[Sensitivity of Pseudomonas aeruginosa to beta-lactam antibiotics and effects of beta-lactamases].

The activities of carbenicillin, ticarcillin, piperacillin, cefsulodin, ceftriaxone, azlocillin, ceftazidime and apalcillin were evaluated against 128 hospital isolates of Pseudomonas aeruginosa. Against the 85 isolates that were susceptible to carbenicillin, apalcillin had the best minimal inhibitory concentrations, the median MIC being 1, 1 microgram/ml. Against the 43 isolates resistant to carbenicillin, ceftazidime inhibited 90% of organisms at a concentration of 16 micrograms/ml. A constitutive beta-lactamase, was found in 27 of the 43 resistant isolates. All the 128 isolates, but one, were producers of an inducers of an inducible beta-lactamase. Ceftriaxone, piperacillin and azlocillin were highly antagonized by cefoxitin. The extent of antagonism was lower with ticarcillin and carbenicillin.