A 32-year-old man from rural India presents with acute watery diarrhea and cramping abdominal pain for 3 days. Stool culture on Campy food agar at 42°C yields oxidase-positive, gram-negative curved rods. A second patient with chronic epigastric pain and dyspepsia shows a positive rapid urease test on gastric antral biopsy. Which finding best differentiates the first organism from the second?