ANTAGONISM OF BACTERIAL COMPETITORS TOWARD LISTERIA MONOCYTOGENES IN ENRICHMENT CULTURE

A study was made of the competitiveness toward Listeria monocytogenes ( Lm82 ) in Listeria enrichment broth (LEB) by bacteria isolated from foods and by strains of Enterococcus and other Gram-positive bacteria. Competitive (i.e., able to mask during enrichment in LEB for 24 h) and noncompetitive bacteria were tested for production ofanti-Lm82 agents in diffusion zone assays on deMann-Rogosa-Sharpe (MRS) agar with added beta-glycero-phosphate (MRSB) and in Listeria enrichment agar (LEA). Enterococci were the most active competitors. The presence of small (2–6 mm diameter) inhibitory zones on MRSB correlated significantly with competitive activity in LEB; however, the correlation was not due to the metabolic activity that produced inhibitory zones on MRSB. Zone-producing bacteria were more likely to be competitors than were nonzone producers, but not all zone producers were competitors. Similarly, about 15% of bacteria that did not produce zones were competitive. The few inhibitory zones on LEA indicated that competitor activity in the selective enrichment broth may only rarely be due to the production of diffusible inhibitors. The most important factor in competitiveness was the ability of enterococci and some other bacteria to maintain superior numbers in the presence of prolisterial selective agents in LEB. With their superior numbers, competitors significantly decreased the pH of LEB. faster than did noncompetitors. Diffusible inhibitors produced in LEA by bacteria may also contribute significantly to competitiveness .