198 BACTERIOLOGY 



the growing bacteria on the carbohydrates of the potato, 

 are produced. 



A most striking form of development on potato is that 

 often exhibited by the bacillus of typhoid fever and the 

 bacillus of diphtheria. After inoculation of a potato with 

 either of these organisms there is usually no naked-eye 

 evidence of growth, though microscopic examination 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 first of the differential media. 



CHANGES IN THE REACTION OF MEDIA AS A RESULT 

 OF BACTERIAL ACTIVITY. 



For purposes of differentiation, much stress is laid upon 

 the reaction assumed by media as a result of bacterial 

 growth. Under the influence of certain species the medium 

 will become acid, under that of others it is alkaline, while 

 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 begin- 

 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, are commonly employed f0r this purpose. They 

 may be added to the media in the proportions given in the 



