Shigellosis

Shigella spp. are nonmotile, non-spore forming gram negative bacilli that infect the intestinal tract of humans causing bacterial dysentery.  Shigella spp. are transmitted from person to person via fecal-oral route either by direct physical contact, or indirectly by ingestion of food or water contaminated with human feces. Symptoms of Shigellosis include diarrhea, bloody stool, abdominal cramps, and fever. Typically the first sign of Shigellosis is profuse watery diarrhea due to the presence of enterotoxin. The blood and pus in the stool develop as a result of the invasion of Shigella spp into the mucosa of the colon. A variety of adhesins enable the Shigella spp. to come into contact with the intestinal mucosal cells. Once across the mucosa, Shigella spp. use invasins to enter the epithelial cells where they escape from the vacuole into the cytoplasm and multiply. They survive phagocytosis by inducing apoptosis in macrophages.  Here is a video from the CDC with some general information on Shigella:

http://www.youtube.com/watch?v=kufdtZzSn70

Shigella spp. are identified in the clinical microbiology laboratory by growth on MAC and XLD/HE agar, as well as biochemical tests such as KIA, LIA and Urea. Shigella does not ferment lactose and appears clear on MAC agar. Hektoen enteric agar (HE) is a higly selective media that primarily recovers Shigella and Salmonella spp., inhibits common colon flora, and contains indicators to detect hydrogen sulfide production. Shigella appears green without black centers on HE agar b/c it does not produce hydrogen sulfide. When Shigella grows on Xyline-lysine deoxycholate agar it appears red or clear b/c it does not ferment any of these carbohydrates.

Shigella KIA results are Alk/A, LIA results are P/Y, and Urea testing is negative.

KIA- organism ferments glucose, organism does not ferment lactose (Alk/A)

LIA- Pink/Yellow

Urea- Negative

Additional testing may be performed to determine the grouping of Shigella spp. by agglutination of specimen with antisera fro Groups A (S. dysenteriae), B (S. flexneri), C (S. boydii), and the most common D (S. sonnei).

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