Gram-negative rod. Produces β-galactosidase, which breaks down lactose into glucose and galactose. EMB agar – lactose fermenters grow as purple/black colonies. E coli grows colonies with a green sheen.

Virulence factors:- Fimbriae – cystitis and pyelonephritis (P-pili)- K-capsule – pneumonia, neonatal meningitis- LPS endotoxin – septic shock

EIEC: Dysentery. Clinical manifestations similar to Shigella.

ETEC: Enterotoxins. Travelers' diarrhea (watery). (T for traveler)

EPEC: No toxin. Diarrhea, usually in children. (P for pediatrics)

EHEC. O157:H7 is most common serotype. Often transmitted via undercooked meat. Does not ferment sorbitol (distinguishes EHEC from other E. coli).Shiga-like toxin (bacteriophage) causes hemolytic-uremic syndrome: triad of anemia, thrombocytopenia, and acute renal failure → mechanical hemolysis, platelet consumption, ↓ renal blood flow