50 MEDICAL MYCOLOGY 



taken that the vegetable does not cook on the bottom of the container and that 

 water lost by evaporation is replaced. The decoction is then strained to remove 

 the larger particles of vegetable and may be filtered through paper. Potato 

 decoctions are very difficult to filter, as further precipitation may follow 

 sterilization. 



Solid Media.^Some of the first media of this class were slices or plugs 

 of vegetables, either raw or cooked (during the sterilization process). Some 

 of these substances, such as string beans, prunes, squash, potato, and root 

 crops, are still used in the study of plant pathogens and similar vegetable 

 products have been used occasionally by isolated workers in the tropics with- 

 out easy access to the more usual types of media. These are frequently very 

 good for producing abundant vegetative growth but less satisfactory for secur- 

 ing reproductive organs. 



If the vegetable is to be used raw, it must be carefully washed, the out- 

 side thoroughly sterilized by alcohol which is allowed to evaporate or by 

 repeated washing with sterile water in an atmosphere relatively free from 

 dust or spores. Cylinders are then cut out with a sterile cork borer, slightly 

 smaller than the diameter of the test tube to be used, and sliced diagonally. 

 These pieces are then placed either in a special sterile test tube or in a test 

 tube containing one or two glass beads and a small amount of water. If there 

 is no objection to having the vegetable slant cooked, it may be prepared 

 under clean but not necessarily sterile conditions and the whole sterilized 

 together. Glycerol is sometimes added instead of, or in addition to, water in 

 order to insure the surface of the slant remaining moist. Of course the glycerol 

 should also be considered as a possible additional source of carbon. 



Rarely bits of meat or fish have been sterilized and used directly as media 

 (Sawyer 1930, Rewb ridge. Dodge and Ayers 1929). 



The other solid media are all colloidal gels, either silicates, proteins, or 

 carbohydrates. Silicates are somewhat difficult to prepare and are at present 

 principally used where it is essential to know definitely the chemical con- 

 stituents of the medium in physiological studies or in the rare cases in which 

 the organism is capable of attacking and digesting the medium. 



Egg albumen and gelatin are the principal protein media used, mostly 

 in the study of bacteria. Historically, gelatin has been used for a very long 

 time and many of the important early methods and results were obtained with 

 this medium. The directions of Dalmau (1929, 1930) for its preparation are 

 useful, especially for workers in tropical countries. 



To 900 c.c. nutrient broth add 200 gm. of gelatin and heat in a water-bath 

 until dissolved. The Bacto Gelatin and Pfanstiehl's brand are highly acid, 

 about pH 3 or 4. Adjust reaction to about 6.5 or any desirable pH, let cool 

 to 50° C, add whites of 2 eggs shaken in 100 c.c. nutrient broth, and bring 

 it rapidly to a boil at 100° C. for 10 minutes in a double boiler with saturated 

 salt solution in the lower compartment. The coagulum should clear the solu- 

 tion. Filter through cotton, or cotton and gauze if necessary. Distribute in 



