Pseudomembranous colitis is caused by:

Pseudomembranous colitis is caused by Cl. difficile. After the use of antibiotics the normal GI flora gets suppressed and these organisms proliferates, producing cytopathic toxin and enterotoxin. Patients usually suffer from marked diarrhea and necrosis of mucosa with accumulation of inflammatory cells and fibrin, which forms the pseudomembrane. This condition is diagnosed by demonstrating neutralizable cytotoxin in the stool through its cytopathic effect in cell culture or by detecting enterotoxin by immunoassay. Ref: Brooks G.F. (2013). Chapter 21. Infections Caused by Anaerobic Bacteria. In G.F. Brooks (Ed), Jawetz, Melnick, & Adelberg's Medical Microbiology, 26e.