56 
PRINCIPLES OF BACTERIOLOGY 
mere presence, but through the production of different 
poisons, the next question which suggests itself is: 
How do the bacterial poisons act? 
It is one of the most widely accepted facts in the 
whole domain of bacteriology that the bacterial poisons, 
whether exotoxins or endotoxins, have a more or less 
definite selective chemical action on special tissues and 
organs, thus the exotoxins of the tetanus bacillus acts 
specifically on the nervous tissue, the endotoxin of the 
streptococcus, and staphylococcus act on the red blood 
cells, and so forth. 
The special selective action, or, to borrow the term 
from chemistry, affinity, which seems to exist between 
certain bacteria and certain tissues, depends upon the 
physical and chemical ability of the poisons to enter 
into union with the tissue cells. The famous experi¬ 
ment brought forth as a proof of this contention is 
that of Wassermann and Takaki; viz., if brain tissue is 
allowed to remain in the solution of the tetanus exo¬ 
toxin, and is then removed, the remaining fluid is free 
from any poisonous effects, while the injection of the 
brain tissue into an animal will result in the produc¬ 
tion of tetanus, thus showing that the brain tissue had 
actually absorbed the tetanus exotoxin from the solu¬ 
tion. 
III. Immunity (from Latin “immunis,” meaning safe) 
When we stop to consider that so many people har¬ 
bor enormous numbers of bacteria while only a small 
number of individuals actually develop infection and 
a still much smaller number dies of infections, we are 
brought face to face with the fact that there is some 
peculiarity about some people that, under similar cir¬ 
cumstances, permits them to escape the infection while 
