Factors that influence the permeability assay of the outer membrane ofPseudomonas aeruginosa

Brief exposure of Pseudomonas aeruginosa to a temperature of 10°C or lower caused a significant leakage of the periplasmic β-lactamase into the medium. The extent of leakage increased as the incubation temperature was lowered to 4°C and reached a maximum at 0°C. Cells grown in the presence of β-lactamase inducers were unsuitable for the permeability assay. It was found that the diffusion rates of β-lactams through the outer membrane of P. aeruginosa were much lower than those previously reported, as assayed under refined conditions. The diffusion rates of β-lactams in one of the mutants tested were an order of magnitude lower than those of the other strains, despite the fact that the outer membrane protein profile of the strain appeared to be indistinguishable from those of the others. These results suggest that β-lactam antibiotics diffuse through the outer membrane of P. aeruginosa, at least partly, through a non-porin pathway.