This antibiotic injection is used to treat infections caused by bacteria, such as urinary tract infections. Colistimethate injection is a medication that belongs to a family of drugs known as antibiotics. It works by destroying microorganisms on the surface of the skin. Colds, the flu, and other viral diseases will not be treated with antibiotics such as colistimethate injection, for example. Taking antibiotics when they are not necessary raises your chances of having an illness that is resistant to antibiotic treatment later on in life.