CHANGES IN THE REACTION OF MEDIA 215 



Cultures on Potato. A useful factor in the identification 

 of an organism is its growth on sterilized potato. Many 

 organisms present appearances under this method of cul- 

 tivation which alone can almost be considered characteristic. 

 In some cases coarsely lobulated, elevated, dry or moist 

 patches of development occur after a few hours; again, the 

 growth may be finely granular and but slightly elevated 

 above the surface of the potato; at one time it will be dry 

 and dull in appearance, again it may be moist and glisten- 

 ing. Sometimes bubbles, due to the fermentative action of 

 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- 



