Shigella Dysenteriae - Diagnosis

Diagnosis

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 a non-lactose fermentor. Inoculation of a TSI slant shows an alkaline slant and acidic but 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).

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