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Updated: Jun 25, 2026
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Published on: March 2, 2020
Ciprofloxacin selects for multidrug resistance in Salmonella enterica serovar Typhimurium mediated by at least two
Vito Ricci1, Laura J V Piddock
1University of Birmingham, UK.
Objectives:
The aim of this study was to understand the role of ramA in conferring multidrug resistance (MDR) in Salmonella enterica serovar Typhimurium.
Methods:
Two phenotypically distinct isogenic MDR laboratory mutants derived from Salmonella Typhimurium SL1344 and three human clinical isolates from a patient that failed antibiotic therapy, including with ciprofloxacin, were investigated for the cause of MDR. MICs were determined by agar dilution and efflux activity assessed by monitoring the accumulation of Hoechst 33342. The combination of specific genes and MDR was assessed by inactivation, and complementation RT-PCR was used to assess gene expression and DNA sequencing to identify mutations within genes of interest.
Results:
Mutation in ramR and the consequent over-production of ramA and acrB were revealed in one laboratory mutant selected with ciprofloxacin and this was associated with cyclohexane tolerance. Complementation of SL1344 ramR::aph with pUC19 ramR(mutant) conferred MDR and cyclohexane tolerance. However, analysis of a second ciprofloxacin-selected MDR mutant, which was susceptible to cyclohexane, revealed no mutation in ramR or altered expression of marA, soxS or rob. There was a mutation in ramR in both the pre- and post-therapy clinical isolates and no difference between the isolates in the level of expression of ramA.
Conclusions:
These data show that ciprofloxacin exposure can select for mutations within ramR and consequently mediate MDR. These data also indicate that there is at least one further unidentified gene in addition to marA, soxS, rob and ramA that confers MDR in S. enterica.
Insights
RamR mutations in Salmonella Typhimurium lead to increased RamA and AcrB production, causing multidrug resistance (MDR). Ciprofloxacin exposure can select for these RamR mutations, highlighting a key mechanism in MDR development.
Area of Science:
- Microbiology
- Molecular Biology
- Genetics
Background:
- Multidrug resistance (MDR) in Salmonella enterica serovar Typhimurium is a significant public health concern.
- Understanding the genetic mechanisms underlying MDR is crucial for developing effective therapeutic strategies.
Purpose of the Study:
- To investigate the role of the ramA gene in conferring MDR in Salmonella Typhimurium.
- To identify genetic alterations responsible for MDR in clinical and laboratory-selected isolates.
Main Methods:
- Analysis of isogenic laboratory mutants and human clinical isolates of Salmonella Typhimurium.
- Determination of Minimum Inhibitory Concentrations (MICs) and assessment of efflux pump activity.
- Gene expression analysis using RT-PCR and identification of mutations via DNA sequencing.
Main Results:
- Mutations in ramR were identified in ciprofloxacin-selected laboratory mutants and clinical isolates, leading to increased ramA and acrB expression and MDR.
- Complementation studies confirmed that ramR mutations confer MDR and cyclohexane tolerance.
- One MDR mutant showed no ramR mutation or altered expression of known resistance genes (marA, soxS, rob), suggesting additional MDR mechanisms.
Conclusions:
- Ciprofloxacin exposure can select for ramR mutations, mediating MDR in Salmonella Typhimurium.
- The study suggests the existence of at least one unidentified gene contributing to MDR in S. enterica, beyond known regulators like marA, soxS, rob, and ramA.
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