Pathogenicity of Listeria monocytogenes grown on crabmeat

The pathogenicity of Listeria monocytogenes as influenced by growth on crabmeat at 5 and 10 degrees C was studied. Crabmeat was inoculated with L. monocytogenes V7 (ca. 10(4) CFU/g) and incubated for up to 14 days at 5 and 10 degrees C. At selected incubation times, L. monocytogenes was removed from crabmeat by washing with 0.1 M potassium phosphate buffer (pH 7.0), and populations were determined by surface plating on LiCl-phenylethanol-moxalactam agar. Buffered suspensions were then centrifuged, and the resulting pellets were suspended in phosphate buffer containing 10% glycerol and stored at -18 degrees C. Thawed, diluted suspensions of cells were tested for pathogenicity by intraperitoneal injection into immunocompromised and nonimmunocompromised mice. L. monocytogenes cells recovered from crabmeat and then recultured in tryptose phosphate broth (TPB), as well as cells which had not been passed through crabmeat but had been cultured in TPB, were likewise harvested, suspended in buffered 10% glycerol, frozen, thawed, diluted, and tested for pathogenicity by intraperitoneal injection. Growth on crabmeat at 5 and 10 degrees C did not have a significant effect on pathogenicity. The population of L. monocytogenes necessary to kill about 50% of the immunocompromised mice in each test set within 7 days was about 10(4) CFU, and this result was not significantly affected by storage temperature of the crabmeat or type of substrate, i.e., crabmeat or TPB, on which it had grown.