Susceptibility of 324 nonfermentative gram-negative rods to 6 cephalosporins and azthreonam

Susceptibility of 324 isolates of nonfermentative gram-negative bacteria to cephalothin, cefamandole, cefoxitin, ceftriaxone, ceftazidime, cefsulodin, and azthreonam was determined by agar dilution and disc diffusion techniques. With the exception of Moraxella species, first- and second-generation cephalosporins were minimally active against nonfermenters tested. Cefsulodin and azthreonam were mainly active against Pseudomonas aeruginosa. In contrast, ceftazidime and ceftriaxone exhibited wider activity spectra. Cefsulodin and ceftazidime were the most active against P. aeruginosa, followed by azthreonam and ceftriaxone. Ceftazidime was the only drug with significant activity against Pseudomonas maltophila and Pseudomonas fluorescens/ Pseudomonas putida, and was also the most active against Pseudomonas cepacia. Acinetobacter calcoaceticus and Alcaligenes strains were most sensitive to ceftazidime, followed by ceftriaxone and axthreonam. Ceftriaxone was the most active against moraxellas, followed by cefamandole/cefoxitin, ceftazidime, cephalothin, cefsulodin, and azthreonam. Ceftazidime was the most active against uncommonly isolated nonfermenters. Results observed in this study reflect a potential use for ceftazidime in therapy of infections caused by most gram-negative nonfermenters; therapy with cefsulodin and azthreonam in nonfermenter infections should be restricted to those caused by P. aeruginosa, and that with ceftriaxone reserved for non-P. aeruginosa infections.