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Shigella: Disease, properties, pathogenesis and laboratory diagnosis

Shigellae are Gram-negative, non-motile, non-spore forming, rod-shaped highly infectious bacteria. Shigella species includes Shigella sonnei, S. boydii, S. flexneri, and S. dysenteriae.


Shigellosis (also called bacillary dysentery).

Routes of transmission: Humans are the only host of Shigella. Fecal-oral route is the primary means of human-to-human transmission. With regard to foods, contamination is often due to an infected food handler with poor personal hygiene.

Pathogenesis: Some strains of Shigella produce enterotoxins and Shiga toxin. Shigella species are tolerant to low pH and are able to transit the harsh environment of the stomach.
Infectious dose (ID): As few as 10 to 200 cells can cause disease, depending on the age and condition of the host.

Laboratory diagnosis:

  1. Shigellae form non-lactose fermenting  pale colored or colorless colonies on MacConkey’s agar or EMB agar or DCA.  S. sonnei forms pink colonies.
  2. Xylose Lysine Desoxycholate (XLD) Agar: Shigellae produce red-pink colonies without black centers.
  3. Salmonella-Shigella (SS) agar: Despite its name, this medium is not suitable for isolating shigellae as it is inhibitory to most strains.

On Triple Sugar Iron (TSI) agar test, they cause an alkaline slant and an acid butt, with no gas and no hydrogen sulphide. Confirmation of the organism as Shigella and determination of its group are done by slide agglutination test.

Biochemical properties of Shigellae

  1. Lactose negative (S. sonnei is a late lactose and sucrose fermenter)
  2. Hydrogen sulphide negative
  3. Urease test: Negative
  4. Oxidase test: Negative
  5. Citrate utilization test: negative
  6. Lysine decarboxylase (LDC) test: Negative
  7. Ornithine decarboxylase (ODC) test: Negative except S. sonnei which is ODC positive.
  8. Beta-galactosidase (ONPG) test: Negative
    S. sonnei and upto 15% Shigella dysenteriae type 1 strains and minority of S. boydii strains are ONPG positive.