THE CULTIVATION OF BACTERIA. 55 



CHAPTER III. 



THE PKOCESS, MEDIA, AND UTENSILS OF THE CULTI- 

 VATION OF BACTERIA. 



THE PROCESS OP THE CULTIVATION OF BACTERIA. 



As mentioned before, bacteria can not be separated from 

 one another by form and appearance under the microscope 

 only. Indeed, in a number of instances, and even with the 

 highest power of the microscope, some very inoffensive bac- 

 teria resemble very much and can not be differentiated from 

 some that are highly pathogenic. Especially is this the case 

 with the group of cocci. 



In all such cases it is necessary to study the properties 

 and mode of growth, and for this purpose the bacteriologist 

 must use and prepare suitable soils, which are known by the 

 name of culture -media. These culture-media must themselves 

 be absolutely free from all live bacteria that is, sterile ; or, 

 if they naturally contain bacteria, or if bacteria have been 

 introduced during their preparation, these must be destroyed, 

 or, in bacteriological language, the media must be sterilized. 



THE MEDIA OF THE CULTIVATION OF BACTERIA. 



All substances that contain carbon and nitrogen compounds in 

 assimilable form associated with water may be used as culture- 

 soils for bacteria. The culture-media used ordinarily are 

 either natural or artificial. They may be liquid or solid ; or, 

 again, they may be solid at the temperature used, and liquefied 

 at a temperature not high enough to destroy bacterial life. 



I. The Most Commonly Used Liquid Culture-Media. 



l. Milk. 



Milk, as contained in the udders of the cow, is an excel- 

 lent culture-medium, and is generally sterile. In its col- 

 lection, however, it usually becomes contaminated that is, 

 bacteria are introduced into the milk : so much so that it is 



