EFFECTS OF BACTERIA IN NATURE. 27 



processes occurring in man, animals, and plants. This 

 means that the fluids and tissues of living bodies are, under 

 certain circumstances, a suitable pabulum for the bacteria 

 involved. The effects of the action of these bacteria are 

 analogous to those taking place in the action of the same 

 or other bacteria on dead animal or vegetable matter. The 

 complex organic molecules are broken up into simpler 

 products. We shall study these processes more in detail 

 later. Meantime we may note that the disease-producing 

 effects of bacteria form the basis of another biological 

 division of the group. Some bacteria are harmless to 

 animals and plants, and apparently under no circumstances 

 give rise to disease in either. These are known as sapro- 

 phytes. They are normally employed in breaking up dead 

 animal and vegetable matter. Others normally live on or 

 in the bodies of plants and animals and produce disease. 

 These are known as parasitic bacteria. Sometimes an 

 attempt is made to draw a hard and fast line between the 

 saprophytes and the parasites, and obligatory saprophytes 

 or parasites are spoken of. This is an erroneous distinc- 

 tion. Some bacteria which are normally saprophytes can 

 be made to produce pathogenic effects (bacillus cedematis 

 maligni), and it is consistent with our knowledge that the 

 best-known parasites may have been derived from sapro- 

 phytes. On the other hand, the fact that most bacteria 

 associated with disease processes, and proved to be the 

 cause of the latter, can be grown in artificial media, shows 

 that for a time at least such parasites can be saprophytic. 

 As to how far such a saprophytic existence of disease- 

 producing bacteria occurs in nature, we are in many 

 instances still ignorant. 



The Methods of Bacterial Action. The processes which 

 bodies being split up by bacteria undergo depend, first, on the 

 chemical nature of the bodies involved and, secondly, on 

 the varieties of the bacteria which are acting. The destruc- 

 tion of albuminous bodies which is mostly involved in the 

 wide and varied process of putrefaction can be undertaken 

 by whole groups of different varieties of bacteria. The 



