Clostridium difficile. Transmission electron micrograph of C. difficile bacterium forming an endospore (lower left). C. difficile, a Gram- positive anaerobic bacillus, normally exists in small numbers in the human gut. It can prolif- erate, producing a toxin that irritates the colon and causes diarrhoea, when antibiotic treatment kills other bacteria inhabiting the intestines. In severe cases this leads to pseudomembranous colitis (PMC), where mucus and dead cells build up in plaques on the walls of the colon. PMC can be treated with an antibiotic but relapses are common because the bacteria forms such highly resistant spores. Magnification: x75, 000 at 10x8inch size.