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Listeria monocytogenes is a foodborne pathogen which can cause lethal infections in immunocompromised individuals. These infections involve meningitis in the elderly or spontaneous abortions of neonates--both scenarios result from Listeria crossing the intestinal barrier. The conditions that promote Listeria invasion during the intestinal phase of infection are not clearly defined. We have evidence that suggests intestinal fermentation acids as potential signals for Listeria virulence regulation. Therefore, we hypothesized that probiotic bacteria, which generate different fermentation acids, will exhibit different levels of inhibition against Listeria virulence. To test how different probiotic bacteria affect Listeria virulence, we used two commercially available probiotics from Phillips, Colon Health and Digestive Health Support, each containing a unique mixture of bacteria. First, a co-culture experiment between probiotic bacteria and Listeria was conducted to determine the probiotics ability to inhibit Listeria growth. Second, we tested Listeria survival in the fermentation products generated by these probiotic bacteria. Finally, we tested how the fermentation products affect Listeria production of the virulence factor listeriolysin O (LLO). Listeria growth was reduced when co-culturing with either of the two probiotics with both probiotics showing similar levels of suppression. After five hours of incubation in the supernatant of probiotic cultures, Listeria survival was significantly reduced in the Digestive Health Support probiotic compared to the Colon Health probiotic. Exposure to supernatant from the Digestive Health Support probiotic also significantly reduced LLO production. Taken together, Digestive Health Support probiotics exhibited stronger overall inhibitory activity against Listeria fitness and virulence. Future investigations will focus on determining the chemical composition of the probiotics fermentation products to explain the different responses in Listeria. Probiotics are quickly gaining popularity which argues for better understanding of their effects. Understanding their effects on foodborne pathogens will pave the way for applying appropriate probiotics as effective preventative and treatment strategies.

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Independent Research

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Yvonne Sun

Primary Advisor's Department



Stander Symposium poster


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