Effects of glutathione on acid stress resistance and symbiosis between Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus

Fermentation characteristics and postacidification of yogurt by Streptococcus thermophilus CICC 6038 and Lactobacillus delbrueckii ssp. bulgaricus CICC 6047 at optimal inoculum ratio

This study aimed to investigate the symbiosis between Streptococcus thermophilus CICC 6038 and Lactobacillus delbrueckii ssp. bulgaricus CICC 6047. In addition, the effect of their different inoculum ratios was determined, and comparison experiments of fermentation characteristics and storage stability of milk fermented by their monocultures and cocultures at optimal inoculum ratio were performed. We found the time to obtain pH 4.6 and ΔpH during storage varied among 6 inoculum ratios (1:1, 2:1, 10:1, 19:1, 50:1, 100:1). By the statistical model to evaluate the optimal ratio, the ratio of 19:1 was selected, which exhibited high acidification rate and low postacidification with pH values remaining between 4.2 and 4.4 after a 50-d storage. Among the 3 groups included in our analyses (i.e., the monocultures of S. thermophilus CICC 6038 [St] and Lb. bulgaricus CICC 6047 [Lb] and their cocultures [St+Lb] at 19:1), the coculture group showed higher acidification activity, improved rheological properties, richer typical volatile compounds, more desirable sensor quality after the fermentation process than the other 2 groups. However, the continuous accumulation of acetic acid during storage showed that acetic acid was more highly correlated with postacidification than d-lactic acid for the Lb group and St+Lb group. Our study emphasized the importance of selecting an appropriate bacterial consortium at the optimal inoculum ratio to achieve favorable fermentation performance and enhanced postacidification stability during storage.

The intricate symbiotic relationship between lactic acid bacterial starters in the milk fermentation ecosystem

Fermentation is one of the most effective methods of food preservation. Since ancient times, food has been fermented using lactic acid bacteria (LAB). Fermented milk is a very intricate fermentation ecosystem, and the microbial metabolism of fermented milk largely determines its metabolic properties. The two most frequently used dairy starter strains are Streptococcus thermophilus (S. thermophilus) and Lactobacillus delbrueckii subsp. bulgaricus (L. bulgaricus). To enhance both the culture growth rate and the flavor and quality of the fermented milk, it has long been customary to combine S. thermophilus and L. bulgaricus in milk fermentation due to their mutually beneficial and symbiotic relationship. On the one hand, the symbiotic relationship is reflected by the nutrient co-dependence of the two microbes at the metabolic level. On the other hand, more complex interaction mechanisms, such as quorum sensing between cells, are involved. This review summarizes the application of LAB in fermented dairy products and discusses the symbiotic mechanisms and interactions of milk LAB starter strains from the perspective of nutrient supply and intra- and interspecific quorum sensing. This review provides updated information and knowledge on microbial interactions in a fermented milk ecosystem.

The effect of glutathione biosynthesis of Streptococcus thermophilus ST-1 on cocultured Lactobacillus delbrueckii ssp. bulgaricus ATCC11842

Streptococcus thermophilus and Lactobacillus delbrueckii ssp. bulgaricus are the main species used for yogurt preparation. Glutathione (GSH) can be synthesized by S. thermophilus and plays a crucial role in combating environmental stress. However, the effect of GSH biosynthesis by S. thermophilus on cocultured L. delbrueckii ssp. bulgaricus is still unknown. In this study, a mutant S. thermophilus ΔgshF was constructed by deleting the GSH synthase. The wild strain S. thermophilus ST-1 and ΔgshF mutants were cocultured with L. delbrueckii ssp. bulgaricus ATCC11842 by using Transwell chambers (Guangzhou Shuopu Biotechnology Co., Ltd.), respectively. It was proven that the GSH synthesized by S. thermophilus ST-1 could be absorbed and used by L. delbrueckii ssp. bulgaricus ATCC11842, and promote growth ability and stress tolerance of L. delbrueckii ssp. bulgaricus ATCC11842. The biomass of L. delbrueckii ssp. bulgaricus ATCC11842 cocultured with S. thermophilus ST-1 or ΔgshF (adding exogenous GSH) increased by 1.8 and 1.4 times compared with the biomass of L. delbrueckii ssp. bulgaricus ATCC11842 cocultured with S. thermophilus ΔgshF. Meanwhile, after H₂O₂ and low-temperature treatments, the bacterial viability of L. delbrueckii ssp. bulgaricus cocultured with S. thermophilus ΔgshF, with or without GSH, was decreased by 41 and 15% compared with that of L. delbrueckii ssp. bulgaricus cocultured with S. thermophilus ST-1. Furthermore, transcriptome analysis showed that the expression levels of genes involved in purine nucleotide and pyrimidine nucleotide metabolism in L. delbrueckii ssp. bulgaricus ATCC11842 were at least 3 times increased when cocultured with S. thermophilus (fold change > 3.0). Moreover, compared with the mutant strain ΔgshF, the wild-type strain ST-1 could shorten the fermented curd time by 5.3 hours during yogurt preparation. These results indicated that the GSH synthesized by S. thermophilus during cocultivation effectively enhanced the activity of L. delbrueckii ssp. bulgaricus and significantly improved the quality of fermented milk.

Quality and Functional Parameters of Fermented Milk Obtained from Goat Milk Fed with Broccoli and Artichoke Plant By-Products

Large amount of vegetal by-products are generated during production and processing steps. Introducing silage from vegetable by-products into dairy goat feed would be of great interest from the point of view of reducing costs and supporting the circular economy. The aim of this research was to study the effect of 40% inclusion of silage broccoli by-products and artichoke plant by-products in the diet of Murciano-Granadina goats throughout the lactation to establish milk suitability for fermented milks production. The novelty of this study is the use of milk from goats fed for a long term with a high inclusion of silages from artichoke plant and broccoli by-products, being the first one on broccoli inclusion. Two starter cultures thermophilic (YO-MIXTM300), and, mesophilic (MA400) were used and fermented milks were analyzed at two storage times after fermentation. Fermentation enhances antioxidant properties of fermented milks from all diets (p < 0.05), especially when mesophilic starter cultures are used. The main findings are that long term inclusion of 40% silage from broccoli and artichoke plant by-products in balanced diets of dairy goats yields milk suitable for fermentation by yogurt and cheese cultures, the inclusion of broccoli silage enhances antioxidant properties (p < 0.05), and, the inclusion of plant artichoke enhances fatty acids health indexes (p < 0.05).

Molecular Analysis of Glutamate Decarboxylases in Enterococcus avium

Enterococcus avium (E. avium) is a common bacterium inhabiting the intestines of humans and other animals. Most strains of this species can produce gamma-aminobutyric acid (GABA) via the glutamate decarboxylase (GAD) system, but the presence and genetic organization of their GAD systems are poorly characterized. In this study, our bioinformatics analyses showed that the GAD system in E. avium strains was generally encoded by three gadB genes (gadB1, gadB2, and gadB3), together with an antiporter gene (gadC) and regulator gene (gadR), and these genes are organized in a cluster. This finding contrasts with that for other lactic acid bacteria. E. avium SDMCC050406, a GABA producer isolated from human feces, was employed to investigate the contribution of the three gadB genes to GABA biosynthesis. The results showed that the relative expression level of gadB3 was higher than those of gadB1 and gadB2 in the exponential growth and stationary phases, and this was accompanied by the synchronous transcription of gadC. After heterologous expression of the three gadB genes in Escherichia coli BL21 (DE3), the K_m value of the purified GAD3 was 4.26 ± 0.48 mM, a value lower than those of the purified GAD1 and GAD2. Moreover, gadB3 gene inactivation caused decreased GABA production, accompanied by a reduction in resistance to acid stress. These results indicated that gadB3 plays a crucial role in GABA biosynthesis and this property endowed the strain with acid tolerance. Our findings provided insights into how E. avium strains survive the acidic environments of fermented foods and throughout transit through the stomach and gut while maintaining cell viability.

Bioactive Peptides from Liquid Milk Protein Concentrate by Sequential Tryptic and Microbial Hydrolysis

Recently, bioactive peptides as a health-promoting agent have come to the forefront of health research; however, industrial production is limited, possibly due to the lack of the required technological knowledge. The objective of the investigation was to prepare bioactive peptides with hypoallergenic properties from liquid milk protein concentrate (LMPC), through sequential enzymatic and microbial hydrolysis. LMPC was produced from ultra-heat-treated (UHT) skimmed cow’s milk using a nanofiltration membrane. The effect of the concentration of trypsin (0.008–0.032 g·L−1) on the hydrolysis of LMPC was studied. Subsequently, the hydrolysis of tryptic-hydrolyzed LMPC (LMPC-T) with lactic acid bacteria was performed, and the effect of glucose in microbial hydrolysis was studied. Aquaphotomic analysis of the hydrolysis of LMPC was performed using the spectral range of 1300–1600 nm (near-infrared spectra). Changes in antioxidant capacity, anti-angiotensin-converting enzyme activity, and antibacterial activity against Bacillus cereus, Staphylococcus aureus and Listeria monocytogenes were noted after the sequential tryptic and microbial hydrolysis of LMPC. Allergenicity in LMPC was reduced, due to sequential hydrolysis with 0.016 g·L−1 of trypsin and lacteal acid bacteria. According to the aquaphotomic analysis result, there was a dissociation of hydrogen bonds in compounds during the initial period of fermentation and, subsequently, the formation of compounds with hydrogen bonds. The formation of compounds with a hydrogen bond was more noticeable when microbial hydrolysis was performed with glucose. This may support the belief that the results of the present investigation will be useful to scale up the process in the food and biopharmaceutical industries.

Peptide transporter-related protein 2 plays an important role in glutathione transport of Streptococcus thermophilus

Streptococcus thermophilus is widely used as a starter culture in the fermentation of yogurt. Glutathione (GSH; γ-glutamyl-cysteinyl-glycine), as a tripeptide, has an important physiological role for Strep. thermophilus. However, the scope of the GSH transport proteins is still unexplored in this species. In the present study, 5 peptide transporter-related proteins (Ptrp) of Strep. thermophilus strain ST-1 were selected and then inactivated by gene insertion, respectively. Through detection and comparison of intracellular GSH content of mutant strain and wild strain, we identified 2 proteins, named Ptrp-2 and Ptrp-4, that might be related to GSH transport. Reverse-transcriptase quantitative PCR was performed to verify the gene expressions of these 2 possible GSH transport-related proteins, and it was finally determined that Ptrp-2 plays an important role in GSH transport of Strep. thermophilus. Milk fermentation experiments were further conducted to test the effect of Ptrp-2 on the characteristics of yogurt. The results showed that the fermented milk hardly curds using the mutant strain, indicating that Ptrp-2 is important for Strep. thermophilus as a yogurt starter.

Influence of guabiroba pulp (campomanesia xanthocarpa o. berg) added to fermented milk on probiotic survival under in vitro simulated gastrointestinal conditions

In fermented milks inoculated with two thermophilic strains (Lactobacillus bulgaricus and Streptococcus thermophilus), guabiroba pulp (Campomanesia xanthocarpa O. Berg) was added in different concentrations: 5% (I5 sample) and 10% (I10 sample), compared to a control sample, with no pulp addition. In these fermented milks, Bifidobacterium BB-12 was added and the samples were submitted to a progressive gastrointestinal simulation in vitro. The cells count was performed, including the survival rates for all the progressive steps of the simulated digestion. Total phenolic content (TPC) and antioxidant activity analysis by FRAP (Ferric Reducing Antioxidant Power) and DPPH (2,2-diphenyl-1-picrylhydrazyl) were performed in all the gastrointestinal steps. Before and during the entire gastrointestinal tract, the Bifidobacterium BB-12 count was 8-9 log CFU g^-1, above the recommended for a probiotic product, with a highlight in intestinal colon steps. The I10 sample showed the highest viable cell count, the highest total phenolic content and antioxidant activity throughout the entire gastric steps (p < 0.05). The fermented milk proved to be an effective matrix for the probiotic stability and incorporation of guabiroba components. Bioactive compounds present in the guabiroba pulp may have occasioned a prebiotic and protective effect on Bifidobacterium BB-12 after gastric conditions. The possible bioconversion of these compounds in more active forms can contribute to the absorption in epithelial cells, enhancing fermented milks with guabiroba pulp as important sources of dietary accessible bioactive compounds.

Characteristics of oxidative stress and antioxidant defenses by a mixed culture of acidophilic bacteria in response to Co2+ exposure

During bioleaching of Cobalt from waste lithium-ion batteries, the biooxidation activity of acidophilic bacteria is inhibited by a high concentration of Co ion in the liquid phase. However, the mechanism for Co²⁺ toxicity to acidophilic bacteria has not been fully elucidated. In this study, the effects of Co²⁺ concentration on the biooxidation activity for Fe²⁺, intracellular reactive oxygen species (ROS) level and antioxidant defense systems in a mixed-culture of acidophilic bacteria (MCAB) were investigated. The results showed that the biooxidation activity of the MCAB was inhibited by Co²⁺. Furthermore, it was indicated that the intracellular ROS contents of the MCAB under conditions of 0.4 M and 0.6 M Co²⁺ were 2.60 and 3.34 times higher than that under the condition of 0 M Co²⁺. The increase in intracellular malondialdehyde content indicated that the oxidative damage was induced by Co²⁺. Moreover, the antioxidant systems in MCAB were affected by Co²⁺. It was observed that the Co²⁺ exposure increased the catalase and glutathione peroxidase activities while reducing the superoxide dismutase activity and the intracellular glutathione (GSH) content. It was found that an exogenous GSH supplementation eliminated excess intracellular ROS and improved the biooxidation activity of the MCAB.

Oxidative Stress Induced by Metal Ions in Bioleaching of LiCoO2 by an Acidophilic Microbial Consortium

An acidophilic microbial consortium (AMC) was used to investigate the fundamental mechanism behind the adverse effects of pulp density increase in the bioleaching of waste lithium ion batteries (WLIBs). Results showed that there existed the effect of metal-ion stress on the bio-oxidative activity of AMC. The Li+ and Co2+ accumulated in the leachate were the direct cause for the decrease in lithium and cobalt recovery yields under a high pulp density. In a simulated bioleaching system with 4.0% (w ⋅v-1) LiCoO2, the intracellular reactive oxygen species (ROS) content in AMC increased from 0.82 to 6.02 within 24 h, which was almost three times higher than that of the control (2.04). After the supplementation of 0.30 g⋅L-1 of exogenous glutathione (GSH), the bacterial intracellular ROS content decreased by 40% within 24 h and the activities of intracellular ROS scavenging enzymes, including glutathione peroxidase (GSH-Px) and catalase (CAT), were 1.4- and 2.0-folds higher in comparison with the control within 24 h. In the biofilms formed on pyrite in the bioleaching of WLIBs, it was found that metal-ion stress had a great influence on the 3-D structure and the amount of biomass of the biofilms. After the exogenous addition of GSH, the structure and the amount of biomass of the biofilms were restored to some extent. Eventually, through ROS regulation by the exogenous addition of GSH, very high metal recovery yields of 98.1% Li and 96.3% Co were obtained at 5.0% pulp density.

Comparative Genomics of Streptococcus thermophilus Support Important Traits Concerning the Evolution, Biology and Technological Properties of the Species

Streptococcus thermophilus is a major starter for the dairy industry with great economic importance. In this study we analyzed 23 fully sequenced genomes of S. thermophilus to highlight novel aspects of the evolution, biology and technological properties of this species. Pan/core genome analysis revealed that the species has an important number of conserved genes and that the pan genome is probably going to be closed soon. According to whole genome phylogeny and average nucleotide identity (ANI) analysis, most S. thermophilus strains were grouped in two major clusters (i.e., clusters A and B). More specifically, cluster A includes strains with chromosomes above 1.83 Mbp, while cluster B includes chromosomes below this threshold. This observation suggests that strains belonging to the two clusters may be differentiated by gene gain or gene loss events. Furthermore, certain strains of cluster A could be further subdivided in subgroups, i.e., subgroup I (ASCC 1275, DGCC 7710, KLDS SM, MN-BM-A02, and ND07), II (MN-BM-A01 and MN-ZLW-002), III (LMD-9 and SMQ-301), and IV (APC151 and ND03). In cluster B certain strains formed one distinct subgroup, i.e., subgroup I (CNRZ1066, CS8, EPS, and S9). Clusters and subgroups observed for S. thermophilus indicate the existence of lineages within the species, an observation which was further supported to a variable degree by the distribution and/or the architecture of several genomic traits. These would include exopolysaccharide (EPS) gene clusters, Clustered Regularly Interspaced Short Palindromic Repeats (CRISPRs)-CRISPR associated (Cas) systems, as well as restriction-modification (R-M) systems and genomic islands (GIs). Of note, the histidine biosynthetic cluster was found present in all cluster A strains (plus strain NCTC12958^T) but was absent from all strains in cluster B. Other loci related to lactose/galactose catabolism and urea metabolism, aminopeptidases, the majority of amino acid and peptide transporters, as well as amino acid biosynthetic pathways were found to be conserved in all strains suggesting their central role for the species. Our study highlights the necessity of sequencing and analyzing more S. thermophilus complete genomes to further elucidate important aspects of strain diversity within this starter culture that may be related to its application in the dairy industry.

The Effect of Salt and Temperature on the Growth of Fresco Culture

The effect of environmental factors, including temperature and water activity, has a considerable impact on the growth dynamics of each microbial species, and it is complicated in the case of mixed cultures. Therefore, the aim of this study was to describe and analyze the growth dynamics of Fresco culture (consisting of 3 different bacterial species) using predictive microbiology tools. The growth parameters from primary fitting were modelled against temperature using two different secondary models. The intensity of Fresco culture growth in milk was significantly affected by incubation temperature described by Gibson’s model, from which the optimal temperature for growth of 38.6 °C in milk was calculated. This cardinal temperature was verified with the Topt = 38.3 °C calculated by the CTMI model (cardinal temperature model with inflection), providing other cardinal temperatures, i.e., minimal Tmin = 4.0 °C and maximal Tmax = 49.6 °C for Fresco culture growth. The specific growth rate of the culture under optimal temperature was 1.56 h−1. The addition of 1% w/v salt stimulated the culture growth dynamics under temperatures down to 33 °C but not the rate of milk acidification. The prediction data were validated and can be used in dairy practice during manufacture of fermented dairy products.

Bactericidal Effects of Silver Nanoparticles on Lactobacilli and the Underlying Mechanism

While the antibacterial properties of silver nanoparticles (AgNPs) have been demonstrated across a spectrum of bacterial pathogens, the effects of AgNPs on the beneficial bacteria are less clear. To address this issue, we compared the antibacterial activity of AgNPs against two beneficial lactobacilli ( Lactobacillus delbrueckii subsp. bulgaricus and Lactobacillus casei) and two common opportunistic pathogens ( Escherichia coli and Staphylococcus aureus). Our results demonstrate that those lactobacilli are highly susceptible to AgNPs, while the opportunistic pathogens are not. Acidic environment caused by the lactobacilli is associated with the bactericidal effects of AgNPs. Our mechanistic study suggests that the acidic growth environment of lactobacilli promotes AgNP dissolution and hydroxyl radical (•OH) overproduction. Furthermore, increases in silver ions (Ag⁺) and •OH deplete the glutathione pool inside the cell, which is associated with the increase in cellular reactive oxygen species (ROS). High levels of ROS may further induce DNA damage and lead to cell death. When E. coli and S. aureus are placed in a similar acidic environment, they also become more susceptible to AgNPs. This study provides a mechanistic description of a pH-Ag⁺-•OH bactericidal pathway and will contribute to the responsible development of products containing AgNPs.

Glutathione biosynthesis and activity of dependent enzymes in food-grade lactic acid bacteria harbouring multidomain bifunctional fusion gene (gshF)

AIMS

To assess glutathione (GSH) biosynthesis ability and activity of dependent enzymes in food-grade lactic acid bacteria (LAB) and correlating with genomic information on GSH system in LAB.

METHODS AND RESULTS

Whole-genome sequences of 26 food-grade LAB were screened for the presence/absence of a set of genes involved in de novo GSH system. Multiple strains of Streptococcus thermophilus (37), Lactobacillus casei (37), Lactobacillus rhamnosus (4), Lactobacillus paracasei (8) Lactobacillus plantarum (23) and Lactobacillus fermentum (22) were screened for biochemical evidence of the GSH system. Multiple sequence analysis of GshF sequences was carried out for comparing the genomic signatures between GSH-producing and nonproducing species.

CONCLUSIONS

Streptococcus thermophilus was found to have de novo GSH biosynthesis as well as import ability. Lactobacillus sp. were negative for GSH synthesis but could import it from the medium. All the species exhibited prolific GSH reductase and peroxidase activity. Sequence analysis revealed the absence of key amino acid residues as well as a truncated N-terminal region in lactobacilli.

SIGNIFICANCE AND IMPACT OF THE STUDY

The study provides a comprehensive view on the status of an important antioxidative system (the GSH system) in LAB and is expected to serve as a primer for future work on the mechanistic role of GSH in the group.