340 SELMAN A. WAKSMAN 



agar, which is acid in reaction maj^ be used, or a drop of lactic 

 acid added to each tube of the common media. 



A medium has therefore been devised, havdng a reaction acid 

 enough to prevent the development of the actinomycetes and the 

 great majority of bacteria. At first, raisin agar was used, but, 

 in view of that fact that this medium is not definite in composi- 

 tion and its reaction depends on the acid content of the raisins, 

 the following synthetic medium has been developed. 



Glucose 10 grams 



Peptone 5 grams 



KHjPOj 1 grams 



MgSC-THjO .5 grams 



Distilled water 1000 cc. 



Dissolve by boiUng, add enough ^ acid (H2SO4 or H3PO4) to 

 bring the reaction to a pH = 3.6 to 3.8. This will require from 

 12 to 15 cc. of N acid per liter of medium. Add 25 gm. of agar, 

 dissolve by boiling, filter, tube and steriUze as usual. The final 

 reaction should be pH = 4.0. 



The soil is now diluted, in the regular way, to only 5V to -^h^ 

 of the highest dilution used for the determination of bacteria 

 and plates are prepared in the regular way. The plates are 

 incubated for seventy-two hours at 25°C. To obtain an accurate 

 count and a low probable error, 10 plates should be prepared. 

 The colonies may be counted after 48 hours, then after 72 hours, 

 due to the fact that in some soils, rich in mucorales, the spreading 

 forms will tend to overgrow the plate in 72 hours. 



The following table gives a comparison of the numbers of 

 fungi obtained from the same soil by the regular bacterial plate 

 and the special method hereby suggested. A cultivated field 

 soil was used for this purpose. 



Instead of an impossible figure of 460,000 fungi per gram of 

 soil, only 29,400 have actually been found by the modified method. 

 The variability of the common method is so great as to make it 

 valueless. Further details on the appUcation of the new pro- 

 cedure will be pubUshed elsewhere (1922 b). 



This method can also be appUed to the determination of the 

 number of fungi (molds) in various food preparations. 



