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escapes from the bowel wall, especially when the latter is injured, and may give rise to peritonitis. It becomes virulent during the presence of typhoid fever, and is the common cause of the suppurative processes occurring in the later stages of this disease.

The close relationship between the typhoid and colon bacilli makes it highly probable that both organisms are derived from a common ancestor. As a matter of fact,

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Fig. 15.—A, Bacillus Typhosus. B, Bacillus Coli Communis.

[From Curtis's Essentials of Practical Bacteriology.]

they may be said to form one great group, out of which the typhoid bacilli have arisen by natural selection. How far the B. typhosus is helped by colon bacilli in the production of enteric fever is a question on which there is a considerable diversity of opinion. But it may be affirmed that the B. coli communis does bring about some of the symptoms of this disease, as it is met with in practice.

TUBERCULOSIS.

TUBERCULOSIS was regarded as an infectious disease even by the older physicians; but it was Koch who, in 1882, discovered the specific organism and established the fact that without the B. tuberculosis there could be no tubercle.

Tubercle bacilli (Frontispiece, Fig. IV.) are fine rods having a length of about half the diameter of a red-blood corpuscle, and occur singly or in pairs. They take up the ordinary stains very slowly, but after being stained they are not decolorised even by mineral acids (“ acid-fast”). When stained, they are frequently beaded, the unstained portions probably representing the vacuoles rather than spores. In old cultures the bacilli tend to be filamentous, and may also show true branching, or they may be swollen at one extremity. These appearances have been variously interpreted; Metschnikoff holding that “the bacillus, as ordinarily met with, is not the end-stage, but only a stage in the developmental cycle of a filamentous fungus”. Other observers, on the contrary, regard these aberrant forms as mere degenerative changes which normally occur in the life-history of an organism.

Being a typical parasite, the organism grows on blood serum, but not on ordinary media. These, however, can be made available for its culture by the addition of glycerine. It will then grow on bouillon, agar, potato, and even on such substances as carrot and macaroni.

[graphic]

The growth on glycerine-agar is highly distinctive : small dry scales appear at the end of two weeks, which finally coalesce and form a white wrinkled membrane. The growth on blood serum is similar, but proceeds more slowly.

Although spore formation has not been demonstrated, tubercle bacilli are fairly resistant outside the body. They remain alive in dried sputum for two to three months, and resist the action of gastric juice and putrefactive bacteria. On the other hand, they are easily destroyed by sunlight and various germicides. Like other pathogenic germs which exist in milk, they are killed by the heat of “pasteurisation(70° C. for thirty minutes).

Pathogenesis. — Tuberculosis can be readily produced in various animals by subcutaneous or intraperitoneal injections, or by feeding the animals with cultures, or by making them inhale the dried bacilli. Subcutaneous injection, however, is the mode commonly employed for diag

FIG. 16.–Bacillus Tubercu

losis. Glycerine-agar culture, several months old. [From Curtis's Essentials of

Practical Bacteriology.]

nostic purposes in the case of sputum or urine suspected to be tuberculous. Thus, when tuberculous sputum is introduced beneath the skin of a guinea-pig there occurs a local tuberculosis at the seat of inoculation, followed by infiltration of the lymphatic glands, which progresses in a definite order. The animal becomes emaciated, and finally succumbs to general tuberculosis in the course of six to twelve weeks.

In the case of intraperitoneal injections there are no local manifestations, and death is more rapid.

Besides guinea-pigs, rabbits, birds and most domestic animals can be infected with tuberculosis, although man, the monkey and cattle are most subject to the disease.

In man the disease mostly results from inhalation of the dried tubercular sputum. Such sputum usually contains living bacilli for several months, and is very infective. It has been demonstrated that in the dust of consumption hospitals bacilli are present in sufficient numbers to induce tuberculosis in guinea-pigs. According to Flügge, an important source of infection is to be found in the fine atoms of moisture discharged by consumptive patients during coughing and sneezing. Their existence can be readily demonstrated by holding a mirror before the face ; but they do not generally pass further than about twenty inches from the patient.

Another mode of infection is from the ingestion of tuberculous milk or meat. But Koch has recently declared that human tuberculosis is different from the bovine, and that man is so rarely infected from the latter that it is unnecessary to take any measures against it. These views have not yet been confirmed—they are,

to
grave

doubt-and in the meanwhile it will be best not to relax our precautions in this matter.

Toxins.-It has been found that intravenous injections

indeed, open

of dead tubercle bacilli in rabbits give rise to pulmonary tuberculosis, which closely resembles the lesion produced by the living organisms. It thus appears that the toxins of B. tuberculosis are chiefly intracellular, that is to say, are contained in the bacterial cell, and do not diffuse into the surrounding medium.

Koch's original tuberculin," which is the filtered glycerine-broth culture of the organism, probably contains certain toxins derived from the bacilli. It has no effect on healthy persons; but, if tuberculosis or lupus is present, fever and local necrosis round tubercular deposits follow its injection. The reaction is not specific, for it is also produced when the person is suffering from syphilis, and when milk and ricin are substituted for tuberculin. The tuberculin reaction, however, is extremely useful in detecting latent tuberculosis in cattle, and, for this purpose, is largely employed on the Continent.

Koch has recently introduced other tuberculins, which are a considerable improvement on his original preparation. The most important of these is the “ tuberculin R." (T. R.), which is prepared by repeatedly crushing the dried bacilli, so as to extract their intracellular toxins. This tuberculin has been used in early phthisis and lupus, but without any satisfactory results. In the laboratory, however, it has proved successful in conferring immunity on guinea-pigs experimentally inoculated with the tubercle bacillus.

RELATION OF THE HUMAN TUBERCLE BACILLUS TO

ALLIED ORGANISMS.

(1) Avian Tubercle Bacillus.—The organism of avian tuberculosis is apparently similar to the human tubercle bacillus, but differs from it in some essential particulars.

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