In-vitro activity of ceftazidime against Pseudomonas aeruginosa: and its stability to pseudomonal -lactamases

In-vitro activity of ceftazidime was compared with five other recently developed cephalosporins: cefsulodin (Ciba-Geigy), cefoperazone (Pfizer), cefatriaxon (Roche), moxalactam (chemically a 1-oxa beta-lactam) (Lilly), cefotaxime (Roussel) and also with carbenicillin. Against 92 Pseudomonas aeruginosa isolates, not elaborating plasmid mediated beta-lactamases, mode MICs (mg/l) were: ceftazidime, 1; cefsulodin, 2; cefoperazone, 2; ceftriaxone, 4; moxalactam, 8; cefotaxime, 16; and carbenicillin, 64. There was close correlation between MICs of carbenicillin and MICs of the new cephalosporins (including ceftazidime) for these strains. Bioassay and spectrophotometric assay indicated ceftazidime was completely stable to the Sabath and Abrahams beta-lactamase (Id enzyme) which is present in all Ps. aeruginosa strains. All the other compounds tested were also stable excepting cefatriaxon and cefoperazone, where slow hydrolysis was detectable on bioassay. TEM and PSE beta-lactamases significantly hydrolysed cefoperazone: PSE enzymes also destroyed cefsulodin. PSE-2 and 3 enzymes possessed slight activity against cefatriaxon and cefotaxime. TEM enzymes also slowly hydrolysed cefotaxime. Of the enzymes only PSE-3 showed some low activity against ceftazidime and moxalactam. Strains producing this enzyme remained susceptible to ceftazidime.