A 42-year-old man presents with acute diarrhea, abdominal cramps, and bloody stools for 3 days. He recently consumed undercooked ground beef at a restaurant. On examination, he has mild fever (38.1°C) and diffuse abdominal tenderness. Stool culture grows a gram-negative bacillus that is sorbitol-fermenting, motile, and produces Shiga toxin. Which of the following is the most likely causative organism?