SIMULTANEOUS GROWTH OF LISTERIA-MONOCYTOGENES AND LISTERIA-INNOCUA

Listeria spp. have been isolated from a wide variety of sources, and in many situations Listeria innocua is more commonly found than Listeria monocytogenes. Growth of three L. monocytogenes strains was studied when inoculated simultaneously with a rhamnose negative L. innocua strain into culture media and cheese sauce. Fraser broth (FB), Trypticase(TM) soy broth plus 0.6% yeast extract (TSB-YE), University of Vermont medium (UVM) modified Listeria enrichment broth, and cheese sauce were inoculated (ca. 10(2) cells per ml) and incubated for 24 h; FB, TSB-YE, and cheese sauce at 35-degrees-C, UVM at 30-degrees-C. Growth of four rhamnosepositive. L innocua strains was also studied in culture media. Growth of L monocytogenes was similar to that for L. innocua in TSB-YE or cheese sauce. However, in FB and UVM, L. innocua populations were significantly higher than L. monocytogenes. This occurred when media were inoculated individually or simultaneously. This may explain in part why L innocua is isolated more frequently than L. monocytogenes from foods and environmental samples.