Clostridium sp. bacteria, 3D computer illustration based on scanning electron microscopy (SEM) imagery. This is a Gram- positive, rod-shaped, spore-forming bacterium that is found in soil, and human and animal intestines. The club-shaped bacteria have entered the endospore phase, which is a tough, dormant phase, during which the bacteria are resistant to heat and ultraviolet (UV) radiation. Although some species are harmless several cause human disease, including C. botulinum, which causes botulism, C. tetani, which causes tetanus, and C. difficile, which causes pseudomembranous colitis.