Evidence for an efflux pump mediating multiple antibiotic resistance in Salmonella enterica serovar Typhimurium
L J Piddock1, D G White, K Gensberg
1Antimicrobial Agents Research Group, Division of Infection and Immunity, University of Birmingham, Birmingham, B15 2TT, United Kingdom. l.j.v.piddock@bham.ac.uk
Abstract:
The mechanism of multiple antibiotic resistance in six isolates of Salmonella enterica serovar Typhimurium recovered from a patient treated with ciprofloxacin was studied to investigate the role of efflux in the resistance phenotype. Compared to the patient's pretherapy isolate (L3), five of six isolates accumulated less ciprofloxacin, three of six isolates accumulated less chloramphenicol, and all six accumulated less tetracycline. The accumulation of one or more antibiotics was increased by carbonyl cyanide m-chlorophenylhydrazone to concentrations similar to those accumulated by L3 for all isolates except one, in which accumulation of all three agents remained approximately half that of L3. All isolates had the published wild-type sequences of marO and marR. No increased expression of marA, tolC, or soxS was observed by Northern blotting; however, three isolates showed increased expression of acrB, which was confirmed by quantitative competitive reverse transcription-PCR. However, there were no mutations within acrR or the promoter region of acrAB in any of the isolates.
Insights
Investigating antibiotic resistance in Salmonella Typhimurium, researchers found that increased expression of the acrB gene, not mutations, played a key role in the reduced accumulation of antibiotics like ciprofloxacin.
Area of Science:
- Microbiology
- Molecular Biology
- Pharmacology
Background:
- Multiple antibiotic resistance (MAR) is a growing public health concern.
- Efflux pumps are a significant mechanism contributing to MAR.
- Salmonella enterica serovar Typhimurium is an important foodborne pathogen.
Purpose of the Study:
- To investigate the mechanism of MAR in Salmonella Typhimurium isolates from a patient treated with ciprofloxacin.
- To determine the role of efflux in the observed antibiotic resistance phenotype.
Main Methods:
- Antibiotic accumulation assays were performed on patient isolates.
- The effect of carbonyl cyanide m-chlorophenylhydrazone (an efflux inhibitor) was assessed.
- Gene expression analysis (Northern blotting, quantitative competitive reverse transcription-PCR) of relevant genes (marA, tolC, soxS, acrB) was conducted.
- Sequence analysis of marO, marR, acrR, and the acrAB promoter region was performed.
Main Results:
- Five of six isolates showed reduced accumulation of ciprofloxacin, three of six of chloramphenicol, and all six of tetracycline compared to the pretherapy isolate.
- Efflux inhibition partially restored antibiotic accumulation in most isolates.
- Increased expression of acrB was observed in three isolates, confirmed by qRT-PCR.
- No mutations were found in marO, marR, acrR, or the acrAB promoter region.
Conclusions:
- Increased expression of the acrB gene is implicated in the reduced antibiotic accumulation and MAR phenotype in these Salmonella Typhimurium isolates.
- Efflux mechanisms, specifically related to AcrB, contribute significantly to ciprofloxacin resistance in this context.
- Further research into AcrB-mediated efflux is warranted for understanding and combating MAR in Salmonella.
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