Bacillus (= a little rod), one of the divisions of the group of Bacteria (q.v.). A bacillus may be roughly characterised by saying that it is at least twice as long as it is broad, and it thus differs from those forms of bacteria, cocci, which possess a more or less rounded shape. Bacilli may be capable or non-capable of movement; they often grow into long threads, and in these rounded or oval spores may be developed. These spores are very important bodies; they offer much greater resistance to heat and other destructive influences than do the rods from which they are developed. A spore may readily be distinguished from a coccus by its high refrangibility, and its peculiar behaviour with staining reagents; it is not, however, always so easy to distinguish a spore from a vacuole, or from other abnormal developments in the bacterial protoplasm; in cases of doubt the test of resistance to heat must be applied, or it must be ascertained whether the supposed spore is capable of sprouting and producing a bacterium by germination.
Certain bacilli have been shown to be the cause of diseases affecting man and animals. The bacillus anthracis produces the disorder known as anthrax (woolsorter's disease of man, splenic fever or splenic apoplexy of animals); the bacillus tuberculosis is the cause of consumption, the bacilli of glanders and leprosy have certainly been isolated, and probably those of tetanus, diphtheria, and typhoid. Among bacilli causing disease in animals, those of swine fever, mouse septicaemia, rabbit septicaemia, and fowl cholera may be mentioned. Other well-known bacilli are the hay bacillus, the bacilli of lactic and butyric acid fermentations, the bacillus of blue pus, and the bacillus prodigiosus. A curved form is often found associated with cases of cholera, and may be the cause of that disease; it is known as the comma bacillus of Koch, but is simply a curved rod, so that the expression comma bacillus is misleading. It really belongs to the Spirilla, and not to the group of bacilli at all.