Virulence Heterogeneity of Listeria monocytogenes Strains From Various Sources (Food, Human, Animal) in Immunocompetent Mice and its Association With Typing Characteristics.

One hundred and twenty-five Listeria monocytogenes strains were screened for their ability to infect immunocompetent white Swiss mice. Mice were infected by intravenous injection of 2.5 × 10 to 7.5 × 10 CFU. Virulence was evaluated by counting viable bacteria in the mouse spleen 2 d after inoculation. Splenic bacterial counts ranged from less than 10 to 4 × 10 CFU per organ; values were between 1 × 10 and 4 × 10 for 88% of the strains. No systematic differences in virulence were noticed among strains of different origins, serovars, phagovars, ribovars, or DNA macrorestriction patterns. All strains isolated from human infections were found to be virulent within this assay. Among the 63 strains isolated from food, two were not virulent (<10 CFU per spleen). Results of this study suggest that L. monocytogenes strains are potential hazards for human health, regardless of their origin and certain strain-specific characteristics, such as serovar, phagovar, ribovar, and DNA macrorestriction patterns.