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Mutations of the Quorum Sensing-Dependent Regulator VjbR Lead to Drastic Surface Modifications in Brucella melitensis

Sophie Uzureau, Marie Godefroid, Chantal Deschamps, Julien Lemaire, Xavier De Bolle, and Jean-Jacques Letesson^*

Unité de Recherche en Biologie Moléculaire, Laboratoire d'Immunologie-Microbiologie, Facultés Universitaires Notre-Dame de la Paix, Namur, Belgium

Received 16 February 2007/ Accepted 21 May 2007

Successful establishment of infection by bacterial pathogens requires fine-tuning of virulence-related genes. Quorum sensing (QS) is a global regulation process based on the synthesis of, detection of, and response to small diffusible molecules, called N-acyl-homoserine lactones (AHL), in gram-negative bacteria. In numerous species, QS has been shown to regulate genes involved in the establishment of pathogenic interactions with the host. Brucella melitensis produces N-dodecanoyl homoserine lactones (C₁₂-HSL), which down regulate the expression of flagellar genes and of the virB operon (encoding a type IV secretion system), both of which encode surface virulence factors. A QS-related regulator, called VjbR, was identified as a transcriptional activator of these genes. We hypothesized that VjbR mediates the C₁₂-HSL effects described above. vjbR alleles mutated in the region coding for the AHL binding domain were constructed to test this hypothesis. These alleles expressed in trans in a vjbR background behave as constitutive regulators both in vitro and in a cellular model of infection. Interestingly, the resulting B. melitensis strains, unable to respond to AHLs, aggregate spontaneously in liquid culture. Preliminary characterization of these strains showed altered expression of some outer membrane proteins and overproduction of a matrix-forming exopolysaccharide, suggesting for the first time that B. melitensis could form biofilms. Together, these results indicate that QS through VjbR is a major regulatory system of important cell surface structures of Brucella and as such plays a key role in host-pathogen interactions.

Published ahead of print on 8 June 2007.

Current address: Departement de Biologie Cellulaire, Institut Cochin, F-75014 Paris, France.

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