Penetration of beta-lactam antibiotics into their target enzymes in Pseudomonas aeruginosa: comparison of a highly sensitive mutant with its parent strain

Pseudomonas aeruginosa K 799/WT and a mutant of this strain, P. aeruginosa K 799/61 ("mutant 61"), that is very sensitive to most beta-lactam antibiotics tested were used to assess the importance of penetration barriers in the resistance of P. aeruginosa to penicillins and cephalosporins. The affinities of various beta-lactams to the penicillin-binding proteins found in membranes prepared from both strains were compared by measuring their competition for the binding of benzyl[14C] penicillin to each of these proteins. Only minor differences between the wild type and the mutant 61 were found. The high sensitivity of the mutant therefore cannot be attributed to drastic alterations of these target proteins, nor can the resistance of the wild type be ascribed to penicillin-binding proteins with low affinities for beta-lactams. Experiments in which the ease of penetration of beta-lactams into the penicillin-binding proteins was measured with exponentially growing intact cells instead of membranes, however, clearly demonstrated an easy access of beta-lactam antibiotics to these proteins in the mutant and an efficient exclusion from the same targets in the wild type.