title: Azithromycin exhibits bactericidal effects on Pseudomonas aeruginosa through interaction with the outer membrane.
abstract: 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.
authors.affiliation: Second Department of Internal Medicine, Nagasaki University School of Medicine, Nagasaki 852-8501, Japan.
authors.forename: Yoshifumi
authors.initials: Y
authors.lastname: Imamura
authors.forename: Yasuhito
authors.initials: Y
authors.lastname: Higashiyama
authors.forename: Kazunori
authors.initials: K
authors.lastname: Tomono
authors.forename: Koichi
authors.initials: K
authors.lastname: Izumikawa
authors.forename: Katsunori
authors.initials: K
authors.lastname: Yanagihara
authors.forename: Hideaki
authors.initials: H
authors.lastname: Ohno
authors.forename: Yoshitsugu
authors.initials: Y
authors.lastname: Miyazaki
authors.forename: Yoichi
authors.initials: Y
authors.lastname: Hirakata
authors.forename: Yohei
authors.initials: Y
authors.lastname: Mizuta
authors.forename: Jun-Ichi
authors.initials: J
authors.lastname: Kadota
authors.forename: Barbara H
authors.initials: BH
authors.lastname: Iglewski
authors.forename: Shigeru
authors.initials: S
authors.lastname: Kohno
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D017963:Azithromycin / Q000494:pharmacology*; D001425:Bacterial Outer Membrane Proteins / Q000187:drug effects; D002462:Cell Membrane / Q000187:drug effects*; D002463:Cell Membrane Permeability / Q000187:drug effects; D008826:Microbial Sensitivity Tests; D011550:Pseudomonas aeruginosa / Q000187:drug effects* / Q000254:growth & development*
chemical_list: D000900:Anti-Bacterial Agents; D001425:Bacterial Outer Membrane Proteins; D017963:Azithromycin