Inability of a reserpine-based screen to identify strains overexpressing efflux pump genes in clinical isolates of Staphylococcus aureus

Overexpression of efflux pump genes conferring multidrug resistance (MDR) in Staphylococcus aureus results in reduced susceptibility to select biocides, dyes and fluoroquinolones. Reserpine is commonly used as an inhibitor of MDR efflux pumps and previous work from our laboratory using a reserpine-based screen to identify clinical isolates with an efflux phenotype revealed that nearly one-half overexpressed norA-B-C, mepA or mdeA. The accuracy of reserpine in predicting efflux pump gene overexpression in clinical strains was examined in detail. Bloodstream isolates of S. aureus previously classified as non-effluxing strains by the reserpine screen underwent gene expression analysis using quantitative real-time reverse transcription polymerase chain reaction (qRT-PCR). The reserpine screen failed to identify many strains shown by qRT-PCR to overexpress one or more MDR efflux pump genes. Microdilution susceptibility testing with and without reserpine also failed to predict efflux pump activity. Although gene expression does not always correlate with protein translation, our results indicate that in clinical S. aureus isolates the use of reserpine to predict the contribution of efflux to reduced susceptibility is not dependable. All strains used in studies designed to assess MDR efflux pump gene expression in clinical isolates should be evaluated by a method independent of in vitro susceptibility testing.