what is the Colistin sulfate ?

Colistin is a polymyxin antibiotic produced by certain strains of Bacillus polymyxa var. colistinus. Colistin is a mixture of cyclic polypeptides colistin A and B. Colistin is effective against most Gram-negative bacilli and is used as a polypeptide antibiotic.
Colistin is a decades-old drug that fell out of favor due to its nephrotoxicity.

Colistin sulfate and colistimethate sodium may both be given intravenously, but the dosing is complicated. The very different labeling of the parenteral products of colistin methanesulfonate in different parts of the world was first revealed by Li et al.  Colistimethate sodium manufactured by Xellia (Colomycin injection) is prescribed in international units, but colistimethate sodium manufactured by Parkdale Pharmaceuticals (Coly-Mycin M Parenteral) is prescribed in milligrams of colistin base:
Colomycin 1,000,000 units is 80 mg colistimethate;
Coly-mycin M 150 mg "colistin base" is 360 mg colistimethate or 4,500,000 units.
Because colistin was introduced into clinical practice over 50 years ago, it was never subject to the regulations that modern drugs are subject to, and therefore there is no standardised dosing of colistin and no detailed trials on pharmacology or pharmacokinetics: The optimal dosing of colistin for most infections is therefore unknown. Colomycin has a recommended intravenous dose of 1 to 2 million units three times daily for patients weighing 60 kg or more with normal renal function. Coly-Mycin has a recommended dose of 2.5 to 5 mg/kg colistin base a day, which is equivalent to 6 to 12 mg/kg colistimethate sodium per day. For a 60 kg man, therefore, the recommended dose for Colomycin is 240 to 480 mg of colistimethate sodium, yet the recommended dose for Coly-Mycin is 360 to 720 mg of colistimethate sodium. Likewise, the recommended "maximum" dose for each preparation is different (480 mg for Colomycin and 720 mg for Coly-Mycin). Each country has different generic preparations of colistin, and the recommended dose will depend on the manufacturer. This complete absence of any regulation or standardisation of dose makes intravenous colistin dosing difficult for any physician.