RE A CTIONS PR OD UCED BY BA CTERIA. 189 



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. 



Reactions produced by Bacteria during their 

 Growth. — The reactions produced in the media by 

 different species of bacteria 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 pro- 

 duce at one period of their growth an alkaline, at 

 another period an acid reaction. This is seen in the 

 cultures of the bacillus diphthenas of Lceffler. 



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 indi- 

 cated by the play of colors. Such substances as litmus 

 in the form of the so-called " litmus tincture," and co- 

 ralline (rosolic acid) in alcoholic 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 alterations in their colors studied with different 

 bacteria. Milk and litmus tincture or peptone solution 

 to which rosolic acid has been added are very favorable 

 media for this experiment. 



In milk coagula will now and then appear as a result 

 of the influence of acids, produced from milk sugar by 

 bacterial action, upon the casein of the milk, while 



9* 



