[Pathogenicity of Listeria]

20 different Listeria strains which hitherto have been all classed with the same serovar 3 of L. monocytogenes because of common surface markers, differ markedly in respect to their pathogenic capacities. Some strains are virulent, because they are able to multiply within a normal, adult mouse. Others show only weak virulence, since they survive within the host for several days. Multiplication, however, does not occur. Still other strains are rapidly eliminated and thus avirulent. These characteristics are still more pronounced in the compromised host. The avirulent strains are even not able to multiply in mice deprived of macrophage function, in T-lymphocyte-deficient mice or in newborn mice. These avirulent strains have to be classified into the new species of L. innuocua because of biochemical markers, whereby the absence of hemolysin production is crucial. Whereas priming of protective immunity is only possible by strains of high and intermediate virulence of L. monocytogenes, boosting of pre-existing immunity could also be achieved by the avirulent strain of L. innocua.