THE TUBERCLE BACILLUS 197 



Varieties of Tubercle Bacilli. At least four types of tubercle 

 bacilli are recognized : human, bovine, avian, and fish. The human 

 and bovine varieties closely resemble each other. The former 

 appear somewhat longer and more slender than the latter, show a 

 greater tendency to irregularities in staining, and grow more 

 luxuriantly on culture media. The important difference lies in 

 the fact that the human type is very pathogenic for man, but is 

 considerably less so for cattle and other animals. On the other 

 hand, the bovine type is very pathogenic for almost all mammalians 

 except man ; it is pathogenic for man, but much less so than the 

 human type. The critical laboratory test for differentiating the 

 two varieties is made on rabbits ; a one hundredth of a gram of 

 a young bovine culture injected intravenously into a rabbit will 

 cause generalized tuberculosis in about six weeks, whereas fifty 

 to one hundred times the amount of the human variety produces 

 at most a slight tuberculous lesion. 



About 10 per cent of all cases of tuberculosis in young children 

 under five years of age is due to bovine tubercle bacilli. The fact 

 that such infections are usually localized in the cervical or abdom- 

 inal lymph nodes strongly suggest that the portal of entry is the 

 tonsils or small intestines and that cows' milk is the source of 

 origin. 



Avian tubercle bacilli correspond in morphology and staining 

 reactions with the above types. They differ in that they grow 

 luxuriantly on culture media at 45 C. and can even multiply at 

 a temperature as high as 50 C. On glycerin agar or blood 

 serum an abundant growth appears within ten days, white, 

 moist, and fat-like, and totally different to the dried and 

 wrinkled appearance of the human type. Chickens, pigeons, 

 and pheasants are very susceptible ; geese and ducks appear to 

 be immune. 



The tubercle bacillus of fish was first isolated from lesions in 

 a carp. Microscopically it resembles the other forms. On culture 

 media growth is thick and moist like that of the avian type. Its 

 temperature requirements, however, are very different to the vari- 

 eties found in warm-blooded animals; growth occurs between 



