Experimental validation of low virulence in field strains of [i]Listeria monocytogenes[/i]

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 werehypovirulent and 17 were avirulent, based on the number of mice contaminated and the colonization of theirspleens after subcutaneous inoculation. All these strains possessed the known virulence genes. We have nowassessed the low virulence of these strains in other assays before determining how they differ from virulentstrains. We have shown that the low-virulence strains exhibited a phenotypic stability and were not a mixtureof virulent and avirulent bacteria. They did not recover virulence after many passages in mice and colonizedthe spleens of mice more poorly than virulent strains after i.v. inoculation. Their lethal capacities, determinedby 50% lethal dose (LD50), were lower than those of virulent strains. Like Listeria innocua, 14 of 17 avirulentstrains 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