Lisa Orfe¹, Edith Orozco², and Douglas R. Call¹

Listeria monocytogenes a gram-positive food-borne bacterial pathogen, is responsible for approximately 500 deaths a year in the United States.  However, not all strains of L. monocytogenes are created equal. For example, during an epidemic outbreak of Listeriosis some strains will cause severe disease while other, co-disseminated strains do not appear linked to any disease. Therefore, determining what makes epidemic strains of L. monocytogenes pathogenic is critically important to devising improved detection, subtyping, and intervention methods and strategies. Our group recently described a collection of epidemic strains of L. monocytogenes that were more infective in an oral mouse challenge model compared with their environmental counterparts. One explanation for this difference is that epidemic strains are better able to withstand exposure to the low pH and bile encountered in the gastrointestinal tract as compared to most environmental strains. In this study we are evaluating survivorship and gene expression of epidemic and non-epidemic strains in response to both acid and bile stresses. Thirteen strains (7 epidemic and 6 environmental) have been analyzed for their response to both stresses (3 biological replicates per strain); it was found that there was essentially no difference in survivorship following acid stress.  However, contrary to expectations, the epidemic strains seemed more susceptible to bile stress than the environmental isolates. We are currently analyzing gene expression data from 115 microarray hybridizations in the hopes of identifying a unique transcriptome for either acid and/or bile stress.