Respiratory Physiology of Lactococcus lactis in Chemostat Cultures and Its Effect on Cellular Robustness in Frozen and Freeze-Dried Starter Cultures

Screening of Lactic Acid Bacteria Strains for Potential Sourdough and Bread Applications: Enzyme Expression and Exopolysaccharide Production

Twenty-eight strains of lactic acid bacteria (LAB) were characterized for the ability to express enzymes of interest (including protease, xylanase, α-amylase, laccase, and glucose oxidase) as well as the ability to produce exopolysaccharide (EPS). The screening of enzyme capability for all LAB strains proceeded in a progressive 3-stage manner that helps to profile the efficiency of LAB strains in expressing chosen enzymes (Stage 1), highlights the strains with affinity for flour as the substrate (Stage 2), and discerns strains that can adapt well in a simulated starter environment (Stage 3). The theoretical ability of LAB to express these enzymes was also assessed using Basic Local Alignment Search Tool (BLAST) analysis to identify the underlying genes in the whole genome sequence. By consolidating both experimental data and information obtained from BLAST, three LAB strains were deemed optimal in expressing enzymes, namely, Lb. delbrueckii subsp. bulgaricus (RBL 52), Lb. rhamnosus (RBL 102), and Lb. plantarum (ATCC 10241). Meanwhile, EPS-producing capabilities were observed for 10 out of 28 LAB strains, among which, Lactococcus lactis subsp. diacetylactis (RBL 37) had the highest total EPS yield (274.15 mg polysaccharide/L culture) and produced 46.2% polysaccharide with a molecular mass of more than 100 kDa.

Fermentation properties and functional stability of dough starter Jiaozi and Laomian after frozen storage

Purpose

This study aims to investigate the effects of frozen storage on the stability of traditional dough starters in China.

Methods

The microbial community structure and abundance of related metabolic genes in different fermented sourdough prepared by Jiaozi (JZ) and Laomian (LM) starters before and after frozen storage at -20°C for half a year were analyzed using the shotgun metagenomic sequencing method, and differences in characteristics of texture in steamed bread were also compared by formal methods.

Results

The fermentation ability (FA) and metabolic activities of yeast in the JZH sourdough (started by JZ which was stored at -20°C for half a year) were better than those of LMH sourdough (started by LM which was stored at -20°C for half a year). The dominant genera of Acetobacter were found to be increased in the JZH0 sourdough (started by JZH and fermented for 0 h) and those of Lactobacillus were found to be decreased. Lactobacillus (98.72%), Pediococcus (0.37%), Saccharomyces (0.27%), and Acetobacter (0.01%), were dominant in sourdough LMH0 (started by LMH and fermented for 0 h). The abundances of "oxidative phosphorylation-related enzymes" and the "biosynthesis of glutamate"-related enzymes and genes related to "biosynthesis of glutamate" and "unsaturated fatty acid" were higher in JZH0 than in the JZ0 sourdough (started by JZ without being frozen and fermented for 0 h). The good FA of yeast, the acid production capacity of bacteria in the sourdough, and the quality of the JZH steamed bread (made by the JZH starter) indicated the better freezing tolerance of the microorganisms in JZ than in LM.

Conclusion

The conclusion of this study suggests the better application potential of the JZ as the fermentation starter in actual production.

The stressostat: A novel approach in adaptive laboratory evolution to improve end-product resistance

End-product inhibition in pH-controlled batch cultures, is the major limiting factor for bacterial biomass formation in the starter culture industry as well as in many other biotechnological processes. Adaptive laboratory evolution (ALE) has emerged over the past decades as a powerful tool for phenotype optimization, but none of the existing ALE methods could select for improved end-product resistance. Therefore, we developed the stressostat (STress Resistance Evolution in Substrate Surplus) as a novel continuous ALE method. Stressostat cultivation applies end-product concentrations as constant evolutionary pressure on microorganisms in the presence of substrate surplus. In this study, we improved the lactate resistance of Lactococcus lactis FM03P in 35 days of stressostat cultivations. The lactate concentrations increased over time from 530 to 675 mM, indicating the successful selection for variants with improved lactate resistance. Thirty-four variants were isolated and grouped into four clusters based on their growth rates at high lactate concentrations. In the high-throughput screening without pH control, most isolated variants could grow at high lactate concentrations (870-928 mM), while the wild type was completely inhibited. The variants grew slower than wild type at low lactate media indicating possible evolutionary trade-off. However, in pH-controlled batch cultivations, most variants produced more biomass than the wild type. In conclusion, stressostat cultivation is a valuable method to obtain L. lactis variants with improved end-product resistance and further characterization is needed to elucidate underlying resistance mechanisms and potential industrial applications.

Simple & better - Accelerated cheese ripening using a mesophilic starter based on a single strain with superior autolytic properties

In the manufacture of rennet-coagulated cheese, autolysis is a rate-limiting step for ripening. Previously, a highly autolytic and thermotolerant Lactococcus lactis strain, RD07, was generated, which in preliminary laboratory cheese trials demonstrated great potential as a cheese ripening accelerant. RD07 is proteinase positive (Prt+) and capable of metabolizing citrate (Cit+). In this study, we obtained two derivatives of RD07: EC8 lacking the citrate plasmid, and EC2 lacking the proteinase plasmid. EC2 and EC8 retained the autolytic properties of RD07, and autolyzed 20 times faster than Flora Danica (FD) and SD96, where the latter is the parent of RD07. The three strains EC2, EC8 and RD07 were used in a ratio of 90:8:2, to create a simple starter termed ERC. ERC was less sensitive to cooking when cultured in milk and autolyzed well after entering the stationary phase upon facing sugar starvation. The ERC starter was benchmarked against FD and SD96 in laboratory cheese trials. The free amino acid content in cheese prepared using the ERC culture was 31 % and 34 % higher than in cheese prepared using FD and SD96, respectively. Overall, the ERC culture resulted in a more rapid release of free amino acids. A large-scale (5000 L) Gouda cheese trial at a Danish dairy demonstrated that the single strain ERC starter was comparable in performance to FD + an adjunct Lactobacillus helveticus culture. Furthermore, a large-scale Danbo cheese trial demonstrated that ERC could reduce the ripening period by 50 % for long-term ripened (25 weeks) cheese, resulting in better cheese.

Analysis of the effects of specific growth rate of Lactococcus lactis MG1363 on aerobic metabolism and its application to high-density culture

Lactic acid bacteria (LAB) are known to produce a large amount of lactate when cultured under non-aerated conditions, which inhibits their growth at high concentrations. Our previous studies have shown that LAB can be cultured without lactate production under aerated conditions at a low specific growth rate. In this study, we investigated the effects of specific growth rate on cell yield and the specific production rates of metabolites in aerated fed-batch cultures of Lactococcus lactis MG1363. The results showed that lactate and acetoin production could be suppressed at specific growth rates below 0.2 h^-1, whereas acetate production was the highest at a specific growth rate of 0.2 h^-1. When LAB was cultured at a specific growth rate of 0.25 h^-1 with the addition of 5 mg/L heme to assist ATP production by respiration, lactate and acetate production was suppressed, and cell concentration reached 19 g-dry-cell/L (5.6 × 10ˆ10 cfu/mL) with a high cell yield of 0.42 ± 0.02 g-dry-cell/g-glucose.

Adaptation of Lacticaseibacillus rhamnosus CM MSU 529 to Aerobic Growth: A Proteomic Approach

The study describes the effect of aerobic conditions on the proteome of homofermentative lactic acid bacterium Lacticaseibacillus rhamnosus CM MSU 529 grown in a batch culture. Aeration caused the induction of the biosynthesis of 43 proteins, while 14 proteins were downregulated as detected by label-free LC-MS/MS. Upregulated proteins are involved in oxygen consumption (Pox, LctO, pyridoxine 5'-phosphate oxidase), xylulose 5-phosphate conversion (Xfp), pyruvate metabolism (PdhD, AlsS, AlsD), reactive oxygen species (ROS) elimination (Tpx, TrxA, Npr), general stress response (GroES, PfpI, universal stress protein, YqiG), antioxidant production (CysK, DkgA), pyrimidine metabolism (CarA, CarB, PyrE, PyrC, PyrB, PyrR), oligopeptide transport and metabolism (OppA, PepO), and maturation and stability of ribosomal subunits (RbfA, VicX). Downregulated proteins participate in ROS defense (AhpC), citrate and pyruvate consumption (CitE, PflB), oxaloacetate production (AvtA), arginine synthesis (ArgG), amino acid transport (GlnQ), and deoxynucleoside biosynthesis (RtpR). The data obtained shed light on mechanisms providing O₂-tolerance and adaptation to aerobic conditions in strain CM MSU 529. The biosynthesis of 39 from 57 differentially abundant proteins was shown to be O₂-sensitive in lactic acid bacteria for the first time. To our knowledge this is the first study on the impact of aerobic cultivation on the proteome of L. rhamnosus.

Probiotic Effects and Metabolic Products of Enterococcus faecalis LD33 with Respiration Capacity

Respiration metabolism could improve the long-term survival of lactic acid bacteria (LAB); however, its effect on potential probiotic traits of LAB was not reported. The difference made by Enterococcus faecalis LD33 that was cultured under respiration-permissive and fermentation conditions, such as the biomass, metabolites, antimicrobial activity, tolerance to acid and bile salt, adhesion capabilities, and the ability to inhibit the proliferation of cancer cells were studied. Under a respiration-permissive condition, the final biomass of the culture was about twice as compared to that of fermentation condition. When the metabolites were measured, glucose was exhausted within 8 h. Two-folds of acetic acid, triple of both acetoin and diacetyl, and less than half of lactic acid, were accumulated under the respiratory-permissive condition. No discrimination of growth inhibition on Salmonella enterica serovar Typhimurium ATCC 14028 and Shigella sonnei ATCC 25931 was observed when Enterococcus faecalis LD33 was cultured under both conditions; however, under respiration-permissive condition, the strain presented significant antimicrobial activities to Listeria monocytogenes ATCC19111 and Staphylococcus aureus ATCC6538P. Enterococcus faecalis LD33 displayed relatively strong bile salt tolerance and adherence capability but weaker acid tolerance when undergoing respiration metabolism. There was no significant difference in the anti-cancer effect of the viable bacterial cells on both growth modes; however, the supernatant showed a higher inhibition effect on HT-29 cells than the live bacteria, and there was no significant difference between the supernatant and the 5-Fluorouracil (7 μg/mL). Consequently, the Enterococcus faecalis LD33 undergoing respiration metabolism could bring higher biomass, more flavor metabolites, and better antimicrobial and anti-cancer activities. This study extends our knowledge of respiratory metabolism in LAB and its impact on probiotic traits. E. faecalis LD33 qualifies as a suitable strain against foodborne pathogens, cancer therapy, and eventual application in the food and pharmaceutical industries.