A 28-year-old woman from rural Maharashtra presents with acute onset bloody diarrhea and abdominal cramps for 3 days. She reports consuming contaminated water from a community well. On examination, she is febrile (38.5°C) with mild dehydration. Stool microscopy shows RBCs and WBCs; culture on MacConkey agar yields non-lactose-fermenting colonies. The isolate is oxidase-negative, catalase-positive, and produces acid without gas from glucose. Further testing shows it is Shiga toxin-positive. What is the most likely organism?