Characterization of Listeria monocytogenes isolated from channel catfish (Ictalurus punctatus)

Objective-To characterize Listeria monocytogenes from tissues of channel catfish for their ability to cause hemolysis and grow intracellularly in mouse macrophages. Samples-15 isolates from processed fillets and 15 isolates from the brain, spleen, and kidneys. Procedure-Serotype and hemolytic activity of L monocytogenes isolates were evaluated, using plate agglutination and CAMP tests, respectively Invasiveness of L monocytogenes was determined by inoculating each strain or isolate on J774A.1 macrophage cells. Infected cells were incubated for 0 or 3 hours and lysed; then 100 mu l of the lysate was plated onto a brain heart infusion agar plate. Colony counts for each strain or isolate were analyzed statistically. Results-Of 30 isolates, 19 were serotype 1 and 11 were serotype 4. Mouse J774A.1 macrophages were inoculated with catfish isolates, a wild-type (EGD) or a nonhemolytic strain of L monocytogenes. Seventy-three percent (11/15) of isolates originating from catfish organs and 100% (15/15) of isolates originating from fillets were not significantly different from the wild-type EGD strain. The nonhemolytic L monocytogenes strain used as a negative control failed to replicate. Intracellular growth of all L monocytogenes isolates decreased after an additional 3-hour incubation period with medium containing 50 mu g/ml of gentamicin. Conclusions-Similar to the wild-type EGD strain, most channel catfish L monocytogenes isolates were hemolytic, serotype 1 or 4, and were invasive for mouse J774A.1 macrophages. Clinical Relevance-L monocytogenes growth in mouse macrophages may serve as an in vitro model for determining virulence of isolates from food products or environments.