TEXT-BOOK OP BACTERIOLOGY. 19 



surroundings exercises an unfavorable influence on their vital 

 powers. 



These are called aerobic bacteria. River water, the air, and 

 upper portions of the soil are rich in representatives of this class, 

 and the majority of the pigment-forming- kinds seem to require 

 oxygen for the development of coloring matter. 



Others are not so dependent on the presence of oxygen; they 

 grow, indeed, in an atmosphere rich in oxygen, and often better 

 than in one poor in it, but even the entire want of this gas does 

 not suffice to stop their development completely. These are semi- 

 aerobic, and to them belong most of the pathogenic species. In 

 the interior of living bodies, except in the lungs, there is no ox^'gen 

 for them ; that introduced by haemoglobin being quickly absorbed 

 by the cells and appropriated by the tissues. 



All micro-organisms that are to exist as parasites must, there- 

 fore, be able to live without oxygen, at least under certain circum- 

 stances. 



We may premise this as necessary, even though our experiments 

 and observations may seem to show the contrary. In fact, some of 

 the most important pathogenic bacteria, for instance, can scarcely 

 be grown by our artificial cultures with exclusion of oxygen. But 

 it is to be understood that we do not argue from this as to their 

 natural conditions. 



The ability of the cell-protoplasm to exist without oxygen is, 

 in many micro-organisms, dependent also on the other conditions 

 of nourishment, and is therefore not an invariable property. 



If these bacteria find in their neighborhood an opportunity for 

 decomposing higher combinations, they can thus obtain their need 

 of oxygen, taking it from the molecule. In such conditions they 

 manage to live without free oxygen, which they cannot do where 

 the opportunity of obtaining it }3j creating decomposition fails them. 



Besides those micro-organisms which can thrive without oxygen, 

 there are also others which can only thrive in the absence of oxy- 

 gen. These strange bodies, which differ from all other known or- 

 ganic beings, were first discovered by Pasteur. Eecently special 

 attention has been directed to these strictly anaerobic bacteria, and 

 it has been found that their occurrence is by no means so rare as 

 might be supposed. 



It seems, on the contrary, that these species have an important 

 duty to perform in 'the household of Nature. The atmospheric con- 

 ditions necessary for their welfare are produced by the simultane- 

 ous presence of the aerobic bacteria, which consume the oxygen 

 and, therefore, create a zone without oxygen. 



