114 BACTERIOLOGY. 



A most striking form of development on potato is 

 that possessed by the organism of typhoid fever and the 

 bacillus of diphtheria. After the inoculation of a potato 

 with either of these organisms there is no uaked-eye 

 evidence of a growth in either instance, though micro- 

 scopic 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 most important differential media which we possess 

 for this work. 



Reaction Produced by Organisms in their 

 Growth. — The reactions produced in .the media by 

 diiferent organisms in the course of their growth are 

 very valuable as means of differentiation. 



In some cases these changes are so marked that they 

 are readily detected by the coarser reagents ; again they 

 are so slight as to require the employment of the most 

 delicate indicators. They are sometimes seen to produce 

 at one period of their growth an alkaline, at another 

 period an acid reaction. This is seen in the cultures of 

 the bacillus diphtherise of Lotfler. 



These differences are best seen after the addition to the 

 media in which the organisms are to grow of some of 

 the chemical substances which do not interfere with the 

 development of the organisms, but which under one 

 reaction are of one color, and with an alteration of the 

 reaction become a different color, the change being in- 

 dicated by the play of colors. Such substances as 

 litmus, in the form of tincture, and coralline (rosolic 

 acid) in alcoholic solution have been employed for this 

 purpose. They may be added to the media in the pro- 

 portions given in the chapter on media, and the altera- 

 tions in their colors studied with different bacteria. 



