

ANILINE D YES FOR DIFFERENTIAL DIA GNOSIS. 181 



REACTION PRODUCED BY ORGANISMS IN THEIR 



ROWTH. The reactions produced in the media by 

 different 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 

 bacillus diphtherice of Loeffler. 



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 maybe 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 



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