RamA, a member of the AraC/XylS family, influences both virulence and efflux in Salmonella enterica serovar Typhimurium.

The transcriptomes of Salmonella enterica serovar Typhimurium SL1344 lacking a functional ramA or ramR, or with plasmid-mediated high-level over-expression of ramA were compared to their wildtype parental strain. Inactivation of ramA led to increased expression of 14 SPI-1 genes and decreased expression of three SPI-2 genes, and altered expression of ribosomal biosynthetic genes and several amino acid biosynthetic pathways. Furthermore, disruption of ramA led to decreased survival within RAW 264.7 mouse macrophages and attenuation within the BALB/c ByJ mouse model. Highly over-expressed ramA led to increased expression of genes encoding multidrug resistance (MDR) efflux pumps including acrAB, acrEF and tolC. Decreased expression of 34 Salmonella Pathogenicity Island- (SPI) 1 and 2 genes, decreased SipC production, decreased adhesion to, and survival within, macrophages and decreased colonisation of Caenorhabditis elegans was also seen. Disruption of ramR led to the increased expression of ramA, acrAB and tolC, but not to the same level as when ramA was over-produced on a plasmid. Inactivation of ramR caused a more limited effect on pathogenicity gene expression. In silico analysis of a suggested RamA-binding consensus sequence identified target genes including ramR, acrA, tolC, sipABC and ssrA. This study demonstrates that the regulation of a mechanism of MDR and expression of virulence genes show considerable overlap and we postulate that such a mechanism is dependent on transcriptional activator concentration and promoter sensitivity. However, we have no evidence to support the hypothesis that increased MDR via RamA regulation of AcrAB-TolC gives rise to a hyper-virulent strain.