The aim of the present study was to elucidate the effect of the
macrolide antibiotic azithromycin on Pseudomonas aeruginosa. We studied the susceptibility to
azithromycin in P. aeruginosa PAO1 using a killing assay. PAO1 cells at the exponential growth phase were resistant to
azithromycin. In contrast, PAO1 cells at the stationary growth phase were sensitive to
azithromycin. The
divalent cations Mg2+ and Ca2+ inhibited this activity, suggesting that the action of
azithromycin is mediated by interaction with the outer membranes of the cells, since the
divalent cations exist between adjacent
lipopolysaccharides (LPSs) and stabilize the outer membrane. The divalent
cation chelator EDTA behaved in a manner resembling that of
azithromycin;
EDTA killed more PAO1 in the stationary growth phase than in the exponential growth phase. A
1-N-phenylnaphthylamine assay showed that
azithromycin interacted with the outer membrane of P. aeruginosa PAO1 and increased its permeability while Mg2+ and Ca2+ antagonized this action. Our results indicate that
azithromycin directly interacts with the outer membrane of P. aeruginosa PAO1 by displacement of
divalent cations from their binding sites on LPS. This action explains, at least in part, the effectiveness of sub-MICs of
macrolide antibiotics in pseudomonal chronic airway
infection.