Influence of environmental conditions on the expression of virulence factors by Listeria monocytogenes and their use in species identification.

The hemolytic, lecithinase or phosphatidylinositol-specific phospholipase C activities of Listeria monocytogenes can be used to differentiate this pathogenic bacteria from L. innocua, apathogenic, frequently isolated from environmental sources and food. However, the interpretation of these characteristics is problematic because of the variation in the expression of virulence factors by L. monocytogenes, which can be influenced by environmental conditions. We used a cheap, simple plate assay to monitor this expression in strains obtained from various sources and grown under different culture conditions. The results were increasingly significant and were obtained adding activated charcoal and different salts to the culture media, and in some cases changing the culture temperature, all with a rigorous control on the process of media sterilization.