1. A mixture of antibiotic substances obtained from cultures of Bacillus polymyxa (B. serosporus), an organism found in water and soils and obtainable as a crystalline hydrochloride; all are polypeptides containing various amino acids and a branched-chain fatty acid, usually (+)-6-methyloctanoic acid. There are several polymyxins, (designated A, B1, C, D, E, M, T), which are about equally effective against gram-negative bacteria, but which differ in toxicity, polymyxin E (colistin) and polymyxin B being the least toxic.

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