CHAPTER III 



STAINING REACTIONS OF BACTERIA^ 



JOHN W. CHURCHMAN 



Cornell University Medical College, New York 



CHEMISTRY OF DYES 



Bacteriological stains belong almost entirely to the group known as "aniline 

 dyes." Since a number of these, however, are not derived from aniline and bear no 

 direct relation to it, and since all are derivatives of the hydrocarbon, benzene (CeHo), 

 "coal-tar dyes" is a better term. 



Coal-tar dyes are monacid salts of color bases or alkali salts of color acids. "Ba- 

 sic," "acidic," "neutral" — the descriptive terms usually applied to them — are not 

 particularly fortunate terms since the dyes are not necessarily bases or acids, and 

 even if called "basic" may have an H-ion concentration on the acid side of neutrality. 

 Basic dyes are usually encountered as dye salts of a colorless acid such as hydrochloric, 

 sulphuric, oxalic, or acetic acid; acid dyes as sodium, potassium, calcium, or am- 

 monium salts of dye acids. The terms "acidic" and "basic" as applied to dyes really 

 refer to the affinity of the chromogenic radicle for acidic or basic groups as the case 

 may be. The term "chromogenic radicle" leads directly to a consideration of the cur- 

 rently accepted theory as to the molecular structure of the dyes. 



The basis of this structure is the benzene ring of Kekule from which — as is well 

 known — an almost infinite number of derivatives can be formed. When the deriva- 

 tives contain certain groups of elements known as "chromophores," these groups 

 impart the property of color. Benzene derivatives containing chromophore radicles 

 are known as "chromogens." Chromogens, although colored, are not dyes since they 

 may have little or no affinity for fibers or tissues ; the color they impart to fibers or tis- 

 sues is a superficial coat easily removed by mechanical processes — the color does not 

 "take." In order that a chromogen become a dye the chromogen derivative must 

 contain, in addition to the chromophore, auxiliary groups which are known as "aux- 

 ochromes." These have themselves little or no color and are not the cause of the color 

 of the dye; but they impart to the compound the property of electrolytic dissociation, 

 •furnish it with salt-forming properties, and thus convert it into a dye. 



The formation of the yellow dye, picric acid, from the yellow chromogen, tri- 

 nitro-benzene, by the addition of the auxochrome, hydroxyl ( — OH), illustrates 

 present-day conceptions of the chemical structure of coal-tar dyes. When three H- 

 atoms in the benzene ring are replaced with the chromophore (— NOj), tri-nitro- 

 benzene results: O.N. .. .NO, 



NO. 

 ' The detailed history of staining in bacteriology may be found in Unna's articles, "Die Ent- 

 wicklung der Bakterienfarbung," Centralbl.f. BakierioL, 3, 22-345. 1888. 



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