56 THE CULTIVATION OF BACTERIA. 



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

 pose what 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 

 covered and protected from dust, is put into an ice-box, and 

 at the end of twenty-four hours the clot, consisting of fibrin 

 and of blood-corpuscles, is firm and sinks to the bottom of 

 the vessel, leaving a clear serum above and around it. This 

 clear serum may be siphoned or pipetted out and distributed 

 among sterilized test-tubes, which, after being plugged with 

 absorbent cotton-batting, are sterilized in Koch's serum steril- 

 izer by the low temperature process to be described later. 



Loeffler's modification of this method is generally used in all 

 municipal laboratories for the cultivation and diagnosis of 

 the bacillus of diphtheria ; 



