It is derived from bacteria of the genus Paenibacillus. Medical uses
Antibacterial spectrum
Colistin has been effective in treating infections caused by Pseudomonas, Escherichia, and Klebsiella species. The following represents minimum inhibitory concentration (MIC) susceptibility data for a few medically significant microorganisms:
Escherichia coli: 0.12–128 μg/mL
Klebsiella pneumoniae: 0.25–128 μg/mL
Pseudomonas aeruginosa: ≤0.06–16 μg/mLFor example, colistin in combination with other drugs is used to attack P. aeruginosa biofilm infection in lungs of patients with cystic fibrosis. Biofilms have a low-oxygen environment below the surface where bacteria are metabolically inactive, and colistin is highly effective in this environment. However, P. aeruginosa reside in the top layers of the biofilm, where they remain metabolically active. This is because surviving tolerant cells migrate to the top of the biofilm via pili and form new aggregates via quorum sensing. Administration and dosage
Forms
Two forms of colistin are available commercially: colistin sulfate and colistimethate sodium (colistin methanesulfonate sodium, colistin sulfomethate sodium). Colistin sulfate is cationic; colistimethate sodium is anionic. Colistin sulfate is stable, whereas colistimethate sodium is readily hydrolysed to a variety of methanesulfonated derivatives. Colistin sulfate and colistimethate sodium are eliminated from the body by different routes. With respect to Pseudomonas aeruginosa, colistimethate is the inactive prodrug of colistin. The two drugs are not interchangeable. Colistimethate sodium may be used to treat Pseudomonas aeruginosa infections in patients with cystic fibrosis, and it has come into recent use for treating multidrug-resistant Acinetobacter infection, although resistant forms have been reported.
PET-CT may also be beneficial in detecting extramedullary tumours in individuals diagnosed with SPB. CT imaging may be better than plain film radiography for assessing bone damage.An important distinction to be made is that a true plasmacytoma is present and not a systemic plasma cell disorder, such as multiple myeloma. The difference between plasmacytoma and multiple myeloma is that plasmacytoma lacks increased blood calcium, decreased kidney function, too few red blood cells in the bloodstream, and multiple bone lesions (collectively termed CRAB). Classification
Plasmacytoma is a tumor of plasma cells. The cells are identical to those seen in multiple myeloma, but they form discrete masses of cells in the skeleton (solitary plasmacytoma of bone; SPB) or in soft tissues (extramedullary plasmacytoma; EP). They do not present with systemic disease, which would classify them as another systemic plasma cell disorder.The International Myeloma Working Group (IMWG) has published criteria for the diagnosis of plasmacytomas. They recognise three distinct entities: SPB, extramedullary plasmacytoma and multiple solitary plasmacytomas (+/- recurrent). The proposed criteria for SPB is the presence of a single bone lesion, normal bone marrow (less than 5% plasma cells), small or no paraprotein, no related organ involvement/damage and a normal  skeletal survey (other than the single bone lesion). The criteria for extramedullary plasmacytoma are the same but the tumor is located in soft tissue. No bone lesions should be present. Criteria for multiple solitary plasmacytomas (+/- recurrent) are the same except either multiple solitary bone or soft tissue lesions must be present.