306 



THE TUBERCLE BACILLUS 



thing like a moustache due to the regular interlocking of the bacilli with a 

 common orientation. 



This arrangement of the bacilli can be readily shown by lightly pressing a cover- 

 glass on the surface of a glycerin-agar culture and lifting it off without friction. 

 The film should be fixed by heat and stained by one or other of the methods described 

 below. On examining with an oil-immersion lens the appearances reproduced in 

 fig. 194 will be seen. 



FIG. 194. Tubercle bacillus : impression preparation. (After Koch.) 



The bacillus must be stained before it can be found in fluids and tissues 

 and it will be necessary to describe the various methods of staining before 

 embarking upon a description of its characteristics. 



Staining methods. 



Special methods have to be adopted in order to stain the tubercle bacillus. 



The tubercle bacillus is difficult to stain with the basic aniline dyes but once 

 stained it resists the decolourizing action of such powerful agents as dilute mineral 

 acids. Only two other pathogenic bacilli share this characteristic with the 

 tubercle bacillus viz. the leprosy bacillus from which it is easily distinguished 

 and the bacillus of Verruga peruana. 1 



This acid-fast property of the tubercle bacillus serves as a means of diagnosing 

 the organism in fluids and tissues in which it is present. The property of resisting 

 the decolourizing action of acids seems to be due to the presence of a fatty or waxy 

 substance insoluble in alcohol and ether (Koch, Tavel, Viquerat). By treating the 

 bacilli with warm xylol, Borrel extracted a waxy substance which was acid-fast 

 while the bacilli had lost this property. 2 



1 Besides these two there are a few other bacilli, like the tubercle bacillus, capable 

 when deeply stained of resisting the decolourizing action of dilute acids. Such, for 

 instance, are the smegma bacillus and the' bacillus of Tavel (the so-called syphilis bacillus 

 of Lustgarten) but these, unlike the tubercle bacillus, are decolourized by absolute 

 alcohol or ether likewise the various acid-fast bacilli of Bienstock, Gottstein, Moller, 

 Rabinowitsch, etc. (Vide infra, The acid-fast bacilli.} 



2 In opposition to the opinion expressed by H. Aronson, Sabrazes has shown that by 

 treating tissues for sections with ether, xylol'and chloroform the characteristic staining 

 properties of the tubercle bacillus are in no way interfered with. And the same is true 

 of picric acid, carbolic acid and perchloride of mercury none of which prevent subse- 

 quent staining by the Ziehl-Neelsen method. On the other hand, undiluted mineral 

 acids, 2 per cent, chromic acid, formalin, bichromates, osmic acid, etc. either interfere with 

 or entirely prevent subsequent staining by the carbol-fuchsin method. [Eastwood, 

 however, working for the English Commission hardened tissues in formalin (p. 338).] 





