BACILLUS, a member of the group of rod-shaped forms con stituting one variety of the non-chlorophyllaceous vegetable tnicro-organisms known as bacteria. Certain bacilli cause disease in man and animals, e.g., tuberculosis, diphtheria, plague ; but the majority are non-pathogenic, though they occasion various putre factive and other changes in vegetable and animal materials. Loosely and incorrectly the term "bacilli" is sometimes used to denote bacteria in general. (See BACTERIOLOGY BACTERIA AND DISEASE.)