Which antibiotic is not active against anaerobes?

• A. Metronidazole
• B. Clindamycin
• C. Amoxicillin
• D. Aztreonam

Answer: (D)

Understanding which antibiotics cover anaerobes is essential for choosing the right drug in infections where anaerobic bacteria are suspected. Some agents have strong activity against anaerobes, while others do not.

Aztreonam is not active against anaerobes. It is a monocyclic beta-lactam that primarily targets gram-negative aerobic bacteria by binding specific PBPs and inhibiting cell wall synthesis. It does not have meaningful activity against anaerobic organisms, so it’s not suitable for anaerobic infections. In contrast, metronidazole and clindamycin are well known for anaerobic coverage, and amoxicillin can have anaerobic activity, especially when used with a beta-lactamase inhibitor or in certain mixed infections.