Pseudomembranous colitis (PMC) is a term used for a group of colitides in which there is necrosis of the mucosa followed by abundant exudation of necrotic tissue, fibrin and inflammatory cells. When viewed grossly or endoscopically, these exudates resemble dirty, grey-yellow membranes on the surface of the mucosa, hence the term pseudomembranes.
PMC is marked pathologically by the presence of pseudomembranes on a background of inflamed mucosa. Histologically, there is an acute colitis with the membranes representing sloughed, necrotic exudate. There are three major causes of PMC: infection with Clostridium difficile or Escheria coli 0157:H7, as well as in ischemic colitis. C. difficile-induced PMC typically arises following antibiotic therapy. Antibiotics typically suppress the normal colonic flora allowing C. difficile to proliferate out of proportion to the other organisms present. C. difficile produces a toxin which damages the colonic mucosa by binding to the epithelial cell and causing the actin filaments in the cell to disaggregate and retract. The toxin can be detected in the stool.
E. Coli 0157:H7, which is typically found in undercooked hamburger meat, also elaborates a virulent toxin which damages the mucosa. This is the organism responsible for several deaths in Southwestern Ontario in 2000; severe infections with E. coli 0157:H7 may produce hemolytic uremic syndrome with intravascular hemolysis and renal failure.