Escherichia coli (E. coli)
Escherichia coli is a gram-negative, rod-shaped bacterium that normally inhabits the human intestine. Most strains are harmless, but pathogenic strains cause urinary tract infections, traveler's diarrhea, neonatal meningitis, and hemolytic uremic syndrome.
Escherichia coli is a gram-negative bacillus in the family Enterobacteriaceae and a normal resident of the human gut. It is a facultative anaerobe, ferments lactose (producing pink colonies on MacConkey agar), and is catalase-positive. Lactose fermentation is the feature that separates it from non-lactose fermenters such as Salmonella and Shigella; catalase positivity is shared across the Enterobacteriaceae.
Pathogenic strains are grouped by mechanism. ETEC (enterotoxigenic) causes traveler's diarrhea through heat-labile and heat-stable toxins that trigger watery diarrhea without invading tissue. EHEC (enterohemorrhagic), notably serotype O157:H7, produces a Shiga-like toxin causing bloody diarrhea and, especially in children, hemolytic uremic syndrome — the triad of hemolytic anemia, thrombocytopenia, and acute kidney injury; it is classically acquired from undercooked ground beef and does not ferment sorbitol. Other important presentations include urinary tract infections — E. coli is the most common cause, aided by P fimbriae that adhere to uroepithelium — and neonatal meningitis, associated with the K1 capsule.
Beyond the clinic, E. coli is molecular biology's workhorse model organism: much of what is known about prokaryotic gene expression, operons, and recombinant DNA technology comes from studying it.
USMLE Step 1 tests E. coli's lab identification, virulence factors, and strain-specific diseases, and its role in small intestine pathology. The MCAT uses E. coli as the model for prokaryotic chromosome structure and gene expression, and the CCMA exam covers it as a common cause of urinary and digestive system infections.
Key takeaways
- E. coli is a lactose-fermenting, gram-negative rod that is normal gut flora but has several pathogenic strains.
- ETEC causes traveler's diarrhea via heat-labile and heat-stable enterotoxins.
- EHEC O157:H7 produces Shiga-like toxin, causing bloody diarrhea and hemolytic uremic syndrome.
- E. coli is the most common cause of urinary tract infections, and K1-encapsulated strains cause neonatal meningitis.
- USMLE Step 1, the MCAT, and the CCMA each test E. coli — as pathogen, model organism, and common infection cause respectively.