Clostridium difficile. Coloured Transmission electron micrograph of C. difficile bacterium forming an endospore (lower right, red oval). C. difficile, a Gram-positive anaerobic bacillus, exists in small numbers in the human gut. It can proliferate when intestinal resistance is low, producing a toxin that irritates the colon and causes diarrhoea. 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: x18,000 at 6x6.45cm size.