58 THE CULTIVATION OF BACTERIA. 



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. 



1. 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 

 necessary to sterilize the same before using, and for this pur- 

 pose Avhat is known as the discontinuous or fractional steril- 

 ization by steam is resorted to. 



Mode of Preparing Sterilized Milk. Sterilized test-tubes 

 from 5 to 7 inches in length, and about from 1 to 1J inches 

 in diameter, are filled to one-third their capacity with raw 

 milk. The test-tubes are plugged tightly with ordinary 

 cotton-batting, and are submitted to live steam in the steam 

 sterilizer, at 100 C., for twenty minutes each time, on three 

 consecutive days. 



Before sterilization tincture of blue litmus may be added to 

 the milk, and in this way the generation of acids by bacteria 

 may easily be ascertained. 



Milk prepared in the foregoing manner offers an excellent 

 culture-soil for nearly all forms of bacteria ; it serves also for 

 differentiating between certain species accordingly as these 

 have the property of coagulating the casein in the milk 

 rapidly, slowly, or not at all. 



2. Animal Blood-Serum. 



Animal blood-serum obtained from a slaughter-house is an 

 exceedingly useful culture -medium. 



Its mode of preparation is as follows : In large cylindrical 

 jars the fresh fluid blood is collected and allowed to remain 

 untouched for a half-hour or an hour. After this, with a 

 clean sterilized glass rod the coagulum that begins to form is 

 detached from the sides of the vessel. The vessel then, well 



