CHAPTER VII. 

 GELATIN AND POTATO MEDIA.— CULTIVATION OF BACTERIA. 



The study of bacteria is not limited to the mere obser- 

 vation of these organisms under the microscope. Indeed, 

 as long as this was of necessity tiie case bacteriology was 

 far from being an exact science. A drop of blood in an- 

 thrax, or a drop of a putrid liquid, may teem with bacteria. 

 If the examination is limited to the use of a microscope, no 

 positive conclusion can be formulated as to the relation of 

 these organisms to the condition in which they occur. An 

 exact knowledge of their function was possible, when meth- 

 ods were discovered for artificially cultivating these organ- 

 isms. 



The first requirement, in order to accomplish this ob- 

 ject, is the preparation of suitable, sterile, nutrient media. 

 The soil on which the bacteria in question are to be planted, 

 must be absolutely free from other forms of bacterial life. 

 Otherwise, foreign adventitious organisms may develop and 

 outgrow the particular form under observation. The use 

 of sterile infusions, or nutrient liquid media was introduced 

 at an early date by Pasteur. 



When the material mentioned above is planted in a ster- 

 ile, liquid medium, it is evident that each of the different 

 kind of bacteria that may be present will multiply. The 

 result is a "mixed culture." With liquid media, it is ex- 

 tremely difficult to separate the several bacteria that may 

 be present in a mixture. The introduction of nutrient gela- 

 tin by Koch, in 1881, rendered this a relatively easy task. 

 The remarkable progress in bacteriology during the past 

 two decades, is largely the result of this simple transforma- 



