Polypeptide Antibiotics : Uses, Dosage, Side Effects, Interactions, & Contraindications

Polypeptide Antibiotics : Definition, Classification, Mechanism of action, Clinical uses, Indications, Side Effects, Interactions, Precautions, & Contraindications


  • Polypeptide antibiotics are low molecular weight cationic polypeptide antibiotics.
  • All are powerful bactericidal agents, but not used systemically due to toxicity. All are produced by bacteria.


Clinically used ones are:

Mechanism of action

  • Polymyxins have high affinity for phospholipids. They have detergent-like action on the cell membrane & bind to phospholipids on the bacterial cell membrane of gram-negative bacteria causing membrane distortion or pseudopore formation i.e. disrupts cell membrane integrity. It leads to leakage of cellular components (ions, amino acids, etc.) & ultimately cell death.
  • Bacitracin acts by inhibiting cell wall synthesis at a step earlier than that inhibited by penicillin. Subsequently, it increases the efflux of ions by binding to cell membrane.
  • Tyrothricin acts on cell membrane causing leakage and uncouples oxidative phosphorylation in the bacteria.

Antibacterial spectrum

  • Polymyxins are active against gram-negative bacteria only. They are active against most clinically important gram-negative bacteria except Proteus, Serratia and Neisseria.
  • Tyrothricin is active against grampositive and a few gram-negative bacteria.
  • In contrast to polymyxin, Bacitracin is active mainly against gram-positive organisms (both cocci and bacilli). Neisseria, H. influenzae and few other bacteria are also affected.

Therapeutic uses

Polymyxins have following uses :

  • Topically : Usually in combination with other antimicrobials for skin infections, burns, otitis externa, conjunctivitis, corneal ulcer-caused by gram-negative bacteria including Pseudomonas.
  • Orally : Gram-negative bacillary (E. coli, Salmo11ella Shigella) diarrhoeas, especially in infants and children, Pseudomonas superinfection enteritis.

Bacitracin use is restricted to topical application for infected wounds, ulcers, eye infections-generally in combination with neomycin, polymyxin, etc.

Side effects

Polypeptide antibiotics are not used systemically due to toxicity. Consult with your pharmacist.


  • Contraindicated in persons having hypersensitivity to any of polypeptide antibiotic.

For detailed query or in case of uncertainty, Always consult your doctor or pharmacist.

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