The effect of lactic acid production of Lactobacillus casei on the growth characteristics of Listeria monocytogenes was examined in milk. The effect of temperature (7, 13, 20 °C) and the co-ratios with Lb. casei at different cell count ratios (10:1, 100:1, 10 000:1) on the growth of L. monocytogenes were studied. At a 10 000:1 ratio induction of an early stationary phase of L. monocytogenes occurred. An exponentially decreasing correlation was observed between the logarithmic maximum population of L. monocytogenes and the initial log counts of lactobacilli. The Baranyi model was fitted to the obtained growth curves. The Baranyi equation provided good fit for the lag and exponential phase of L. monocytogenes. According to our observations, pH decrease does not seem to be the main factor of early stationary phase induction of L. monocytogenes in milk.
Lactic acid bacteria isolated from commercially produced alfalfa sprouts were screened for activity against Listeria monocytogenes F4258. Most active isolates were identified as Lactococcus lactis subsp. lactis. The isolates fell into two categories, strains that inhibited by acid production only, and strains that appeared to have additional inhibitory activity. An acid-only isolate, SP26, was used to evaluate the effect of initial pH (5, 6, 7, 8) and temperature (10, 20, and 30 °C) on the interaction between the lactic acid bacterium and L. monocytogenes using “sprout juice” as a model system. The model system was inoculated with an initial level of approx. 103 CFU ml-1L. monocytogenes in both mono-culture controls and the co-cultures and the co-cultures with L. lactis (103-104 CFU ml-1). The primary inhibitory effect attributable to L. lactis was a 2 to 3 log cycle decrease in the maximum population density obtained by L. monocytogenes. The extent of the inhibition was decreased at 10 °C, but was only slightly affected by pH in the range of 6.0 to 8.0. L. monocytogenes did not grow in the sprout broth at pH 5.0 at any of the incubation temperatures.