The permeability parameter of the outer membrane of Pseudomonas aeruginosa varies with the concentration of a test substrate, cephalosporin C.

The permeability parameter (C) for the movement of cephalosporin C across the outer membrane of Pseudomonas aeruginosa was measured using the widely accepted method of Zimmermann & Rosselet. In one experiment, the value of C varied continuously from 4.2 to 10.8 cm3 min-1 (mg dry wt cells)-1 over a range of concentrations of the test substrate, cephalosporin C, from 50 to 5 microM. Dependence of C on the concentration of test substrate was still observed when the effect of a possible electric potential difference across the outer membrane was corrected for. In quantitative studies of beta-lactam permeation the dependence of C on the concentration of beta-lactam should be taken into account.