FIG. 3.

Autoinducers are reduced in the presence of azithromycin and partially restore rhlAB expression and elastase production. (A) Strain PAO1 cultures were grown for 12 h in Luria-Bertanimedium, either in the absence (azithromycin −) or presence (azithromycin +) of 2 μg of azithromycin/ml. The supernatants were extracted with ethyl acetate, and the 3-oxo-C₁₂-HSL and C₄-HSL concentrations were measured using specific bioassays. Results are the mean ± SD of three independent experiments. (B) Cells were grown for 10 h in the absence (azithromycin −) or presence (azithromycin +) of 2 μg of azithromycin/ml and in the absence (autoinducers −) or presence (autoinducers +) of exogenous autoinducers. The expression of the rhlAB gene was determined by measuring the β-Gal activity of the rhlA′-lacZ reporter fusion pECP60. The production of elastase was determined by the elastin Congo red assay performed on culture supernatants. Results are expressed as the percentage of the maximum observed in each individual experiment and represent the mean ± SD of three independent experiments performed in duplicate.