Shigella dysenteriae

Shigella dysenteriae is a species of the rod-shaped bacterial genus Shigella.^[1] Shigella can cause shigellosis (bacillary dysentery). Shigellae are Gram-negative, non-spore-forming, facultatively anaerobic, non-motile bacteria.^[2]

S. dysenteriae, spread by contaminated water and food, causes the most severe dysentery because of its potent and deadly Shiga toxin, but other species may also be dysentery agents.^[3] Contamination is often caused by bacteria on unwashed hands during food preparation.

Diagnosis

A stool specimen is Gram-stained to show Gram-negative rods, with no particular arrangement. Enrichment is performed by growing the organisms on Selenite-F broth. Then, since the specimen is not sterile, the use of selective plates is mandatory. XLD agar, DCA agar, or HE agar are inoculated and colonies are colorless on all of them as the organism is non-lactose a fermentor. Inoculation of a TSI slant shows an alkaline slant and acidic butt with no gas or H₂S production. Following incubation on SIM, the culture appears non-motile with no H₂S production. Addition of Kovac's reagent to the SIM tube following growth typically indicates no indole formation (serotypes 2, 7 and 8 produce indole^[4]).

It's noteworthy that Shigella flexneri will produce acid and gas from glucose, and Shigella sonnei is mannitol and ornithine positive, and is also late lactose fermentor (ONPG positive). Some Shigella species are capable of producing indole.

References

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