YEAST-LIKE FUNGI OF HUMAN INTESTINAL TRACT 19 



MEDIUMS EMPLOYED FOR SPECIFIC DIFFERENTIATION 



Sugar Medium. Hansen has separated the genus Saccharomyces into 6 

 subgroups based on their fermentative action toward 4 sugars : glucose, sucrose, 

 maltose, and lactose. Guilliermond ('12) has also based his preliminary sepa- 

 ration of the genus on these reactions. Castellani ('16) has used a much larger 

 number of sugars and other carbohydrates in differentiating his 'Monilia' species 

 and he emphasizes the importance of acid-formation in the mediums. He 

 goes to the extreme in this direction in that he separates species in the genus 

 on the basis of slight differences in reaction with sugars alone. Ashford ('15d)., 

 criticizes Castellani and doubts the value of the use of sugar mediums alone. 

 He recognizes the value of such mediums, however, but insists that the results 

 are to be used only in connection with other differentiating characters. Castel- 

 lani's results should be confirmed by other investigators or a more detailed 

 account of the methods employed by him should be given before his con- 

 clusions are accepted. Geiger ('10) has shown that in various sugar mediums, 

 during 3 months, with different yeasts, there may be a decrease, then an increase 

 in acidity, or an increase followed by a decrease, or a steady increase or 

 decrease. Recording the length of time the cultures are grown and the tem- 

 perature at which they are inoculated is of great importance. 



The sugars selected in the present study after preliminary experiments were 

 glucose, sucrose, lactose, maltose, galactose, levulose, and rafiinose. Dextrin 

 was used for studies in acid-production and other cultural characters. None 

 of the yeasts investigated produced gas with either lactose or rafiinose, but 

 the former was included because it has been generally used by other investi- 

 gators and the latter because it is a trisaccharid. The importance of using 

 very pure sugars cannot be overemphasized, and the lack of this precaution 

 undoubtedly accounts for the many positive results reported by some investi- 

 gators for sugars that are not ordinarily fermented. 



Fermentation tubes of the Smith type were used in all cases to determine 

 the presence or absence of gas production. The Durham tube-within-a-tube 

 method was tried but proved unsatisfactory. In the determination of acidity 

 a 200 c.c. flask with 100 cc. of medium was used. The cotton plugs in the 

 flasks were covered with waxed paper held by a rubber band in order to pre- 

 vent excessive evaporation. The large amount of medium was necessary to 

 allow for the 3 or 4 series of titrations made. The formation of rings, films, 

 and other cultural characters could also be better studied in flasks. 



The basic substance employed in the sugar medium may have an important 

 bearing on the nature of the reaction obtained. Since yeast water forms an 

 ideal medium for. the growth of all yeasts studied, and since it has been com- 

 monly employed by Will and others working on this group of asporogenic 

 yeasts, it was thought best to employ it as the main basic solution. The results 

 obtained from the use of this medium were not different in most respects 

 from those obtained by the use of 'nutrient broth' as the basic substance, except 

 in the acidity tests. It is important that some standard solution be selected 

 for the use of all investigators, such as is employed by. all bacteriologists. The 

 bacteriologic mediums are not entirely satisfactory and are more complex than 

 is necessary for the cultivation of the yeast-like organisms. Since the yeast 

 water solution is easily made and the material used in its composition is gen- 

 erally available, it is recommended that it be made the standard for future 

 investigations on this group. 



The yeasts used for inoculation were taken from young cultures on glucose 

 agar slants. The tubes and flasks were run in duplicate with a control set 



