FERMENTA TION. 211 



several others, as well as combinations of these dyes, 

 has been recommended as a means of differentiation of 

 bacteria. The changes that occur as a result of bacterial 

 growth in media so treated consist of alterations in the 

 color of the media due to the oxidizing or reducing 

 properties of the growing bacteria. It is doubtful if 

 this is, in general, an important differential method ; at 

 all events, it has been pretty well abandoned after hav- 

 ing enjoyed at one time some degree of popularity. 



Behavior toward Staining-reagexts. — -The be- 

 havior of certain bacteria toward the different dyes 

 and their reactions under special methods of after- 

 treatment aid materially in their identification. With 

 very few exceptions bacteria stain readily with the com- 

 mon aniline dyes ; but they differ markedly in the te- 

 nacity with which they retain these colors under the 

 subsequent treatment with decolorizing-agents. 



The tubercle bacillus and the bacilhis of leprosy, for 

 example, are difficult to stain ; but when once stained 

 they retain their color under the action of such energetic 

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

 etc. Certain other organisms Avhen stained with a solu- 

 tion 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 washed in water, or but 

 for a few seconds in alcohol, without losing their 

 color. It is essential that all these peculiarities should 

 be carefully noted in studying an organism. 



Fermentation. — The production of gas as an indi- 

 cation of fermentation is an accompaniment of the 

 growth of certain bacteria. This is best studied in 



