64 STUDYING MOLDS, YEASTS, AND ACTINOMYCETES 



Lodder ^^ has presented a simple procedure (originally devised by 

 Beijerinck and called by him an auxanographic method) for deter- 

 mining the utilization of various substrates. For sugars, the basic 



medium is 



Ammonium sulphate 5.0 grams 



Monopotassium phosphate 1.0 gram 



Magnesium sulphate 0.5 gram 



Agar 20.0 grams 



Water 1 liter 



This medium, melted and cooled, is heavily seeded with the yeast to 

 be tested and poured into Petri dishes. When solidified, the plate 

 cultures are incubated for a few hours to dry the surface. Then small 

 quantities of the dried sugar are deposited on the surface of the agar 

 in labeled areas. Glucose is added to one spot in every case to serve 

 as a control, since all yeasts can utilize this source of carbon. As 

 many as six sugars may be tested on one plate, in addition to the 

 glucose. The sugars dissolve and diffuse into the agar and, if util- 

 izable, the yeasts will grow in that part of the plate culture. 



Similarly, auxanographs of nitrogen utilization may be made. 

 Here the basic medium is 



Peptone, ammonium sulphate, asparagine, urea, and potassium 

 nitrate are the test substances used. Some difficulty may be antici- 

 pated in these highly purified liquid media, for the reason that 

 growth-promoting factors are necessary for some yeasts. Nicker- 

 son ^^ used as criterion a medium including growth-promoting sub- 

 stances, glucose, and nitrates, and tested for nitrate reduction, rather 

 than nitrate utilization. If further work shows that all organisms 

 which are able to utilize nitrates as a sole nitrogen source also reduce 

 nitrates to nitrites, this will be an improvement over previous meth- 

 ods. Nickerson used 



Dipotassium phosphate 3 grams 



Magnesium sulphate 0.25 gram 



Calcium chloride 0.25 gram 



Potassium nitrate 6 grams 



Glucose 20 grams 



Yeast extract 0.1 gram 



Water 1 liter 



