446 APPLICATION OF METHODS OF BACTERIOLOGY 



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 1 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 ; both 

 of which observations have been repeatedly confirmed. 



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, and rats 

 possess immunity from the disease. 



Tuberculin. The filtered sterile products of growth from 

 old fluid cultures of the tubercle bacillus represent what is 

 known as tuberculin a solution containing a group of 

 protein substances possessing most interesting properties. 

 When injected subcutaneously into healthy subjects tuber- 

 culin has no effect; but when introduced into the body of 

 a tuberculous person or animal a pronounced systemic 



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



