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 × 104 to 7.5 × 104 CFU. Virulence was evaluated by counting viable bacteria in the mouse spleen 2 d after inoculation. Splenic bacterial counts ranged from less than 103 to 4 × 108 CFU per organ; values were between 1 × 106 and 4 × 108 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 (<103 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.

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