THE MICROBE 
27 
oxygen in order to live. This is also true of 
most microbes, and for a long time it was be¬ 
lieved to be true of all of them. But it has Do they 
been discovered that a few kinds can grow per- ox ys en t0 
fectly well without air, that is, without free 
oxygen. In fact, many of this kind are so con¬ 
structed that they cannot live in air. 
The microbes that must have free oxygen are 
known as aerobes. They are by far the most 
numerous group. Most of the common air and 
water microbes and the majority of the disease 
germs, such as diphtheria and cholera, belong 
to this group. We know little about how the 
second group, which are called anaerobes and 
are able to live without oxygen, but the fact 
that they exist has been proved by many scien¬ 
tists who have studied them. Many of the soil 
microbes belong to this group. 
need 
1 live? 
