EFFECTS OF BACTERIA IN NATURE. 27 



substances which form the bodies of plants and animals, 

 or which are excreted by them. In some cases we know 

 some of the stages of disintegration, but in most cases we 

 know only general principles and sometimes only results. 

 In the case of milk, for instance, we know that lactic acid 

 is produced from the lactose by the action of the bacillus 

 acidi lactici and of other bacteria. From urea we know 

 that ammonium carbonate is produced by the micrococcus 

 ureae. That the very complicated process of putrefaction 

 is due to bacteria is absolutely proved, for any organic 

 substance can be preserved indefinitely from ordinary putre- 

 faction by the adoption of some method of killing all bacteria 

 present in it, as will be afterwards described. This state- 

 ment, however, does not exclude the fact that molecular 

 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 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. 



