Investigation of the Mechanisms by Which Listeria monocytogenes Grows in Porcine Gallbladder Bile

The food-borne pathogen Listeria monocytogenes is known to colonize the lumen of the gallbladder in infected mice and to grow rapidly in this environment (J. Hardy et al., Science 303: 851-853, 2004). However, relatively little is known about the mechanisms utilized by the pathogen to survive and grow in this location. We utilized gallbladder bile (GB bile) isolated directly from porcine gallbladders as an ex vivo model of gallbladder growth. We demonstrate that GB bile is generally nontoxic for bacteria and can readily support growth of a variety of bacterial species including L. monocytogenes, Lactococcus lactis, Salmonella enterica serovar Typhimurium, and Escherichia coli. Significantly, L. monocytogenes grew at the same rate as the nonpathogenic species Listeria innocua, indicating that the pathogen does not possess specialized mechanisms that enable growth in this environment. However, when we reduced the pH of GB bile to pH 5.5 in order to mimic the release of bile within the small intestine, the toxicity of GB bile increased significantly and specific resistance mechanisms (Sigma B, BSH, and BilE) were essential for survival of the pathogen under these conditions. In order to identify genetic loci that are necessary for growth of L. monocytogenes in the gallbladder, a mariner transposon bank was created and screened for mutants unable to replicate in GB bile. This led to the identification of mutants in six loci, including genes encoding enzymes involved in purine metabolism, amino acid biosynthesis, and biotin uptake. Although GB bile does not represent a significant impediment to bacterial growth, specific metabolic processes are required by L. monocytogenes in order to grow in this environment.