A 25-year old, previously healthy woman entered the emergency room for evaluation of bloody diarrhea and diffuse abdominal pain of 24 hours' duration. She complained of nausea and had vomited twice. She denied a history of inflammatory bowel disease, previous diarrhea, or contact with other individuals with diarrhea. Onset of symptoms began 24 hours after eating an undercooked hamburger at a local fast food restaurant. Rectal examination revealed watery stool with gross blood present. Sigmoidoscopy showed diffuse mucosal erythema and petechiae with modest exudation but no ulceration or pseudomembranes. Cultures of Salmonella, Shigella, Yersinia, and Campylobacter were negative as was the examination for enteric parasites.
a. What is the most likely organism responsible for episodes of hemorrhagic colitis? Which other Enterobacteriaceae can present as colitis?