Abstract

In a single quantitative study, we measured acrA, acrB, tolC, mdfA, and norE expression in Escherichia coli clinical isolates by using real-time PCR. acrA and acrB overexpression strongly correlated with fluoroquinolone and multidrug resistance; tolC, mdfA, and norE expression did not. The order of abundance of efflux pump transcripts in all fluoroquinolone-susceptible isolates was tolC (highest), then acrA and acrB, and then mdfA and norE. Our findings suggest acrAB overexpression is an indicator of multidrug resistance. Multidrug resistance (MDR) is an increasing public health concern worldwide (7, 11). There is a growing epidemic of multidrug-resistant Gram-negative pathogens and a dwindling arsenal of antibiotic options. MDR is most commonly defined as resistance to 3 classes of antibiotics (4). Increased efflux pump expression has been documented in association with resistance to several antibiotic classes, including the fluoro-quinolones (reviewed in reference 11). Of more than 40 puta-tive transporters in Escherichia coli, acrAB-tolC, mdfA, and norE affect fluoroquinolone MICs when expressed with their