CHAPTER III. 

 METHODS OF STUDYING BACTERIA. 



24. Necessity of bacterial masses for study. Bac- 

 teria are so infinitesimally small that it is impossible to 

 study individual germs separately without the aid of first- 

 class microscopes. 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 organism could be cultivated and the progeny of 

 this cell increased to such an extent in a short course of 

 time, that they would be visible to the unaided eye. 



25. Culture methods. The system of cultivating 

 bacteria, known as the pure culture method, is based 

 upon the supposition that the food medium in which the 

 organism is grown is first freed completely from all pre- 

 existing forms of life, or in other words, is perfectly 

 sterile. The pure culture processes of the bacteriologist 

 may be said to be in a sense, refined methods of seeding, 

 such as the agriculturist employs . Just as the seed grain will 

 in due season bring forth a harvest after its kind, so any 

 kind of bacteria planted in a favorable food medium will 

 produce a crop of its own. If the farmer's seed is foul, 

 it shows in%is crop, and the same is true with bacterial 

 farming. 



Bacteria, however, are so universally distributed that 

 it becomes an impossibility to grow any special kind, 

 unless the soil is first freed from all existing forms of 

 germ life. To accomplish this, it is necessary to subject 

 the nutrient medium used for a culture to some method 

 of sterilization, such as by heat or filtration, whereby all 



2-B. I 



