Wild strains of Listeria monocytogenes, Listeria ivanovii, Listeria seeligeri, Listeria innocua, and Listeria welshimeri were isolated from infected animals and foodstuffs. Their virulence was tested in Swiss mice after intraperitoneal injection of a fixed number of organisms. The presence of hemolysin was determined using the CAMP test. Bacteria were enumerated in peritoneal lavage fluid, liver, and spleen. Spleen weights were measured, and the presence of L. monocytogenes in the brain was also investigated. L. innocua, L. seeligeri, and L. welshimeri were not found to be pathogenic for mice. L. ivanovii was detected in liver, spleen, and peritoneal lavage fluid but at lower levels than L. monocytogenes (p<0.001). The pathogenic capabilities of four different serovars of L. monocytogenes (4b, 1/2a, 1/2b, 1/2c) were compared. Serovars l/2b and l/2c, which are frequently isolated from foodstuffs, were found to colonize the liver and spleen to a lesser extent than serovar 4b (p<0.01 and <0.001 respectively). The behavior of serovar l/2a, the most commonly isolated from foodstuffs, was strain dependent. Two out of the four strains tested were strongly hemolytic and were as virulent as strains of serovar 4b, while the other two were weakly hemolytic, and avirulent like L. innocua. These results could account for the relatively small number of human Listeria infections due to L. monocytogenes serogroup 1/2, despite the very frequent occurrence of this serovar in foodstuffs.
Use of the CAMP test for identification of Listeria monocytogenes.
