In vitro activity of ceftazidime, cefotaxime and gentamicin against 11,521 clinical isolates of bacteria

The in vitro activity of ceftazidime has been compared with those of another third-generation cephalosporin, cefotaxime, and the aminocyclitol aminoglycoside, gentamicin. A total of 11,521 clinical isolates of aerobic bacteria were employed, and an agar diffusion method was used for sensitivity testing. The MIC-values were calculated from regression lines. The mean inhibition zones for ceftazidime against Gram-positive organisms were significantly less than those against Gram-negative isolates (23 mm vs. 33 mm, p less than 0.0001). Cefotaxime inhibited 74.0%, gentamicin 66.3% and ceftazidime 20.4% of the Gram-positive isolates at a concentration of less than or equal to 2 mg/ml. Ceftazidime and cefotaxime were equally active against fermentative Gram-negative rods, inhibiting 92.7% of each of these isolates at 2 mg/l. Against Ps. aeruginosa, ceftazidime (MIC90 2.2 mg/l) was found to be almost as active as gentamicin (MIC90 1.2 mg/l), and far more active than cefotaxime (MIC90 434 mg/l). Gentamicin was the most active agent against Acinetobacter sp. (MIC90 6.0 mg/l), followed by ceftazidime (MIC90 18 mg/l) and cefotaxime (MIC90 83 mg/l).