Overexpression of RamA, which regulates production of the multidrug resistance efflux pump AcrAB-TolC, increases mutation rate and influences drug resistance phenotype

Elizabeth M Grimsey, Natasha Weston, Vito Ricci, Jack W Stone, Laura J V Piddock

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6 Citations (Scopus)
164 Downloads (Pure)


In Enterobacteriales, the AcrAB-TolC efflux pump exports substrates, including antimicrobials, from the cell. Overexpression of AcrAB-TolC can occur after exposure to fluoroquinolones, leading to multidrug resistance. The expression of AcrAB-TolC in Salmonella is primarily regulated by the transcriptional activator RamA. However, other transcriptional activators, such as MarA, SoxRS, and Rob, can influence AcrAB-TolC expression. This study determined whether the overproduction or absence of RamA influences the mutation rate or the phenotype of mutants selected in Salmonella enterica serovar Typhimurium SL1344 after ciprofloxacin exposure. The absence of RamA (SL1344 ramA:: aph) resulted in mutation frequencies/rates similar to those of wild-type Salmonella Typhimurium SL1344. However, the overproduction of RamA (SL1344 ramR:: aph) and, consequently, AcrB resulted in a significantly higher mutation frequency and rate than for wild-type Salmonella Typhimurium SL1344. Whole-genome sequencing revealed that in addition to selecting gyrA mutants resistant to quinolones, SL1344 and SL1344 ramA:: aph also produced multidrug-resistant (MDR) mutants, associated with mutations in soxR Conversely, mutations in SL1344 ramR:: aph occurred in gyrA only. Although transcriptional regulators such as SoxRS are believed to play a minor role in AcrAB-TolC regulation under antibiotic selective pressure, we show that soxR mutants can be selected after exposure to ciprofloxacin, including when RamA is absent. This demonstrates that under selective pressure, Salmonella can respond to increased efflux pump expression by mutating other AcrAB-TolC regulatory genes, allowing for the evolution of MDR. Understanding how Salmonella responds to antibiotic pressure in the absence/overproduction of RamA is important if targeting transcriptional regulators to alter efflux is to be considered an avenue for future drug discovery.

Original languageEnglish
Article numbere02460
Number of pages19
JournalAntimicrobial Agents and Chemotherapy
Issue number4
Publication statusPublished - 24 Mar 2020

Bibliographical note

Copyright © 2020 American Society for Microbiology.


  • AcrB
  • Multidrug resistance
  • Mutation rate
  • RamA
  • SoxR

ASJC Scopus subject areas

  • Pharmacology
  • Pharmacology (medical)
  • Infectious Diseases


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