Polymyxin B

Poly-Rx (generic)

Polymyxin B is a cyclic cationic lipopeptide antibiotic produced by the soil bacterium Paenibacillus polymyxa. Unlike the glycopeptides, which act on Gram-positive cell walls, polymyxin B targets Gram-negative bacteria. Its positively charged peptide ring is drawn to the negatively charged lipid A component of lipopolysaccharide (LPS) in the Gram-negative outer membrane; a fatty-acid tail then inserts into the membrane, displacing stabilizing cations and disrupting the membrane's integrity. The result is a leaky, destabilized envelope and bacterial death. This detergent-like action is why polymyxins work against organisms that resist many other antibiotics. Polymyxin B was largely set aside for decades because of concerns about kidney and nerve toxicity, but the global rise of multidrug-resistant Gram-negative infections — including carbapenem-resistant Enterobacterales, Pseudomonas aeruginosa, and Acinetobacter baumannii — has brought it back as a critical last-resort intravenous antibiotic. It is also a familiar ingredient in topical antibiotic ointments and some eye and ear preparations, where systemic toxicity is not an issue. Systemic polymyxin B is a specialist, reserve antibiotic. Its main dose-limiting toxicities are nephrotoxicity (kidney injury) and neurotoxicity, so it is dosed carefully — increasingly with efforts to optimize exposure — and used under infectious-disease guidance, often in combination with other agents. Emerging plasmid-borne resistance (such as the mcr genes) is a serious public-health concern precisely because it threatens one of the last available options. Polymyxin B is an approved prescription medicine, not a research or consumer peptide, and systemic use belongs strictly within hospital care.