The Global Regulatory Cyclic AMP Receptor Protein (CRP) Controls Multifactorial Fluoroquinolone Susceptibility in Salmonella enterica Serovar Typhimurium.

Fluoroquinolone antibiotics are prescribed for the treatment of infections, but resistance to this family of antibiotics is growing. Here we report that loss of the global regulatory protein cyclic AMP (cAMP) receptor protein (CRP) or its allosteric effector, cAMP, reduces susceptibility to fluoroquinolones. A Δ mutation was synergistic with the primary fluoroquinolone resistance allele , thus able to contribute to clinically relevant resistance. Decreased susceptibility to fluoroquinolones could be partly explained by decreased expression of the outer membrane porin genes and with a concomitant increase in the expression of the ciprofloxacin resistance efflux pump gene in Δ cells. Expression of , which encode the DNA supercoiling enzyme GyrAB, which is blocked by fluoroquinolones, and expression of , which encodes the dominant supercoiling-relaxing enzyme topoisomerase I, were unchanged in Δ cells. Yet Δ cells maintained a more relaxed state of DNA supercoiling, correlating with an observed increase in topoisomerase IV () expression. Surprisingly, the Δ mutation had the unanticipated effect of enhancing fitness in the presence of fluoroquinolone antibiotics, which can be explained by the observation that exposure of Δ cells to ciprofloxacin had the counterintuitive effect of restoring wild-type levels of DNA supercoiling. Consistent with this, Δ cells did not become elongated or induce the SOS response when challenged with ciprofloxacin. These findings implicate the combined action of multiple drug resistance mechanisms in Δ cells: reduced permeability and elevated efflux of fluoroquinolones coupled with a relaxed DNA supercoiling state that buffers cells against GyrAB inhibition by fluoroquinolones.