Colistin for Multidrug-Resistant Gram-Negative Infections: The Last-Resort Peptide

The clinical scenario

Some of the hardest infections in medicine are caused by multidrug-resistant Gram-negative bacteria — organisms like carbapenem-resistant Klebsiella, Pseudomonas aeruginosa, and Acinetobacter baumannii that have become resistant to nearly all standard antibiotics. When the usual options are exhausted, colistin (polymyxin E) can be one of the few remaining active drugs. This use case explains its last-resort role; such infections are managed strictly within hospital infectious-disease care.

Why the mechanism fits

Colistin is a polymyxin — a lipopeptide antibiotic that attacks the very feature making Gram-negatives so hard to treat: their outer membrane. Its positively charged ring binds the negatively charged lipid A of the membrane's lipopolysaccharide, and its fatty tail inserts and destabilizes the envelope, permeabilizing it in a detergent-like way. Because it targets the outer membrane directly, colistin often remains active against organisms resistant to antibiotics that work by other mechanisms.

The prodrug complication

A defining challenge is that colistin is given as an inactive prodrug — colistimethate sodium (CMS) — that the body must convert to the active drug. This conversion is gradual and variable, so active levels build slowly and differ between patients. Historically this caused underdosing and treatment failure. Modern practice addresses it with:

• loading doses to reach effective levels faster, and
• careful attention to exposure targets and kidney function.

Understanding the prodrug step is essential to using colistin correctly — it's not a drug where a simple fixed dose reliably works.

The toxicity trade-off

Colistin was largely abandoned for decades because of its main dose-limiting toxicities:

• Nephrotoxicity (kidney injury), and
• Neurotoxicity (nerve-related effects).

It came back only out of necessity — because resistance left few alternatives. So it's reserved, dosed carefully, closely monitored, and often used in combination with other agents to improve effectiveness and limit resistance. Inhaled colistimethate has a separate role in chronic Pseudomonas lung infection (for example in cystic fibrosis), where airway irritation is the relevant concern.

The resistance threat: mcr

Because colistin is so often a last line of defense, resistance to it is alarming. The mcr genes (first identified as mcr-1) are transferable, plasmid-carried resistance elements that modify the bacterial membrane so polymyxins bind poorly — and because they're mobile, they can spread between bacteria and species. Their global emergence is a major reason for careful stewardship of the polymyxins and strong infection-prevention efforts.

Colistin vs polymyxin B

Colistin's close relative polymyxin B shares its mechanism and toxicity but is given directly as the active drug, giving somewhat more predictable levels for bloodstream infections. Colistin retains a distinct inhaled role. See the colistin vs polymyxin B comparison for how they're chosen.

Realistic expectations

• Colistin is a rescue option, used when better-tolerated drugs no longer work — not a first-line antibiotic.
• Success depends on correct dosing (accounting for the prodrug), source control, and often combination therapy.
• It's entirely specialist-managed, and preventing resistant infections in the first place matters as much as treating them.

The takeaway

Colistin embodies the double edge of last-resort medicine: a membrane-cracking peptide that still works against the most resistant Gram-negatives, but one that's toxic, hard to dose because of its prodrug, and threatened by spreading mcr resistance. Its careful, reserved use is a direct reflection of how precious the remaining options for these infections have become.

This article is educational and does not constitute medical advice. Resistant infections are managed by qualified specialists.