THE TUBERCLE BACILLUS 499 



confirming Smith's original work. Morphologically, Smith ^ found that 

 the bovine baciUi were usually shorter than those of the human type and 

 grew less luxuriantly than these upon artificial media. He determined, 

 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. Fluctuations, 

 after this, do not exceed 0.1 or 0.2 per cent on either side of neutrahty. 

 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,^ Vagedes,' and others. 



The cultural differences between the two types have been studied 

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

 Heuss.^ 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 qualities of these bacilli toward various 

 animal species. 



Guinea-pigs inoculated with the bovine type ^ 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, 

 of course, attaches to the question as to whether the bovine and the 

 human disease are mutually intercommunicable. Extensive attempts to 

 infect cattle with bacilli of the hmnan type have been made,'' for the most 

 part with very little or no success. Infections of human beings with 



1 Th. Smith, Jour. Exp. Med., 1905. 



2 Ravenel, Lancet, 1901; Univ. Penn. Med. Bull., 1902. 

 = Vagedes, Zeit. f. Hyg., 1898. 



* Wolbach and Ernst, "Studies from the Rockefeller Inst.," 11, 1904. 

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

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

 ^Beck, "Festsch. R. Koch," 1902; Smith, loc. cit. 



