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Merck Manual

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Brian J. Werth

, PharmD, University of Washington School of Pharmacy

Last full review/revision Jul 2020| Content last modified Jul 2020
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Polypeptides are a class of antibiotics used to treat several types of infections. Polypeptides include the following:

  • Bacitracin

  • Colistin

  • Polymyxin B

Most bacteria have an outer covering (cell wall) that protects them. Bacitracin works by preventing bacteria from forming this wall. Colistin and polymyxin B work by disrupting the cell membrane underlying this wall in some bacteria. As a result, the bacteria die.

Bacitracin is used mainly to treat superficial skin infections caused by Staphylococcus aureus. It is applied directly to the skin (topically).

Colistin and polymyxin B commonly cause kidney damage, so they are used only for serious infections in which bacteria are resistant to all other antibiotics and no safer alternatives are available. When colistin or polymyxin B is used, it is given by vein (intravenously). In some cases, colistin may be inhaled through a nebulizer.




Common Uses

Some Side Effects



Polymyxin B

Ear, eye, or skin infections

Bladder infections (polymyxin B may be used for bladder irrigation)

Serious infections caused by susceptible bacteria that are very resistant to many other antibiotics (colistin and polymyxin B given by injection)

Kidney and nerve damage (when given by injection)*

* Polypeptide antibiotics are usually applied directly to the ears, skin, or eyes, but colistin and polymyxin B are sometimes given by injection when bacteria are resistant to all other antibiotics and no safer alternatives are available. Bacitracin is not available for injection.

Use of Polypeptides During Pregnancy and Breastfeeding

With bacitracin, the risk to the fetus during pregnancy and to the newborn during breastfeeding is thought to be slight because bacitracin is applied topically and little of the drug is absorbed into the bloodstream. However, its safety has not been established. The same is true for polymyxin B and colistin when they are applied topically.

Polymyxin B and colistin should be taken by injection during pregnancy only when the benefits of treatment outweigh the risks. With polymyxin B, no harmful effects on the fetus have been observed in animal studies, but no well-designed studies have been done in pregnant women. With colistin, harmful effects on the fetus have been observed in animal studies. (See also Drug Use During Pregnancy.)

Whether use of polymyxin B or colistin by injection during breastfeeding is safe is unknown. (See also Drug Use During Breastfeeding.)

Drugs Mentioned In This Article

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No US brand name
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