THE TUBERCLE BACILLUS 587 



against drying and entrance of stains. This membrane gives a cellulose 

 reaction and is believed to contain most of the waxy substances which 

 can be extracted from the cultures. 



Various observers, notably Nocard and Roux, 4 Mafucci, 5 and Klein, 6 

 have demonstrated branched forms of the tubercle bacillus. These 

 observations, variously extended and confirmed, make it probable that 

 Bacillus tuberculosis is not a member of the family of schizomycetes, 

 but belongs rather to the higher bacteria, closely related to the actino- 

 myces. 



Staining. Tubercle bacilli do not stain easily with the ordinary 

 anilin dyes; to these they are made permeable only by long exposure 

 or by heating of the staining solution. Once stained, however, the dye 

 is tenaciously retained in spite of treatment with alcohol and strong 

 acids. For this reason, this bacillus, together with some other bacteria 

 to be mentioned later, is spoken of as "acid-fast." The acid-fast nature 

 of the bacillus seems to depend upon the fatty substances contained in 

 it, 7 and has furnished the basis for differential staining methods. All 

 the staining methods devised for the recognition of the tubercle bacillus 

 thus depend upon the use of an intensely penetrating staining solution, 

 followed by vigorous decolorization which deprives all but the acid-fast 

 group of their color. Counterstains of any of the weaker dyes may 

 then be used to stain the decolorized elements. One of the first of the 

 staining solutions to be of practical use was the anilin-water-gentian- 

 violet solution of Ehrlich 8 (11 c.c. saturated alcoholic gentian-violet 

 to 89 c.c. 5 per cent anilin water). This dye, although of sufficient 

 penetrating power, has the disadvantage of deteriorating rapidly and 

 has in practice been almost entirely displaced by Ziehl's 9 carbol-fuchsin 

 solution. (Fuchsin 1 gm. in 10 c.c. alcohol absolute, added to 90 c.c. 

 5 per cent carbolic.) This staining solution is the one now in general 

 use and is employed as follows: Thin smears, on slides or cover-slips, 

 are covered with the dye and gently heated. In the case of cover-glasses, 

 these may be floated, face downward, in staining dishes filled with the 

 dye. The dye is allowed to act for about three minutes, steaming but not 

 allowed to boil. At the end of this time the preparation is washed 



4 Nocard et Roux, Aim. <le 1'inst. Pasteur, 1887. 



* Mafucci, Zeit. f. Hyg., ii. 

 'Klein. (Vnt. f. Uakt., 1890. 



7 />'/r;/.v/or/,, 1'WI. (1. M<M!., l,XS(j; /JY///. Dciil. nicd. \Vorli., 1SP1. 



* Khi-lifli, Di-iil. tuc.l. Woch., 18SL> ; ll'cif/crl, I>out. uio.l. \Vnch., .Iss.l. 



9 Xichl, Deut, nied. Woch., 1883; Ncelscn, "Lehrb. <1. allff. Path.," 1894, 



