Pseudomembranous colitis is severe inflammation of the colon in which raised, yellowish plaques, or pseudomembranes, develop on the mucosal lining. The plaques consist of clumps of dead epithelial cells from the colon, white blood cells, and fibrous protein.

Pseudomembranous colitis is usually associated with antibiotic use. When the normal balance of the flora in the colon is disturbed, pathogenic strains of the bacillus Clostridium difficile may proliferate out of control and produce damaging amounts of cytotoxins known as cytotoxins A and B.

C. difficile toxins often cause diarrhea and mild inflammation of the colon. Less frequently, the condition may progress further, causing ulceration and formation of the pseudomembranous plaques. Pseudomembranous colitis is most common in health care facilities such as hospitals and nursing homes, where an individual is most likely to be immune-compromised and to come into contact with persistent, heat-resistant C. difficile spores by the fecal-oral route. Thus, the best way to prevent it is meticulous cleanliness, coupled with avoiding the overuse of antibiotics.

Mild symptoms such as diarrhea often disappear spontaneously soon after the antibiotics are discontinued. Ironically, severe antibiotic-associated colitis must generally be treated with additional antibiotics to target the C. difficile pathogen. Benign intestinal flora such as lactobacillus or non-pathogenic yeast may be administered orally or rectally. Supportive therapies such as intravenous fluids are used as in other cases of ulcerative colitis. In rare cases, surgery to remove the damaged section of colon may be required.

While antibiotic use is the most common precipitating cause of pseudomembranous colitis, occasionally the condition may result from chemotherapy, bone marrow transplantation, or other causes.