Abstract
The bacteriocin-producing strain Enterococcus faecium ST10Bz, isolated from boza, a Bulgarian cereal-based beverage, exhibited strong activity against Listeria strains, vancomycin-resistant and other Enterococcus strains, but not against most of the other lactic acid bacteria (LAB) strains included in the test panel. Bacteriocin ST10Bz was proven as a stable antimicrobial, even after exposure to various environmental conditions, including varying pH values, temperatures, and commonly used chemicals in industry and laboratory practice. Bacteriocin activity against L. monocytogenes ATCC®15313™ was recorded at 25,600 AU/mL when the producer strain was cultured in MRS broth at 25 °C and 30 °C, and 19,200 AU/mL, when cultured at 37 °C. Additionally, bacteriocin ST10Bz exhibited bactericidal mode of action when added to actively growing cultures of L. monocytogenes ATCC®15313™ and Enterococcus faecalis 200A. E. faecium ST10Bz was susceptible to the antibiotics kanamycin, gentamycin, ampicillin, streptomycin, tylosin, chloramphenicol, clindamycin, tetracycline, and vancomycin; with no evidence for vanA, B, C, D, E, or G genes. PCR analysis of DNA from strain ST10Bz generated positive results for presence of some bacterial adhesion genes, including map, mub and ef-tu, as well as the gamma aminobutyric acid (GABA) production-related gene, gad. Under simulated gastrointestinal conditions in single and co-culture with L. monocytogenes ATCC®15313™ and E. faecalis 200A, E. faecium ST10Bz showed a high survival rate and the ability to reduce the viable numbers of the two test strains.
Highlights
Several species belonging to the group of Lactic acid bacteria (LAB) are Generally Recognized As Safe (GRAS) [1], having different biotechnological applications such as in the food and feed fermentation processes, and can produce metabolites of commercial importance, including organic acids, antimicrobial and aromatic compounds, exopolysaccharides, vitamins, and bio-active peptides, that may contribute to the sensory, nutritional and safety properties of fermented food products [2]
Application of the triple layer method in the selection of potential bacteriocinogenic strains from boza resulted in the selection of a total 71 strains exhibiting inhibitory activity against the applied test strains, L. monocytogenes ATCC®15313TM and E. faecalis 200A
From all the isolates, only four were found to be of bacteriocinogenic nature. This property of the produced inhibitory substance was confirmed by treating cell free supernatants (CFS) with proteolytic enzymes, and the loss of antimicrobial activity was considered as confirmation of the proteinaceous nature of the antimicrobial substance
Summary
Several species belonging to the group of Lactic acid bacteria (LAB) are Generally Recognized As Safe (GRAS) [1], having different biotechnological applications such as in the food and feed fermentation processes, and can produce metabolites of commercial importance, including organic acids, antimicrobial and aromatic compounds, exopolysaccharides, vitamins, and bio-active peptides, that may contribute to the sensory, nutritional and safety properties of fermented food products [2]. Microbial metabolites, including the bioactive compounds produced during fermentation processes, as well as cell structures, are collectively termed as postbiotics [4]. These comprise all the non-viable structures and components of microbial cells. Antibiotics, described as of low-molecular weight and mostly non-proteinaceous, are among the most popular antimicrobial metabolites produced by microorganisms. These antimicrobial compounds were even used as growth promoters in animal farming, were eventually banned in several countries, as these were considered to selectively promote antibiotic resistance of bacterial populations [5]. The lack of urgency in addressing this problem may lead to a post-antibiotic era where common microbial infections may become highly problematic, probably like the time before the discovery of antibiotics
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