CHAPTER IV 
BACTERIA 
THE world is everywhere pervaded by bacteria; minute 
microbes are carried by the atmosphere, they float in 
the water and occupy the soil. When we or indeed 
any other animal take our walks abroad, we create and 
are surrounded by an invisible halo of bacteria ; some 
are stirred up by the dust of our going, others shaken 
from our clothes, and others are even distributed by our 
breath. They are scattered over the food which we 
eat ; the water which we drink is full of them, and as 
a matter of fact it would seem detestably vapid if it was 
germ-free. 
These germs form a very mixed multitude : some are 
distinctly useful and even necessary forms, others are 
harmless, and a few are malignant and fatal disease- 
bacteria. 
In order to study any particular variety, it is necessary 
to keep it from being contaminated by some of the 
numerous wild forms. The methods in use require very 
careful manipulation ; the germ is grown upon gelatine, 
on peptone broth, or on agar-agar (a seaweed prepara- 
tion), and atmospheric germs are carefully excluded 
from the test-tube or glass shells in which the “culture” 
is preserved, usually by a cork of cotton-wool. 
Dr. Kienitz Gerloff* made an interesting experiment 
with one of his sterilised or germ-free agar-agar saucers. 
He persuaded a healthy young maiden (gesundes junges 
Méddchen) to imprint a hearty kiss upon this unfeeling 
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