Genetic bases of the rifampin resistance phenotype in Brucella spp.

Rifampin is one of the most potent and broad-spectrum antibiotics against bacterial pathogens. Its bactericidal activity is due to its ability to bind to the beta subunit of the DNA-dependent RNA polymerase encoded by the rpoB gene. Mutations of the rpoB gene have been characterized in rifampin-resistant (Rif(r)) strains of Escherichia coli and Mycobacterium tuberculosis. The genetic bases of Rif(r) in Brucella spp. are still unknown. In the present study, the nucleotide sequences of the rpoB gene of the Rif(r) vaccine strain Brucella abortus RB51 and of 20 Rif(r) clones derived in our laboratory from two Brucella melitensis isolates were determined. These sequences were then compared to those of the respective rifampin-susceptible (Rif(s)) parental strains and to the published B. melitensis strain 16M. All Rif(r) strains carried one or more missense mutations mapping in two regions of the rpoB gene. These two "hot" regions were investigated in eight additional Rif(r) Brucella laboratory mutants and in 20 reference Rif(s) Brucella strains. rpoB mutations were found in all Rif(r) mutants. In contrast, no missense mutations were found in any analyzed Rif(s) strains. Our results represent the first from a study of the molecular characterization of rpoB mutations in resistant Brucella strains and provide an additional proof of the association of specific rpoB mutations with the development of the Rif(r) phenotype in prokaryotes. In addition, because of the relationship between Rif(r) and the attenuation of virulence in Brucella spp., studies of virulence in these mutants may provide useful information about the genetic basis of pathogenesis in Brucella.