Activity of beta-lactamase inhibitors in combination with new beta-lactam antibiotics against resistant gram-negative organisms

Thirty-eight isolates of gram-negative bacilli resistant to four new beta-lactam antibiotics, aztreonam, moxalactam, ceftazidime, and cefoperazone, were tested in the presence of two beta-lactam inhibitors, clavulanic acid and sulbactam. Microorganisms tested included 22 isolates of Pseudomonas species, 5 of Klebsiella species, and 11 of Enterobacter species. A 2- to 10-fold decrease in minimum inhibitory concentration was noted when antibiotics and beta-lactamase inhibitors were combined compared to antibiotics alone. Four strains of Pseudomonas aeruginosa showed increased resistance to the combination of antibiotics and beta-lactamase inhibitors.