190 BACTERIOLOGY. 



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, 

 has been recommended as a means of differentiation 

 of bacteria. The differences that are said to be pro- 

 duced consist of alterations in the color of the media due 

 to oxidizing or reducing properties of the growing bac- 

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

 work. It cannot at present be recommended as a reli- 

 able means of diagnosis. 



Behavior toward Staining-re agents. — The 

 behavior of certain bacteria toward the different dyes 

 and their reactions under special methods of after- 

 treatment serve as aids to their diagnosis. With very 

 few exceptions 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 decolori zing-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 

 decolorizing-agents as alcohol, nitric acid, oxalic acid, etc. 



Certain other organisms when stained with a solu- 

 tion of gentian-violet in aniline-water retain tlieir color 

 when treated with such decolorizing-bodies as iodine 

 solution and alcohol (Gram's method), while again 

 others are completely decolorized by this method. 



Many of them can only be treated with water, or but 

 for a few seconds with alcohol, without losing their 

 color. 



