CULTURE MEDIA 13 



synthetic medium which has only one source of carbon and one source of 

 nitrogen, it must synthesize many complex chemical compounds from 

 constituents present in the medium. It may be suspected that these 

 biochemical syntheses are slowed up under these conditions. When a 

 mixture of many carbon and nitrogen sources is present, the fungus may 

 function more efficiently, because the biochemical syntheses are easier 

 since some of the intermediates are furnished. These speculations 

 receive some support from evidence to be presented in Chaps. 6 and 7_ 



CHOICE AND PREPARATION OF MEDIA 



Considerable care is needed in the selection of a suitable medium. A 

 medium may be excellent for growth and unsuitable for reproduction or 

 the production of an antibiotic. The method of preparation may influ- 

 ence the composition of a medium in unsuspected ways. 



Choice of media. In selecting a medium the purpose for which it is to 

 be used should be kept in view. For many purposes a natural medium is 

 the one of choice. This is especially true for routine maintenance of 

 cultures, for isolations, and for preliminary investigations. The composi- 

 tion of natural media may be varied by choosing different substrates. 

 Frequentl}^, a combination of natural products may be used to advantage, 

 e.g., malt and yeast extracts. In addition, these natural substrates may 

 be fortified with one or more pure chemical compounds. The constitu- 

 ents of natural media are fixed by the substances used, but the amounts 

 used may be changed at will. 



More judgment enters into the selection of synthetic media. The 

 essentials of a synthetic medium may be stated as follows : sources of car- 

 bon and nitrogen in utilizable forms; phosphate and sulfate ions; the 

 metallic ions potassium, magnesium, iron, zinc, manganese, and others 

 which are usually present as impurities in the chemicals used. These are 

 the essential elements and will be considered at length in later chapters. 

 Most fungi utilize glucose, so this sugar is frequently used as the carbon 

 source. More fungi utilize nitrogen in organic combinations than in 

 inorganic compounds. The question of specificity enters into the choice 

 of the carbon and nitrogen sources, and this can be determined only by 

 experiment. In order to cultivate deficient fungi on synthetic media, the 

 specific metabolites for which the fungi are deficient must be added. 

 Since synthetic media are used to study nutrition, the development of a 

 suitable synthetic medium for a specific fungus may require considerable 

 investigation. In our laboratory we commonly first use a glucose-casein 

 hydrolysate medium containing the essential inorganic elements. This 

 medium has been very useful in vitamin studies. Its composition is 

 given in Chap. 10. 



