There are two bacilli by whose action starch, milk, sugar, and lactic acid are made to yield butyric acid. They also dissolve coagulated casein. One of them is strictly anaerobic (B. butyricus of Prazmowsky) and the other is aerobic (B. butyricus of Hueppe). They are both large bacilli, both bear spores and both are motile. The anaerobic gives a deep indigo-blue colour with iodine (hence the name B. amylobacter). In this form also the spores swelling out the body of the bacilli and the ends tapering off give rise to a spindle-shaped appearance. This appearance, which is not limited to this bacillus, has received the name of Clostridium ( = spindle), and the bacillus is sometimes called Clostridium butyricum.

The anaerobic form presents, under certain circumstances, a peculiar reaction with watery solution of iodine, certain parts of the cells taking on a deep indigo blue colour, especially when cultivation has occurred in starchy media.