- 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.
- 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.
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.
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.