Diffusion of macrolide antibiotics through the outer membrane of Moraxella catarrhalis.

We reported previously that the high susceptibility of Moraxella catarrhalis to macrolide antibiotics and other hydrophobic antimicrobial agents was related to the hydrophobicity of the cell surface. Electrophoretic analysis of lipopolysaccharide (LPS) extracted from M. catarrhalis revealed a deep rough-type profile similar to that of an LPS Re type mutant of Salmonella typhimurium, which also exhibits high susceptibility to macrolides. Moreover, treatment of 32P-labeled cells of M. catarrhalis by phospholipase C induced the release of radioactive materials. These results suggested that hydrophobic agents such as macrolides readily access the cell surface exposed by the deep rough-type LPS and phospholipids, and permeate into the cell interior through the lipid bilayer. In fact, M. catarrhalis cells rapidly accumulated large amounts of the macrolide antibiotics, erythromycin and rokitamycin, whereas no accumulation of the macrolides was observed in cells having smooth-type or Rc type LPS under the same conditions.