Colistin, also known as polymyxin E, is an antibiotic medication used as a last-resort treatment for multidrug-resistant Gram negative infections including pneumonia. These may involve bacteria such as "Pseudomonas aeruginosa", "Klebsiella pneumoniae", or "Acinetobacter". It comes in a form which can be injected into a vein or into a muscle, or inhaled, known as colistimethate sodium and one which is applied to the skin or taken by mouth, known as colistin sulfate. Resistance to colistin began to appear as of 2015.