Which bacterium is the best-known cause of antibiotic-associated pseudomembranous colitis?

Clostridioides difficile is the best-known cause of antibiotic-associated pseudomembranous colitis.

C. difficile is a Gram-positive, spore-forming, anaerobic bacterium. Antibiotics can disrupt the normal gut microbiota, allowing C. difficile to multiply and produce toxins. These toxins damage intestinal cells and can cause watery diarrhea, abdominal pain, fever, and inflammation.

Severe disease may produce pseudomembranous colitis, in which raised inflammatory plaques form on the colon lining. The infection can recur because hardy spores survive for long periods on surfaces and resist many ordinary cleaning methods.

The organism was formerly called Clostridium difficile, and that older name remains common. It is different from Clostridium perfringens, which is associated with gas gangrene and some forms of food poisoning.