88 BACTERIOLOGY. 
blood-serum media, but later on it may be transplanted 
to agar, and still later to bouillon. After the bacilli 
have become accustomed to the bouillon they grow with 
great luxuriance, but only when carefully floated on the 
surface of the liquid. If submerged in the slightest 
degree they will not grow. : 
Many bacteria which demand free access of oxygen 
grow only in the superficial portion of the nutrient 
agar jelly, where there is plenty of air. 
It is evident, therefore, that for each variety of organ- 
ism there are special conditions requisite for growth, 
and that a temperature, degree of acidity, supply of 
oxygen, immersion in fluid, etc., suitable for one may 
be utterly unsuitable for another; that, still further, 
when two organisms grow together one may so alter 
some of these conditions as to render unsuitable ones 
suitable, and vice versa. Since, therefore, bacteria vary 
greatly as to the amount of toxin which they produce, 
and their ability to develop under different conditions 
outside of the body, we should certainly expect even 
greater variations in the living bodies of men and ani- 
mals where not only in different individuals, but even 
in the same individual at different times, there is a 
varying suitableness for such growth. 
Let us now consider some of the facts which have 
been observed concerning the growth of bacteria in the 
body, and then endeavor, as far as possible, to explain 
them. 
In the first place, there are some bacteria which find 
it impossible to grow in the living body. This is true 
of the great mass of bacteria occurring in the air, water, 
and soil. These bacteria cannot, therefore, produce 
infectious diseases. Some of them, however, produce 
