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 



