Patients treated with antibiotics develop diarrhea that, in most cases, is self-limiting. However, in some instances, paicularly in those patients treated with ampicillin or clindamycin, a severe, life-threatening pseudomembranous enterocolitis develops. This disease has characteristic histopathology, and membranous plaques can be seen in the colon by endoscopy. Pseudomembranous enterocolitis and antibiotic-associated diarrhea are caused by an anaerobic Gram-positive rod, Clostridium difficile. It has been recently shown that C. difficile produces a protein toxin with a molecular weight of about 250,000. The "toxin" is, in fact, two toxins, toxin A and toxin B. Both toxins are always present in fecal samples, but there is approximately one thousand times more toxin B than toxin A. Toxin A has enterotoxic activity--that is, it elicits a positive fluid response in ligated rabbit ileal loops-- whereas toxin B appears to be primarily a cytotoxin. Reference: Ananthanarayan and Paniker&;s Textbook of Microbiology Tenth edition