Colistimethate Sodium is used to treat acute and chronic infections caused by susceptible strains of certain gram-negative bacilli. It is especially effective against infections caused by Pseudomonas aeruginosa. However, it is not effective for infections caused by Proteus or Neisseria species. This drug shows strong activity against several gram-negative bacteria, including Enterobacter aerogenes, Escherichia coli, Klebsiella pneumoniae, and Pseudomonas aeruginosa.