Specific osmolyte transporters mediate bile tolerance in Listeria monocytogenes

The food-borne pathogenic bacterium Listeria monocytogenes has the potential to adapt to an array of suboptimal growth environments encountered within the host. The pathogen is relatively bile tolerant and has the capacity to survive and grow within both the small intestine and the gallbladder in murine models of oral infection. We have previously demonstrated a role for the principal carnitine transport system of L. monocytogenes (OpuC) in gastrointestinal survival of the pathogen (R. Sleator, J. Wouters, C. G. M. Gahan, T. Abee, and C. Hill, Appl. Environ. Microbiol. 67:2692-2698, 2001). However, the mechanisms by which OpuC, or indeed carnitine, protects the pathogen in this environment are unclear. In the current study, systematic analysis of strains with mutations in osmolyte transporters revealed a role for OpuC in resisting the acute toxicity of bile, with a minor role also played by BetL, a secondary betaine uptake system which also exhibits a low affinity for carnitine. In addition, the toxic effects of bile on wild-type L. monocytogenes cells were ameliorated when carnitine (but not betaine) was added to the medium. lux-promoter fusions to the promoters of the genes encoding the principal osmolyte uptake systems Gbu, BetL, and OpuC and the known bile tolerance system BilE were constructed. Promoter activity for all systems was significantly induced in the presence of bile, with the opuC and bilE promoters exhibiting the highest levels of bile-dependent expression in vitro and the betL and bilE promoters showing the highest expression levels in the intestines of orally inoculated mice. A direct comparison of all osmolyte transporter mutants in a murine oral infection model confirmed a major role for OpuC in intestinal persistence and systemic invasion and a minor role for the BetL transporter in fecal carriage. This study therefore demonstrates a previously unrecognized function for osmolyte uptake systems in bile tolerance in L. monocytogenes.