What is the main virulence factor of E. coli related to severe diarrheal disease?

• A. Fimbriae
• B. Capsule
• C. Shiga toxin
• D. Flagella

Answer: (C)

The main virulence factor of E. coli associated with severe diarrheal disease is Shiga toxin. This potent cytotoxin is produced by certain strains of E. coli, notably Enterohemorrhagic E. coli (EHEC), such as the infamous strain O157:H7. Shiga toxin inhibits protein synthesis in host cells by inactivating the ribosome, leading to cell death. This mechanism of action contributes significantly to the severity of the symptoms associated with infections, including bloody diarrhea and hemolytic uremic syndrome (HUS), which can lead to serious complications and even renal failure in affected individuals.

While fimbriae, the capsule, and flagella are all important factors for bacterial adherence, protection, and motility, they do not directly cause the severe pathogenic effects that Shiga toxin does. Fimbriae help E. coli adhere to intestinal epithelium, aiding colonization but not directly causing the toxic effects. A capsule can protect bacteria from phagocytosis but does not contribute to the severity of diarrheal disease overtly. Flagella provide motility, assisting in movement through fluids, yet again, they do not have a direct role in the virulence associated with severe diarrheal manifestations