Enterohemorrhagic Escherichia coli (EHEC) can cause diarrhea and hemorrhagic colitis, it sometimes progresses to hemolytic uremic syndrome. E.coli O157:H7 infection is common in some area, but non-O157:H7 strain is also important. It occurs mostly at the terminal ileum or right colon. Endoscopic findings include severe mucosal swelling, erythema, friability, and superficial ulcers. It appears like ischemic colitis and may need to be discriminated from that, but there is a difference in frequently involved site [5] (Fig. 13.2).

CMV (cytomegalovirus) belongs to the herpes virus group, and can cause various diseases including gastrointestinal diseases [3]. The large intestine is one of the most frequently involved sites [3]. CMV colitis commonly occurs in immunocompromised states such as organ transplantation, and patients with hematologic malignancy, AIDS, or inflammatory bowel diseases. However, it also occurs rarely in immunocompetent patients [2, 3]. It may cause exacerbation of ulcerative colitis and failure of intensive treatment in acute severe ulcerative colitis.

Frequent symptoms are fever, diarrhea, or abdominal pain. CMV colitis can cause fatal complications such as severe hematochezia, toxic megacolon, or perforation, adequate diagnosis and management are important [3]. Endoscopic findings are varied and nonspecific. Erosions or ulcers with diffuse subepithelial hemorrhagic spots can be appeared, and ulcers [3, 5, 10]. Also, these lesions are fused and advanced to well-defined, large ulcers whose adjacent mucosa is nearly normal [3]. Sometimes, in the colon, severe erythematous and friable mucosa losing vascularity, without ulcers, appears, and can seem like ulcerative colitis [3, 5]. When CMV colitis occurs in patients with underlying severe ulcerative colitis, it can be difficult to grossly distinguish from ulcerative colitis. So, in severe ulcerative colitis, we should make sure to perform biopsy. It is important to perform the biopsy in the deep portion of ulcer bases and confirm inclusion body [2, 3, 10].

Use of antibiotics can break the balance of intestinal microbiota. Then, Clostridium difficile can overgrow and their toxins induces intestinal mucosal cell necrosis and change intestinal permeability. Finally, antibiotic-associated colitis with ulcer can occur [2, 3, 11]. Pseudomembranous colitis is a leading and severe form of antibiotic-associated colitis, and a common cause of nosocomial diarrhea. It also aggravates clinical symptoms in patients with inflammatory bowel disease. Clinical features may appear as fever, diarrhea, and abdominal pain [12]. When diarrhea occurs in patients who recently used antibiotics, we can diagnose pseudomembranous colitis through stool toxin assay A or B, stool culture for Clostridium difficile, and colonoscopy [12].