162 METHODS OF CULTURE AND ISOLATION 



consists of filling an ordinary moist chamber with a drop of yeast 

 water containing the microorganisms. By means of a sterile plati- 

 num wire a little of the carbohydrate which one wishes to study, is 

 added to the drop. It may be necessary to pulverize the sugar in 

 order that, as far as possible, equal amounts of the sugar may be trans- 

 ferred. The moist chamber, thus prepared, is covered with a cover 

 slip and placed in an incubator at 25. The next day the preparation 

 is examined. If a fermentation has taken place, the cover slip may 

 be forced up from the glass collar upon which it rested and a bubble 

 of gas may be seen. (Fig. 69.) In order to make certain that the 

 ^T ILL - ui Th bubble is made up of carbon dioxide, in 



V//'/>/' l r^i4%5v ^\ part it is sufficient to allow a few drops of 

 Fig. 69. -Fermentation in an caustic potash to fall on the cover slip. If 



Ordinary Moist Chamber the bubble is CO 2 it will contract and 



disappear. If, on the other hand, there is 



no fermentation, the cover slip will not have changed place. It will 

 be adherent to the glass slide. 



Bronfenbrenner and Schlesinger's Method for Determining the 

 Action of Microorganisms on Carbohydrates : These investigators l 

 have proposed a method for studying the action of bacteria to- 

 wards carbohydrates which may be of value for the yeasts. The 

 method may be outlined as follows: One prepares medium contain- 

 ing 1.5% of agar, 0.5% NaCl, and 1% peptone. This mixture is 

 brought to boiling and the reaction not adjusted. At this point a 

 suitable amount of indicator is added and the medium distributed 

 into small tubes containing 1 or 2 cc. of medium, autoclaved and stored 

 on ice. When used, the medium is melted and to it is added 0.1 or 

 0.2 cc. of a 20% lactose solution. While hot, this medium is de- 

 posited in drops on the inner surface of the bottom of a sterile Petri 

 dish. This may be placed symmetrically by marking the outside of 

 the dish. Each of these drops is inoculated from the suspected col- 

 onies or material, leaving two drops uninoculated on each plate as 

 controls. After this, a fresh drop of the medium is placed over the 

 inoculated drops, giving conditions of slightly lowered O tension favor- 

 able to carbohydrate metabolism of bacteria. In order to prevent the 

 volatilized acids formed in some drops from causing a color change in 

 other drops, filter paper saturated with NaOH was placed in the top of 

 the Petri dishes. When desired, sterile slides with a concave well may 

 be used. The hollow of the slide is filled with lactose agar, prepared as 

 outlined above, and a sterile cover glass placed over it. This method 

 greatly decreases the length of time required for the formation of gas. 



1 Bronfenbrenner, J., and Schlesinger, M. J. A Rapid Method for the Identifica- 

 tion of Bacteria Fermenting Carbohydrates. Amer. J. Pub. Health, 8 (1918) , 922-923. 



