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SUMMARY : In the classification of yeasts it is customary to use an infusion of bakers' yeast as the basal medium for fermentation tests. This extract frequently contains variable amounts of trehalose. A number of yeasts were observed to ferment yeast ex- tract and trehalose. The fermentationof yeast extract is serious from a taxonomic point of view, since it gives the impression of positive fermentation of a sugar which actually may not be fermentable. Dilute yeast autolysate should be used as the basal fermenta- tion medium since during autolysis trehalose is destroyed. The fermentation of yeast extract (without added sugar) is easily observed in Durham tubes by the collection of gas in the inserts, but when Einhorn fermentation tubes are used gas production is seldom apparent. One culture (N-18) isolated from spoiled apricots, and identified as Candida tropicalis showed adaptive trehalose fermentation. The ability of various yeasts to ferment trehalose was investigated, using 133 cultures, representing twenty genera and seventyTthree species. Sixteen species representing seven genera fermented yeast extract and trehalose. The fermentation of trehalose is worthy of consideration as a character for use in differentiating certain species of Candida, and perhaps other yeasts. In identifying yeasts it is necessary to use a number of natural media the compositions of which are uncertain. Although the method of preparation may be well standardized, these natural media undoubtedly vary in composition from batch to batch. These variations were considered of sufficient importance by Bedford (1942) to warrant the use of a chemically defined medium rather than liquid wort for studying cell and film characteristics of Hansenula spp. The use of chemically defined media has not been extended to the other genera of yeasts, principally because most yeasts grow poorly in such media, unless fortified with growth factors. Stelling-Dekker (1931) used fermentation media prepared by adding 2 yo of a given sugar to a water extract of bakers' yeast. This method has also been used for a number of years in our laboratory. During recent investigations it was observed that a strain N-18, isolated from home-canned apricots, and later identified as Candida tropicalis, appeared to ferment both maltose and lactose when the yeast-extract medium of Stelling- Dekker (1931) was employed in Durham fermentation tubes. In view of the observation of Kluyver (1931), that in general, yeasts fermenting maltose do not ferment lactose and vice versa, this finding seemed of interest. Custers's (1940) report, that Brettanomyces claussenii fermented maltose and lactose, is the only known exception to this rule. B. claussenii also ferments maltose and lactose when the auxanographic plate method of Lodder (1934) is used; but culture N-18 was found to ferment maltose and not lactose.

The Fermentation of Trehalose by Yeasts and its Taxonomic Implications