FERMENTA TION. 21 5 



in the color of the media, due to the oxidizing or reduc- 

 ing 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 

 having enjoyed at one time some degree of popularity, 

 though a number of investigators still regard saffranin 

 and neutral red as useful agents for the differentiation 

 of allied species, and as handy aids in the identification 

 of those species capable of reducing them. 



BEHAVIOR TOWARD STAINING-REAGENTS. 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 bacillus 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 when 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 



