Which of the following is not true of Shigella sonnei?

• A. Large numbers of organisms must be ingested to produce disease
• B. The organism produces an inflammatory condition in the large intestine with bloody diarrhea
• C. The organism produces disease most commonly in the pediatric population
• D. The organism is a delayed lactose fermenter

Answer: (A)

Shigella infections are highly infectious due to a remarkably low infectious dose—only a small number of organisms can cause disease. This makes the idea that large numbers must be ingested incorrect. After entry, Shigella invades the colonic mucosa, especially through M cells over Peyer’s patches, and triggers a strong inflammatory response that damages the lining of the large intestine. The resulting inflammation and ulceration lead to dysentery, i.e., inflammatory diarrhea with blood and mucus. Shigella sonnei infections are most common in children, reflecting higher susceptibility and close-contact transmission in pediatric settings. In the lab, Shigella species do not ferment lactose, so they are lactose non-fermenters; the description of them as “delayed lactose fermenters” does not fit their typical profile.