Comparative analysis ofLactococcus lactis bacteriocins and preliminary characterisation of a new proteinase K resistant lactococcin member

Detection of lactic acid bacteria (LAB) bacteriocins producers is of great significance for food industry to establish starter bacterial association and to improve food safety. Eighty oneLactococcus lactis strains, isolated from traditional Tunisian dairy products, were screened for their antibacterial activity. Bacteriocin production in the supernatant was demonstrated for twelve strains by the well diffusion assay, protease susceptibility and by direct detection of the activity on SDS-PAGE. By using PCR with primers targeting structural genes of nisin and lactoccocin 481, we were able to predict their presence in 10 and one strain respectively. No amplifications were recorded with primers targeting lactococcin A, lactococcin 972 and bacteriocin J46 ofL. lactis. The remaining unidentified bacteriocin produced by strain BMG 6.25 and designed as lactococcin IAF 25, was further characterised. IAF 25 was shown to be a heat-stable proteinaceous inhibitory factor sensitive to papain and trypsin but resistant to proteinase K treatment. IAF 25 has an apparent molecular weight of 6 kDa and showed a narrow antimicrobial activity spectrum against closely related bacteria and genera. These original characteristics amongL. lactis bacteriocins coupled with cross inhibition tests with bacteriocin producers reference strains, led to the assumption of the novelty of lactococcin IAF 25.