541 
and the number and kinds of hosts a pathogenic bacterium may 
infect ; hence, there is no reason why the tubercle bacillus, which has 
received more attention and which affects more species of animals 
and more individuals than any other bacterium, should not have been 
found to include many different types, the extremes of which would 
leave us in doubt as to their specific classification if they were not 
connected by a chain of transition forms. 
Mohler and Washburn,® after a comparison of many tubercle bacilli 
from different sources and a careful search of the literature, con- 
cluded that the more the subject is studied the more numerous the 
instances become in 'which bacilli of special types are found occurring 
naturally in animals far removed from the species which may be 
supposed to be their natural host. They obtained cultures of tubercle 
bacilli from human lesions that were morphologically and biologic- 
ally bovine types, and in their summary of the investigations of 
others show that bovine types have frequently been obtained from 
man and human types from cattle. These investigators,^ after a 
prolonged study of the susceptibility of tubercle bacilli to modifica- 
tion, draw the conclusion “ that the morphology of tubercle bacilli 
is their most variable characteristic.” They successfully changed the 
morphology and also the virulence of tubercle bacilli in the course 
of their investigations and found it possible both to reduce and in- 
crease the virulence of tubercle bacilli for different species of animals. 
As examples of changes in morphology, the following are instruct- 
ive as well as interesting: A tubercle culture isolated from sputum 
was given a more perfect so-called “ human ” morphological charac- 
ter than it originally possessed by passing it through cats. The same 
culture was given a perfect so-called “ bovine ” morphological char- 
acter by passing it through cattle. A culture isolated from a tuber- 
culous boy was found to be morphologically a bovine type; after 
fifteen generations on artificial media it was still bovine in character ; 
by passage through cats it became, morphologically, a human type. 
A culture isolated from bovine tuberculous lesions was found to be 
morphologically a bovine type; it became morphologically a human 
type by growth on solidified human blood serum. It is reasonable to 
assume if human blood serum can effect this change in a morpho- 
logically bovine tubercle bacillus from a bovine source that the 
residence of tubercle bacilli from bovine lesions in the human body 
may likewise cause a change from so-called “ bovine ” to so-called 
“ human ” morphology. 
Mohler and Washburn are not the only investigators who have ob- 
tained results to prove that tubercle bacilli may be made to vary in 
® Bureau of Animal Industry Bulletin 96. 
^Annual Report, Bureau of Animal Industry, 1906, pp. 113-163. 
