Abstract
Nisin A belonging to the class I bacteriocins and produced by Lactococcus lactis subsp. lactis is widely used in many countries as highly efficient and safe preservative preventing growth of undesirable bacteria in food products. Though this compound is efficient at very low concentrations, reduction of its manufacturing cost is still relevant problem. An increased nisin A production requires improved resistance of its producer to nisin. According to some studies, mechanisms of microbial resistance to nisin A and bacitracin have a similar basis, and the same transporters are used to export these antibiotics from cells. To obtain strains with improved growth rate and nisin A productivity, selection of spontaneous bacitracin-resistant L. lactis mutants followed by examination of their stability as well as physiological and fermentation characteristics was carried out. Spontaneous mutants were obtained by culturing of L. lactis VKPM B-2092 strain on selective bacitracin-containing agar medium. The obtained bacitracin-resistant strain FL-75 was characterized by accelerated growth rate, doubled biomass accumulation, and improved nisin A resistance. The nisin A productivity of FL-75 exceeded that of the parental strain by 25% reaching 8902 U/mL after 14-h cultivation. In addition, FL-75 was characterized by the improved resistance to oxidative stress that has never been reported earlier for bacitracin-resistant microorganisms. Based on the performed characterization of FL-75, we can consider it as a new independent strain promising for the industrial production of food and feed biopreservatives. Comparison of published data and the obtained results allowed us to suppose that the bacitracin resistance mutation in FL-75 is determined rather by an increased expression of a gene homologous to the bcrC gene of Bacillus sp. than by the activation of multidrug resistance mechanisms. The revealed resistance of FL-75 to bacitracin and oxidative stress can be regulated by a common transcription factor activating in response to various environmental stresses.
Highlights
Many lactic acid bacteria possess antimicrobial activity caused by the production of bacteriocins [1]
Nisin A belonging to the class I bacteriocins and produced by Lactococcus lactis subsp. lactis is widely used in many countries as highly efficient and safe preservative preventing growth of undesirable bacteria in food products
Nisin A belongs to the subgroup of lantibiotics, low-molecular bacteriocins containing lanthionine and methyl lanthionine, which are produced by the binding of rare amino acids with cysteine residues via thioether bridges [2]
Summary
Many lactic acid bacteria possess antimicrobial activity caused by the production of bacteriocins [1]. Bacteriocins represent heterogeneous group of proteins consisting of both small peptides and lowmolecular proteins with the molecular mass up to 30 kDa. Nisin A belongs to the subgroup of lantibiotics, low-molecular bacteriocins containing lanthionine and methyl lanthionine, which are produced by the binding of rare amino acids (dehydrobutyrine or dehydroalanine) with cysteine residues via thioether bridges [2]. Nisin A belongs to the subgroup of lantibiotics, low-molecular bacteriocins containing lanthionine and methyl lanthionine, which are produced by the binding of rare amino acids (dehydrobutyrine or dehydroalanine) with cysteine residues via thioether bridges [2] Due to such structure, lantibiotics provide high antimicrobial activity even at nanomolar concentrations [3]. Nisin A is able to inhibit some Gram-negative bacteria in the case of their previous treatment with chelators, such as EDTA, or pyrophosphate, which are able to increase the permeability of the thick cell wall of these bacteria [7,8]
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