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 screen ed for their ability to infect immunocompetent white Swiss mice. Mice were infected by intravenous injection of 2.5 x 10(4) to 7.5 x 10(4) C FU. Virulence was evaluated by counting viable bacteria in the mouse s pleen 2 d after inoculation. Splenic bacterial counts ranged from less than 10(3) to 4 x 10(8) CFU per organ; values were between 1 x 10(6) and 4 x 10(8) for 88% of the strains. No systematic differences in vir ulence were noticed among strains of different origins, serovars, phag ovars, ribovars, or DNA macrorestriction patterns. All strains isolate d from human infections were found to be virulent within this assay. A mong the 63 strains isolated from food, two were not virulent (<10(3) CFU per spleen). Results of this study suggest that L. monocytogenes s trains are potential hazards for human health, regardless of their ori gin and certain strain-specific characteristics, such as serovar, phag ovar, ribovar, and DNA macrorestriction patterns.