Polymyxins



Polymyxins
Polymyxin B & E (Colistin) are two clinically important polypeptide antibiotics obtained from Bacillus polymyxa and Bacillus colistinus respectively.
Both are narrow spectrum, rapidly bactericidal antibiotics.
Highly active against G -ve orgs. Such as E. Coli, Salmonella spp. And P. Aeruginosa.
They are ineffective against G +ve orgs.
These have detergent like action on the Bac.’s cell membrane.
They bind with bac. Cell membrane phospholipids with high affinity, distort it. As a result ions, aminoacids, etc leak out and bac. Cells die.
They are neither absorbed topically nor orally and do not penetrate BBB.
For systemic they are administered parenterally.
Systemically rarely used due to serious toxicity (nephrotoxicity, neurotoxicity and neuromuscular blockade) and only used in life threatening infections caused by G –ve bacilli or Pseudomonas.
Orally it is used in G – ve enteritis in calves and pigs, topically in skin infections (along with neomycin / bacitracin), otitis externa (with OTC), in bovine mastitis (with OTC for Klebsiella spp.) and as semen additive (in bull semen containing Pseudomonas org.)
Dose:
Oral: 20000 units/kg, bid.
IM: 5000 units/kg, bid.
Intramammary: 50000 – 1L units.
Intrauterine: 1L units.

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