Clostridium perfringens: A bacterium that is the most common cause of gas gangrene, a lethal infection of soft tissue, especially muscle. C. perfringens bacteria are toxin- and gas-producing bacteria. Before the introduction of antibiotics, a significant percentage of battlefield injuries were complicated by gas gangrene. C. perfringens also causes food poisoning and a fulminant form of bowel disease called necrotizing colitis. Formerly known as C. welchii.