UTILIZATION OF SPECIFIC COMPOUNDS 63 



the upper portion of the plaster of Paris slant with a sterile pipet. 

 About 3 ml. of water made to pB. 4.0 with acetic acid is pipetted 

 into the lower half of the slant. The inoculated plaster of Paris 

 slants are incubated for 1 to 2 days. 



Fermentation by Yeasts. With yeasts and yeast-like fungi, if 

 sugars are fermented the fermentation is usually alcoholic, and al- 

 though acid is formed it is not of great significance in identification; 

 an indicator is therefore optional, and greatest attention is given to 

 gas production. In our experience, the observation of fermentation 

 by yeasts requires higher concentrations of sugar and larger amounts 

 of medium than are usually used in studying bacterial fermentation. 

 With 1 per cent sugar in the customary small sugar tubes, results 

 are likely to be variable. A medium containing 3 per cent of the 

 sugar to be tested and 1 per cent of peptone, placed in 30-ml. amounts 

 in 25 by 150 mm. tubes, with inverted 78 by 11 mm. tubes for gas 

 traps, are suitable. Stelling-Dekker ^^ used a 2 per cent solution of 

 the test sugars in yeast, infusion, placed in Einhorn fermentation 

 tubes. To identify yeasts by Stelling-Dekker's key in many in- 

 stances, it is necessary to determine whether the trisaccharide raf- 

 finose is fermented completely or only one third. For this purpose 

 she uses a 4 per cent solution of raffinose in yeast infusion, and the 

 fermentation is studied quantitatively in the van Iterson-Kluyver 

 apparatus. Raffinose is a trisaccharide and each molecule is made 

 up of one molecule of the disaccharide melibiose (%) and one mole- 

 cule of the monosaccharide, fructose (%). If tubes of both raflfinose 

 and melibiose broth are used, it can be determined without the use 

 of special chemical equipment whether all or only one third of the 

 raflfinose is fermented. If both raflfinose and melibiose are fermented, 

 obviously all the raflfinose is fermentable; if raflfinose and not meli- 

 biose, only one third, that is, the fructose portion of the molecule. 

 Wickerham^^ has given details for making fermentation tests with 

 melibiose. 



Utilization of Specific Compounds. In addition to studying fer- 

 mentation, the identification of yeasts, especially the asporogenous 

 species, requires in many cases a determination of the carbohydrates 

 or nitrogen sources which can be utilized by the organism. A medium, 

 is prepared which contains all the necessary ingredients for growth 

 except a source of carbon, and to this medium the test sugars are 

 added; or a n^edium is prepared which contains a known utilizable 

 sugar (all yeasts can utilize glucose), but with no nitrogen source, to 

 which various test nitrogen compounds may be added. 



