Aztreonam: antibacterial activity, beta-lactamase stability, and interpretive standards and quality control guidelines for disk-diffusion susceptibility tests.

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.