Comparative in-vitro activity of ceftazidime (GR 20263) and other -lactamase stable cephalosporins against pseudomonas: Effect of inoculum size and divalent cation supplementation

The anti-pseudomonal activity of ceftazidime was determined by an agar-dilution technique against 92 strains of Pseudomonas aeruginosa and 22 non-aeruginosa isolates and compared with that of cefoperazone, moxalactam and ceftizoxime. Against Ps. aeruginosa, ceftazidime and cefoperazone were comparable in activity and both were significantly more active than moxalactam and ceftizoxime (P < 0001). At 16 mg/l, 95% of strains were inhibited by ceftazidime, compared to 94% by cefoperazone, 80% by moxalactam and 25% by ceftizoxime. A three- to four-fold increase in geometric mean MIC for ceftazidime, moxalactam and ceftizoxime, and a 23-fold increase for cefoperazone was observed with a larger inoculum of 10(6) cfu, demonstrating a marked inoculum effect. Additionally, divalent cation (calcium and magnesium) concentrations of the test medium also influenced the in-vitro results for ceftazidime and cefoperazone, but not for moxalactam or ceftizoxime. Against non-aeruginosa strains, cefoperazone was most active while ceftizoxime was less effective.