90 TEXT-BOOK OF BACTERIOLOGY. 



By far the most important, however, is a modification of the 

 ordinary bouillon agar, which contains from 4 to &^ of pure neutral 

 glycerin. By the investigations of Nocard and ^oux, the very im- 

 portant fact has been elicited that on a food agar thus prepared, 

 those micro-organisms are able to thrive which formerly required 

 far more complicated mixtures to live upon. Such, for instance, 

 are the tubercle bacilli. While on agar without glycerin their 

 development is very insignificant, they form a particularly luxuri- 

 ant growth on glycerin agar, and the growth of many other par- 

 asitic bacteria may be greatly aided by a little glycerin. 



In consequence of this discovery, glycerin agar is very largely 

 employed, and has superseded the use of solid blood-serum, which 

 was formerly much used in these cases. 



As is well known, blood when it is removed from the influences 

 of the living vascular wall coagulates, and that in the further de- 

 velopment of this process a separation takes place between the red 

 clots of blood and the liquid, almost colorless or light, amber-colored 

 serum. 



The latter is rich in albuminous substances, which coagulate 

 when heated. The principal mass of the serum albumin consoli- 

 dates at about 70° C. If we do not heat much above that temper- 

 ature and do not allow it to continue too long, the serum is trans- 

 formed into a solid, uniform substance, which is scarcely less 

 transparent than the common food gelatin, and which, in like man- 

 ner, can be employed as a culture medium for bacteria. 



Koch had introduced its employment because he sav/ that cer- 

 tain strictly parasitical micro-organisms were unable to thrive on 

 those rough imitations of animal juices which our liquid and solidir 

 fled bouillons were supposed to give. He therefore endeavored to 

 procure for these bacteria a nourishment which might more nearly 

 resemble their natural conditions, and in this way he came to em- 

 ploy serum. 



Without doubt, this ingredient of blood is an excellent medium 

 for the culture of bacteria, and if at the present day it is used 

 comparatively seldom, the only reason is that its preparation and 

 application are attended with considerable difficulty. 



The serum is best prepared in the following manner: 



The blood which flows from the jugular vein of animals when 

 slaughtered is collected in large previously-sterilized glass cylin- 

 ders. They must stand about forty-eight hours undisturbed, if 

 possible in the refrigerator; here the separation of serum and 

 coagulum takes place. The yellowish, sometimes also slightly^ 

 reddish, serum is removed with sterilized pipettes and placed in 



