Disclosed herein is a composition comprising at least one polymyxin or a salt thereof represented by formula (I) wherein R1 is an aliphatic linear or branched C6-C10 acyl group, or (I′) R5 is —CH(CH3)2, —CH2CH(CH3)2, —CH(CH3)CH2CH3, or —CH2C6H5; R6 is —CH(CH3)2, —CH2CH(CH3)2, or —CH(CH3)CH2CH3; each of R2, R3, R4, R7 and R8 is either —(CH2)XCH2NH2 or —(CH2)xCH2N(CH2SO3M)2; wherein x is 0 or 1; wherein M is a monovalent cation; and wherein at least three of R2, R3, R4, R7 and R are —(CH2)xCH2N(CH2SO3M)2.