Expression of the putative RND efflux system genes amrB and amrR in clinical CF and in vitro antibiotic or hydrogen peroxide exposed B. vietnamiensis isolates. Expression was determined by real-time reverse transcription PCR and compared (A) between the early, aminoglycoside-susceptible isolate C8395, and the late, aminoglycoside-resistant D0774, in various types of media and stages of growth, and (B) between C8395 before and after its exposure to various antimicrobials, peroxide, or passage alone in CAMHB. The averages of three technical repeats were taken for each biological replicate. Fold change means were calculated by comparing the mean expression in C8395 to each biological replicate of (A) D0774 or (B) condition. Data points represent the averages of three biological replicates ± standard errors. (A) **, P < 0.01; *** P < 0.001 by unpaired Student’s t-test. (B) *, P < 0.05; **, P < 0.01 by Dunnett’s Multiple Comparison Test after one-way ANOVA. Abbreviations: amrB, Bcep1808_1575; amrR, Bcep1808_1573; RND, resistance-nodulation-division; CAMHB, cation-adjusted Mueller-Hinton broth; LB, Luria-Bertani medium; SCFM, synthetic cystic fibrosis sputum medium; OD600, optical density at 600 nm.