Functional roles and engineering strategies to improve the industrial functionalities of lactic acid bacteria during food fermentation

In order to improve the flavor profiles, food security, probiotic effects and shorten the fermentation period of traditional fermented foods, lactic acid bacteria (LAB) were often considered as the ideal candidate to participate in the fermentation process. In general, LAB strains possessed the ability to develop flavor compounds via carbohydrate metabolism, protein hydrolysis and amino acid metabolism, lipid hydrolysis and fatty acid metabolism. Based on the functional properties to inhibit spoilage microbes, foodborne pathogens and fungi, those species could improve the safety properties and prolong the shelf life of fermented products. Meanwhile, influence of LAB on texture and functionality of fermented food were also involved in this review. As for the adverse effect carried by environmental challenges during fermentation process, engineering strategies based on exogenous addition, cross protection, and metabolic engineering to improve the robustness and of LAB were also discussed in this review. Besides, this review also summarized the potential strategies including microbial co-culture and metabolic engineering for improvement of fermentation performance in LAB strains. The authors hope this review could contribute to provide an understanding and insight into improving the industrial functionalities of LAB.

Halophilic lactic acid bacteria - Play a vital role in the fermented food industry

Halophilic lactic acid bacteria have been widely found in various high-salt fermented foods. The distribution of these species in salt-fermented foods contributes significantly to the development of the product's flavor. Besides, these bacteria also have the ability to biosynthesize bioactive components which potentially apply to different areas. In this review, insights into the metabolic properties, salt stress responses, and potential applications of these bacteria have been have been elucidated. The purpose of this review highlights the important role of halophilic lactic acid bacteria in improving the quality and safety of salt-fermented products and explores the potential application of these bacteria.

Transcriptomic profiling reveals differences in the adaptation of two Tetragenococcus halophilus strains to a lupine moromi model medium

BACKGROUND

Tetragenococcus (T.) halophilus is a common member of the microbial consortia of food fermented under high salt conditions. These comprises salty condiments based on soy or lupine beans, fish sauce, shrimp paste and brined anchovies. Within these fermentations this lactic acid bacterium (LAB) is responsible for the formation of lactic and other short chain acids that contribute to the flavor and lower the pH of the product. In this study, we investigated the transcriptomic profile of the two T. halophilus strains TMW 2.2254 and TMW 2.2256 in a lupine moromi model medium supplied with galactose. To get further insights into which genomic trait is important, we used a setup with two strains. That way we can determine if strain dependent pathways contribute to the overall fitness. These strains differ in the ability to utilize L-arginine, L-aspartate, L-arabinose, D-sorbitol, glycerol, D-lactose or D-melibiose. The lupine moromi model medium is an adapted version of the regular MRS medium supplied with lupine peptone instead of casein peptone and meat extract, to simulate the amino acid availabilities in lupine moromi.

RESULTS

The transcriptomic profiles of the T. halophilus strains TMW 2.2254 and TMW 2.2256 in a lupine peptone-based model media supplied with galactose, used as simulation media for a lupine seasoning sauce fermentation, were compared to the determine potentially important traits. Both strains, have a great overlap in their response to the culture conditions but some strain specific features such as the utilization of glycerol, sorbitol and arginine contribute to the overall fitness of the strain TMW 2.2256. Interestingly, although both strains have two non-identical copies of the tagatose-6P pathway and the Leloir pathway increased under the same conditions, TMW 2.2256 prefers the degradation via the tagatose-6P pathway while TMW 2.2254 does not. Furthermore, TMW 2.2256 shows an increase in pathways required for balancing out the intracellular NADH/NADH⁺ ratios.

CONCLUSIONS

Our study reveals for the first time, that both versions of tagatose-6P pathways encoded in both strains are simultaneously active together with the Leloir pathway and contribute to the degradation of galactose. These findings will help to understand the strain dependent features that might be required for a starter strain in lupine moromi.

Evaluating the effect of lactic acid bacterial fermentation on salted soy whey for development of a potential novel soy sauce-like condiment

There were two main objectives of this study: (1) to understand the effect of salt concentration on the growth of four lactic acid bacteria (LAB) in soy whey and determine the non-volatile and volatile profiles generated after fermentation; (2) to evaluate the potential of using salted soy whey to develop a sauce-like condiment through LAB fermentation. The four LAB included non-halophilic Lactiplantibacillus plantarum ML Prime, Limosilactobacillus fermentum PCC, Oenococcus oeni Enoferm Beta and halophilic Tetragenococcus halophilus DSM20337. At 2% salt, all LAB grew remarkably from day 0 to day 1, except for T. halophilus, while at 6% salt, the growth of L. plantarum, L. fermentum and O. oeni was suppressed. Conversely, the higher salt concentration enhanced the growth of T. halophilus in soy whey as the cell count only increased from 6.36 to 6.60 log CFU/mL at 2% salt but it elevated from 6.61 to 7.55 log CFU/mL at 6% salt. Similarly, the higher salt content negatively affected the sugar and amino acids metabolism and organic acids production by non-halophilic LAB. L. plantarum and O. oeni generated significantly (p < 0.05) more lactic acid (3.83 g/L and 4.17 g/L, respectively) than L. fermentum and T. halophilus (2.02 g/L and 0 g/L, respectively) at 2% salt. In contrast, a higher amount of acetic acid was generated by L. fermentum (0.72 g/L at 2% salt) and T. halophilus (0.51 g/L at 6% salt). LAB could remove the green and beany off-flavours in soy whey by metabolizing C6 and C7 aldehydes. However, to develop a novel soy sauce-like condiment, yeast fermentation and Maillard reaction may be required to generate more characteristic soy sauce-associated aroma compounds.

•

Soy whey with 2% and 6% NaCl supported the growth of lactic acid bacteria (LAB).

•

At 6% NaCl, T. halophilus grew better while the growth of other LAB was impeded.

•

T. halophilus and L. fermentum produced significant amounts of acetic acid.

•

Through LAB fermentation, green and beany off-odour of soy whey could be removed.

•

Yeast fermentation and heating are required to produce key aroma soy sauce compounds.

Heat Adaptation Induced Cross Protection Against Ethanol Stress in Tetragenococcus halophilus: Physiological Characteristics and Proteomic Analysis

Ethanol is a toxic factor that damages membranes, disturbs metabolism, and may kill the cell. Tetragenococcus halophilus, considered as the cell factory during the manufacture of traditional fermented foods, encounters ethanol stress, which may affect the viability and fermentative performance of cells. In order to improve the ethanol tolerance of T. halophilus, a strategy based on cross protection was proposed in the current study. The results indicated that cross protection induced by heat preadaptation (45°C for 1.5 h) could significantly improve the stress tolerance (7.24-fold increase in survival) of T. halophilus upon exposure to ethanol (10% for 2.5 h). Based on this result, a combined analysis of physiological approaches and TMT-labeled proteomic technology was employed to investigate the protective mechanism of cross protection in T. halophilus. Physiological analysis showed that the heat preadapted cells exhibited a better surface phenotype, higher membrane integrity, and higher amounts of unsaturated fatty acids compared to unadapted cells. Proteomic analysis showed that a total of 163 proteins were differentially expressed in response to heat preadaptation. KEGG enrichment analysis showed that energy metabolism, membrane transport, peptidoglycan biosynthesis, and genetic information processing were the most abundant metabolic pathways after heat preadaptation. Three proteins (GpmA, AtpB, and TpiA) involved in energy metabolism and four proteins (ManM, OpuC, YidC, and HPr) related to membrane transport were up-regulated after heat preadaptation. In all, the results of this study may help understand the protective mechanisms of preadaptation and contribute to the improvement of the stress resistance of T. halophilus during industrial processes.

Heat preadaptation improved the ability of Zygosaccharomyces rouxii to salt stress: a combined physiological and transcriptomic analysis

Zygosaccharomyces rouxii plays important roles in the brewing process of fermented foods such as soy sauce, where salt stress is a frequently encountered condition. In this study, effect of heat preadaptation on salt tolerance of Z. rouxii and the protective mechanisms underlying heat preadaptation were investigated based on physiological and transcriptomic analyses. Results showed that cells subjected to heat preadaptation (37 °C, 90 min) prior to salt stress aroused many physiological responses, including maintaining cell surface smooth and intracellular pH level, increasing Na⁺/K⁺-ATPase activity. Cells subjected to heat preadaptation increased the amounts of unsaturated fatty acids (palmitoleic C16:1, oleic C18:1, linoleic C18:2) and decreased the amounts of saturated fatty acids (palmitic C16:0, stearic C18:0) which caused the unsaturation degree (unsaturated/saturated = U/S ratio) increased by 2.4 times when compared with cells without preadaptation under salt stress. Besides, salt stress led to increase in contents of 5 amino acids (valine, proline, threonine, glycine, and tyrosine) and decrease of 2 amino acids (serine and lysine). When comparing the cells pre-exposed to heat preadaptation followed by challenged with salt stress and the cells without preadaptation under salt stress, the serine, threonine, and lysine contents increased significantly. RNA sequencing revealed that the metabolic level of glycolysis by Z. rouxii was weakened, while the metabolic levels of the pentose phosphate pathway and the riboflavin were enhanced in cells during heat preadaptation. Results presented in this study may contribute to understand the bases of adaptive responses in Z. rouxii and rationalize its exploitation in industrial processes.Key points• Heat preadaptation can improve high salinity tolerance of Z. rouxii.• Combined physiological and transcriptomic analyses of heat preadaptation mechanisms.• Provide theoretical support for the application of Z. rouxii.

Genomic and metabolic features of Tetragenococcus halophilus as revealed by pan-genome and transcriptome analyses

The genomic and metabolic diversity and features of Tetragenococcus halophilus, a moderately halophilic lactic acid bacterium, were investigated by pan-genome, transcriptome, and metabolite analyses. Phylogenetic analyses based on the 16S rRNA gene and genome sequences of 15 T. halophilus strains revealed their phylogenetic distinctness from other Tetragenococcus species. Pan-genome analysis of the T. halophilus strains showed that their carbohydrate metabolic capabilities were diverse and strain dependent. Aside from one histidine decarboxylase gene in one strain, no decarboxylase gene associated with biogenic amine production was identified from the genomes. However, T. halophilus DSM 20339^T produced tyramine without a biogenic amine-producing decarboxylase gene, suggesting the presence of an unidentified tyramine-producing gene. Our reconstruction of the metabolic pathways of these strains showed that T. halophilus harbors a facultative lactic acid fermentation pathway to produce l-lactate, ethanol, acetate, and CO₂ from various carbohydrates. The transcriptomic analysis of strain DSM 20339^T suggested that T. halophilus may produce more acetate via the heterolactic pathway (including d-ribose metabolism) at high salt conditions. Although genes associated with the metabolism of glycine betaine, proline, glutamate, glutamine, choline, and citrulline were identified from the T. halophilus genomes, the transcriptome and metabolite analyses suggested that glycine betaine was the main compatible solute responding to high salt concentration and that citrulline may play an important role in the coping mechanism against high salinity-induced osmotic stresses. Our results will provide a better understanding of the genome and metabolic features of T. halophilus, which has implications for the food fermentation industry.

A metabolomics and proteomics study of the Lactobacillus plantarum in the grass carp fermentation

Background

Lactobacillus plantarum, a versatile lactic acid-fermenting bacterium, isolated from the traditional pickles in Ningbo of China, was chosen for grass carp fermentation, which could also improve the flavor of grass carp. We here explored the central metabolic pathways of L. plantarum by using metabolomic approach, and further proved the potential for metabolomics combined with proteomics approaches for the basic research on the changes of metabolites and the corresponding fermentation mechanism of L. plantarum fermentation.

Results

This study provides a cellular material footprinting of more than 77 metabolites and 27 proteins in L. plantarum during the grass carp fermentation. Compared to control group, cells displayed higher levels of proteins associated with glycolysis and nucleotide synthesis, whereas increased levels of serine, ornithine, aspartic acid, 2-piperidinecarboxylic acid, and fumarate, along with decreased levels of alanine, glycine, threonine, tryptophan, and lysine.

Conclusions

Our results may provide a deeper understanding of L. plantarum fermentation mechanism based on metabolomics and proteomic analysis and facilitate future investigations into the characterization of L. plantarum during the grass carp fermentation.

Electronic supplementary material

The online version of this article (10.1186/s12866-018-1354-x) contains supplementary material, which is available to authorized users.