CHAPTER II. 

 METHODS OF STUDYING BACTERIA. 



Necessity of bacterial masses for study. The bacteria 

 are so extremely small that it is impossible to study indi- 

 vidual germs separately without the aid of first-class micro- 

 scopes. For this reason, but little advance was made in 

 the knowledge of these lower forms of plant life, until the 

 introduction of culture methods, whereby a single organ- 

 ism could be cultivated and the progeny of this cell in- 

 creased to such an extent in a short course of time, that 

 they would be visible to the unaided eye. 



This is done by growing the bacteria in masses on vari- 

 ous kinds of food media that are prepared for the purpose, 

 but inasmuch as bacteria are so universally distributed, it 

 becomes an impossibility to cultivate any special form, 

 unless the medium in which they are grown is first freed 

 from, all pre-existing forms of germ life. So accomplish 

 this, it is necessary to subject the nutrient medium to 

 some method of sterilization, such as heat or filtration, 

 whereby all life is completely destroyed or eliminated. 

 Such material after it has been rendered germ-free is kept 

 in sterilized glass tubes and flasks, and is protected from 

 infection by cotton stoppers. 



Culture media. For culture media, many different 

 substances are employed. In fact, bacteria will grow on 

 almost any organic substance whether it is solid or fluid, 

 provided the other essential conditions of growth are fur- 

 nished. The food substances that are used for culture 

 purposes are divided into two classes; solids and liquids. 



