192 BA CTERIOLOG Y. 



its frequency or constancy under varying conditions 

 determined. 



CULTURES ON POTATO. A very important feature 

 in the study of an organism is its growth on sterilized 

 potato. Many organisms present appearances under 

 this method of cultivation which alone can almost be 

 considered characteristic. In some cases coarsely lob- 

 ulated, 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 ap- 

 pearance, again it may be moist and glistening. Some- 

 times 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 examina- 

 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 REACTION OF MEDIA AS A RESULT 

 OF BACTERIAL ACTIVITY. 



For purposes of differentiation, much stress is iaid 

 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, while some cause little or no change. In 

 media of particular composition i. e., those containing 



