Aztreonam is the sole marketed monobactam, so named due its monocyclic -lactam
ring resistant to many -lactamases. Aztreonam
is active against gram-negative organisms (including Pseudomonas)
but has no activity against gram-positive organisms or anaerobes.
Aztreonam resembles ceftazidime in its gram-negative activity. Thus, it is generally strong against E coli, Klebsiella, Proteus, and Pseudomonas spp, but unpredictable against Acinetobacter, Enterobacter, and Citrobacter spp. Clinical
uses of aztreonam are limited because of the availability of third-generation
cephalosporins with a broader spectrum of activity and minimal toxicity.
Despite the structural similarity of aztreonam to penicillin, IgE cross-reactivity does not exist, and it can therefore be used in patients with IgE-mediated penicillin allergy.