<prompt> Shigella dysentery due to exotoxin is by:S. dysenteriaeS. flexneriS. sonneiS. shigellae  <response>: Ans. a S. dysenteriaeRef. Harrison, Medicine, 18th ed., Chapter 154. ShigellosisSHIGELLA 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. 