The pathogenesis of Listeria monocytogenes depends on its ability to attach to and invade the gastrointestinal epithelium and subsequently withstand the host immune response. Despite a thorough understanding of the intracellular phase of infection, relatively little is known about how the pathogen behaves in the gastrointestinal tract and whether it is affected by the presence of host commensal microbiota. Lactobacillus and Bifidobacterium are two important genera of the human gut microbiota proposed to possess probiotic effects. Here we demonstrate that probiotic bacteria significantly inhibit subsequent listerial infection in an in vitro C2Bbe1 epithelial cell model. In the case of Lactobacilli, inhibition was due to a combination of acid production and secretion of an as yet unidentified protein. In the case of Bifidobacterium, inhibition was attributable to an extracellular proteinaceous secreted compound. In addition, we observed a significant reduction in interleukin-8 and an increase in IL-10 cytokines secreted from epithelial cells following probiotic pretreatment and subsequent infection with Listeria. A reduction in the infection of epithelial cells and an altered mucosal immune response suggests that probiotic bacteria could be of therapeutic benefit against listerial infection. This study infers a role for probiotic bacteria as an antagonist of Li. monocytogenes infection.
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