432 



MYCOBACTERIUM 



TABLE 28. (Modified from A. S. Griffith 1924.) 

 Showing the Differential Characteristics of Mammalian Tubercle Bacilli. 



occurs, on which a few large isolated warty and pigmented colonies may eventually 

 develop. Similarly on glycerine broth there is only a thin layer of growth with an 

 occasional raised island of irregular appearance. In virulence this variety resembles 

 the standard human type, being high for the guinea-pig and low for the rabbit. 

 These dysgonic human strains are most likely to be confused with the attenuated 

 bovine strains ; differentiation can be efltected by making a sufficiently wide and 

 prolonged series of tests. The dysgonic bovine strains do not give a yellow pig- 

 mented growth on bovine serum ; and their virulence is lowered not differentially 

 but uniformly ; that is to say, their virulence is lowered not only for the more 

 resistant calf and rabbit, but also for the highly susceptible guinea-pig and monkey. 



Whenever an aberrant strain is encountered, the possibility of the cultures 

 being impure must always be considered. Thus the Royal Commission (Report 

 1911) examined 2 cultures that were eugonic on glycerine media and were highly 

 virulent to calves and rabbits. By plating out these cultures they were able to 

 separate off a dysgonic virulent strain from a eugonic strain of slight virulence for 

 the calf and rabbit ; in other words, the original cultures contained a mixture of 

 bovine and human types. 



Considerable attention has been paid to these atypical strains, because they 

 have been regarded by some workers as evidence of instabihty of type. Griffith, 

 who has had the greatest experience of these atypical strains, is, however, 



