Shigella sp.

General: Composed of four species "subgroups," A through D. Shigellosis is usually mild (fever, cramps, watery diarrhea; may progress to bloody/mucousy diarrhea with penetration of mucosa) or asymptomatic; humans are a natural reservoir, with spread via direct contact or contaminated food or water. Neutrophils abundant in stool specimens. As few as 10 organisms can cause infection; it is the most common infection among laboratory workers.

Resistance:

Growth characteristics: Generally nonaerogenic and nonmotile. Most do not ferment lactose (except (Shigella sonnei)), which ferments late). Form colorless colonies on MacConkey, HE, and XLD agars. Do not ferment lactose, salicin, sucrose, or xylose. Produce an alkaline slant and an acid butt with no gas on TSI agar. May be similar to E. coli on IMViC testing. Serologic testing required to distinguish Shigella flexneri from Shigella boydii.

Common pathogens:

• (A) Shigella dysenteriae
• (B) Shigella flexneri-- second most common cause of shigellosis in developed countries
• (C) Shigella boydii
• (D) Shigella sonnei — most common cause of shigellosis in developed countries