CULTURE MEDIA 49 



Liquid Media. — The earlier media were mostly naturally occurring liquids, 

 such as tap water, milk, urine, etc., often with one or more other substances 

 added. Physiologic studies then developed a series of solutions of known chemi- 

 cal composition usually consisting of a basal solution containing all the metallic 

 elements needed for growth and one or more organic compounds containing 

 the requisite sources of carbon and nitrogen. Perhaps the solution known as 

 Czapek's or Dox's solution is more frequently used by mycologists. 



Czapek's basal solution: 



For other useful formulae see Levine and Schoenlein (1930). 



Besides a host of formulae of liquid media of definite chemical composition 

 which have been developed in connection with physiologic studies on com- 

 paratively narrow ranges of organisms, there are many, both solid and liquid, 

 in common use, in which one or more of the principal constituents are aqueous 

 extracts. These extracts may be classified as infusions and decoctions. Infu- 

 sions are prepared by allowing the material to be extracted, more or less finely 

 divided, to remain in contact with water, either cold or lukewarm, for a con- 

 siderable length of time. Hay infusion was one of the very early media of 

 this class but has practically disappeared. Meat infusions are still greatly 

 used in the cultivation of bacteria, although much less important in the cul- 

 tivation of most groups of fungi. The following directions may be taken as 

 a sample of this type : 



IMacerate 1 part finely chopped lean meat with 2 parts distilled water in 

 an ice box for 18 hours, stirring occasionally. Strain while cold through a 

 fine cloth. Add 1.0% peptone and 0.5% NaCl to the filtrate. Heat until solu- 

 tion is complete. Add NaOH until the reaction is slightly alkaline (practically 

 neutral to phenolphthalein). Heat on a water-bath for 30 minutes and boil 

 for 5 minutes over a free flame. Filter while hot through paper or cotton and 

 cloth. Add 1.0% of desired nutrients. Adjust reaction as necessary (see p. 

 38). For the many variants of this method, consult Levine and Schoenlein. 

 Most of the commonly employed media of this type may now be obtained in 

 dehydrated form. In the preparation of these media the directions furnished 

 by the manufacturer should be followed. 



Decoctions are usually employed with vegetable substances, as the process 

 is more rapid and rarely are there suflScient proteins to cause trouble. Duggar 

 (1909) proposes that for every 1000 c.c. of water in the decoction the equiva- 

 lent of 50 gm. dry weight should be used. The plant product is washed, peeled 

 if necessary, thinly sliced, and the necessary water added. It is boiled in a 

 steam sterilizer for 2 hours or placed in the autoclave at 115° C. for 20 min- 

 utes, or may be boiled over a free flame for a corresponding period, care being 



