P-010. Listeria monocytogenes Strains with Invasion Attenuating Mutations in inlA and Strains from Outbreaks of Human Disease Establish Similar Resultant Populations in Host-and Food Associated Environments

J. L. Corron, J. M. Simpson, S. K. Williams, K. K. Nightingale;
Colorado State Univ., Fort Collins, CO.

Listeria monocytogenes is a foodborne pathogen that causes the potentially fatal disease listeriosis. L. monocytogenes appears to contain two distinct subpopulations, including (i) epidemic clone strains linked to the majority of human epidemic and sporadic listeriosis cases and (ii) invasion attenuated strains that carry premature stop codons (PMSCs) in inlA, which represent a significant proportion of L. monocytogenes isolated from ready-to-eat foods (> 30%) but have only been linked to 2% of human listeriosis cases. We thus hypothesize that epidemic clone strains are adapted to infect human hosts, while invasion attenuated strains with a PMSC in inlA are better adapted to a food niche. We determined the Caco-2 cell invasiveness of 15 human clinical isolates from listeriosis outbreaks worldwide and identified four isolates with enhanced invasiveness. We also selected four food isolates that demonstrated significantly (P = 0.0004) attenuated invasion due to PMSCs in inlA. We assessed the ability of epidemic clone strains with enhanced invasiveness and invasion attenuated strains from food to survive and proliferate in the host using an intracellular growth assay and in food environments using a low-inoculum cold growth assay. Similar initial populations of epidemic clone and invasion attenuated strains (P > 0.05) were phagocytized by J774 mouse macrophages, yet the epidemic clone strains grew to significantly higher levels (P = 0.002) by 9 hours, but by 12 hours no significant differences existed between the two groups (P > 0.05). Low initial populations of epidemic clone and invasion attenuated strains (2x102 CFU/ml) were inoculated into brain heart infusion broth and maintained at 7oC for 18 d to simulate refrigerated storage and all isolates grew similarly and exceeded a level of 2x109 CFU/ml by day 12. Our findings support the conclusion that strains responsible for the majority of human disease and invasion attenuated strains show similar abilities to proliferate in host and food associated environments, providing a possible explanation for the ability of L. monocytogenes strains with a PMSC in inlA to still cause disease.