NBME Microbiology Practice Test

Enterohemorrhagic E. coli is a Shiga-toxin–producing strain that causes hemorrhagic (bloody) diarrhea and can lead to serious complications like hemolytic uremic syndrome, especially in children. It is classically associated with ingestion of undercooked ground beef, making that link a hallmark clinical clue. A helpful distinguishing lab feature is that the O157:H7 strain often does not ferment sorbitol on SMAC agar, so it appears sorbitol nonfermenting rather than fermenting rapidly. E. coli, including this pathotype, is oxidase-negative, so oxidase-positive would not fit. It typically causes bloody diarrhea rather than purely nonbloody, watery diarrhea.