| Shigella dysenteriae|
Castellani & Chalmers 1919
Shigella dysenteriae is a species of the rod-shaped bacterial genus Shigella. Shigella species can cause shigellosis (bacillary dysentery). Shigellae are Gram-negative, nonspore-forming, facultatively anaerobic, nonmotile bacteria.
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. Contamination is often caused by bacteria on unwashed hands during food preparation, or soiled hands reaching the mouth.
Signs and symptoms
The most commonly observed signs associated with Shigella dysentery include colitis, malnutrition, rectal prolapse, tenesmus, reactive arthritis, and central nervous system problems. Further, S. dysenteriae is associated with the development of hemolytic-uremic syndrome, which includes anemia, thrombocytopenia, and renal failure.
Since the typical fecal specimen is not sterile, the use of selective plates is mandatory. XLD agar, DCA agar, or Hektoen enteric agar are inoculated; all give colorless colonies as the organism is not a lactose fermenter. Inoculation of a TSI slant shows an alkaline slant and acidic, but with no gas, or H
2S production. Following incubation on SIM, the culture appears nonmotile with no H
2S production. Addition of Kovac's reagent to the SIM tube following growth typically indicates no indole formation (serotypes 2, 7, and 8 produce indole).
Shigella flexneri will produce acid and gas from glucose, and Shigella sonnei is mannitol and ornithine positive, and is also a late lactose fermenter (ONPG positive). Some Shigella species can produce indole.
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