Antibiotic diffusion pathways in the outer membrane of Pseudomonas aeruginosa

We investigated the effect of a temperature shift from 37°C to 17°C on the steady-state diffusion rate of imipenem and cephalothin by evaluating periplasmic drug concentrations in intact cells of Pseudomonas aeruginosa, which overexpresses the extended spectrum β-lactamase. We found that the ratio of periplasmic imipenem concentration at 17°C relative to that of 37°C was 1.03 ± 0.1, whereas that of cephalothin was 0.43 ± 0.09. Accumulation rates of cell-associated tetracycline and fluoroquinolone at 17°C were roughly 1/16 and 1/8, respectively, compared with that at 37°C. We concluded from these data that cephalothin and possibly most other antibiotics excepting carbapenems cross the outer membrane of P. aeruginosa mainly by dissolving in the lipid phase but probably not passing through the porin channel. This may explain why the outer membrane of P. aeruginosa is a tight barrier against the penetration of antibiotics.