The colistin, also known as fosfomycin and polypeptide E, is a kind of polypeptide antibiotic, which is a mixture of two kinds of cyclic polypeptide--A and B. It can be produced by a variety of strains of Bacillus (polymyxa var. colistinus). It is effective for most gram-negative bacteria, but has liver toxicity and neurotoxicity, so it is considered as "the last line drug".