210 PHYSIOLOGY OF THE FUNGI 



The basal medium should be essentially free from the vitamin or other 

 factor under test. The response of the test organism to the basal 

 medium should be slight; this value is known as the blank, or control. 

 The size of the blank depends upon the residual concentration of the 

 factor in the basal medium and the amount and kind of inoculum used. 

 The degree to which a basal medium should be freed of the substance 

 under test depends upon the sensitivity of the test organism. 



The best basal medium for any test organism can be determined only 

 after a prolonged investigation of the nutritional requirements of the 

 organism. This arduous task is too infrequently attempted. Fre- 

 quently, it is desirable to use some natural material in the medium. A 

 complex medium which supplies several sources of carbon and nitrogen 

 as well as other organic compounds may support more rapid growth than 

 a simple minimal medium. 



The sample being analyzed may contain compounds which stimulate 

 or depress growth. Stimulation of growth due to the presence of acces- 

 sory factors, is perhaps more often encountered than growth depression. 

 The adequacy of the basal medium may be tested by comparing the 

 growth curve obtained on the sample with the standard curve. If the 

 response of the test organism to the sample is due solely to the factor con- 

 tained in the sample, the two curves will be identical. The presence of 

 inhibiting substances in the sample is detected when the sample curve 

 falls below the standard curve. Stimulating substances are revealed by 

 an upward drift of the sample curve. 



If biologically pure compounds were available, the preparation of basal 

 media for assay purposes would be greatly simplified. No general 

 method of purification is useful for all purposes. Riboflavin is destroyed 

 by light, and media can be freed of this vitamin by exposure to strong 

 illumination. Activated charcoal (Norit or Darco) is very useful in 

 adsorbing residual traces of many vitamins. Recrystallization of sugars, 

 asparagine, and mineral salts is helpful in some instances. Casein is 

 extracted with hot alcohol to remove vitamins. The essential micro 

 elements may be removed in the ways discussed in Chap. 5. Frequently 

 reagents made by one manufacturer are purer in certain respects than 

 those of another. 



Three basal media which have been used for fungi in microbiological 

 assays are given below. 



Glucose-Asparagine 



Glucose 30 e, 



Asparagine 1 g. 



MgS04-7HoO 0.5 g. 



KH2PO4 1.5 g. 



Distilled water to make 1 liter 



