The global regulatory protein CRP controls multifactorial fluoroquinolone susceptibility in Salmonella enterica serovar Typhimurium [PublishAheadOfPrint]

Fluoroquinolone antibiotics are prescribed for the treatment of Salmonella enterica infections, but resistance to this family of antibiotics is growing. Here we report that loss of the global regulatory protein, CRP, or its allosteric effector, cAMP, reduces susceptibility to fluoroquinolones. A crp mutation was synergistic with the primary fluoroquinolone resistance allele gyrA83, thus can contribute to clinically-relevant resistance. Decreased susceptibility to fluoroquinolones could be partly explained by decreased expression of the outer membrane porin genes ompA and ompF with a concomitant increase in the expression of the ciprofloxacin-resistance efflux pump gene acrB in crp cells. Expression of gyrAB, which encode the DNA supercoiling enzyme GyrAB that is blocked by fluoroquinolones, and expression of topA, which encodes the dominant supercoiling-relaxing enzyme topoisomerase I, were unchanged in crp cells. Yet crp cells maintained a more relaxed state of DNA supercoiling, correlating with an observed increase in topoisomerase IV (parCE) expression. Surprisingly, the crp mutation had the unanticipated effect of enhancing fitness in the presence of fluoroquinolone antibiotics, which can be explained by the observation that exposure of crp cells to ciprofloxacin had the counterintuitive effect of restoring wildtype levels of DNA supercoiling. Consistent with this, crp cells did not become elongated nor induce the SOS response when challenged with ciprofloxacin. These findings implicate the combined action of multiple drug resistance mechanisms in crp cells: reduced permeability and elevated efflux of fluoroquinolones coupled with a relaxed DNA supercoiling state that buffers cells against GyrAB-inhibition by fluoroquinolones.

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