20 Dairy Bacteriology. 



CHAPTER II. 



METHODS OF STUDYING BACTERIA. 



Necessity of artificial cultivation. The bacteria are 

 so extremely small, that it is impossible to study individ- 

 ual germs separately without the aid of powerful micro- 

 scopes. Little advance was made in the knowledge of 

 these lower forms of plant life until the introduction of 

 culture methods, whereby a single organism could be 

 cultivated, and the progeny of this cell increased to 

 such an extent in a short course of time that the result- 

 ing 'mass of cells would be visible to the unaided eye. 

 This is done by growing the bacteria on various kinds 

 of nutrient media that are prepared for the purpose, 

 but inasmuch as bacteria are so universally distributed, 

 it becomes an impossibility to cultivate any special form 

 alone, unless, the medium in which they are grown is 

 first freed from all pre-existing forms of germ life. 



Food materials. Many kinds of food substances are 

 used for the cultivation of bacteria in the laboratory. 

 In fact, bacteria will grow on almost any organic sub- 

 stance, whether it is solid or liquid, provided the other 

 essential conditions of growth are furnished. The food 

 substances that are used for culture purposes are di- 

 vided into two classes, solids and liquids. 



Solid culture media may be either permanently solid, 

 like potatoes and coagulated egg, or they may retain 

 their solid properties only at certain temperatures, like 

 gelatin or agar. The latter two, which were devised by 

 Robert Koch, are of utmost importance in bacteriolog- 



