Carbapenems

Carbapenems are β-lactam antibiotics — the same family as the penicillins and cephalosporins already covered — and they kill the same way: they bind penicillin-binding proteins and cripple bacterial cell-wall synthesis, so the cell bursts. What sets them apart is spectrum: carbapenems are the broadest-spectrum β-lactams available, covering gram-positives, gram-negatives (including Pseudomonas, except ertapenem), and anaerobes, and resisting many of the β-lactamase enzymes that defeat other β-lactams — notably ESBL-producing organisms.

That power is exactly why they are held in reserve. Using carbapenems for routine infections breeds carbapenem-resistant organisms (CRE) — among the most feared, nearly untreatable pathogens. So they are the "last-resort big guns," deployed for multidrug-resistant or severe infection, not first-line.

The hallmark toxicity of carbapenems is CNS excitation → seizures, greatest with imipenem and in patients with renal impairment (drug accumulates), high doses, or a prior seizure/CNS disorder. A closely related, high-yield interaction: carbapenems dramatically lower valproic acid (valproate) levels, which can break seizure control in an epileptic patient — so the combination is avoided.

Imipenem is always paired with cilastatin for a clever reason: the kidney enzyme dehydropeptidase-I would otherwise destroy imipenem and create nephrotoxic byproducts. Cilastatin inhibits that enzyme, protecting the drug (and the kidney) — it has no antibacterial action of its own. As β-lactams, carbapenems share the class allergy concern, but cross-reactivity with penicillin allergy is low (<1%).