CULTURE MEDIA 51 



tubes and sterilize at 100° C. for 15 minutes on each of three successive days. 

 Cool rapidly after withdrawal from the sterilizer to obtain a hard gelatin, 

 which will not liquefy at tropical room temperatures (28° C). 



At present it is little used except in studies of the ability of organisms to 

 attack and digest protein. Sawyer (1930) has recently used egg yolk with 

 very good results in his work on the Entomophthoraceae, fungous parasites 

 of insects. 



The carbohydrate media are chiefly starches and agar. Commercial starches 

 are used, either corn, potato (laundry) or inulin. Ten per cent starch is com- 

 monly used and a colloidal solution obtained by short boiling. The medium 

 is tubed and sterilized with or without the addition of salts or other nutrients. 

 The hydrogen ion concentration of the added material should be carefully 

 considered, as any considerable acidity or alkalinity will hydrolyze the starch, 

 defeating the purpose of the medium by preventing solidification and provid- 

 ing sugar. These media have not been widely used on this account. A variant 

 of this medium, in which com meal mush is prepared, has been employed, al- 

 though perhaps less frequently than the related corn meal agar. The container 

 is partially filled with corn meal which is then thoroughly moistened with hot 

 water (it is difficult to wet it thoroughly with cold water) and sterilized in 

 the usual manner. This medium is never clear as the starch media may be, 

 but it is easy to prepare, has enough of the inorganic compounds and; sources 

 of nitrogen to support growth without further addition, and provides a medium 

 rich in starch. 



The agars and similar compounds are complex substances which in part 

 hydrolyze to galactose. They are produced during the metabolism of the 

 marine algae, especially the Rhodophyceae (red algae), and comparatively 

 little is known of their chemical structures. The agar of commerce is largely 

 the product of various species of the Gelidiaceae found on the coast of Japan. 

 It solidifies at a comparatively low temperature (about 40° C.) and melts at 

 a very high one (about 95° C). The modern sources of supply have improved 

 the quality very much, so that it seldom contains undesirable salts or nitrog- 

 enous substances. Naturally, for very careful physiologic work, this point 

 would be checked up before using it. Agar may usually be had in shreds, 

 chopped shreds, or in powdered form. The variant methods of preparing agar 

 found in various laboratories give about equally satisfactory final products. 

 Since the medium carries practically no available nutrient, this is added in 

 the form of a solution, infusion or decoction, or a combination of these. The 

 formulae for these are almost infinite, Levine and Schoenlein recording 803 

 formulae, not counting the numerous variants in proportions and procedures. 



Of these, one of the simplest and perhaps the most widely used in culti- 

 vating human pathogens is Sabouraud's conservation agar: 



