Effects of inducible beta-lactamase and antimicrobial resistance upon the activity of newer beta-lactam antibiotics against Pseudomonas aeruginosa.

The activity of carbenicillin, ticarcillin, piperacillin, cefotaxime, moxalactam, and N-formimidoyl thienamycin was evaluated against 262 clinical isolates of Pseudomonas aeruginosa. There were 242 (92%) of the isolates that were susceptible to carbenicillin or ticarcillin by an agar dilution method. Against this population of susceptible isolates, the median MICs were 1.56 microgram/ml of N-formimidoyl thienamycin, 3.13 microgram/ml of piperacillin, 25 microgram/ml of ticarcillin, 25 microgram/ml of cefotaxime, 50 microgram/ml of carbenicillin and 50 microgram/ml of moxalactam. N-Formimidoyl thienamycin was the only beta-lactam antibiotic not affected by an inducible beta-lactamase detected in 24 randomly selected susceptible isolates by a disk approximation assay, while cefotaxime was inactivated to a greater extent than any of the other beta-lactam antibiotics. Resistance to carbenicillin and ticarcillin was noted in 20 isolates (8%); these were susceptible to N-formimidoyl thienamycin, but cross-resistance with piperacillin, cefotaxime, and moxalactam was frequent. Only four of these resistant isolates were found to have a constitutive beta-lactamase. Gentamicin resistance occurred in 51 isolates (19%) and was an independent variable of resistance to the beta-lactam drugs.