GABA-Producing Lactococcus lactis Strains Isolated from Camel's Milk as Starters for the Production of GABA-Enriched Cheese

New Insight into Microbial Exploitation to Produce Bioactive Molecules from Agrifood and By-Products' Fermentation

Consumers are increasingly interested in a healthy lifestyle, and choosing foods and ingredients with proven human health benefits has become a current trend. Recently, scientific evidence has proven that the use of microorganisms in different food matrices appears to play a key role in the production of bioactive molecules with biological effects on human health. In particular, selected microorganisms with specific traits can be exploited for the production of specific molecules with high nutraceutical value that can be used in the food industry. This review aims to explore the most recent studies that correlate the use of microorganisms to produce high-value molecules through fermentation and synthetic biology, confirming their strategic role in obtaining nutraceuticals for human consumption with health-promoting effects.

Tryptamine accumulates in cheese mainly via the decarboxylation of tryptophan by lactic acid bacteria

Few microorganisms are known to decarboxylate L-tryptophan, thereby producing tryptamine, a neuromodulator biogenic amine (BA) that can accumulate in cheese. Since lactic acid bacteria (LAB) are largely responsible for the production of other BA in this product, it was hypothesised that they would also be the main agents of tryptamine production. Using a rapid test based on the natural fluorescence of tryptamine, thousands of bacterial isolates from several cheese samples were screened. Only 1.4% of all isolates (43 out of 2982) were able to synthesise tryptamine and secrete it into the culture medium, highlighting the rarity of tryptophan-decarboxylating activity in this food source. Moreover, over 90% of these isolates were identified as belonging to Loigolactobacillus coryniformis, Enterococcus durans, or the Latilactobacillus sakei group (all lactic acid bacteria). No strain belonging to either of the first two species has previously been described as a tryptamine producer. Strains of the non-LAB species Staphylococcus epidermidis, Klebsiella pneumonia and Corynebacterium flavescens were also identified as tryptamine producers for the first time. Further typification of the producers based on their tryptamine yield was performed, and their potential applications as technological adjuncts for use in the dairy industry, as cell factories, or even as psychobiotics, are discussed.

Selection of GABA-Producing Lactic Acid Bacteria Strains by Polymerase Chain Reaction Using Novel gadB and gadC Multispecies Primers for the Development of New Functional Foods

Gamma-aminobutyric acid (GABA) has been attributed to health-promoting properties and has received attention from the food industry as an attractive bioactive compound for the development of functional foods. Some lactic acid bacteria (LAB) produce GABA through a glutamate decarboxylase encoded by gadB and a glutamate/GABA antiporter encoded by gadC. In this study, we develop a molecular screening method based on a polymerase chain reaction able to detect those genes in different LAB species through the use of novel multispecies primers. PCR was performed in 92 LAB strains of six different species. The primer pair designed for gadB allowed its identification in Lactiplantibacillus plantarum, Lactococcus cremoris, Lactococcus lactis, Levilactobacillus brevis, Limosilactobacillus fermentum, and Limosilactobacillus reuteri strains. For gadC, two different primer pairs were designed for its detection in different species. Glutamate decarboxylase activity (GAD assay) and GABase enzymatic quantification were also assessed. Among those strains showing glutamate decarboxylase activity, 93.2% harbored the gadB gene, and those showing GABA production had the gadB gene and exhibited glutamate decarboxylase activity. PCR detection of gadB correlates strongly with GABA production and constitutes a good strategy for the selection of LAB with high yields (>18 mM) that could be used for the development of GABA-enriched functional foods.

Microbial gamma-aminobutyric acid synthesis: a promising approach for functional food and pharmaceutical applications

Gamma-aminobutyric acid (GABA) is a non-protein amino acid that is a main inhibitory neurotransmitter in the mammalian central nervous system. This mini-review emphasis on the microbial production of GABA and its potential benefits in various applications. Numerous microorganisms, including lactic acid bacteria, have been identified as efficient GABA producers. These microbes utilize glutamate decarboxylase enzymes to convert L-glutamate to GABA. Notable GABA-producing strains include Lactobacillus brevis, Lactobacillus plantarum, and certain Bifidobacterium species. Microbial GABA production offers numerous benefits over chemical synthesis, including cost-effectiveness, sustainability, and the potential for in situ production in fermented foods. Recent research has optimized fermentation conditions, genetic engineering approaches, and substrate utilization to enhance GABA yields. The benefits of GABA extend beyond its neurotransmitter role. Studies have shown its potential to reduce blood pressure, assuage anxiety, improve sleep quality, and improve cognitive function. These properties make microbial GABA production particularly attractive for developing functional foods, nutraceuticals, and pharmaceuticals. Future research directions include exploring novel GABA-producing strains, improving production efficiency, and investigating additional health benefits of microbially produced GABA.

Optimization of fermentation conditions for the production of γ-aminobutyric acid by Lactobacillus hilgardii GZ2 from traditional Chinese fermented beverage system

γ-Aminobutyric acid (GABA) is a crucial neurotransmitter with wide application prospects. In this study, we focused on a GABA-producing strain from a traditional Chinese fermented beverage system. Among the six isolates, Lactobacillus hilgardii GZ2 exhibited the greatest ability to produce GABA in the traditional Chinese fermented beverage system. To increase GABA production, we optimized carbon sources, nitrogen sources, temperature, pH, and monosodium glutamate and glucose concentrations and conducted fed-batch fermentation. The best carbon and nitrogen sources for GABA production and cell growth were glucose, yeast extract and tryptone. Gradual increases in GABA were observed as the glucose and monosodium glutamate concentrations increased from 10 g/L to 50 g/L. During fed-batch fermentation, lactic acid was used to maintain the pH at 5.56, and after feeding with 0.03 g/mL glucose and 0.4 g/mL sodium glutamate for 72 h, the GABA yield reached 239 g/L. This novel high-GABA-producing strain holds great potential for the industrial production of GABA, as well as the development of health-promoting functional foods and medical fields.

Phenotypic, Technological, Safety, and Genomic Profiles of Gamma-Aminobutyric Acid-Producing Lactococcus lactis and Streptococcus thermophilus Strains Isolated from Cow's Milk

Gamma-aminobutyric acid (GABA)-producing lactic acid bacteria (LAB) can be used as starters in the development of GABA-enriched functional fermented foods. In this work, four GABA-producing strains each of Lactococcus lactis and Streptococcus thermophilus species were isolated from cow's milk, and their phenotypic, technological, and safety profiles determined. Genome analysis provided genetic support for the majority of the analyzed traits, namely, GABA production, growth in milk, and the absence of genes of concern. The operon harboring the glutamate decarboxylase gene (gadB) was chromosomally encoded in all strains and showed the same gene content and gene order as those reported, respectively, for L. lactis and S. thermophilus. In the latter species, the operon was flanked (as in most strains of this species) by complete or truncated copies of insertion sequences (IS), suggesting recent acquisition through horizontal gene transfer. The genomes of three L. lactis and two S. thermophilus strains showed a gene encoding a caseinolytic proteinase (PrtP in L. lactis and PrtS in S. thermophilus). Of these, all but one grew in milk, forming a coagulum of good appearance and an appealing acidic flavor and taste. They also produced GABA in milk supplemented with monosodium glutamate. Two L. lactis strains were identified as belonging to the biovar. diacetylactis, utilized citrate from milk, and produced significant amounts of acetoin. None of the strains showed any noticeable antibiotic resistance, nor did their genomes harbor transferable antibiotic resistance genes or genes involved in toxicity, virulence, or pathogenicity. Altogether these results suggest that all eight strains may be considered candidates for use as starters or components of mixed LAB cultures for the manufacture of GABA-enriched fermented dairy products.

Natural diversity of lactococci in γ-aminobutyric acid (GABA) production and genetic and phenotypic determinants

BACKGROUND

γ-aminobutyric acid (GABA) is a bioactive compound produced by lactic acid bacteria (LAB). The diversity of GABA production in the Lactococcus genus is poorly understood. Genotypic and phenotypic approaches were therefore combined in this study to shed light on this diversity. A comparative genomic study was performed on the GAD-system genes (gadR, gadC and gadB) involved in GABA production in 36 lactococci including L. lactis and L. cremoris species. In addition, 132 Lactococcus strains were screened for GABA production in culture medium supplemented with 34 mM L-glutamic acid with or without NaCl (0.3 M).

RESULTS

Comparative analysis of the nucleotide sequence alignments revealed the same genetic organization of the GAD system in all strains except one, which has an insertion sequence element (IS981) into the PgadCB promoter. This analysis also highlighted several deletions including a 3-bp deletion specific to the cremoris species located in the PgadR promoter, and a second 39-bp deletion specific to L. cremoris strains with a cremoris phenotype. Phenotypic analysis revealed that GABA production varied widely, but it was higher in L. lactis species than in L. cremoris, with an exceptional GABA production of up to 14 and 24 mM in two L. lactis strains. Moreover, adding chloride increased GABA production in some L. cremoris and L. lactis strains by a factor of up to 16 and GAD activity correlated well with GABA production.

CONCLUSIONS

This genomic analysis unambiguously characterized the cremoris phenotype of L. cremoris species and modified GadB and GadR proteins explain why the corresponding strains do not produce GABA. Finally, we found that glutamate decarboxylase activity revealing GadB protein amount, varied widely between the strains and correlated well with GABA production both with and without chloride. As this protein level is associated to gene expression, the regulation of GAD gene expression was identified as a major contributor to this diversity.

The Effect of Probiotic Bacteria on Composition and Metabolite Production of Faecal Microbiota Using In Vitro Batch Cultures

Probiotic supplements are increasingly being used to target the gut microbiome with a view to improving cognitive and psychological function via the gut-brain axis. One possible mechanism behind the effect of probiotics is through alterations to microbially-derived metabolites including short-chain fatty acids (SCFA) and neurotransmitters. However, research to date has largely been conducted in animal models or under conditions irrelevant to the human gastrointestinal tract (GIT). The aim of the current work was therefore to use anaerobic, pH controlled in vitro batch cultures to (a) assess the production of neuroactive metabolites in human faecal microbiota under conditions relevant to the human GIT, and (b) to explore how several pre-selected probiotic strains may affect bacterial composition and metabolite production. Enumeration of bacteria was assessed using fluorescence in situ hybridisation with flow cytometry, and concentrations of SCFAs and neurotransmitters were measured using gas chromatography and liquid chromatography mass spectroscopy, respectively. GABA, serotonin, tryptophan, and dopamine were successfully detected, suggesting some level of microbial derivation. The addition of Lactococcus lactis W58 and Lactobacillus rhamnosus W198 resulted in a significant increase in lactate after 8 h of fermentation, while no significant effect of probiotics on bacterial composition or neurotransmitter production was found.

Molecular evolution and population genetics of glutamate decarboxylase acid resistance pathway in lactic acid bacteria

Glutamate decarboxylase (GAD) pathway (GDP) is a major acid resistance mechanism enabling microorganisms' survival in low pH environments. We aimed to study the molecular evolution and population genetics of GDP in Lactic Acid Bacteria (LAB) to understand evolutionary processes shaping adaptation to acidic environments comparing species where the GDP genes are organized in an operon structure (Levilactobacillus brevis) versus lack of an operon structure (Lactiplantibacillus plantarum). Within species molecular population genetic analyses of GDP genes in L. brevis and L. plantarum sampled from diverse fermented food and other environments showed abundant synonymous and non-synonymous nucleotide diversity, mostly driven by low frequency changes, distributed throughout the coding regions for all genes in both species. GAD genes showed higher level of replacement polymorphism compared to transporter genes (gadC and YjeM) for both species, and GAD genes that are outside of an operon structure showed even higher level of replacement polymorphism. Population genetic tests suggest negative selection against replacement changes in all genes. Molecular structure and amino acid characteristics analyses showed that in none of the GDP genes replacement changes alter 3D structure or charge distribution supporting negative selection against non-conservative amino acid changes. Phylogenetic and between species divergence analyses suggested adaptive protein evolution on GDP genes comparing phylogenetically distant species, but conservative evolution comparing closely related species. GDP genes within an operon structure showed slower molecular evolution and higher conservation. All GAD and transporter genes showed high codon usage bias in examined LAB species suggesting high expression and utilization of acid resistance genes. Substantial discordances between species, GAD, and transporter gene tree topologies were observed suggesting molecular evolution of GDP genes do not follow speciation events. Distribution of operon structure on the species tree suggested multiple independent gain or loss of operon structure in LABs. In conclusion, GDP genes in LABs exhibit a dynamic molecular evolutionary history shaped by gene loss, gene transfer, negative and positive selection to maintain its active role in acid resistance mechanism, and enable organisms to thrive in acidic environments.

Screening of lactic acid bacteria strains isolated from Iranian traditional dairy products for GABA production and optimization by response surface methodology

A total of 50 lactic acid bacteria (LAB) isolates from Iranian traditional dairy products (Motal and Lighvan cheeses, and artisanal yogurt) were screened for gamma-aminobutyric acid (GABA) production. Firstly, a rapid colorimetric test was performed to evaluate the glutamate decarboxylase (GAD) activity among the LAB isolates examined. Thin layer chromatography (TLC) was then performed on selected strains to identify isolates with high/moderate GABA producing capacity, and a GABase micro-titer plate assay was employed to quantify GABA. Finally, two Lactococcus (Lac.) lactis strains were selected for GABA production optimization via Response Surface Methodology (RSM) following Central Composite Design (CCD). Forty-one out of the 50 isolates showed GAD activity according to the colorimetric assay. Eight isolates displayed strong GAD activity, while nine showed no activity; low to moderate GAD activity was scored for all other isolates. GABA production was confirmed by TLC in all isolates with high GAD activity and in four selected among isoaltes with moderate activity. Among the Lactococcus strains tested, Lac. lactis 311 and Lac. lactis 491 were the strongest GABA producers with amounts of 3.3 and 1.26 mM, respectively. These two strains were subjected to GABA production optimization applying RSM and CCD on three key variables: Monosodium glutamate concentration (MSG) (between 25 and 150 mM), incubation temperature (between 25 and 37 °C), and pH (between 4.0 and 5.0). Optimal conditions for GABA production by Lac. lactis 311 and Lac. lactis 491 of temperature, pH and MSG concentration were, respectively, 35.4 and 30 °C, pH 4.5 and 4.6, and MSG concentration of 89 and 147.4 mM, respectively. Under the above conditions, the amount of GABA produced by Lac. lactis 311 and Lac. lactis 491 was 0.395 and 0.179 mg/mL, respectively. These strains and the optimal culture conditions determined in this study could be used for the biotechnological production of GABA or applied in food fermentations for the development of naturally GABA-enriched foods.

Phenotypic and Safety Assessment of the Cheese Strain Lactiplantibacillus plantarum LL441, and Sequence Analysis of its Complete Genome and Plasmidome

This work describes the phenotypic typing and complete genome analysis of LL441, a dairy Lactiplantibacillus plantarum strain. LL441 utilized a large range of carbohydrates and showed strong activity of some carbohydrate-degrading enzymes. The strain grew slowly in milk and produced acids and ketones along with other volatile compounds. The genome of LL441 included eight circular molecules, the bacterial chromosome, and seven plasmids (pLL441-1 through pLL441-7), ranging in size from 8.7 to 53.3 kbp. Genome analysis revealed vast arrays of genes involved in carbohydrate utilization and flavor formation in milk, as well as genes providing acid and bile resistance. No genes coding for virulence traits or pathogenicity factors were detected. Chromosome and plasmids were packed with insertion sequence (IS) elements. Plasmids were also abundant in genes encoding heavy metal resistance traits and plasmid maintenance functions. Technologically relevant phenotypes linked to plasmids, such as the production of plantaricin C (pLL441-1), lactose utilization (pLL441-2), and bacteriophage resistance (pLL441-4), were also identified. The absence of acquired antibiotic resistance and of phenotypes and genes of concern suggests L. plantarum LL441 be safe. The strain might therefore have a use as a starter or starter component in dairy and other food fermentations or as a probiotic.

Phenotype testing, genome analysis, and metabolic interactions of three lactic acid bacteria strains existing as a consortium in a naturally fermented milk

This work reports the characterization of three lactic acid bacteria (LAB) strains -Lactococcus lactis LA1, Lactococcus cremoris LA10, and Lactiplantibacillus plantarum LA30- existing as a stable consortium in a backslopping-inoculated, naturally fermented milk (NFM). This study aimed at uncovering the biochemical and genetic basis of the stability of the consortium and the cooperativity among the strains during milk fermentation. All three strains were subjected to phenotyping, covering the utilization of carbohydrates, enzyme activity, and antibiotic resistance. The strains were grown in milk individually, as well as in all possible combinations, and the resulting fermented product was analyzed for sugars, organic acids, and volatile compounds. Finally, the genomes of the three strains were sequenced and analyzed for genes associated with technological and safety properties. As expected, wide phenotypic diversity was seen between the strains. Lactococcus cremoris LA10 was the only strain to reach high cell densities and coagulate milk alone after incubation at 22°C for 24 h; congruently, it possessed a gene coding for a PrtP type II caseinolytic protease. Compared to any other fermentation, acetaldehyde concentrations were greater by a factor of six when all three strains grew together in milk, suggesting that its production might be the result of an interaction between them. Lactococcus lactis LA1, which carried a plasmid-encoded citQRP operon, was able to utilize milk citrate producing diacetyl and acetoin. No genes encoding virulence traits or pathogenicity factors were identified in any of the strains, and none produced biogenic amines from amino acid precursors, suggesting them to be safe. Lactiplantibacillus plantarum LA30 was susceptible to tetracycline, although it harbors a disrupted antibiotic resistance gene belonging to the tetM/tetW/tetO/tetS family. All three strains contained large numbers of pseudogenes, suggesting that they are well adapted ("domesticated") to the milk environment. The consortium as a whole or its individual strains might have a use as a starter or as starter components for dairy fermentations. The study of simple consortia, such as that existing in this NFM, can help reveal how microorganisms interact with one another, and what influence they may have on the sensorial properties of fermented products.

Sheep’s milk cheeses as a source of bioactive compounds

Since ancient times, sheep`s milk cheeses have been a part of a human diet. Currently, their consumption is of great interest due to its nutritional and health values. The aim of the article was to review the chemical composition of sheep’s milk cheeses and its main bioactive ingredients in the context of nutritional and health values. Sheep’s milk cheeses are rich in functionally and physiologically active compounds such as: vitamins, minerals, fatty acids, terpenes, sialic acid, orotic acid and L-carnitine, which are largely originate from milk. Fermentation and maturation process additionally enrich them in other bioactive substances as: bioactive peptides, γ-aminobutyric acid (GABA) or biogenic amines. Studies show that sheep’s milk cheese consumption may be helpful in the prevention of civilization diseases, i.e. hypertension, obesity or cancer. However, due to the presence of biogenic amines, people with metabolic disorders should be careful of their intake.

Probiotic Properties, Prebiotic Fermentability, and GABA-Producing Capacity of Microorganisms Isolated from Mexican Milk Kefir Grains: A Clustering Evaluation for Functional Dairy Food Applications

Isolation and functional characterization of microorganisms are relevant steps for generating starter cultures with functional properties, and more recently, those related to improving mental health. Milk kefir grains have been recently investigated as a source of health-related strains. This study focused on the evaluation of microorganisms from artisanal Mexican milk kefir grains regarding probiotic properties, in vitro fermentability with commercial prebiotics (lactulose, inulin, and citrus pectin), and γ-aminobutyric acid (GABA)-producing capacity. Microorganisms were identified belonging to genera Lactococcus, Lactobacillus, Leuconostoc, and Kluyveromyces. The probiotic properties were assessed by aggregation abilities, antimicrobial activity, antibiotic susceptibility, and resistance to in vitro gastrointestinal digestion, showing a good performance compared with commercial probiotics. Most of isolates maintained a concentration above 6 log colony forming units/mL after the intestinal phase. Specific isolates of Kluyveromyces (BIOTEC009 and BIOTEC010), Leuconostoc (BIOTEC011 and BIOTEC012), and Lactobacillus (BIOTEC014 and BIOTEC15) showed a high fermentability in media supplemented with commercial prebiotics. The capacity to produce GABA was classified as medium for L. lactis BIOTEC006, BIOTEC007, and BIOTEC008; K. lactis BIOTEC009; L. pseudomesenteroides BIOTEC012; and L. kefiri BIOTEC014, and comparable to that obtained for commercial probiotics. Finally, a multivariate approach was performed, allowing the grouping of 2-5 clusters of microorganisms that could be further considered new promising cultures for functional dairy food applications.