ACTION OF BACTEKIAL FERMENTS 21 



changes take place spontaneously in the passing of the organic 

 body from life to death. Many processes not usually referred to 

 as putrefactive are also bacterial in their origin. The souring 

 of milk, already referred to, the becoming rancid of butter, the 

 ripening of cream and of cheese, are all due to bacteria. 



A certain comparatively small number of bacteria have been 

 proved to be the causal agents in some disease processes 

 occurring in man, animals, and plants. This means that the 

 fluids and tissues of living bodies are, under certain circum- 

 stances, 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 saprophytes. They are normally engaged 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 distinction. Some bacteria 

 which are normally saprophytes can produce pathogenic effects 

 (e.g. bacillus oedematis maligni), and it is consistent with our 

 knowledge that the best-known parasites may have been derived 

 from saprophytes. 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 undergo in being split up by bacteria depend, first, on 

 the chemical nature of the bodies involved and, secondly, on the 

 varieties of the bacteria which are acting. The destruction 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 action of the 

 latter on such substances is analogous to what takes place when 

 albumins are subjected to ordinary gastric and intestinal digestion. 



