CHAPTER II. 

 CULTURE MEDIA. 



WHILE there are certain advantages in sterilizing the glass test- 

 tubes prior to filling them with media, yet this may be dispensed with 

 the sterilization after the media has been tubed being sufficient. If a 

 dressing sterilizer is at hand, this is preferable for sterilizing such media 

 as bouillon, potato, and agar (10 to 15 pounds' pressure for fifteen min- 

 utes). Milk should be sterilized with the Arnold, subjecting the media 

 to three steamings for twenty minutes on three successive days. Gelatin 

 may be sterilized in either way, but preferably in the autoclave at 7 

 pounds' pressure for fifteen minutes. As soon as taken out of the ster- 

 ilizer it should be cooled as quickly as possible in cold water. This 

 procedure tends to prevent the lowering of the melting-point of the fin- 

 ished gelatin and also preserves its spissitude. 



Blood-serum is preferably solidified as slants in a blood-serum inspissator. This 

 requires one to two hours. The subsequent sterilization in the autoclave or Arnold 

 should not be done immediately after making the solidified slants, but on the subse- 

 quent day. If done on the same day, many of the slants are ruined by being dis- 

 rupted by bubbles. The preparation of blood-serum slants or slants of egg media 

 can be conveniently carried out in a rice cooker (double boiler). Place the tubes 

 in the inner compartment of the cooker, obtaining the slant desired B"y manipulating 

 an empty test-tube, or with a towel or cotton batting on the bottom. Then cover 

 the tubes with another towel. The outer compartment should contain water alone 

 (no 25% salt solution). The inner compartment should be weighted down so that 

 it is surrounded by water the light tubes not being sufficient to sink it. Allowing 

 the water in the outer compartment to boil one or two hours will inspissate or solidify 

 the slants satisfactorily. The sterilization on subsequent days may be carried out 

 in the same apparatus, although it is more efficient if done in an Arnold or an auto- 

 clave. (This sterilization in the rice cooker makes the media too dry.) 



In making media a rice cooker is almost essential; at any rate, it is so if ease, 

 expedition, and unfailing success in preparation are to be achieved. As it is neces- 

 sary to make the contents of the inner compartment boil, the temperature of the 

 water in the outer compartment must be raised. This is done by using a 25% 

 solution of common salt or a 20% solution of calcium chloride in the outer compart- 

 ment instead of plain water. Should CaCla be carried over to media in inner com- 

 partment (as by thermometer) coagulation of albumin and clearing of medium will 

 be prevented. 



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