Behavior of Listeria monocytogenes in the Presence of Streptococcus lactis in a Medium with Internal pH Control.

Growth of Listeria monocytogenes strains V7, Scott A, and California (initial inoculum 10/ml) at 21 or 30°C in the presence of Streptococcus Iactis (initial inoculum 0.25 or 1.0%) was determined using a medium with internal pH control (IPCM-1). The pH of the uninoculated medium (control) was 7.0 before and after incubation. Populations of L. monocytogenes in IPCM-1 without S. lactis after 30 h at 21°C were ca. 10/ml for strains V7 and Scott A and ca. 10/ml for strain California, and at 30°C they were ca. 10/ml for all three strains. When data were plotted, areas of graphs between curves representing controls and treatments were calculated to quantitate the extent of inhibition of L. monocytogenes caused by S. lactis . Each such area is called the "area of inhibition" (AI). Growth of the pathogen was inhibited by S. lactis ; the degree was dependent on temperature and concentration of lactic culture and, in some instances, strain of Listeria . Greatest inhibition of each strain occurred with the largest inoculum of S. lactis and at the highest temperature. No significant difference (p>0.05) in AI or pH among the three strains was observed at 21°C. At 30°C, strain California was inhibited significantly more (p<0.05) than V7 or Scott A by both concentrations of S. lactis at 24 and 30 h of incubation. No significant difference (p>0.05) in pH was found at this temperature regardless of concentration of S. lactis or strain of Listeria . IPCM-1 inoculated with a lactic starter culture is ready for use at pH 5.5 after 15-18 h of incubation. Inhibition of Listeria was not complete at this pH under any of the experimental conditions. Substantial numbers of L. monocytogenes (10-10 CFU/ml) were present when this medium was ready for use to produce cultured dairy foods.