56 AGRICULTURAL BACTERIOLOGY 



portion of the walls inclosing any space is exposed to the 

 sun. 



Chemicals. Many chemicals, both inorganic and organic, 

 are toxic to animals. The same is true of microorganisms, 

 and in a general way the same chemicals are injurious to 

 both forms of life. The term 'disinfectant is applied to 

 those substances that have a marked action on microorgan- 

 isms ; to those that have a less pronounced effect the term 

 antiseptic is applied. The action of the latter may be 

 limited to the prevention of growth. When such substances 

 are used in foods, they are called preservatives. A discus- 

 sion of the various chemicals used, in one connection or 

 another, will be included later. 



Food. The food of bacteria, yeasts, and molds must be 

 soluble in water, otherwise it can not pass into the cell. All 

 naturally occurring organic matter serves as food for some 

 kind of an organism; and yet, much of it is insoluble in 

 water. The question then arises as to how the insoluble 

 material can be used. 



The animal ingests insoluble food and prepares it for 

 passage through the intestinal wall by pouring into the 

 alimentary tract at various levels agents that change the 

 insoluble food into soluble compounds. These digestive 

 agents are known as enzymes. One acts upon starch, an- 

 other on protein, still another on fats. In fact, each animal 

 is provided with such an array of enzymes as will enable it 

 to use all the compounds of its normal food. 



The cell of the microorganism may be compared to the 

 animal in this respect. It forms enzymes that diffuse into 

 the material in which the cell finds itself, and that change 

 the insoluble compounds into soluble ones. This is a proc- 

 ess of decomposition. It is a form of decomposition, how- 

 ever, in which energy is not dissipated. If it were, it 

 would be a most wasteful process as far as the organism is 



