70 PRACTICAL BACTERIOLOGY 



milk ; again acids may be produced without coagulation being 

 noticed. 



Rosolic acid in alcoholic solutions (see | 83, page 51) is also 

 added to culture media to test the above reactions. 



CXXX. Fermentation^ or the production of gas as an indication 

 of fermentation, is associated with the growth of some bacteria, and 

 is best seen in cultures grown in media containing 1 to % per cent, of 

 grape sugar. Proceed as follows : 



1. Liquefy the medium, and reduce to the proper temperature. 



2. Place a small quantity of a pure culture of the organism under 

 investigation in the liquid medium with a sterile platinum loop, and 

 distribute it equally. 



3. Place the tube in ice-water and solidify rapidly in a vertical 

 position. 



FIG. 21. 



FIG. 20. 



Smith's Fermentation Tube. 



Dunbar's Fermentation Tube. 



4. When solid, place in the incubator. 



5. In twenty-four to thirty-six hours, if the organism causes 

 fermentation of glucose, the medium will be dotted all over with 

 small cavities containing the gas formed. 



The property of fermentation with production of gas has now 

 assumed such an important role as a means of differential diagnosis 

 that not only the amount of gas or gases produced by an organism 

 under consideration, but also the nature and quality, are determined. 

 For this purpose special fermentation tubes are necessary (see Figs. 

 20 and 21.) 



It is a tube bent at an acute angle, closed at one end, enlarged 

 into a bulb at the other, and attached to a glass foot, so that it may 

 stand upright. The tube is only used with fluid media as follows : 



