Ciprofloxacin selects for RNA polymerase mutations with pleiotropic antibiotic resistance effects.

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Citation

Pietsch F, Bergman JM, Brandis G, Marcusson LL, Zorzet A, Huseby DL, Hughes D

Ciprofloxacin selects for RNA polymerase mutations with pleiotropic antibiotic resistance effects.

J Antimicrob Chemother. 2017 Jan;72(1):75-84. doi: 10.1093/jac/dkw364. Epub 2016 Sep 12.

PubMed ID
27621175 [ View in PubMed
]
Abstract

OBJECTIVES: Resistance to the fluoroquinolone drug ciprofloxacin is commonly linked to mutations that alter the drug target or increase drug efflux via the major AcrAB-TolC transporter. Very little is known about other mutations that might also reduce susceptibility to ciprofloxacin. We discovered that an Escherichia coli strain experimentally evolved for resistance to ciprofloxacin had acquired a mutation in rpoB, the gene coding for the beta-subunit of RNA polymerase. The aim of this work was to determine whether this mutation, and other mutations in rpoB, contribute to ciprofloxacin resistance and, if so, by which mechanism. METHODS: Independent lineages of E. coli were evolved in the presence of ciprofloxacin and clones from endpoint cultures were screened for mutations in rpoB. Ciprofloxacin-selected rpoB mutations were identified and characterized in terms of effects on susceptibility and mode of action. RESULTS: Mutations in rpoB were selected at a high frequency in 3 out of 10 evolved lineages, in each case arising after the occurrence of mutations affecting topoisomerases and drug efflux. All ciprofloxacin-selected rpoB mutations had a high fitness cost in the absence of drug, but conferred a competitive advantage in the presence of ciprofloxacin. RNA sequencing and quantitative RT-PCR analysis showed that expression of mdtK, encoding a multidrug efflux transporter, was significantly increased by the ciprofloxacin-selected rpoB mutations. The susceptibility phenotype was shown to depend on the presence of an active mdtK and a mutant rpoB allele. CONCLUSIONS: These data identify mutations in RNA polymerase as novel contributors to the evolution of resistance to ciprofloxacin and show that the phenotype is mediated by increased MdtK-dependent drug efflux.

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