Bacillus Tuberculosis is non-motile, non-flagellate, non-spo-rogenous, non-liquefying, non-chromogenic, non-aŽrogenic, distinctly aerobic, and acid resisting. It commonly occurs in the form of slender, slightly curved rods with rounded ends, not infrequently showing branches. For this reason it may not be a bacillus, but an organism belonging to the higher bacteria. It measures from 1.5 to 3.5 u by 0.2 to 0.5 u.

Staining

This bacillus belongs to a group of organisms known as "acid fast," on account of their ability to resist decolorization by acids. The tubercle bacillus is very difficult to stain, and special methods have to be employed. The Ziehl-Neelson being the most satisfactory. This resistance to staining is probably due to a surrounding capsule that consists of a fatty or waxy substance, inasmuch as it can be colored by the fat stains, such as Sudan III. Under some conditions this bacillus may not possess its acid-fast properties, yet when inoculated into animals the same organisms will produce tuberculosis, and acid-fast bacteria will be found.

Cultures

This bacillus only grows in artificial cultures, provided that the medium contains serum, glycerin, yolk of egg, or fragments of tissue. It is an aerobic organism and grows only at temperatures above 300 C. In the case of human tubercle bacilli growth ceases at 41° C, and in the case of the bovine-bacilli, at 440 to 450 C. The optimum temperature is 38° C. In order to obtain a pure growth it is best to first inoculate a guinea-pig with the suspected material. In the course of a couple of weeks the lymph-nodes will be enlarged, due to the disease. These should be removed with 14 aseptic precautions, portions carefully taken, and culture-tubes be inoculated. Many tubes should be used, as some will either show no growth or else may be contaminated.

After inoculating blood-serum the growth will be apparent to the naked eye after about twelve days in the form of small, white, round, scaly, dry looking colonies scattered over the surface of the medium. On further incubation the colonies become raised, but maintain their scaly appearance, and the margins are irregular in outline. This medium is not very satisfactory and is not much used.