Membrane permeability of Pseudomonas aeruginosa to 4-quinolones.

The outer membrane-disorganizing agent polymyxin B nonapeptide sensitized Pseudomonas aeruginosa strains 2-40fold to ciprofloxacin, norfloxacin, and ofloxacin, and 80-200fold to nalidixic acid. Sensitized P. aeruginosa strains were nearly as susceptible to nalidixic acid as Escherichia coli strains. The data suggest that the outer membrane permeability is of critical importance for the antipseudomonal activity of 4-quinolones. The low penetration of the outer membrane is the major cause of the nalidixic acid resistance of P. aeruginosa.