CULTURE-MEDIA AND THEIR PREPARATION 95 



the slanted tubes to 76 for an hour or more on five or six consecu- 

 tive days. An opaque medium is secured by heating to a tempera- 

 ture of 95. 



Loeffler's blood-serum is a mixture of three parts of the serum 

 with one part of neutral 1 per cent, dextrose broth. It is solidi- 

 fied in the same manner as the simple serum. 



Egg Medium. This medium was developed by Dorset, of the 

 U. S. Bureau of Animal Industry. It has come into common 

 use for the growth of the Bacillus tuberculosis and has been used in 

 recent years as a satisfactory substitute for blood-serum. Dorset's 

 description of the method of preparation follows: " The egg 

 shell is broken carefully, and the entire contents dropped into a 

 wide-mouthed sterile flask. The yolk may be broken with a 

 sterile platinum wire. Gentle shaking of the flask will serve 

 to mix the white and yolk of the egg quite thoroughly. Care 

 should be taken, however, not to shake the flask so that a foam will 

 be produced, otherwise an uneven and unsatisfactory surface will 

 be obtained when the medium is hardened. When the mixing 

 is complete, the egg is poured into tubes, care being taken to avoid 

 foaming, and the tubes containing about 10 c.c. of the medium are 

 then inclined in a blood-serum oven and hardened at a temperature 

 of 70 C. This hardening will usually require two days, four or 

 five hours each day. Sterilization will be accomplished at the 

 same time. A higher temperature may be used and the. medium 

 will be hardened more quickly. The growth of tubercle bacillus 

 seems to be more vigorous when the egg is hardened at 70 to 

 74 C., and, in addition, the prolonged heating probably insures 

 a more certain sterilization. The medium after hardening is 

 opaque and yellowish in color, and usually dry, there being 

 practically no water of condensation in the tube. The egg tubes 

 should be kept in an ice-box to prevent further drying. Just before 

 inoculation, three or four drops of sterile distilled water should 

 be added to each tube to supply the moisture required for the 

 satisfactory development of the tubercle bacillus." 



