Most common organism, responsible for pseudomembranous colitis is:
Pseudomembranous colitis refers to swelling or inflammation of the large intestine (colon) due to an overgrowth of Clostridium difficile (C. difficile) bacteria. 
This infection is a common cause of diarrhea after antibiotic use.
Clostridium difficile
Clostridium botulism
Clostridium bifermentans
Clostridium histolyticum