ANILINE DYES. 171 



Behavior towaed 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 



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

 for a few seconds with alcohol, without losing their 

 color. 



It is essential that these peculiarities should be care- 

 fully noted in studying an organism. 



Fermentation. — The production of gas as an indica- 

 tion of fermentation is an accompaniment of the growth 

 of some organisms. This is best studied in media to 

 which 1 to 2 per cent, of grape sugar has been added. 



In this experiment the test-tube should be filled to 

 about one-half its volume with agar-agar. The me- 

 dium is then liquefied, and when at the proper tem- 

 perature, a small quantity of a pure culture of the 

 organism under consideration should be carefully dis- 

 tributed through it. The tube is then placed into ice- 

 water and rapidly solidified in the vertical position. 



