ANILINE DYES. 115 



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 cornbi nations of these dyes, 

 have been recommended as a means of differentiation of 

 organisms, the differences consisting in alterations in the 

 color of the media due to oxidizing or reducing proper- 

 ties 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. 



BEHAVIOR TOWARD STAINING REAGENTS. The 

 behavior of certain organisms toward the different dyes 

 and their reactions under special methods of after-treat- 

 ment serve as aids to their diagnosis. With very few 

 exceptions all bacteria stain readily with the common 

 aniline dyes, but they differ materially in the tenacity 

 with which they retain these colors under the subse- 

 quent treatment with decolorizing agents. 



The tubercle bacillus and the bacillus of leprosy, for 

 example, are difficult to stain, but when once stained 

 retain their color under the action of such energetic de- 

 colorizing agents as alcohol, nitric acid, oxalic acid, etc. 



Certain other organisms when stained with a solution 

 of gentian violet in aniline- water, retain their color when 

 treated with such decolorizing bodies as iodine solution 

 and alcohol (Gram's method), while again others are 

 completely decolorized by this method. 



