This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Microbiology
The endotoxin of the following gram negative bacteria does not play any part in the pathogenesis of the natural disease. 
Select and explain. 
A. E. coli, B. Klebsiella, C. Vibrio cholerae, D. Pseudomonas
C: Vibrio cholerae
Answer: C: Vibrio cholerae 
Explanation: Cholera Vibrios also possess lipopolysaccharide O antigen (LPS, endotoxin) as in other gram-negative intestinal bacilli. But unlike other gram-negative bacilli like E.coli, Klebsiella and Pseudomonas, etc. (where endotoxin is the principle virulence factor), LPS apparently plays no role in pathogenesis of cholera but is responsible for the immunity induced by killed vaccine.
For Vibrio cholerae the principle virulence factor is the exotoxin, i.e. cholera toxin.