152 BACTERIA IN RELATION TO DISEASE 



The fact that the diphtheria bacillus deviates in shape from the cylin- 

 drical renders it easily recognizable, but absolute proof of its identity is 

 given only by experiment upon animals. The question of toxines and serum 

 treatment will be considered in the next chapter. 



5. TUBERCLE (Figs. 28,^-; 27, tf; 14,^). The tubercle bacillus discovered 

 and isolated by Koch is, like the diphtheria organism, a true parasite (134). 

 Thanks to Koch's investigations, the demonstration and differentiation of the 

 bacillus in sputum and diseased tuberculous tissues is now a matter of no 

 difficulty, but the isolation and pure culture is still an uncertain and delicate 

 operation. Even on the most suitable media, blood-serum or glycerine agar 

 at optimal temperature (38 C.), the organisms grow excessively slowly, and 

 need three or four weeks to form colonies as large as those produced by 

 other bacteria in three or four days. It seems as though artificial media 

 can never replace the living tissues of the host, and possibly we shall never 

 succeed in getting the tubercle bacillus to grow more rapidly in our cultures. 

 It may be, however, that a happy chance may show us some food, widely 

 differing from the stereotyped media, that will offer optimal conditions to 

 the organism. The fact that less ' nutritious ' media, media with glycerine 

 as a carbon, and ammonia as a nitrogen, source, permit slow growth, as do 

 also potatoes and other vegetable media, may lead to further investigations. 



In nature the tubercle bacillus has never been detected growing and 

 multiplying, nor is it found in any situations where the possibility of 

 contamination by tuberculous secretions is excluded. The commonest 

 mode of infection for man is probably by the dried resting-stage, in 

 which the organisms preserve their virulence for months. For children 

 the use of the milk of tuberculous cows is a fruitful source of infection ; 

 the portal of infection in this case being the intestine. For adults 

 the inhalation of tuberculous dust and the infection of wounds is the 

 commoner occurrence. The infection of the lung is doubtlessly in all 

 cases preceded and conditioned by a ' predisposition ' which permits the 

 inhaled bacteria to develop. Possibly minute lesions play a part in this. 

 The so-called heredity of tuberculosis is in many cases undoubtedly nothing 

 but heredity of the ' predisposition,' although infection of the embryo /// 

 titero has been observed and induced experimentally. Transportation of 

 the bacilli by the spermatozoa is impossible, and infection through the 

 ovum as yet unproven (135). Tuberculosis may attack any part of the 

 body. The commonest form of the disease is pulmonary phthisis, but it 

 frequently makes its appearance in the joints, in bones, glands, and other 

 organs. Nodular inflammatory centres arise, which change later on into 

 cheesy, necrotic masses. These contain enormous numbers of bacilli, the 

 proliferation of which in the cells causes the degeneration and the other 

 pathological changes that precede it. 



The tubercle bacillus, like many other much-studied organisms, has been 



