Pharmacology · Class
Polymyxins !
Old, detergent-like antibiotics that punch holes in the gram-negative membrane; a last resort for multidrug-resistant bacteria, toxic to kidney and nerves.
Updated 2026-11-01
What they are
Cationic cyclic peptides (polymyxin B and colistin, also called polymyxin E). They fell out of use because of toxicity and returned as a last resort against multidrug-resistant Pseudomonas, Acinetobacter and Enterobacterales.
Mechanism of action
Polymyxins are surface-active, amphipathic peptides that act like detergents. They bind the lipid A portion of lipopolysaccharide in the gram-negative outer membrane, interact with membrane phospholipids and disrupt the membrane, so the cell leaks and dies. Binding lipid A also neutralizes endotoxin.
Target: Lipid A of the lipopolysaccharide in the gram-negative outer membrane
Drugs in this class
- Colistin (colistimethate)A last-resort polymyxin for multidrug-resistant Pseudomonas, Acinetobacter and Klebsiella; it can injure the kidney…Lipid A of the lipopolysaccharide, gram-negative outer membrane
- Polymyxin BA polymyxin given IV, IM or intrathecally for resistant gram-negatives, and used topically in eye and ear drops…Lipid A of the lipopolysaccharide, gram-negative outer membrane
Test yourself
Think of the answer, then tap the question to see it.
Which bacterial structure do polymyxins attack?
The gram-negative outer membrane: they bind the lipid A of lipopolysaccharide and disrupt the membrane like a detergent.
Which two organisms are naturally resistant to polymyxins?
Proteus and Serratia (Burkholderia as well).
Why are polymyxins combined with aminoglycosides only with great caution?
Both damage the kidney and both block the neuromuscular junction, adding the risk of kidney failure and apnea.
Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed., ch. 59 (McGraw Hill, 2023); DailyMed - COLISTIMETHATE- colistimethate sodium injection, powder, for solution (DailyMed (U.S. National Library of Medicine), 2026); DailyMed - POLYMYXIN B- polymyxin b sulfate injection, powder, for solution (DailyMed (U.S. National Library of Medicine), 2026).
Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake
Representative model, for educational purposes only.