Outer membrane permeability barrier to azithromycin, clarithromycin, and roxithromycin in gram-negative enteric bacteria.

Mutations which severely affect the function of the outer membrane of Escherichia coli and Salmonella typhimurium (lpxA and firA mutations of lipid A synthesis and rfaE mutation of the lipopolysaccharide inner-core synthesis) were found to decrease the MICs of erythromycin, roxithromycin, clarithromycin, and azithromycin by factors of 32 to 512, 32 to 1,024, 64 to 512, and 16 to 64, respectively. The sensitization factors for three other hydrophobic antibiotics (rifampin, fusidic acid, and mupirocin) ranged from 16 to 300. The outer membrane permeability-increasing agents polymyxin B nonapeptide (3 micrograms/ml) and deacylpolymyxin B (1 microgram/ml) sensitized wild-type E. coli to azithromycin by factors of 10 and 30, respectively. Quantitatively very similar sensitization to the other macrolides took place. Polymyxin-resistant pmrA mutants of S. typhimurium displayed no cross-resistance to azithromycin. Proteus mirabilis mutants which were sensitized to polymyxin by a factor of > or = 300 to > or = 1,000 had a maximal two- to fourfold increase in sensitivity to azithromycin. These results indicate that azithromycin and the other new macrolides use the hydrophobic pathway across the outer membrane and that the intact outer membrane is an effective barrier against them. Furthermore, the results indicate that azithromycin, in contrast to polymyxin, does not effectively diffuse through the outer membrane by interacting electrostatically with the lipopolysaccharide.