20 STAINING REACTIONS OF BACTERIA 



This yellow substance, which is insoluble in water, is neither an acid nor a base. 

 Since it cannot dissociate electrolytically, it is incapable of forming salts. It is not 

 therefore a dye. If, however, one more H-atom in the benzene ring be replaced with 

 the auxochrome hydroxyl (—OH), picric acid is formed: 



This yellow substance is an acid, capable of electrolytic dissociation and of form- 

 ing salts with alkalies. It is a dye. 



Some auxochromes (e.g., the amine group, — NH) are basic; others (e.g., the hy- 

 droxyl group, — OH) are acidic. The acidity or basicity of a dye — as these terms are 

 used in the expressions "acidic" or "basic" dyes — is determined by the character of 

 its auxochromes. According as the chromogen is united with acidic or basic groups, 

 the dyes are known as "acidic" or "basic." If basic groups are united to an acid chro- 

 mophore, the dye is more weakly basic than if the same basic groups were united with 

 a basic chromophore. A dye retains its color only so long as its affinities for hydrogen 

 are not completely satisfied. When they are satisfied, reduction occurs and colorless 

 leukobodies are formed. 



The polychrome stains stand in a class somewhat by themselves. In principle, all 

 depend upon a combination of eosin and methylene blue, these elements not only stain- 

 ing as units, but acting together in combination. It is assumed that these compound 

 dyes act on the protoplasm as follows: Certain parts of the cell have an affinity for the 

 neutral stain and take it up as such. Others have an affinity for the basic dye and 

 break up the neutral stain so as to obtain the basic portion of it or, if dissociation has 

 taken place, take up the basic ion directly. Other parts of the cell, with an affinity for 

 acid dyes, similarly combine with the acid portion of the stain. These three types of 

 cell structures are known as "neutrophile," "basophile," and "oxyphile" elements, 

 respectively.' Polychrome stains are used in bacteriology chiefly for the study of 

 spirochetes, Vincent's spirilla, and protozoa; and for the demonstration of chromatin 

 (Zettnow).* 



Two other types of substance besides dyes are used in staining, namely, mordants 

 and decolorizers. Mordants are chemical substances which have the power of making 

 dyes stain material which they would not stain otherwise. This method of staining, in 

 which the presence of a third substance besides dye and material to be stained is re- 

 quired, is called by Mann the "adjective or indirect" method in contradistinction to 

 the "substantive or direct" method in which the chemical and physical natures of 

 dye and material to be stained are so interrelated that the material acquires the color 



' Conn, H. J.: Biological Slains. Geneva, N.Y., 1925. 

 ^Zettnow: Zlschr.f. Hyg. u. Injektionskrankh., 30, i. 1889. 



