A PrfA-regulated bile exclusion system (BilE) is a novel virulence factor in Listeria monocytogenes

The ability to colonize the gall bladder has recently been shown to be an important feature of virulent Listeria monocytogenes (J. Hardy, K. P. Francis, M. DeBoer, P. Chu, K. Gibbs, C. H. Contag. Science 303: 851-853, 2004). We suggest that the cytotoxic effects of bile may be increased upon release from the gall bladder into the upper small intestine, and report the identification of a novel bile exclusion system which plays an essential role in intestinal colonization and virulence of L. monocytogenes. In silico analysis of the L. monocytogenes EGDe genome revealed a two-gene operon (formerly opuB) exhibiting significant sequence similarity to members of the betaine carnitine choline transporter (BCCT) family. The operon, herein designated bilE (bile Exclusion) is preceded by consensus sigma(A)- and sigma(B)-dependent promoter-binding sites and is transcriptionally upregulated at elevated osmolarities and reduced temperatures (stresses known to induce sigB). Furthermore, a significant reduction in the level of bilE transcription was observed in the absence of sigma(B). In addition, we demonstrate an important role for PrfA, the master regulator of virulence potential in L. monocytogenes, in coordinating bilE expression. Computational structural analysis suggests that, rather than functioning as a compatible solute uptake system as was previously believed, BilE is more likely to be an exclusion system, a conclusion substantiated by radiolabelled bile accumulation studies. In addition, functionally inactivating BilE resulted in a five-log reduction in the ability of the bacterium to tolerate lethal concentrations of bovine bile (oxgall) and also significantly increased sensitivity to physiological concentrations of human bile, a phenotype which translates to a significant reduction in virulence potential when administered to a murine model by the oral route. Thus, this novel bile exclusion locus bilE, coordinately regulated by sigma(B) and PrfA, represents a new and important virulence factor in L. monocytogenes.