CsrA-mediated regulation of a virulence switch in Acinetobacter baumannii.

CsrA is an RNA binding protein that functions as a global regulator in bacteria. We demonstrate that, in Acinetobacter baumannii, CsrA acts as a positive regulator of the switch from virulent (VIR-O) to avirulent (AV-T) subpopulations. This regulation is mediated, in part, by CsrA interfering with Rho-dependent termination in the mRNA leader region of the ABUW_1645 gene, encoding the primary TetR-type transcriptional regulator that drives cells from the VIR-O to the AV-T state. We demonstrate that CsrA directly binds to the ABUW_1645 mRNA leader region and interferes with Rho binding. We identify three small RNAs (sRNAs) designated CsrB, CsrC, and CsrD that bind to CsrA and inhibit its activity. Individual overexpression of each sRNA greatly decreased the rate of VIR-O to AV-T switching. Individual deletions of each sRNA increased the frequency of VIR-O to AV-T switching, with loss of CsrB giving the highest increase at 2.4-fold. The expression of each sRNA was strongly dependent on the GacA response regulator and the expression of each sRNA was higher in VIR-O cells than in AV-T variants. This regulation provides a mechanism for the differential control of CsrA activity between VIR-O and AV-T variants. IMPORTANCE: The World Health Organization has ranked Acinetobacter baumannii atop its "priority pathogens" list highlighting the urgent need for new therapeutics against this pathogen. Many A. baumannii strains including AB5075 can rapidly switch between cell subpopulations that are virulent or avirulent. In this study, we identify the RNA binding protein CsrA as an important regulator of this switch. Since this switch represents an "Achilles Heel" for pathogenesis, our work may shed light on new mechanisms to lock cells into the avirulent state to block disease.