Promoter activity of acrD from Erwinia amylovora determined by transcriptional fusions with the reporter egfp . Fluorescence was determined 24 h after incubation of the bacteria with various transporter substrates. Substrates were added to a final concentration of 1:10 of the determined MIC values; deoxycholate (50 μg/ml), zinc sulfate (15.6 μg/ml), tetracycline (0.16 μg/ml), naringenin (31.2 μg/ml), novobiocin (1.2 μg/ml), fusidic acid (0.31 μg/ml) and tannin (500 μg/ml). The dotted line indicates the basal acrD promoter activity. Statistically significant differences (P < 0.05) are indicated by asterisks (*) and were determined by a two-sided t-test with equal variances. a.u., arbitrary units.