This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Cell Wall Synthesis Inhibitors, Subject name: Pharmacology
Colistin is not active against? 
Select and explain. 
A. Pseudomonas aeruginosa, B. Serratia, C. Klebsiella pneumoniae, D. Burkholderia
B: Serratia
Answer: B: Serratia 
Explanation: 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.