FIG. 1.

Multidrug efflux pump expression in E. coli clinical isolates. Transcript levels of acrA (A), acrB (B), tolC (C), mdfA (D) and norE (E) were determined by qPCR and are shown normalized to their expression in the standard E. coli strain ATCC 25922, which had a norfloxacin MIC of 0.032 μg/ml. The housekeeping gene rpsL was used to calculate relative expression. Data are displayed relative to the MIC (μg/ml) of the historically relevant fluoroquinolone norfloxacin as measured in our laboratory. Isolates were classified as either susceptible (⋄) or resistant (×) to fluoroquinolones as determined by the hospital. Each point is the average of three experiments. Lines represent the average relative expression values for all of the isolates in the norfloxacin-susceptible (⋄) and -resistant (×) groups. Overexpression was defined as greater than two standard deviations above the mean for the 24 fluoroquinolone-susceptible isolates.