170 BACTERIOLOGY. 



delicate indicators. Tiiey 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 Loffler. 



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 the so-called " litmus tincture," and 

 coralline (rosolic acid) in alcoholic solution have been 

 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 acids produced during the bacterial life, while again 

 acids may be produced and yet no coagulation be noticed. 



Aniline Dyes for Differential Diagnosis. — 

 The addition to solid media of some of the aniline 

 dyes, fuchsin, methylene-blue, methylene-green, and 

 several others, as well as combinations of these dyes, 

 have been recommended as a means of differentiation of 

 organisms, the differences claimed to be produced con- 

 sisting of alteratibns in the color of the media due to 

 oxidizing or reducing properties of the growing bacteria. 

 As yet but little has come from this method of work. 

 It cannot at present be recommended as a reliable means 

 of diagnosis. 



