What is the hallmark colonizing mechanism of EPEC and its typical epidemiology?

• A. Invades mucosa causing bloodstream infection.
• B. Aggregative adherence pattern via fimbriae.
• C. Attaching and effacing lesions via a type III secretion system; common in infants in developing countries causing watery diarrhea.
• D. Produces Shiga toxin leading to HUS.

Answer: (C)

EPEC’s hallmark is attaching and effacing lesions on enterocytes, driven by a type III secretion system that injects effectors into host cells. This causes cytoskeletal rearrangements, loss of microvilli, and pedestal-like attachment without invading the bloodstream, leading to watery diarrhea. Epidemiology-wise, EPEC is classically seen in infants in developing countries, transmitted fecal-orally, often in settings with crowded or poor sanitation. This sets it apart from pathogens that invade mucosa and cause septicemia, from those with an aggregative adherence pattern, or from toxin producers like Shiga toxin–producing strains that lead to HUS.