210 SACTEBlOLOar, 



tion of scrapings from the surface of the potato reveals 

 an active multiplication of the organisms which had 

 been planted there. The potato is one of the most im- 

 portant differential media that we possess for this work. 



CHANGES IN THE EEACTION OP MEDIA AS A RESULT 

 OP BAOTEEIAL ACTIVITY. 



For purposes of differentiation, much stress is laid 

 upon the reaction assumed by media as a result of bac- 

 terial growth. Under the influence of certain species 

 the medium will become acid, under that of others it is 

 alkaline, wliile some cause little or no change. In 

 media of particular composition — -i. e., those containing 

 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 mediimi 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. 



Aniline Dyes foe Diffeeential Diagnosis. — 

 The addition to soli<l media of some of the aniline 

 dyes, fuchsin, methylene-blue, methylene-green, and 



