THE PREPARATION OF CULTURE MEDIA 139 



organisms which are sensitive to their food environment, it is often neces- 

 sary or advisable to add to the ordinary media enriching substances, 

 which empirical study has shown to favor the growth of the organism 

 in question. The substances most commonly used for such enrichment 

 are glucose, nutrose (sodium caseinate), glycerin, sodium formate, and 

 unsolidified animal proteids. As animal and blood serum and whole 

 blood must frequently be used in this way, an understanding of the 

 methods employed in obtaining these substances is necessary. 



Method of Obtaining Blood and Blood Media. — Blood serum from 

 beef and sheep may be collected in the manner recommended for the 

 collection of such serum in the preparation of Loeffler's medium, pipetted 

 into test tubes, and sterilized in the fluid state by exposure to tempera- 

 tures ranging from 60° to 65° C, for one hour upon six consecutive days. 

 It is not a simple matter to sterilize serum in this way and requires much 

 time and care. 



The method most commonly employed, in laboratories which have 

 access to hospitals, for obtaining clear serum depends upon the collection 

 of exudate or transudate fluids by sterile methods directly from the 

 pleiural cavity, the abdominal cavity, or the hydrocele cavity. Sterile 

 flasks or test tubes are prepared and the fluid is allowed to flow directly 

 out of the cannula into these. It is necessary to avoid carbolic acid or 

 other disinfectants in sterilizing instruments and rubber tubing used 

 during the operation. These should be brought into the ward in the 

 water in which they have been boiled and not in strong antiseptic solu- 

 tions, as is frequently done. The fluid so obtained may be incubated 

 and the contaminated tubes discarded. The serum may then be- added, 

 in proportions of one to three, to sterile broth or melted agar. 



Agar thus used is melted and cooled to 60° C, or below. One-third 

 of its volume of warmed exudate fluid is added, and the plates are 

 poured. 



Serum may be rendered free of bacteria by filtration through a 

 Berkefeld or Pasteur-Chamberland filter. This is an effectual method, 

 but requires much time and care. 



Whole blood may be obtained for cultural purposes by bleeding 

 rabbits or dogs or other animals directly from a blood-vessel into tubes of 

 melted agar. In the case of a rabbit, after the administration of an anes- 

 thetic (ether), an incision is made directly over the trachea, and, by 

 careful section, the carotid artery is isolated, lying close to the side of 

 the trachea. 



