Pancolitis with a history of diarrhea and no PR bleeding is highly suggestive of an infective etiology. While other differentials of colitis should be considered, infection (particular C. diff.) needs to be actively excluded.
The stool was proven to contain C. difficile toxin and oral (via PEG tube) antibiotics (vancomycin) started. It is worth noting that the antibiotic therapy must be administered by the oral route in order to be effective.