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Applied and Environmental Microbiology, October 2005, p. 6039-6048, Vol. 71, No. 10
0099-2240/05/$08.00+0     doi:10.1128/AEM.71.10.6039-6048.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Investigation of Specific Substitutions in Virulence Genes Characterizing Phenotypic Groups of Low-Virulence Field Strains of Listeria monocytogenes

S. M. Roche,1*,{dagger} P. Gracieux,1,{dagger} E. Milohanic,2,6 I. Albert,3 I. Virlogeux-Payant,1 S. Témoin,1 O. Grépinet,1 A. Kerouanton,4 C. Jacquet,5 P. Cossart,6 and P. Velge1

Institut National de la Recherche Agronomique, Pathologie Infectieuse et Immunologie, 37380 Nouzilly, France,1 Institut Pasteur, Laboratoire de Génomique des Microorganismes Pathogènes, 28 rue du Docteur Roux, 75015 Paris, France,2 Institut National de la Recherche Agronomique, Institut National Agronomique de Paris-Grignon, Département Mathématiques et Informatique Appliquées, 16 rue Claude Bernard, 75365 Paris Cedex 5, France,3 AFSSA-LERQAP, Unité de Caractérisation et d'Epidémiologie Bactérienne, 23 avenue du Général de Gaulle, 94706 Maisons-Alfort Cedex 06, France,4 Institut Pasteur, Laboratoire des Listeria, Centre National de Référence des Listeria, WHO Collaborating Center for Foodborne Listeriosis, 28 rue du Docteur Roux, 75724 Paris Cedex 15, France,5 Institut Pasteur, Unité des Interactions Bactéries-Cellules, 28 rue du Docteur Roux, 75015 Paris, France6

Received 15 November 2004/ Accepted 25 May 2005

Several models have shown that virulence varies from one strain of Listeria monocytogenes to another, but little is known about the cause of low virulence. Twenty-six field L. monocytogenes strains were shown to be of low virulence in a plaque-forming assay and in a subcutaneous inoculation test in mice. Using the results of cell infection assays and phospholipase activities, the low-virulence strains were assigned to one of four groups by cluster analysis and then virulence-related genes were sequenced. Group I included 11 strains that did not enter cells and had no phospholipase activity. These strains exhibited a mutated PrfA; eight strains had a single amino acid substitution, PrfAK220T, and the other three had a truncated PrfA, PrfA{Delta}174-237. These genetic modifications could explain the low virulence of group I strains, since mutated PrfA proteins were inactive. Group II and III strains entered cells but did not form plaques. Group II strains had low phosphatidylcholine phospholipase C activity, whereas group III strains had low phosphatidylinositol phospholipase C activity. Several substitutions were observed for five out of six group III strains in the plcA gene and for one out of three group II strains in the plcB gene. Group IV strains poorly colonized spleens of mice and were practically indistinguishable from fully virulent strains on the basis of the above-mentioned in vitro criteria. These results demonstrate a relationship between the phenotypic classification and the genotypic modifications for at least group I and III strains and suggest a common evolution of these strains within a group.

* Corresponding author. Mailing address: Institut National de la Recherche Agronomique, Pathologie Infectieuse et Immunologie, 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.

{dagger} S. M. Roche and P. Gracieux contributed equally to this work.

Applied and Environmental Microbiology, October 2005, p. 6039-6048, Vol. 71, No. 10
0099-2240/05/$08.00+0     doi:10.1128/AEM.71.10.6039-6048.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.



This article has been cited by other articles:

  • Neves, E., Silva, A. C., Roche, S. M., Velge, P., Brito, L. (2008). Virulence of Listeria monocytogenes isolated from the cheese dairy environment, other foods and clinical cases. J Med Microbiol 57: 411-415 [Abstract] [Full Text]  
  • Temoin, S., Roche, S. M., Grepinet, O., Fardini, Y., Velge, P. (2008). Multiple point mutations in virulence genes explain the low virulence of Listeria monocytogenes field strains. Microbiology 154: 939-948 [Abstract] [Full Text]  
  • Velge, P., Herler, M., Johansson, J., Roche, S. M., Temoin, S., Fedorov, A. A., Gracieux, P., Almo, S. C., Goebel, W., Cossart, P. (2007). A naturally occurring mutation K220T in the pleiotropic activator PrfA of Listeria monocytogenes results in a loss of virulence due to decreasing DNA-binding affinity. Microbiology 153: 995-1005 [Abstract] [Full Text]  
  • Liu, D. (2006). Identification, subtyping and virulence determination of Listeria monocytogenes, an important foodborne pathogen. J Med Microbiol 55: 645-659 [Abstract] [Full Text]  


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