Transcriptional activation of virulence genes in wild-type strains of Listeria monocytogenes in response to a change in the extracellular medium composition

A panel of 103 Listeria monocytogenes strains of different origins was examined for haemolysin and lecithinase production in brain-heart infusion (BHI). Three distinct phenotypes were observed. Phenotype 1 was characterized by low to undetectable levels of expression and was exhibited by almost all strains tested. Phenotype 2 expressed high levels of haemolysin and lecithinase and was displayed by five strains: one (P14-A) was a spontanous mutant derived from a type 1 isolate (P14); the four others (EGD-A, NCTC 7973, SLCC 2373 and CLIP 545) were all laboratory strains kept under in vitro conditions for a long period. Phenotype 3 was intermediate and was exhibited by another laboratory strain (L028). We therefore concluded that phenotype 1 corresponded to the wild type, whereas phenotypes 2 and 3 represented mutant or variant phenotypes. Interestingly, wild-type strains were able to dramatically increase the expression of virulence factors when cultured in BHI treated with activated charcoal (BHIC), up to levels similar to those constitutively expressed by the hyperhaemolytic/lecithinase variants in BHI. Experiments with P14 and P14-A demonstrated that both charcoal and the hyperhaemolytic/lecithinase mutation exerted their effect by inducing (or derepressing) transcription of prfA, the pleiotropic transcriptional activator of the L. monocytogenes virulence regulon. Moreover, P14 and P14-A were equally virulent for mice despite the different levels of virulence factor expression in BHI. Taken together, these observations indicate that L. monocytogenes turns off virulence gene expression when growing in vitro in a rich medium, and suggest that the increased levels of virulence factors in the hyperhaemolytic/lecithinase mutants and in wild-type strains grown in BHIC might represent the levels of expression needed in vivo by L. monocytogenes for infecting host tissues.