Colistin is not active against?

Colistin* It was obtained from B. colistinus .* It is active against gram-negative bacteria only; all except Proteus, Serratia and Neisseria are inhibited. * It is more potent on Pseudomonas, Salmonella and Shigella. Mechanism of action Rapidly acting bactericidal agent having a detergent-like action on the cell membrane. They have high affinity for phospholipids. It causes membrane distoion or pseudopore formation. It may also inactivate the bacterial endotoxin.