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Antimicrobial Agents and Chemotherapy, October 2008, p. 3604-3611, Vol. 52, No. 10
0066-4804/08/$08.00+0     doi:10.1128/AAC.00661-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

RamA Confers Multidrug Resistance in Salmonella enterica via Increased Expression of acrB, Which Is Inhibited by Chlorpromazine {triangledown}

Andrew M. Bailey,1 Ian T. Paulsen,2,{dagger} and Laura J. V. Piddock1*

Antimicrobial Agents Research Group, Department of Immunity and Infection, The Medical School, The University of Birmingham, Birmingham B15 2TT, United Kingdom,1 The Institute for Genomic Research, 9712 Medical Center Drive, Rockville, Maryland 208502

Received 20 May 2008/ Returned for modification 15 July 2008/ Accepted 29 July 2008

Salmonella enterica serovar Typhimurium SL1344, in which efflux pump genes (acrB, acrD, acrF, tolC) or regulatory genes thereof (marA, soxS, ramA) were inactivated, was grown in the presence of 240 antimicrobial and nonantimicrobial agents in the Biolog Phenotype MicroArray. Mutants lacking tolC, acrB, and ramA grew significantly worse than other mutants in the presence of 48 agents (some of which have not previously been identified as substrates of AcrAB-TolC) and particularly poorly in the presence of phenothiazines, which are human antipsychotics. MIC testing revealed that the phenothiazine chlorpromazine had antimicrobial activity and synergized with common antibiotics against different Salmonella serovars and SL1344. Chlorpromazine increased the intracellular accumulation of ethidium bromide, which was ablated in mutants lacking acrB, suggesting an interaction with AcrB. High-level but not low-level overexpression of ramA increased the expression of acrB; conferred resistance to chloramphenicol, tetracycline, nalidixic acid, and triclosan and organic solvent tolerance; and increased the amount of ethidium bromide accumulated. Chlorpromazine induced the modest overproduction of ramA but repressed acrB. These data suggest that phenothiazines are not efflux pump inhibitors but influence gene expression, including that of acrB, which confers the synergy with antimicrobials observed.

* Corresponding author. Mailing address: Antimicrobial Agents Research Group, Division of Immunity and Infection, University of Birmingham, Birmingham B15 2TT, United Kingdom. Phone: (44) (0)121-414-6966. Fax: (44) (0)121-414-6819. E-mail: l.j.v.piddock{at}bham.ac.uk

{triangledown} Published ahead of print on 11 August 2008.

{dagger} Present address: Department of Chemistry & Biomolecular Science, Macquarie University, Sydney, NSW, Australia 2109.

Antimicrobial Agents and Chemotherapy, October 2008, p. 3604-3611, Vol. 52, No. 10
0066-4804/08/$08.00+0     doi:10.1128/AAC.00661-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.



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