Aztreonam: Antibacterial Activity, β-Lactamase Stability, and Interpretive Standards and Quality Control Guidelines for Disk-Diffusion Susceptibility Tests

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Abstract
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 ⩽8.0 Ilg of aztreonam/ml. Additional studies confirmed the stability of aztreonam in the presence of seven different β-lactamases. For disk-diffusion susceptibility tests, 30-µg disks are recommended, with interpretive breakpoints of ⩽15 mm for resistance (MIC ⩾32 µg/ml), 16–21 mm for intermediate susceptibility (MIC, 16 µg/ml), and ⩾22 mm for susceptibility (MIC Escherichia coli (ATCC 25922) should be 28–36 mm and those for P. aeruginosa (ATCC 27853) should be 23–29 mm.

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