Induction of vap genes encoded by the virulence plasmid of Rhodococcus equi during acid tolerance response

The response of the intracellular pathogen Rhodococcus equi to acid shock, a stress potentially encountered after phagocytosis by macrophages, was analyzed. The wild-type and its avirulent plasmid-cured strain acquired increased acid tolerance during the exponential growth phase upon exposure to sublethal acid stress, a response referred to as the acid tolerance response. Maximal adaptation was observed when cells were pretreated for 90 min at pH 5.0 before exposure to the pH challenge. Search for plasmid-encoded proteins regulated by an acidic pH was performed using two-dimensional gel electrophoresis, and enabled us to detect several membrane and cytoplasmic proteins with altered expression during the adaptation phase, but none of them were plasmid-encoded. However, using a strategy based on plasmid-encoded gene expression, we showed that two operons located on the virulence plasmid of strain 85F were upregulated by acid pHs with a maximal induction at pH 5.0. One operon, containing vapA, was monocistronic whereas the other was polycistronic composed of vapD and an unknown open reading frame. Our combined results suggest that these genes may play an important role in the pathogenicity of R. equi.