420 APPLICATION OF METHODS OF BACTERIOLOGY 



that more energetic and penetrating reagents than the 

 ordinary watery solutions be employed. Experience has 

 taught us that certain substances not only increase the 

 solubility of the aniline dyes, but their penetration as well. 

 Two of these are aniline oil and carbolic acid. They are 

 employed in the solutions to about the point of saturation. 

 (For the methods of staining B. tuberculosis see Chapter 

 on Staining.) 



Under the influence of heat these solutions are seen to 

 stain all bacteria very intensely — the tubercle bacilli as 

 well as other forms. If we subject our preparation, which 

 may contain a mixture of tubercle bacilli and other species, 

 to the action of decolorizing-agents, another peculiarity 

 of the tubercle bacillus will be observed. While all other 

 organisms in the preparation give up their color and become 

 invisible, the tubercle bacillus retains it with marked tenacity. 

 It stains with great difficulty; but once stained it retains 

 the color even under the action of strong decolorizing-agents. 



Variations of B. Tuberculosis. — Theobald Smith^ has called 

 attention to certain very conspicuous differences that may 

 be observed between the bacilli obtained from human and 

 those from bovine tuberculosis; and in a series of inocula- 

 tion experiments Ravenel has shown that for a large number 

 of animal species tubercle bacilli of bovine origin were con- 

 stantly more virulent than those from human sources. 



Susceptibility of Animals to Tuberculosis. — The animals 

 that are known to be susceptible to tuberculosis are man, 

 apes, cattle, horses, sheep, hogs, guinea-pigs, pigeons, rab- 

 bits, cats, and field-mice. White mice, dogs, aiid rats 

 possess immunity from the disease. 



1 Transactions of the Association of American Physicians, 1896, xi, 275. 



