Nutritional requirements and media development for Lactococcus lactis IL1403

The sensory evaluation and antimicrobial efficacy of Lactobacillus acidophilus supernatant on Salmonella enteritidis in milk

Postbiotics are metabolites derived from living probiotic bacteria like Lactobacillus strains, during the fermentation process and/or produced in pure form on laboratory scales. These compounds, depending on the type of probiotic from which they are prepared, have specific antibacterial agents such as: organic acids, bacteriocins, short-chain fatty acids, and peptides. The objective of this study was to investigate the effect of Lactobacillus acidophilus supernatant (LAS) on the growth pattern of Salmonella enteritidis at fluctuating temperatures and the sensory evaluation of milk that contains this probiotic. Baranyi and Roberts's model determined the best-fit curve for the microbial growth. According to mathematical equations, the highest and lowest specific growth (μ max) rates of S. enteritidis were obtained at 0.055 h-1 and 0.0059 h-1 and also highest and lowest maximum generation time (MGT) values were obtained at 20.06 h and 8.85 h, respectively. Sensory evaluation by the Triangel test reveals that LAS could not establish a significant (p > .05) adverse effect on milk perceptible. Regarding the results obtained in the present study, LAS, without causing adverse sensory change, could act as a safe food additive for the control of bacterial pathogens and reducing food waste, particularly in milk and milk-containing food products.

Elucidating the Mechanisms of Cell-to-Cell Crosstalk in Probiotics Co-culture: A Proteomics Study of Limosilactobacillus reuteri ZJ625 and Ligilactobacillus salivarius ZJ614

Limosilactobacillus reuteri ZJ625 and Ligilactobacillus salivarius ZJ614 are potential probiotic bacteria with improved benefits when administered to the host as a multi-strain preparation. To elucidate the mechanisms of cell-to-cell crosstalk between these two strains, we studied their intracellular and extracellular proteomes in co-culture by liquid-chromatography mass-spectrometry (LC-MS) using Dionex Nano-RSLC and fusion mass spectrometer. The experiment consisted of five biological replicates, and samples were collected during the mid-exponential growth phase. The quantitative proteomic profiles revealed several differentially expressed proteins (DEPs), which are down- or up-regulated between and within groups for both the intracellular and extracellular proteomes. These DEPs include proteins synthesising autoinducer-2, a sensor compound for cell-to-cell bacterial crosstalk during quorum sensing in mixed culture. Other important DEPs identified include enolase, phosphoglycerate kinase, and l-lactate dehydrogenase, which play roles in carbohydrate metabolism. Proteins associated with transcription, ATP production and transport across the membrane, DNA repair, and those with the potential to bind to the host epithelium were also identified. The post-translational modifications associated with the proteins include oxidation, deamidation, and ammonia loss. Importantly, this study revealed a significant expression of S-ribosylhomocysteine lyase (luxS) involved in synthesising autoinducer-2 that plays important roles in quorum sensing, aiding bacterial cell-to-cell crosstalk in co-cultures. The proteome of L. salivarius ZJ614 was most affected when co-cultured with L. reuteri ZJ625. In contrast, omitting some medium components from the defined medium exerted more effects on L. reuteri ZJ625 than L. salivarius ZJ614.

Formulation of Chemically Defined Media and Growth Evaluation of Ligilactobacillus salivarius ZJ614 and Limosilactobacillus reuteri ZJ625

Lactic acid bacteria are increasingly becoming important dietary supplements due to their health benefits when consumed in adequate quantity. The increasing attention on these important microbes has necessitated an in-depth understanding of their physiological processes, such as nutritional requirements and growth patterns, to better harness their probiotic potentials. This study was carried out to determine the nutritional requirements for the growth of L. salivarius ZJ614 and L. reuteri ZJ625 from a chemically defined medium and evaluate growth kinetics by fitting different sigmoidal growth models. The complete CDM contains 49 nutritional ingredients such as glucose, Tween 80^®, mineral salts, buffers, amino acids, vitamins, and nucleotides at defined concentrations. In addition, the minimal nutritional requirements of the isolates were determined in a series of single-omission experiments (SOEs) to compose the MDM. Growth curve data were generated by culturing in an automated 96-well micro-plate reader at 37°C for 36 h, and photometric readings (optical density: OD₆₀₀) were taken. The data were summarized in tables and charts using Microsoft Excel, while growth evaluation was carried out using open-source software (Curveball) on Python. The results revealed that omission of the amino acids, vitamins, and nucleotides groups resulted in 2.0, 20.17, and 60.24% (for L. salivarius ZJ614) and 0.95, 42.7, and 70.5% (for L. reuteri ZJ625) relative growths, respectively. Elimination of the individual CDM components also indicates varying levels of growth by the strains. The growth curve data revealed LogisticLag2 and Baranyi–Roberts models as the best fits for L. reuteri ZJ625 and L. salivarius ZJ614, respectively. All the strains showed appreciable growth on the CDM and MDM as observed in de Man–Rogosa–Sharpe (MRS) broth. We also described the growth kinetics of L. reuteri ZJ625 and L. salivarius ZJ614 in the CDM, and the best models revealed the estimated growth parameters.

Positive Interactions Between Lactic Acid Bacteria Could Be Mediated by Peptides Containing Branched-Chain Amino Acids

Lactic acid bacteria (LAB) are responsible for the sanitary, organoleptic, and health properties of most fermented products. Positive interactions between pairs of LAB strains, based on nitrogen dependencies, were previously demonstrated. In a chemically defined medium, using milk and lupin proteins as sole nitrogen source, two proteolytic strains were able to sustain the growth of non-proteolytic strains, but one did not. The objective of the present study was, thus, to determine which specific peptides were implicated in the positive interactions observed. Peptides produced and involved in the bacterial interactions were quantified using tandem mass spectrometry (LC-MS/MS). About 2,000 different oligopeptides ranging from 6 to more than 50 amino acids in length were identified during the time-course of the experiment. We performed a clustering approach to decipher the differences in peptide production during fermentation by the three proteolytic strains tested. We also performed sequence alignments on parental proteins and identified the cleavage site profiles of the three bacterial strains. Then, we characterized the peptides that were used by the non-proteolytic strains in monocultures. Hydrophobic and branched-chain amino acids within peptides were identified as essential in the interactions. Ultimately, better understanding how LAB can positively interact could be useful in multiple food-related fields, e.g., production of fermented food products with enhanced functional properties, or fermentation of new food matrices.

Positive Interactions between Lactic Acid Bacteria Promoted by Nitrogen-Based Nutritional Dependencies

Nutritional dependencies, especially those regarding nitrogen sources, govern numerous microbial positive interactions. As for lactic acid bacteria (LAB), responsible for the sanitary, organoleptic, and health properties of most fermented products, such positive interactions have previously been studied between yogurt bacteria. However, they have never been exploited to create artificial cocultures of LAB that would not necessarily coexist naturally, i.e., from different origins. The objective of this study was to promote LAB positive interactions, based on nitrogen dependencies in cocultures, and to investigate how these interactions affect some functional outputs, e.g., acidification rates, carbohydrate consumption, and volatile-compound production. The strategy was to exploit both proteolytic activities and amino acid auxotrophies of LAB. A chemically defined medium was thus developed to specifically allow the growth of six strains used, three proteolytic and three nonproteolytic. Each of the proteolytic strains, Enterococcus faecalis CIRM-BIA2412, Lactococcus lactis NCDO2125, and CIRM-BIA244, was cocultured with each one of the nonproteolytic LAB strains, L. lactis NCDO2111 and Lactiplantibacillus plantarum CIRM-BIA465 and CIRM-BIA1524. Bacterial growth was monitored using compartmented chambers to compare growth in mono- and cocultures. Acidification, carbohydrate consumption, and volatile-compound production were evaluated in direct cocultures. Each proteolytic strain induced different types of interactions: strongly positive interactions, weakly positive interactions, and no interactions were seen with E. faecalis CIRM-BIA2412, L. lactis NCDO2125, and L. lactis CIRM-BIA244, respectively. Strong interactions were associated with higher concentrations of tryptophan, valine, phenylalanine, leucine, isoleucine, and peptides. They led to higher acidification rates, lower pH, higher raffinose utilization, and higher concentrations of five volatile compounds. IMPORTANCE Interactions of lactic acid bacteria (LAB) are often studied in association with yeasts or propionibacteria in various fermented food products, and the mechanisms underlying their interactions are being quite well characterized. Concerning interactions between LAB, they have mainly been investigated to test antagonistic interactions. Understanding how they can positively interact could be useful in multiple food-related fields: production of fermented food products with enhanced functional properties or fermentation of new food matrices. This study investigated the exploitation of the proteolytic activity of LAB strains to promote positive interactions between proteolytic and nonproteolytic strains. The results suggest that proteolytic LAB do not equally stimulate nonproteolytic LAB and that the stronger the interactions between LAB are, the more functional outputs we can expect. Thus, this study gives insight into how to create new associations of LAB strains and to guarantee their positive interactions.

Nutrient consumption patterns of Lactobacillus plantarum and their application in suancai

The purpose of the present study was to control the fermentation time and nitrite content of suancai prepared with Lactobacillus plantarum. According to analyses of the consumption amount and rate of nutrients, growth-stimulating nutrients, essential nutrients and nutrients accelerating the fermentation process of suancai, Asp, Thr, Glu, Cys, Tyr, Mg²⁺, Mn²⁺ and inosine were selected as additions to suancai prepared with L. plantarum. The fermentation time and nitrite content of suancai supplemented with nutrients and prepared with L. plantarum were shortened by 2 days and 5 days and reduced by approximately 0.1-fold and 0.7-fold, respectively, compared with unsupplemented suancai prepared with L. plantarum at 25 °C and 10 °C. The fermentation time and nitrite content of suancai supplemented with nutrients and prepared with L. plantarum were shortened by 6 days and 15 days and reduced by approximately 0.17-fold and 0.8-fold, respectively, compared with suancai undergoing spontaneous fermentation at 25 °C and 10 °C. Furthermore, no significant differences were observed in sensory properties in suancai. The results of this study indicated that certain nutrients accelerated the growth of L. plantarum and reduced the fermentation time and nitrite content of suancai prepared with L. plantarum. These findings help to establish a foundation for the practical use of nutrients to control the fermentation of suancai.

Evaluation of the Estimation Capability of Response Surface Methodology and Artificial Neural Network for the Optimization of Bacteriocin-Like Inhibitory Substances Production by Lactococcus lactis Gh1

Bacteriocin-like inhibitory substances (BLIS) produced by Lactococcus lactis Gh1 had shown antimicrobial activity against Listeria monocytogenes ATCC 15313. Brain Heart Infusion (BHI) broth is used for the cultivation and enumeration of lactic acid bacteria, but there is a need to improve the current medium composition for enhancement of BLIS production, and one of the approaches is to model the optimization process and identify the most appropriate medium formulation. Response surface methodology (RSM) and artificial neural network (ANN) models were employed in this study. In medium optimization, ANN (R² = 0.98) methodology provided better estimation point and data fitting as compared to RSM (R² = 0.79). In ANN, the optimal medium consisted of 35.38 g/L soytone, 16 g/L fructose, 3.25 g/L sodium chloride (NaCl) and 5.40 g/L disodium phosphate (Na₂HPO₄). BLIS production in optimal medium (717.13 ± 0.76 AU/mL) was about 1.40-fold higher than that obtained in nonoptimised (520.56 ± 3.37 AU/mL) medium. BLIS production was further improved by about 1.18 times higher in 2 L stirred tank bioreactor (787.40 ± 1.30 AU/mL) as compared to that obtained in 250 mL shake flask (665.28 ± 14.22 AU/mL) using the optimised medium.

Revealing nutritional requirements of MICP-relevant Sporosarcina pasteurii DSM33 for growth improvement in chemically defined and complex media

Microbial induced calcite precipitation (MICP) based on ureolysis has a high potential for many applications, e.g. restoration of construction materials. The gram-positive bacterium Sporosarcina pasteurii is the most commonly used microorganism for MICP due to its high ureolytic activity. However, Sporosarcina pasteurii is so far cultivated almost exclusively in complex media, which only results in moderate biomass concentrations at the best. Cultivation of Sporosarcina pasteurii must be strongly improved in order to make technological application of MICP economically feasible. The growth of Sporosarcina pasteurii DSM 33 was boosted by detecting auxotrophic deficiencies (L-methionine, L-cysteine, thiamine, nicotinic acid), nutritional requirements (phosphate, trace elements) and useful carbon sources (glucose, maltose, lactose, fructose, sucrose, acetate, L-proline, L-alanine). These were determined by microplate cultivations with online monitoring of biomass in a chemically defined medium and systematically omitting or substituting medium components. Persisting growth limitations were also detected, allowing further improvement of the chemically defined medium by the addition of glutamate group amino acids. Common complex media based on peptone and yeast extract were supplemented based on these findings. Optical density at the end of each cultivation of the improved peptone and yeast extract media roughly increased fivefold respectively. A maximum OD600 of 26.6 ± 0.7 (CDW: 17.1 ± 0.5 g/L) was reached with the improved yeast extract medium. Finally, culture performance and media improvement was analysed by measuring the oxygen transfer rate as well as the backscatter during shake flask cultivation.

The nutrient requirements of Lactobacillus acidophilus LA-5 and their application to fermented milk

Lactobacillus acidophilus LA-5 is a suitable probiotic for food application, but because of its slow growth in milk, an increase in its efficiency is desired. To shorten the time required for fermentation, the nutrient requirements of L. acidophilus LA-5 were analyzed, including the patterns of consumption of amino acids, purines, pyrimidines, vitamins, and metal ions. The nutrients required by L. acidophilus LA-5 were Asn, Asp, Cys, Leu, Met, riboflavin, guanine, uracil, and Mn²⁺, and when they were added to milk, the fermentation time of fermented milk prepared by L. acidophilus LA-5 alone was shortened by 9 h, with high viable cell counts that were maintained during storage of nutrient-supplemented fermented milk compared with the control. For fermented milk prepared by fermentation with Streptococcus thermophilus, Lactobacillus delbrueckii ssp. bulgaricus, and L. acidophilus LA-5, viable cell counts of L. acidophilus LA-5 increased 1.3-fold and were maintained during storage of nutrient-supplemented fermented milk compared with the control. Adding nutrients had no negative effect on the quality of the fermented milk. The results indicated that suitable nutrients enhanced the growth of L. acidophilus LA-5 and increased its viable cell counts in fermented milk prepared by L. acidophilus LA-5 alone and mixed starter culture, respectively.

Dynamic co-culture metabolic models reveal the fermentation dynamics, metabolic capacities and interplays of cheese starter cultures

In this study, we have investigated the cheese starter culture as a microbial community through a question: can the metabolic behaviour of a co-culture be explained by the characterized individual organism that constituted the co-culture? To address this question, the dairy-origin lactic acid bacteria Lactococcus lactis subsp. cremoris, Lactococcus lactis subsp. lactis, Streptococcus thermophilus and Leuconostoc mesenteroides, commonly used in cheese starter cultures, were grown in pure and four different co-cultures. We used a dynamic metabolic modelling approach based on the integration of the genome-scale metabolic networks of the involved organisms to simulate the co-cultures. The strain-specific kinetic parameters of dynamic models were estimated using the pure culture experiments and they were subsequently applied to co-culture models. Biomass, carbon source, lactic acid and most of the amino acid concentration profiles simulated by the co-culture models fit closely to the experimental results and the co-culture models explained the mechanisms behind the dynamic microbial abundance. We then applied the co-culture models to estimate further information on the co-cultures that could not be obtained by the experimental method used. This includes estimation of the profile of various metabolites in the co-culture medium such as flavour compounds produced and the individual organism level metabolic exchange flux profiles, which revealed the potential metabolic interactions between organisms in the co-cultures.

Understanding the Mechanisms of Positive Microbial Interactions That Benefit Lactic Acid Bacteria Co-cultures

Microorganisms grow in concert, both in natural communities and in artificial or synthetic co-cultures. Positive interactions between associated microbes are paramount to achieve improved substrate conversion and process performance in biotransformation and fermented food production. The mechanisms underlying such positive interactions have been the focus of numerous studies in recent decades and are now starting to be well characterized. Lactic acid bacteria (LAB) contribute to the final organoleptic, nutritional, and health properties of fermented food products. However, interactions in LAB co-cultures have been little studied, apart from the well-characterized LAB co-culture used for yogurt manufacture. LAB are, however, multifunctional microorganisms that display considerable potential to create positive interactions between them. This review describes why LAB co-cultures are of such interest, particularly in foods, and how their extensive nutritional requirements can be used to favor positive interactions. In that respect, our review highlights the benefits of co-cultures in different areas of application, details the mechanisms underlying positive interactions and aims to show how mechanisms based on nutritional interactions can be exploited to create efficient LAB co-cultures.

Partial characterization and purification of phytase from Lactobacillus plantarum CRL1964 isolated from pseudocereals

Cereals and pseudocereals are a rich source of nutrients and trace elements, but their dietary bioavailability is low due to the presence of phytate (IP6), an antinutritional compound with the ability to chelate cations and proteins. Phytase is an enzyme that catalyzes the hydrolysis of IP6 and it is used as an additive improving the nutritional quality of grain-based foods. The aim of this study was to select lactic acid bacteria (LAB) isolated from pseudocereals with phytase activity, characterize their production and activity, and purify the enzyme. LAB strains isolated from grains and spontaneous sourdough of quinoa and amaranth were grown in the Man Rogosa and Sharpe medium where the inorganic phosphate (Pi) was replaced by 1% of IP6. Phytase activity was determined by measuring the Pi released from IP6. Phytase of Lactobacillus (L.) plantarum CRL1964 (PhyLP) showed the highest specific activity from 73 LAB evaluated. IP6 induces PhyLP production, which is at its maximum at the end of the exponential phase. PhyLP was thermostable and maintained its activity under acidic conditions. The enzymatic activity is stimulated by ethylenediaminetetraacetic acid, Co²⁺, and ascorbic acid. PhyLP was partially purified and showed a molecular mass of 55 kDa. L. plantarum CRL1964 and/or PhyLP have the potential to be included in the processing of cereal/pseudocereals based products for animal feed and/or the food industry improving its nutritional value.

Elucidation of auxotrophic deficiencies of Bacillus pumilus DSM 18097 to develop a defined minimal medium

Background

Culture media containing complex compounds like yeast extract or peptone show numerous disadvantages. The chemical composition of the complex compounds is prone to significant variations from batch to batch and quality control is difficult. Therefore, the use of chemically defined media receives more and more attention in commercial fermentations. This concept results in better reproducibility, it simplifies downstream processing of secreted products and enable rapid scale-up. Culturing bacteria with unknown auxotrophies in chemically defined media is challenging and often not possible without an extensive trial-and-error approach. In this study, a respiration activity monitoring system for shake flasks and its recent version for microtiter plates were used to clarify unknown auxotrophic deficiencies in the model organism Bacillus pumilus DSM 18097.

Results

Bacillus pumilus DSM 18097 was unable to grow in a mineral medium without the addition of complex compounds. Therefore, a rich chemically defined minimal medium was tested containing basically all vitamins, amino acids and nucleobases, which are essential ingredients of complex components. The strain was successfully cultivated in this medium. By monitoring of the respiration activity, nutrients were supplemented to and omitted from the rich chemically defined medium in a rational way, thus enabling a systematic and fast determination of the auxotrophic deficiencies. Experiments have shown that the investigated strain requires amino acids, especially cysteine or histidine and the vitamin biotin for growth.

Conclusions

The introduced method allows an efficient and rapid identification of unknown auxotrophic deficiencies and can be used to develop a simple chemically defined tailor-made medium. B. pumilus DSM 18097 was chosen as a model organism to demonstrate the method. However, the method is generally suitable for a wide range of microorganisms. By combining a systematic combinatorial approach based on monitoring the respiration activity with cultivation in microtiter plates, high throughput experiments with high information content can be conducted. This approach facilitates media development, strain characterization and cultivation of fastidious microorganisms in chemically defined minimal media while simultaneously reducing the experimental effort.

Electronic supplementary material

The online version of this article (10.1186/s12934-018-0956-1) contains supplementary material, which is available to authorized users.

Two-step production of anti-inflammatory soluble factor by Lactobacillus reuteri CRL 1098

We have demonstrated previously that a soluble factor (LrS) produced by Lactobacillus (L.) reuteri CRL 1098 modulates the inflammatory response triggered by lipopolysaccharide. In this study, the production of LrS by L. reuteri CRL 1098 was realized through two steps: i) bacterial biomass production, ii) LrS production, where the bacterial biomass was able to live but did not proliferate. Therefore, the simultaneous evaluation of the effect of different factors on the growth and LrS production was performed. Biomass production was found to be dependent mainly on culture medium, while LrS production with anti-inflammatory activity depended on culture conditions of the biomass such as pH, agitation and growth phase. The L. reuteri CRL 1098 biomass and LrS production in the optimized culture media designed for this work reduced the complete process cost by approximately 95%, respectively to laboratory scale cost.

More than just a gut feeling: constraint-based genome-scale metabolic models for predicting functions of human intestinal microbes

The human gut is colonized with a myriad of microbes, with substantial interpersonal variation. This complex ecosystem is an integral part of the gastrointestinal tract and plays a major role in the maintenance of homeostasis. Its dysfunction has been correlated to a wide array of diseases, but the understanding of causal mechanisms is hampered by the limited amount of cultured microbes, poor understanding of phenotypes, and the limited knowledge about interspecies interactions. Genome-scale metabolic models (GEMs) have been used in many different fields, ranging from metabolic engineering to the prediction of interspecies interactions. We provide showcase examples for the application of GEMs for gut microbes and focus on (i) the prediction of minimal, synthetic, or defined media; (ii) the prediction of possible functions and phenotypes; and (iii) the prediction of interspecies interactions. All three applications are key in understanding the role of individual species in the gut ecosystem as well as the role of the microbiota as a whole. Using GEMs in the described fashions has led to designs of minimal growth media, an increased understanding of microbial phenotypes and their influence on the host immune system, and dietary interventions to improve human health. Ultimately, an increased understanding of the gut ecosystem will enable targeted interventions in gut microbial composition to restore homeostasis and appropriate host-microbe crosstalk.

Processed milk waste recycling via thermal pretreatment and lactic acid bacteria fermentation

Processed milk waste (MW) presents a serious problem within the dairy industries due to its high polluting load. Its chemical oxygen demand (COD) can reach values as high as 80,000 mg O₂ L^-1. This study proposes to reduce the organic load of those wastes using thermal coagulation and recover residual valuable components via fermentation. Thermal process results showed that the COD removal rates exceeded 40% when samples were treated at temperature above 60 °C to reach 72% at 100 °C. Clarified supernatants resulting from thermal treatment of the samples at the temperatures of 60 (MW₆₀), 80 (MW₈₀), and 100 °C (MW₁₀₀) were fermented using lactic acid bacteria strains without pH control. Lactic strains recorded important final cell yields (5-7 g L^-1). Growth mediums prepared using the thermally treated MW produced 73% of the bacterial biomass recorded with a conventional culture medium. At the end of fermentation, mediums were found exhausted from several valuable components. Industrial scale implementation of the proposed process for the recycling of industrial MWs is described and discussed.

Development of a simple, low-cost and eurytopic medium based on Pleurotus eryngii for lactic acid bacteria

Lactic acid bacteria (LAB) are a group of important beneficial microorganisms for human, but their growth is restricted to the habitats with rich nutrients. In order to develop a simple, low-cost and efficient medium based on the mushroom Pleurotus eryngii, this study evaluated the effects of different treatment methods for the mushroom, concentration of the mushroom, buffers, tween 80, MgSO4·7H2O, MnSO4·4H2O, CuSO4·5H2O, riboflavin and ascorbic acid on the growth of Lactococcus lactis subsp. lactis SLPE1-3. An optimized medium was developed, which was composed of the mushroom at 200 g/L, the buffer sodium acetate at 5 g/L, and riboflavin at 0.5 mg/L. The mushroom was ground, boiled and filtered for the filtrate in advance. In this optimized medium which was named as PSR medium, the population density of SLPE1-3 sharply reached 2.13 × 10(9) CFU/mL within 18 h of incubation, and still maintained 1.17 × 10(8) CFU/mL at 120 h. In addition, this study found that 6 kinds of LAB could grow almost well, and maintained high survival in PSR medium compared to M17 or MRS medium, including Lactococcus lactis subsp. lactis, Lactobacillus plantarum, Lactococcus lactis subsp. cremoris, Lactobacillus paracasei, Pediococcus pentosaceus and Lactobacillus rhamnosus. These results showed that PSR medium was a simple, low cost and eurytopic medium for the cultivation of LAB, and could replace MRS or M17 medium in the food industry, biomedicine and laboratory.

Systematic development and optimization of chemically defined medium supporting high cell density growth of Bacillus coagulans

With determined components and experimental reducibility, the chemically defined medium (CDM) and the minimal chemically defined medium (MCDM) are used in many metabolism and regulation studies. This research aimed to develop the chemically defined medium supporting high cell density growth of Bacillus coagulans, which is a promising producer of lactic acid and other bio-chemicals. In this study, a systematic methodology combining the experimental technique with flux balance analysis (FBA) was proposed to design and simplify a CDM. The single omission technique and single addition technique were employed to determine the essential and stimulatory compounds, before the optimization of their concentrations by the statistical method. In addition, to improve the growth rationally, in silico omission and addition were performed by FBA based on the construction of a medium-size metabolic model of B. coagulans 36D1. Thus, CDMs were developed to obtain considerable biomass production of at least five B. coagulans strains, in which two model strains B. coagulans 36D1 and ATCC 7050 were involved.

Development of Chemically Defined Media to Express Trp-Analog-Labeled Proteins in a Lactococcus lactis Trp Auxotroph

Chemically defined media for growth of Lactococcus lactis strains contain about 50 components, making them laborious and expensive growth media. However, they are crucial for metabolism studies as well as for expression of heterologous proteins labeled with unnatural amino acids. In particular, the L. lactis Trp auxotroph PA1002, overexpressing the tryptophanyl tRNA synthetase enzyme of L. lactis, is very suitable for the biosynthetic incorporation of Trp analogs in proteins because of its most relaxed substrate specificity reported towards Trp analogs. Here we present two much simpler defined media for L. lactis, which consist of only 24 or 31 components, respectively, and with which the L. lactis Trp auxotroph shows similar growth characteristics as with a 50-component chemically defined medium. Importantly, the expression levels of two recombinant proteins used for evaluation were up to 2-3 times higher in these new media than in the 50-component medium, without affecting the Trp analog incorporation efficiency. Taken together, the simplest chemically defined media reported so far for L. lactis are presented. Since L. lactis also shows auxotrophy for Arg, His, Ile, Leu Val, and Met, our simplified media may also be useful for the biosynthetic incorporation of analogs of these five amino acids.

Relative Rates of Amino Acid Import via the ABC Transporter GlnPQ Determine the Growth Performance of Lactococcus lactis

The GlnPQ transporter from Lactococcus lactis has the remarkable feature of having two substrate-binding domains (SBDs) fused to the N terminus of the transmembrane domain (TMD), and thus four SBDs are present in the homodimeric complex. Although X-ray structures and ligand binding data are available for both SBDs, little is known of how different amino acids compete with each other for transport via GlnPQ. Here we show GlnPQ has a broader substrate specificity than previously thought, with the ability to take up asparagine, glutamine, and glutamic acid, albeit via different routes and with different affinities. Asparagine and glutamine compete with each other at the level of binding to SBD1 and SBD2 (with differences in dissociation constant), but at the same time SBD1 and SBD2 compete with each other at the level of interaction with the translocator domain (with differences in affinity constant and rate of transport). Although glutamine transport via SBD1 is outcompeted by physiological concentrations of asparagine, SBD2 ensures high rates of import of the essential amino acid glutamine. Taken together, this study demonstrates that even in the presence of competing asparagine concentrations, GlnPQ has a high capacity to transport glutamine, which matches the high needs of the cell for glutamine and glutamate.

IMPORTANCE

GlnPQ is an ATP-binding cassette (ABC) transporter for glutamine, glutamic acid, and asparagine. The system is essential in various Gram-positive bacteria, including L. lactis and several pathogens. Here we show how the amino acids compete with each other for binding to the multiple SBDs of GlnPQ and how these SBDs compete with each other for substrate delivery to the transporter. Overall, our results show that GlnPQ has evolved to transport diverse substrates via different paths and to optimally acquire the abundant and essential amino acid glutamine.

Excess of threonine compared with serine promotes threonine aldolase activity in Lactococcus lactis IL1403

Lactococcus lactis is an important lactic acid starter for food production as well as a cell factory for production of food grade additives, among which natural flavour production is one of the main interests of food producers. Flavour production is associated with the degradation of amino acids and comprehensive studies are required to elucidate mechanisms behind these pathways. In this study using chemically defined medium, labelled substrate and steady-state cultivation, new data for the catabolism of threonine in Lc. lactis have been obtained. The biosynthesis of glycine in this organism is associated with the catabolic pathways of glucose and serine. Nevertheless, if threonine concentration in the growth environment exceeds that of serine, threonine becomes the main source for glycine biosynthesis and the utilization of serine decreases. Also, the conversion of threonine to glycine was initiated by a threonine aldolase and this was the principal pathway used for threonine degradation. As in Streptococcus thermophilus, serine hydroxymethyltransferase in Lc. lactis may possess a secondary activity as threonine aldolase. Other catabolic pathways of threonine (e.g. threonine dehydrogenase and threonine dehydratase) were not detected.