The great frequency of tuberculosis both in man and animals lenders the discovery of the bacillus by Koch one of the most important results of science in this century. The merit of this discovery is the greater as the tubercle bacillus is peculiarly difficult both to observe in its usual seats and to cultivate.
The bacillus is a thin, rod-shaped cell, rather shorter than the diameter of a red corpuscle. It is often slightly curved. The bacilli are mostly single, but occasionally in pairs attached so as to form an angle, more rarely in longer threads. They do not possess power of movement. They usually present a beaded appearance, and the beads have sometimes been regarded as spores. The remarkable persistence of the tubercle bacillus, and the manner in which it retains its infective powers when dried or when kept in putrid fluids, indicate that it is capable of offering considerable resistance to external influences, though whether this be in the form of spores or not is not definitely known.
The staining of the tubercle bacillus may be accomplished in various ways. The material to be stained may be either the discharges from tubercular lesions or the tissues affected with tuberculosis. Of the former the sputum from phthisis pul-monalis is most frequently the subject of examination.
In examining sputum it is important to choose a portion which has actually come from the lung, and not merely the clothing of mucus which the sputa obtain from the bronchial mucous glands. A portion of the sputum may be poured into a watch-glass and the latter placed on a black background. In the midst of the sputum will be found yellow rounded masses which have come from the lungs. A small portion of one of these should be separated with needles and placed on a cover glass. Another cover glass is placed on the top of it and the piece of sputum squeezed between the two till it forms a thin film. By gliding the two cover glasses asunder we obtain two preparations, each consisting of a thin film of sputum. The cover glass should be dried in the air and then passed through the flame of a spirit lamp three or four times. It is then ready to be stained.
The primary staining fluid is preferably Ziehl-Neelsen's. It consists of fuchsine 1, alcohol 10, 'crystalline carbolic acid 5, and water 100. The sputum or section is placed in the solution and left for a quarter of an hour in the case of sputum, and several hours in that of sections, but the time may be curtailed (5-10 min.) by heating the solution till steam is seen to rise. The deeply stained material is then treated with nitric acid (1 in 3 of water) or sulphuric acid (20 per cent.) till it has lost its colour and become greenish or brownish. It may then, after washing, be counter-stained with methyl-blue. A very useful modification is that of Gabbett. After removal from the fuchsine solution, the cover glass or section is placed at once in a solution consisting of sulphuric acid (25 per cent.) 100, methyl-blue, 2. In this solution the acid extracts the colour from all except the bacilli, while the blue dye stains the structures which have been acted on by the acid. Gunther recommends that before mounting cover-glass preparations in Canada balsam, the cover-glass should be passed three to ten times through the flame, as this renders the colour more permanent.
Tubercle bacilli retain the stain when treated by Gram's method, and they are also stained by concentrated warm solutions of any of the basic aniline dyes.
In artificial cultures the bacillus is very slow of growth and difficult of cultivation. It grows scarcely at all on ordinary nutrient jelly, and the best medium is solidified blood serum. The more ordinary media are made available for the culture by the addition of 6 to 8 per cent, of glycerine (Nocard and Roux). A specially favourable medium is stated to be a bouillon made from calf's lung, to which 4 per cent, of glycerine has been added (Bonhoff). The bacilli require for their growth a temperature above 30° C. and under 42°, and the best is near the temperature of the body, namely, 37-5°. In. about ten to fourteen days after implantation the growth first appeals as dry whitish scales, which are entirely superficial. under the microscope the scales pre Been to be composed of colonies of bacilli, which, from their arrangement, form curved lines, some of them like the letter S (see Fig. 151). These- bacilli give the characteristic reactions with the Staining fluids mentioned above. The growth goes on for three or four weeks, and remains superficial all the time, the medium not being liquefied. The culture may be propagated through many generations, the bacilli retaining their morphological and pathogenic characters. Sunlight is very inimical to the bacilli. Direct sunlight kills cultures in a few minutes, and even diffused daylight destroys them in a few days.
Fig. 151. - Tubercle bacilli, forming typical growths, x 700.
Tuberculosis is producible in animals by administering either the products of disease (such as dry sputum or portions of caseous structures) or cultures. It has been induced by inoculation under the skin or into the anterior chamber of the eye, by injection into the serous cavities or into the veins, by inhalation and by ingestion with the food. Except when directly introduced into the blood, there is first a local tuber culosis, which may be followed by a generalization of the disease.
The toxine. of tuberculosis has been investigated, and the substance called Koch's tuberculin is a glycerine extract of cultures.
In man the bacilli are introduced accidentally by inhalation, by the food, or by inoculation. When they attack the skin they produce lupus or scrofuloderma, and when they affect the lungs, phthisis pul-monalis; when introduced with the food they rarely affect the intestinal canal, but are usually carried to the lymphatic glands, where they produce tuberculosis. According to Cornet the tubercle bacillus is not commonly present in the dust of dwellings, but is present when the expectoration of consumptives is allowed to dry on the floor or in handkerchiefs, and to become pulverized.
Tuberculosis in fowls is now known to be due to a bacillus which resembles the ordinary tubercle bacillus, but yet presents certain differences both in form and mode of growth in cultures. The spontaneous disease in fowls is said to be transmitted congenitally (like syphilis), and the lesions are chiefly in the liver.
This microbe differs considerably in its pathogenic relations from that of mammalian tuberculosis, though it cannot be considered a distinct species. Thus while fowls, and especially small singing birds, are highly susceptible, they are comparatively immune to the organism of mammalian tuberculosis. It is possible, however, to inoculate fowll with tuberculosis by means of bacilli obtained from a mammalian source, the lesions produced being closely similar to if not identical with, those produced by the organism of avian tuberculosis. On the other hand, the guinea-pig, which is so highly susceptible to infection with the tubercle bacillus, may also be infected with a tuberculosis in all respects typical by means of the organism obtained from cases of this affection in birds.