Microbial interactions in food model systems: In situ antilisterial activity of mundticin KS producing strains

It is known that microbial interactions are important for the success of food fermentations, as well as for the establishment of the safety of the final products. In this study, we characterized extensively three enterococci (WFE3, WFE20 and WFE31) of flour origin, which were found to be active against Listeria monocytogenes, in order to investigate on their ability to produce bacteriocins in complex food model systems for their future industrial/food applications. The identification of the three bacterial isolates was performed by means of a combined 16S rRNA gene sequencing and multiplex PCR approach. Two isolates belonged unequivocally to the species E. mundtii, while one isolate could not be allotted into any described Enterococcus species. The randomly amplified polymorphic DNA (RAPD) analysis recognized three distinct strains. The supernatants were mainly active against Listeria spp., but some lactic acid bacteria were also inhibited. The proteinaceous nature of the three supernatants was detected after treatment with proteinase K, protease B and trypsin. The bacteriocins were found to be heat resistant, stable in a large pH range and in presence of ethanol. The bacteriocins were not adsorbed onto the surface of the producer cells and their mode of action was bactericidal. The production of bacteriocins was higher at neutral pHs and temperatures in the range 30-37°C. The active supernatants did not show cytotoxicity and the three strains were susceptible to the action of common antibiotics. The genetic characterization of the bacteriocin genes showed that all three strains produced mundticin KS. In order to evaluate the effect of different food components on the inhibitory activity of the E. mundtii strains, five food model systems were prepared from fresh vegetables, cereals, cheeses, meats and fishes. The broths were inoculated first singly with the strains WFE3, WFE20 and 6WFE31 at a final concentration of approximately 10 CFU ml-1. We observed that the three strains produced mundticin KS in all the five food model systems. Then the efficacy of the three E. mundtii strains was evaluated in situ against L. monocytogenes. The five model systems obtained from different food matrices were inoculated singly with one of the bacteriocin producing E. mundtii strains and 4 -1L. monocytogenes ATCC 19114 added at approximately 10 CFU mL . The in situ antilisterial activities of the strains WFE3, WFE20 and WFE31 were quantitatively different.