216 BACTERIOLOGY 



ning of growth and subsequently change to alkaline after 

 the supply of fermentable sugar is exhausted. These changes 

 of reaction 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 alcoholic 

 solution, have been commonly employed for this purpose, 

 though many other indicators having definite ranges of 

 usefulness are now being employed. (See paragraphs on 

 Reaction.) They may be added to the media in the pro- 

 portions 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 indication 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 demonstrating 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 incubator. 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 very small cavities containing the 

 gas that has resulted. 



