Aztreonam: antibacterial activity, beta-lactamase stability, and interpretive standards and quality control...

In vitro activity of aztreonam was compared with that of ceftazidime, cefotaxime, cefoperazone, piperacillin, and ticarcillin against 656 representative bacterial pathogens. Aztreonam was not active against gram-positive cocci but was as active as the third-generation cephalosporins against the Enterobacteriaceae, Haemophilus influenzae, and Neisseria gonorrhoeae. Additional data for 5,262 gram-negative bacilli isolated in four separate medical centers documented the low incidence of resistance to aztreonam; 97.2% of 4,312 isolates of Enterobacteriaceae and 79% of 854 isolates of Pseudomonas aeruginosa were inhibited by less than or equal to 8.0 micrograms of aztreonam/ml. Additional studies confirmed the stability of aztreonam in the presence of seven different beta-lactamases. For disk-diffusion susceptibility tests, 30-micrograms disks are recommended, with interpretive breakpoints of less than or equal to 15 mm for resistance (MIC greater than or equal to 32 micrograms/ml), 16-21 mm for intermediate susceptibility (MIC, 16 micrograms/ml), and greater than or equal to 22 mm for susceptibility (MIC less than 8.0 micrograms/ml). For quality control of tests with 30-micrograms disks, zone-size limits for Escherichia coli (ATCC 25922) should be 28-36 mm and those for P. aeruginosa (ATCC 27853) should be 23-29 mm.