Mixture by primarly colistin A and B.. Very potent polymyxin antibiotic. Has only one amino acid difference compared to polymyxin B. Antimicrobial. Targets the lipopolysaccharide (LPS) of the outer membrane of Gram-negative bacteria. Competitively displaces divalent cations (Ca2+ and Mg2+) from the negatively-charged phosphate groups LPS, causing disruption of the membrane. This increases the permeability of the cell envelope, leakage of cell contents, leading to apoptosis and cell death. Used against multidrug-resistant Gram-negative bacterial infections.