THE METHODS OF BACTERIAL ACTION. 23 



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 cedematis 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. 

 In these circumstances, therefore, the production of albumoses, 

 peptones, etc., similar to those of ordinary digestion, can be 

 recognised in putrefying solutions, though the process of destruc- 

 tion always goes further, and still simpler substances, e.g. indol, 

 and, it may be, crystalline bodies of an alkaloidal nature, are 

 the ultimate results. The process is an exceedingly complicated 

 one when it takes place in nature, and different bacteria are 

 probably concerned in the different stages. Many other bacteria, 

 e.g. some pathogenic forms, though not concerned in ordinary 

 putrefactive processes, have a similar digestive capacity. When 

 carbohydrates are being split up, then various alcohols, ethers, 

 and acids are produced. During bacterial growth there is 

 not infrequently the abundant production of such gases as 



