Thirty‐two foodborne bacterial isolates were tested as potential competitors of Listeria monocytogenes strain LM82 during enrichment because of their resistance to the selective agents in Listeria enrichment and isolation media. Competitive ability of each isolate was classified as weak, moderate, or strong by determining the ratio at which it masked identification of LM82 at an inoculation concentration of 10 colony forming units (CFU)/10 mL of Listeria enrichment broth. Of the competitive isolates identified, six were Enterococcus spp., two were Staphylococcus spp., and one was a Corynebacterium sp. Although several strains of Enterococcus faecium were examined, not all were competitive. Of six other bacterial strains associated with food fermentations and tested for competitiveness with LM82, one, a Gram‐positive tetrad, was competitive. This study showed that although food microfloral strains that are able to survive in enrichment and isolation environments are fairly common, they do not necessarily compete with Listeria. Not all strains in a competitive species are necessarily competitive.

The survival of virulent and avirulent Listeria species internalized in cells of a murine macrophage-like cell line, RAW264.7, was monitored. Mouse macrophage cells (ca. 5 × 105/ml) suspended in fresh RPMI medium 1640 containing fetal bovine serum were mixed with 5 × 107 to 5 × 108 Listeria cells per ml and incubated 1 h at 37°C with CO2-enriched air. Gentamicin (10 μg/ml) was added to kill bacteria not internalized by the cells. At 2, 4, and 6 h postinfection, 10-μl amounts of the suspensions were lysed in microtiter plate wells during serial decimal dilution in water. Triplicate dilutions (10-μl each) were plated on trypticase soy agar, and colonies were counted after 48 h incubation at 35°C. About 0.1 to 1% of the added hemolytic pathogen L. monocytogenes Scott A and the avirulent nonhemolytic L. innocua were internalized at 2 h. The number of internal L. monocytogenes cells increased significantly by 6 h, but L. innocua cells showed no significant change. A strain of the hemolytic species L. seeligeri behaved like the nonhemolytic L. innocua. This distinction between the intracellular behavior of pathogenic and nonpathogenic species, if a general phenomenon, may be useful as an in vitro virulence assessment parameter.

Extracellular antimicrobial substances produced by certain enterococci inhibit Listeria monocytogenes. Enterococcus faecium 108, a competitive food isolate, produced a heat‐stable and protease‐sensitive anti‐L. monocytogenes bacteriocin‐like substance (Ef108) in Listeria selective enrichment broth and other media. Ef108 activity was purified to homogeneity by a four‐step procedure including (NH4)2SO4 fractionation, chromatography on anion exchange QSepharose column, and two Superose 12 gel filtration columns. In activities represented by two peaks (Ef108A and Ef108B), 90% of the crude activity was due to peak B. Ef108A is believed to be a variable aggregate of the active moiety in Ef108B. All Listeria spp. and five L. monocytogenes serotypes were inhibited by Ef108B in several media including Listeria enrichment medium. The Ef108 activity may be due to a bacteriocin‐like inhibitory substance produced by E. faecium 108 that may suppress the growth and predominance of Listeria monocytogenes during selective enrichment.

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.