Microbial competition: Effect of culture conditions on the suppression of Listeria monocytogenes Scott A by Carnobacterium piscicola

The effects of temperature (4, 12, and 19 degrees C), pH (5, 6, and 7), and NaCl (5, 25, and 45 g/liter) on the growth of Listeria monocytogenes Scott A in the presence of either Carnobacterium piscicola LK5 or 2762 were studied quantitatively in brain heart infusion broth. Strain LK5 produces a bacteriocin that is released into the environment, whereas 2762 appears to produce a bacteriocin that remains cell associated. The primary effect of both C. piscicola strains was a suppression of the maximum population density (MPD) attained by L. monocytogenes. The extent of this depression was dependent on the three culture variables, and appeared to be a function of their influence on the relative growth rates of the two species. The effects were similar with both strains. However, two bacteriocin-negative strains, 2305 and 2818, also depressed the growth of L. monocytogenes. Little of the C. piscicola isolates' ability to suppress L. monocytogenes appeared attributable to bacteriocin production. The MPD-depressing activity of 2762 could not be attributed to peroxide, pH depression, or oxygen depletion. However, MPD suppression may involve nutrient depletion, since the extent of MPD suppression was decreased in a dose-related manner when the two species were cultured in 3 x and 6 x brain heart infusion broth.