in vivo

•

Metabolite concentrations in the stationary phase are favorable for lactate oxidation

•

Respiratory metabolism is the basis for continual lactate oxidation in Lactococcus

Biotechnological and in situ food production of polyols by lactic acid bacteria

Polyols such as mannitol, erythritol, sorbitol, and xylitol are naturally found in fruits and vegetables and are produced by certain bacteria, fungi, yeasts, and algae. These sugar alcohols are widely used in food and pharmaceutical industries and in medicine because of their interesting physicochemical properties. In the food industry, polyols are employed as natural sweeteners applicable in light and diabetic food products. In the last decade, biotechnological production of polyols by lactic acid bacteria (LAB) has been investigated as an alternative to their current industrial production. While heterofermentative LAB may naturally produce mannitol and erythritol under certain culture conditions, sorbitol and xylitol have been only synthesized through metabolic engineering processes. This review deals with the spontaneous formation of mannitol and erythritol in fermented foods and their biotechnological production by heterofermentative LAB and briefly presented the metabolic engineering processes applied for polyol formation.

Inhibition of acetate accumulation leads to enhanced production of (R,R)-2,3-butanediol from glycerol in Escherichia coli

This work describes the production of (R,R)-2,3-butanediol in Escherichia coli using glycerol by metabolic engineering approaches. The introduction of a synthetic pathway converting pyruvate to (R,R)-2,3-butanediol into wild-type E. coli strain BW25113 led to the production of (R,R)-2,3-butanediol at a titer of 3.54 g/l and a yield of 0.131 g product/g glycerol (26.7 % of theoretical maximum) with acetate (around 3.00 g/l) as the dominant by-product. We therefore evaluated the impacts of deleting the genes ackA or/and poxB that are responsible for the major by-product, acetate. This increased production of (R,R)-2,3-butanediol to 9.54 g/l with a yield of 0.333 g product/g glycerol (68.0 % of theoretical maximum) in shake flask studies. The utilization of low-priced crude glycerol to produce value-added chemicals is of great significance to the economic viability of the biodiesel industry.

Enhanced acetoin production by Serratia marcescens H32 with expression of a water-forming NADH oxidase

Cofactor engineering was employed to enhance production of acetoin by Serratia marcescens H32. 2,3-Butanediol was a major byproduct of acetoin fermentation by S. marcescens H32. In order to decrease 2,3-butanediol formation and achieve a high efficiency of acetoin production, nox gene encoding a water-forming NADH oxidase from Lactobacillus brevis was expressed. Batch fermentations suggested the expression of the NADH oxidase could increase the intracellular NAD(+) concentration (1.5-fold) and NAD(+)/NADH ratio (2.9-fold). Meanwhile, 2,3-butanediol was significantly decreased (52%), and the accumulation of acetoin was enhanced (33%) accordingly. By fed-batch culture of the engineered strain, the final acetoin titer up to 75.2g/l with the productivity of 1.88 g/(lh) was obtained. To the best of our knowledge, these results were new records on acetoin fermentation ever reported.