58 AGRICULTURAL BACTERIOLOGY 



The enzymes are always the product of life. They seem 

 to stand between the living and the dead, in that they have 

 some of the properties usually ascribed to life alone. They 

 are destroyed by heat, and are injured or destroyed by 

 those chemicals that have an injurious action on the cell 

 itself. They are usually compared to the inorganic cata- 

 lysts of the chemists, substances that favor in some way a 

 chemical reaction, and yet do not enter directly into the 

 reaction. This comparison comes from the fact that a mi- 

 nute quantity of an enzyme may act on a large amount of 

 material as, for example, one part of rennet may curdle 

 one million parts of milk in a short time. 



It is usually considered that the unicellular organism has 

 no dependence on other cells of its own kind. This is prob- 

 ably not true. It seems probable that the enzymes formed 

 by a cell do not serve the originating cell, but a later one. 

 Many of the enzymes seem to be set free only on the death 

 of the cell. If this is true, the popular concept of the in- 

 dependent existence of a unicellular organism is not the 

 correct one in all cases. 



In a pure culture of bacteria, the dead cells undergo dis- 

 solution. Since there are no outside sources for the de- 

 composing agents, it is clear that they must come from the 

 cells themselves. Such decomposition by the inherent en- 

 zymes of the cell, or mass of cells, is termed autolysis. It is 

 noted especially in the dissolution of bacterial cells after 

 spores have been formed. It is also met in the tissues of 

 higher plants and animals. 



The food requirements of microorganisms vary widely. 

 Those that live on dead organic matter are called sapro- 

 phytic; those that grow in the bodies of plants and animals 

 are termed parasitic. Many of the bacteria that grow in 

 the animal body are perfectly harmless to the host ; such are 

 called commensal organisms. The disease-producing organ- 



