What are two key laboratory or clinical features that help differentiate Shigella spp. from other causes of gastroenteritis?

• A. Very low infectious dose with inflammatory dysentery; non-lactose-fermenting, non-H2S producing on TSI; elevated fecal leukocytes.
• B. High infectious dose with watery diarrhea; lactose fermentation.
• C. Produces H2S; motile.
• D. Causes persistent diarrhea; positive blood culture.

Answer: (D)

Shigella infections are defined by two practical clues: they invade the colonic mucosa, causing an inflammatory dysentery picture with blood and white blood cells in the stool, and they do this starting from a very small number of organisms (a very low infectious dose). In the lab, a key distinguishing pattern is that Shigella does not ferment lactose and does not produce hydrogen sulfide on standard media like TSI, and the organism is non-motile. So the best way to separate Shigella from many other causes of gastroenteritis is to recognize the combination of inflammatory dysentery (fecal leukocytes and blood) with a lactose-nonfermenting, H2S-negative lab profile. The other choices describe features more typical of other pathogens or disease courses: high infectious dose with watery diarrhea and lactose fermentation, H2S production with motility, or persistent diarrhea with positive blood culture.