Bacteria 
The habits and way of life of some bacteria are very 
remarkable. One great group of them is unable to exist 
in free oxygen, but obtains what oxygen they require by 
the alcoholic fermentation of sugar and allied material, 
They and the other fungi and plant cells which act like 
them are therefore responsible for alcoholism. 
Sunlight at once destroys most bacteria. A young 
(three hours old) Micrococcus prodigiosus dies in one 
minute of sunlight. Old colonies (that is, ten to fifteen 
hours old) may survive longer but are killed in three 
to five minutes, 
A colony or “culture” of bacteria isolated from the 
world in a test-tube or glass shell, and growing in blood- 
serum or peptone broth, not only extracts and absorbs 
from this liquid or jelly whatever food it requires, but 
also excretes out its own waste products. This is just 
as necessary for bacteria as for any other plant or 
animal. As we saw in the last chapter, carbonic acid 
is a waste product excreted by plant roots, and is in 
excess poisonous to them. 
A disease germ living in an animal’s body gives off 
a series of complex waste products or toxins which are 
deadly poisons to the animal cells. The dead substance 
of these poisoned cells is then absorbed as food material 
by the bacterium, which in consequence thrives and 
multiplies. 
When cultivated artificially in test-tubes or glass 
boxes (Petri’s shells) a. colony of disease germs will in 
process of time exhaust all the food material in its 
little “world” or test-tube, but it will become itself 
diseased and die long before it has done so, for its 
own waste excretions will have so pervaded and 
poisoned its world that it becomes uninhabitable. 
Pasteur suggested that when disease germs in an 
animal body eventually die out, as is found to be always 
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