Frequency of beta-lactamases that are markedly active against carbenicillin in the Pseudomonas aeruginosa strains isolated in a medical school hospital.

During one year, 71 carbenicillin-resistant Pseudomonas aeruginosa strains were isolated. The detection of the beta-lactamase activity in these strains was performed by the iodometric method. The beta-lactamases were found in 70% of the carbenicillin-resistant strains and were characterized by isoelectric focusing and by determination of substrate profiles. Seventy-two per cent of the beta-lactamases tested focused in the same way as the PSE-1 type from strain PU21(RPL11). The determination of the substrate profiles by the acidimetric method showed that these enzymes had a high activity towards carbenicillin and that they can vary in their hydrolysis of cephalosporins. Such PSE-1 type beta-lactamases were found in gentamicin-tobramycin-resistant strains whereas other types of enzymes (PSE-4 enzymes and enzymes with pI 7.7) were found in strains sensitive to these antibiotics.