Aztreonam Does NOT Cover Staphylococcus Species
Aztreonam has no clinically useful activity against Staphylococcus aureus or coagulase-negative staphylococci and should never be used as monotherapy when staphylococcal coverage is needed. 1
Microbiological Spectrum
Aztreonam is a monobactam antibiotic with a highly selective gram-negative spectrum. The FDA label explicitly states that aztreonam is:
- Active against: Aerobic gram-negative bacteria only (Enterobacteriaceae, Pseudomonas aeruginosa, Haemophilus influenzae) 1
- Inactive against: All gram-positive bacteria including Staphylococcus species and all anaerobes 1, 2
The mechanism explains this selectivity: aztreonam binds with high affinity to penicillin-binding protein 3 (PBP-3) of gram-negative bacteria but binds poorly to PBP sites of gram-positive bacteria, rendering it ineffective against staphylococci 3.
Critical Clinical Implications
When Aztreonam is Used, Add Staphylococcal Coverage
The 2016 IDSA/ATS hospital-acquired pneumonia guidelines explicitly address this gap 4:
- If aztreonam is selected for gram-negative coverage in patients with severe penicillin allergy, you MUST add separate coverage for methicillin-sensitive Staphylococcus aureus (MSSA)
- Acceptable MSSA coverage options include: piperacillin-tazobactam, cefepime, levofloxacin, imipenem, or meropenem
- For MRSA risk factors, add vancomycin (15 mg/kg IV q8-12h targeting 15-20 mg/mL trough) or linezolid (600 mg IV q12h) 4
Combination Studies Confirm No Anti-Staphylococcal Activity
Research demonstrates that aztreonam does not affect the activity of nafcillin against Staphylococcus aureus in combination studies, confirming its complete lack of staphylococcal coverage 5. When tested against intracellular S. aureus, aztreonam showed no activity while cefamandole demonstrated antibacterial effects 6.
Common Pitfall to Avoid
Never assume aztreonam provides "broad-spectrum" coverage simply because it's a beta-lactam. Its spectrum is intentionally narrow and gram-negative specific. This selectivity means aztreonam has minimal effect on normal gram-positive and anaerobic intestinal flora 7, but also means it leaves staphylococcal infections completely untreated.
In any clinical scenario where staphylococci (either S. aureus or coagulase-negative staphylococci) are potential pathogens—including bloodstream infections, device-related infections, or empiric therapy for serious infections—aztreonam monotherapy is inadequate and requires combination with an anti-staphylococcal agent.