Abstract
Acinetobacter baumannii is associated with multidrug resistant (MDR) infections in healthcare settings, with fluoroquinolones such as ciprofloxacin being currently ineffective. Clinical isolates largely harbor mutations in the GyrA and TopoIV fluoroquinolone targets, as well as mutations that increase expression of drug resistance-nodulation-division (RND) efflux pumps. Factors critical for maintaining fitness levels of pump overproducers are uncharacterized despite their prevalence in clinical isolates. We here identify proteins that contribute to the fitness of FQR strains overexpressing three known RND systems using high-density insertion mutagenesis. Overexpression of the AdeFGH efflux pump caused hypersensitization to defects in outer membrane homeostatic regulation, including lesions that reduced LOS biosynthesis and blocked production of the major A. baumannii porin. In contrast, AdeAB pump hyperexpression, in the absence of elevated adeC expression (the outer membrane component of the pump), was relatively tolerant to loss of these functions, consistent with the outer membrane protein being the primary disruptive component. Surprisingly, overexpression of proton-transporting efflux pumps had little impact on cytosolic pH, consistent with a compensatory response to pump activity. The most striking transcriptional changes were associated with AdeFGH pump overexpression, including the activation of the phenylacetate (PAA) degradation regulon. Disruption of the PAA pathway resulted in cytosolic acidification and defective expression of genes involved in protection from oxidative stress. These results indicate that RND efflux pump overproduction is compensated by maintenance of outer membrane integrity in A. baumannii to facilitate fitness of FQR isolates.