CHAPTER 10 

 FUNGI AS TEST ORGANISMS 



Numerous physiological problems are accessible to investigation 

 through the use of microorganisms. By the proper choice of deficient 

 organisms, it is feasible to detect minute amounts of physiologically active 

 compounds such as the vitamins and amino acids. Knowledge has been 

 gained of the way vitamins and amino acids are synthesized and destroyed 

 by various organisms. The amino-acid composition of proteins and the 

 availability of certain essential elements in soil may be determined by the 

 use of fungi and bacteria. These highly practical studies are based upon 

 a knowledge of the compounds and elements essential for the nutrition 

 of microorganisms. Since these are, in general, the same elements and 

 compounds needed by animals, there is a very close relation between 

 fungus and animal physiology in nutritional problems. Foodstuffs for 

 man and animals are the most common materials analyzed in routine 

 assays. 



Some of the advantages which have contributed to the widespread use 

 of microorganisms for assay purposes are simple technique and apparatus, 

 sensitivity, specificity, and the short time required. Perhaps the most 

 important single factor is the small sample needed and the fact that little 

 or no purification or concentration of the active material is required. 

 These advantages are to be compared with chemical methods or the use 

 of animals for obtaining the same information. All analytical methods 

 have advantages and disadvantages. A knowledge of the limitations of 

 any method is essential for valid results. 



Most microbiological assays depend upon the proportional response of 

 deficient test organisms to the substances for which they are deficient. 

 This proportional response occurs only for a limited range of concentra- 

 tions. The usable range of concentration depends upon the substance 

 being assayed, the test organism, and the basal medium. In theoiy, 

 any organism may be used to assay any substance for which it is deficient, 

 but in practice not all organisms having the same deficiency are equally 

 suitable. For example, Rhizohium trijolii 205 is about 100 times as sen- 

 sitive to biotin as Sordaria fimicola. 



The following are essential to any quantitative microbiological assay: 

 (1) a suitable test organism; (2) the preparation of a basal medium ade- 

 quate in all respects, but essentially free from the substance to be assayed ; 

 (3) liberation, from the material to be analyzed, of the substance to be 



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