Pathogenesis 727 



and the surface growth formed upon liquids closely resembles 

 that upon solids. 



The growth is dry and lusterless, coarsely granular, 

 wrinkled, slightly yellowish, and does not penetrate into the 

 substance of the culture-medium. It sometimes extends 

 over the surface of the medium and spreads out upon the 

 contiguous surface of moist glass. 



When the medium is moist, the bacterial mass may in 

 rare instances be shining in spots, but it is usually lusterless. 

 When the medium is dry, it is apt to be scaly and almost 

 chalky in appearance. 



The organism grows well when once successfully isolated, 

 and, when once accustomed to artificial media, not only 

 lives long (six to nine months) without transplantation, but 

 may be transplanted indefinitely without variation. 



Reaction. The tubercle bacillus will grow upon other- 

 wise appropriate media whether the reaction be feebly 

 acid or feebly alkaline. Human tubercle bacilli scarcely 

 change the reaction of the media in which they grow, but 

 bovine bacilli produce a slight acidity. 



Relation to Oxygen. The tubercle bacillus requires con- 

 siderable oxygen and, therefore, grows only upon the surface 

 of the culture-media. 



Temperature Sensitivity. The bacillus is sensitive to 

 temperature variations, not growing below 29 C. or above 

 42 C. Rosenau* found .that an exposure to 60 C. for 

 twenty minutes destroys the infectiousness of the tubercle 

 bacillus for guinea-pigs. 



Effect of Light. It does not develop well in the light, 

 and when its virulence is to be maintained should always 

 be kept in the dark. Sunlight kills it in from a few minutes 

 to several hours, according to the thickness of the mass of 

 bacilli exposed to its influence. 



Pathogenesis. Channels of Infection. The channels by 

 which the tubercle bacillus enters the body are numerous. A 

 few cases are on record where the micro-organisms have 

 passed through the placenta, a tuberculous mother infecting 

 her unborn child. It is not impossible that the passage of 

 bacilli through the placenta in this manner causes the rapid 

 development of tuberculosis after birth, the disease having 

 remained latent during fetal life, for Birch-Hirschfeld has 

 shown that fragments of a fetus, itself showing no tuberculous 

 * "Hygienic Laboratory," Bulletin No. 24, Jan., 1908. 



