Most common organism, responsible for pseudomembranous colitis is:
Clostridium difficile
Clostridium botulism
Clostridium bifermentans
Clostridium histolyticum
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.