monobactams

Monobactams are a class of beta-lactam antibiotics defined by a single, monocyclic beta-lactam ring. The prototypical and clinically used member is aztreonam. They are structurally distinct from penicillins and cephalosporins because their beta-lactam ring is not fused to another ring system.

Mechanism and spectrum

Monobactams inhibit bacterial cell wall synthesis by binding to penicillin-binding proteins, producing a bactericidal effect. They are primarily active against aerobic Gram-negative bacteria, including many Enterobacteriaceae and Pseudomonas aeruginosa. They generally have little or no activity against Gram-positive cocci or anaerobes, including Bacteroides species. Their activity against some non-Enterobacteriaceae can be limited, and resistance can arise through beta-lactamase production or PBPs with reduced affinity.

Pharmacology and administration

Aztreonam is administered parenterally (intravenous or intramuscular) and is also available as an inhaled formulation for airway infections in cystic fibrosis. It is not orally absorbable. The drug is predominantly excreted unchanged by the kidneys, so dosage adjustments are needed in renal impairment. It tends to have good penetration into many body fluids, supporting use in serious systemic infections caused by susceptible gram-negative pathogens.

Resistance and safety

Monobactams are relatively resistant to many common beta-lactamases but can be hydrolyzed by extended-spectrum beta-lactamases and certain carbapenemases. They do not cover MRSA or most Gram-positive organisms and are not effective against anaerobes. Adverse effects are generally mild but may include hypersensitivity reactions, rash, diarrhea, and elevated liver enzymes. Aztreonam is often considered in patients with penicillin allergy when Gram-negative coverage is needed.