THE TUBERCLE BACILLUS 613 



from cattle and pointed out differences between the two types. The 

 opinion of Smith was fully accepted by Koch 71 in 1901. 



Since that time, the question, because of its great importance to 

 prophylaxis, has been the subject of many investigations, most of them 

 confirming Smith's original work. Morphologically, Smith 72 found 

 that the bovine bacilli were usually shorter than those of the human type 

 and grew less luxuriantly than these upon artificial media. He deter- 

 mined, furthermore, that, grown upon slightly acid glycerin bouillon, 

 the bovine bacillus gradually reduces the acidity of the culture medium 

 until the reaction reaches neutrality or even slight alkalinity. Fluc- 

 tuations after this do not exceed 0.1 to 0.2 per cent on either side of 

 neutrality. In the case of the human bacillus, on the other hand, there 

 is but slight reduction of the acidity during the first weeks of growth; 

 after this acidity increases and, though subject to fluctuations, never 

 reaches neutrality. This behavior is probably due to action exerted 

 upon the glycerin, since on ordinary bouillon no such differences between 

 the two varieties can be noticed. These observations of Smith were 

 confirmed by Ravenel, 73 Vagedes, 74 and others. 



The cultural differences between the two types have been studied 

 with especial care by Wolbach and Ernst, 75 and Kossel, Weber, and 

 Heuss. 76 All of these observers bear out Smith's contention that 

 luxuriance and speed of growth are much more marked in the human 

 than in the bovine variety. Marked differences, furthermore, have been 

 shown to exist in the pathogenic properties of these bacilli toward 

 various animal species. 



Guinea-pigs inoculated with the bovine type 77 die more quickly and 

 show more extensive lesions than those infected with human bacilli. 

 The difference in the pathogenicity of the two organisms for rabbits is 

 sufficiently striking to be of diagnostic value. The bovine bacilli usually 

 kill a rabbit within two to five weeks; the human bacilli produce a mild 

 and slow disease, lasting often for six months, and occasionally fail to 

 kill the rabbits at all. 



The practical importance of distinguishing between the two types, 



iiKoch, Dent. med. Woch., 1901. 



72 T/i. Smith, Jour. Exp. Med., 1905. 



73 Ravenel, Lancet, 1901; Univ. Penn. Med. Bull., 192. 



Vagedes, Zeit. f. Hyg., 1898. 



75 Wolbach and Ernst, ' ' Studies from the Eockef eller Inst., ' ' 11, 1904. 



76 Kossel, Weber, und Heuss, Arb. a. d. kais. Gesundheitsamt, 1904 and 1905. 

 n Smith, loc. cit., and Medical News, 1902. 



