Approach to classification of Listeria monocytogenes isolates on their virulence for the experimental animals

A pathogenicity of Listeria isolates is usually determined in the qualitative kerato-conjunctival test on guinea pigs. That test allows to subdivided the strains into two main groups, i.e. pathogenic and non-pathogenic, but not to carry out an intraspecific typing of the pathogen, which is necessary for establishing the source of infection and the ways of its expansion, and also for the evaluation of epidemiological significance of isolates from the environment and food products. In our study, the pathogenicity of different Listeria monocytogenes cultures was investigated in test with outbred white mice, and the approach to Listeria classification is proposed on the base of the virulence manifested after an intraperitoneal injection of different bacterial culture. It is shown that the seasons, as well as the composition and type of media, used for bacteria cultivation, did not affect significantly the virulence of the isolates for laboratory animals. The LD 100 and LD 50 were estimated, and LD 50 was found to be the most reliable index for intraspecific typing of L. monocytogenes isolates. The LD 50 for white outbred mice was estimated in 30 cultures of L. monocytogenes. According to the LD 50/ min and LD 50/ max, all isolates were divided into five groups, which can be designated as a highly virulent strains (LD 50/ min and LD 50/ max within 10 4 to 10 6 colony-forming unit per animal), virulent strains (LD 50/ min and LD 50/ max of 10 6 to 10 7 CFU per animal), the strains of moderate virulence (LD 50/ min and LD 50/ max of 10 7-10 8 CFU per animal) and low virulence (LD 50/ min and LD 50/ max within 10 8-10 9 CFU per animal), and the avirulent ones, which LD 50 has not been titrated (LD 100 more then 10 9 CFU per animal).