Figure 1. Expression of tolC, mdfA, and ydhE measured by RT-PCR in the absence and presence of ciprofloxacin. Each value is the average of five culture replicates, each of which was evaluated twice. Relative gene expression levels were calculated as 2^–ΔΔCT, where ΔΔCT = –ΔCT (sample) –ΔCT (control) [3]. CT = threshold cycle.