Ethidium bromide MIC screening for enhanced efflux pump gene expression or efflux activity in Staphylococcus aureus

Diixa Patel, Christos Kosmidis, Susan M Seo, Glenn W Kaatz

Research output: Contribution to journalArticlepeer-review


Multidrug resistance efflux pumps contribute to antimicrobial and biocide resistance in Staphylococcus aureus. The detection of strains capable of efflux is time-consuming and labor-intensive using currently available techniques. A simple and inexpensive method to identify such strains is needed. Ethidium bromide is a substrate for all but one of the characterized S. aureus multidrug-resistant (MDR) efflux pumps (NorC), leading us to examine the utility of simple broth microtiter MIC determinations using this compound in identifying efflux-proficient strains. Quantitative reverse transcription-PCR identified the increased expression of one or more MDR efflux pump genes in 151/309 clinical strains (49%). Ethidium bromide MIC testing was insensitive (48%) but specific (92%) in identifying strains with gene overexpression, but it was highly sensitive (95%) and specific (99%) in identifying strains capable of ethidium efflux. The increased expression of norA with or without other genes was most commonly associated with efflux, and in the majority of cases that efflux was inhibited by reserpine. Ethidium bromide MIC testing is a simple and straightforward method to identify effluxing strains and can provide accurate predictions of efflux prevalence in large strain sets in a short period of time.

Original languageEnglish
Pages (from-to)5070-3
Number of pages4
JournalAntimicrobial Agents and Chemotherapy
Issue number12
Publication statusPublished - Dec 2010


  • Anti-Bacterial Agents/pharmacology
  • Bacterial Proteins/genetics
  • Drug Resistance, Multiple, Bacterial/genetics
  • Ethidium/pharmacology
  • Gene Expression Regulation, Bacterial/drug effects
  • Humans
  • Microbial Sensitivity Tests
  • Reverse Transcriptase Polymerase Chain Reaction
  • Staphylococcus aureus/drug effects


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