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Infection and Immunity, June 2003, p. 3429-3436, Vol. 71, No. 6
0019-9567/03/$08.00+0     DOI: 10.1128/IAI.71.6.3429-3436.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

Experimental Validation of Low Virulence in Field Strains of Listeria monocytogenes

S. M. Roche,1* P. Gracieux,1 I. Albert,2 M. Gouali,3,{dagger} C. Jacquet,3 P. M. V. Martin,3 and P. Velge1

Pathologie Infectieuse et Immunologie, Institut National de la Recherche Agronomique, 37380 Nouzilly,1 Unité de Biométrie et d'Intelligence Artificielle, Institut National de la Recherche Agronomique, Domaine de Vilvert, 78352 Jouy-en-Josas,2 Laboratoire des Listeria, Centre National de Référence des Listeria, WHO Collaborating Center for Foodborne Listeriosis, Institut Pasteur, 75724 Paris Cedex 15, France3

Received 2 December 2002/ Returned for modification 13 January 2003/ Accepted 19 March 2003

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.

* Corresponding author. Mailing address: Pathologie Infectieuse et Immunologie, Institut National de la Recherche Agronomique, 37380 Nouzilly, France. Phone: 33.(0)2.47.42.78.76. Fax: 33.(0)2.47.42.77.79. E-mail: sroche{at}tours.inra.fr.

Editor: B. B. Finlay

{dagger} Present address: Institut Pasteur de Bangui, Bangui, République Centrafricaine.

Infection and Immunity, June 2003, p. 3429-3436, Vol. 71, No. 6
0019-9567/03/$08.00+0     DOI: 10.1128/IAI.71.6.3429-3436.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.



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