This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Bacteriology, Subject name: Microbiology
All are catalase positive Except 
Select and explain. 
A. Shigella flexineri, B. Shigella boydii, C. Shigella dysentriae type I, D. Shigella sonnei
C: Shigella dysentriae type I
Answer: C: Shigella dysentriae type I 
Explanation: S.dysenteriae (subgroup A): This species of mannitol non-fermenting bacilli consists of ten serotypes. It is indole negative and is the only member of the family that is always catalase-negative. S.flexineri, S.boydii, and S.sonnei are catalase positive. Ref: Textbook of microbiology; Aanthanarayan and paniker's; 10th Edition; Pg:292