314 



VETERINARY BACTERIOLOGY 



types and somewhat higher for the avian. The thermal death- 

 point is 60 for twenty minutes. This is, therefore, the minimum 

 time and temperature for the efficient 

 pasteurization of milk. Sunlight de- 

 stroys the organism quickly, but it is 

 moderately resistant to desiccation. 

 Most of the physiological characters, 

 such as acid, gas, pigment, ' and indol 

 production, are negative. 



Theobald Smith has called attention 

 to what appears to be a very constant 

 differential character between human and 

 bovine tubercle bacilli. He found that 

 in glycerin bouillon (2 per cent.) acid to 

 phenolphthalein the human bacillus 

 causes a permanent acid reaction, while 

 with the bacillus of bovine origin the 

 acidity diminishes and the reaction be- 

 comes alkaline if the growth environ- 

 ment of the culture is suitable. The 

 tuberculin prepared from the human 

 bacillus is acid, and from the bovine 

 bacillus alkaline. This difference has 

 been noted by other investigators since 

 its first description, and seems to be one 

 of the best methods of differential diag- 

 nosis. 



Pathogenesis. Tuberculosis is char- 

 acteristically a chronic disease. Even 

 in experimental animals months are often 

 required for it to run its course. As 

 stated by Moore, "It does not destroy 

 life by acute toxemia, but by a chronic 

 and long-continued systemic poisoning 

 and b y the morbid changes brought about 

 through the localization of these lesions 

 in the organs necessary to life." 

 Experimental Evidence of Pathogenesis. The laboratory animals 



Fig. 128. Bacillus 

 tuberculosis, glycerin agar 

 slant (Curtis). 



