MEDIA FOR YEAST 61 



high acidities, and the glucose-tartaric acid medium is the most useful 

 plating medium. Where synthetic media are desired, ammonium 

 salts are preferable to nitrates as a source of nitrogen. 



Observation of the spores is essential in identifying yeasts. Many 

 species will not form spores readily on ordinary media. A number 

 of methods have been devised to force yeasts to sporulate. Of these 

 the plaster block method. is most commonly used. Plaster of Paris 

 mixed with water is put in small dishes and allowed to harden. When 

 set, it is removed and placed in a larger dish or a large test tube, 

 and sufficient water or dilute (0.1 per cent) peptone solution added 

 to moisten the block thoroughly but not cover its surface. This is 

 then sterilized in the autoclave. A young culture, say not over 24 

 hours old, is used for inoculation. A generous loopful of the growth 

 is spread over the surface of the plaster. The temperature of spor- 

 ulation is critical, and with unknown species it is advisable to prepare 

 several blocks and incubate them at different temperatures, say 20°, 

 25°, and 30° C. After 24 hours spores should be searched for by 

 microscopic examination. Other materials, as pieces of clay flower 

 pots or blotting paper, may be used in place of the plaster, the idea 

 being to maintain a limited amount of both nutrition and moisture, 

 though there is some evidence that the calcium sulphate has some 

 specific action in the process. 



Many yeasts sporulate freely on corn meal agar and sporulation 

 is said to occur quite regularly on Gorodkowa's medium, which has 

 been widely used. The composition is 



Growth of yeasts on carrot plugs often leads to abundant spore 

 formation. ]\IcKelvey ^^ developed a medium which in our labora- 

 tories has yielded spores more consistently than have others. This 

 consists of a weak carrot infusion (about 150 grams of chopped car- 

 rots to 1 liter of tap water) and 15 grams of agar melted together. 

 Then 3.5 grams of anhydrous calcium sulphate is added and the 

 mixture is thoroughly mixed, tubed, autoclaved, and slanted like 

 any other agar medium. The medium of Mrak and associates ^^ has 

 also given excellent results in our laboratory. 



Some recent work of Nickerson and Thimann ^* on spore formation 

 in a yeast Zygosaccharomyces may well develop into a method which 



