FERMENTATION. 193 



traces of fermentable carbohydrates, notably muscle- 

 sugar, as seen in infusions of fresh meat the reaction 

 may become acid with the beginning of growth and 

 subsequently change to alkaline after the supply of 

 fermentable sugar is exhausted. These changes of reac- 

 tion are most conveniently observed through the use of 

 indicators bodies that either lose or change their usual 

 color as the reaction of the medium to which they are 

 added changes. 



Such substances as litmus, in the form of the so-called 

 " litmus tincture," and coralline (rosolic acid) in alco- 

 holic solution, are commonly employed for this purpose. 

 They may be added to the media in the proportions 

 given in the chapter on Media, and the changes in their 

 colors studied with different bacteria. Milk and litmus 

 tincture or peptone solution to which rosolic acid has 

 been added are excellent media for this experiment. 



FERMENTATION. The production of gas as an indi- 

 cation of fermentation is an accompaniment of the 

 growth of certain bacteria. This is best studied in 

 media to which 1 to 2 per cent, of grape-sugar (glucose) 

 has been added. A convenient method of demonstrat- 

 ing this property is to employ a tube about half full of 

 agar-agar containing the necessary amount of grape- 

 sugar. The medium is to be liquefied on a water-bath, 

 and then cooled to about 42 C., when a small quantity 

 of a pure culture of the organism under consideration 

 should carefully be distributed through it. The tube is 

 then placed in ice-water and rapidly solidified in the 

 vertical position. When solid it is placed in the incu- 

 bator. After twenty-four to thirty-six hours, if the 

 organism possesses the property of causing fermentation 

 of glucose, the medium will be dotted everywhere with 



18 



