Analysis of tissue invasiveness of adherent strains of Listeria monocytogenes by in vivo mouse assay

Listeria monocytogenes is an important foodborne pathogen that is often present on raw and ready-to-eat (RTE) meat and has been a recurring problem in meat processing facilities. L. monocytogenes is capable of strong adherence to equipment and/or surfaces in meat processing facilities, resulting in the formation of biofilms. We examined select strains of L. monocytogenes isolated from raw and RTE meats that were divided into weakly (CW34, CW35, CW72, and SM3) or strongly (CW50, 99-38, CW77, and CW62) adherent phenotypes based on our fluorescent microplate adherence assay. A/J mice were intragastrically inoculated with 108 CFU of L. monocytogenes and the invasiveness of these strains was evaluated by counting viable Listeria recovered from the spleen and liver of mice 3 days after inoculation. Bacterial counts recovered from the spleen were approximately 5 log CFU/g for strongly adherent strains and ∼ 4 log CFU/g for weakly adherent strains. Bacterial recovery was obtained from both spleen and liver of mice inoculated with strongly adherent strains, whereas bacteria were not recovered from the liver when using weakly adherent strains. Histopathological examination of liver tissue from mice inoculated with strongly adherent strains showed very large and effacing lesions (range 30–400 μm) when compared with lesions observed with weakly adherent strains which exhibited discrete non-effacing lesions (range 30–80 μm). These observations suggest that strongly adherent strains may possess factors that enhance their invasiveness compared to weakly adherent strains. The elimination of L. monocytogenes from the food processing environment is vital as strongly adherent strains may not only persist and present a greater risk of food contamination, but perhaps also a greater degree of invasiveness.