Chapter IV 
THE USEFUL MICROBES 
O NLY a certain number of the countless 
microbes around us are harmful to 
human beings and animals. Most of them are 
harmless, and it has been found that some of 
the harmless microbes do useful work. Some¬ 
times they help Nature in ways that are very 
useful in the lives of human beings. Even some 
of the things they do that seem troublesome to 
us at times, are in other ways valuable to us. 
It is the action of a certain group of microbes 
that sours milk, without which we could not 
have butter or cheese. The rotting away result¬ 
ing from their action in dead plant and animal 
matter is unpleasant but necessary, for it puts 
the dead matter into a condition in which it 
can be used as food by various plants. And 
if it were not for this rotting away, the world 
What microbes 
are useful? 
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