Plasmid profile of lactic acid bacteria with antifungal properties.

The present investigation was carried out to determine if metabolites of lactic acid bacteria can be used as a natural preservative in the dairy industry. Four standard lactic cultures Lactococcus lactis ssp lactis (94), Lactococcus lactis var. diacetylactis (60), Lactobacillus acidophilus (014) and Lactobacillus acidophilus (015) were found to have inhibitory activity towards one or two of the mould cultures tested. Among the lactic cultures tested, Lactobacillus acidophilus (015) was found to be the most promising culture. Plasmid profile analysis showed Lactococcus lactis ssp.lactis (94) and L. lactis var diacetylactis (60) to have a single plasmid of size 26.69kb. Lactobacillus acidophilus (014) and Lactobacillus acidophilus (015) showed a single plasmid of size 19.91 kb. In order to establish the involvement of a particular plasmid with a specific phenotype in the lactic cultures in vitro curing experiments were conducted by physical and chemical methods. In physical method of curing, the cultures were grown at elevated temperature (45 0 C) and it was found that there was loss of plasmid in all the cultures. Subsequently the cell free supernatant of the cured cultures did not show inhibitory activity towards the moulds tested. In the chemical method of curing using intercalating dye ethidium bromide at 4, 6, 8, 10 µg / ml of broth the cell free supernatant of all the cured cultures did not exhibit antimycotic activity. Intergeneric mating by conjugation was tried between L. lactis var. diacteylactis (60) also showing antibacterial activity and L. acidophilus (015) showing antifungal activity using filter mating technique. The results revealed non transfer of plasmid between the two strains suggesting that probably the plasmid responsible for that particular phenotype is not of conjugative nature.