The contribution of bacteriocin to inhibition of Listeria monocytogenes by Carnobacterium piscicola strains in cold-smoked salmon systems

Aims: To study the importance of bacteriocin production for the antilisterial effect of a bacteriocinogenic Carnobacterium piscicola strain A9b on growth of Listeria monocytogenes in broth and cold-smoked salmon systems. Methods and Results: Acriflavin treatment of strain A9b resulted in loss of bacteriocin production and of immunity to carnobacteriocin B2. Two plasmids present in the wild-type were lost in the variant that was also more sensitive to bavaricin and leucocin A than the wild-type indicating cross-resistance to class IIa bacteriocins. The growth rate of the bac(-) mutant was higher than that of the wild-type at 5 and 37degreesC but not at 25 or 30degreesC. In salmon juice the maximum cell density of L. monocytogenes was suppressed 3 and 6 log by co-culture with C. piscicola A9b bac(-) and bac(+), respectively, as compared with the control. Sterile filtered cultures of C. piscicola A9b bac(-) caused a limited suppression of the maximum cell density of L. monocytogenes similar to that observed when sterile buffer was added in equal amounts. Semi-purified carnobacteriocin B2 caused a 3.5 log decline in viable cell count after 6 day of incubation in cold-smoked salmon juice at 5degreesC. High resistance level to carnobacteriocin B2 was observed for L. monocytogenes cells exposed to semi-purified and in situ produced carnobacteriocin B2. Conclusions: The presence of bacteriocin production in C. piscicola enhances its inhibition of L. monocytogenes Significance and Impact of the Study: Due to the emergence of resistance, a bacteriocin negative lactic acid bacteria may be more suited for practical use as a bioprotective agent against L. monocytogenes in ready-to-eat foods.