peptide colistin Colistin A multi-component mixture comprising mostly of colistin A (R = Me) and B (R = H), with small amounts of colistin C and other polymyxins, produced by certain strains of Bacillus polymyxa var. colistinus. An antibiotic, it is used as its sulfate salt (for oral or topical use) or as the sodium salt of the N-methylsulfonic acid derivative (the injectable form) in the treatment of severe Gram-negative infections, partiularly those due to Pseudomonas aeruginosa. Colistin