Shigella dysentery due to exotoxin is by:

Ans. a (S. dysenteriae)(Ref. Harrison, Medicine, 18th ed., Chapter 154. Shigellosis)SHIGELLA# S. dysenteriae does not produce catalase & does not ferment mannitol.# S. Sonnie is late lactose fermentor.# S. Fiexinerri produces gas.# S. dysenteriae Type 1 forms a toxin, the earliest example of a exotoxin produced by a gram-negative bacillus.# All except S. dysenteriae Type 1 produces catalase.# Colonies on MacConkey agars are colorless due to absence of lactose fermentation except S. sonnie, which ferments lactose late and forms pink colonies.# DCA and Wilson and Blair selective media are the selective media.# Infective dose of Shigella is 10-100 bacteria, while that of Salmonella is 102-105.