Experimental validation of low virulence in field strains of [i]Listeria monocytogenes[/i]
Résumé
Several reports have described Listeria monocytogenes strains which were nonpathogenic or weakly pathogenic,
but little is known about these low-virulence strains. We found that 9 field L. monocytogenes strains were
hypovirulent and 17 were avirulent, based on the number of mice contaminated and the colonization of their
spleens after subcutaneous inoculation. All these strains possessed the known virulence genes. We have now
assessed the low virulence of these strains in other assays before determining how they differ from virulent
strains. We have shown that the low-virulence strains exhibited a phenotypic stability and were not a mixture
of virulent and avirulent bacteria. They did not recover virulence after many passages in mice and colonized
the spleens of mice more poorly than virulent strains after i.v. inoculation. Their lethal capacities, determined
by 50% lethal dose (LD50), were lower than those of virulent strains. Like Listeria innocua, 14 of 17 avirulent
strains had no LD50 and were eliminated by the lymph nodes after subcutaneous inoculation. The virulent,
hypovirulent, and avirulent strains were always significantly different, whatever the tests of virulence used,
confirming the importance of these low-virulence field strains in identifying the proteins involved in virulence.