230 TUBERCULOSIS. 



regard them as indicating a special phase in the life history 

 of the organism, allying it with the higher bacteria. This 

 latter view, however, has many facts against it, especially 

 the circumstance that these aberrant forms are chiefly met 

 with when the organisms are undergoing retrogressive 

 change. The question, however, is one which at present is 

 not definitely settled. 



Staining Reactions. The tubercle bacillus takes up the 

 ordinary stains with great slowness, and very faintly, and 

 for successful staining one of the most powerful solutions 

 ought to be employed, e.g., gentian-violet or fuchsin, along 

 with aniline oil water or solution of carbolic acid. Further, 

 such staining solutions require to be applied for a long time, 

 or the staining must be accelerated by heat, the solution 

 being warmed till steam arises and the specimen allowed to 

 remain in the hot stain for two or three minutes. This 

 resistance to staining Koch attributes to the presence in the 

 bacterial protoplasm of two monatomic* fatty acids. As 

 stated above, Koch at first used a solution of methylene- 

 blue with caustic potash added, but even this method stains 

 somewhat faintly, and he afterwards abandoned it in favour 

 of the combination of aniline oil with gentian-violet, intro- 

 duced by Ehrlich. One of the best and most convenient 

 methods is the Ziehl-Neelsen method (see p. 113). The 

 bacilli present this further peculiarity, however, that after 

 staining has taken place they resist decolorising by solutions 

 which readily remove the colour from the tissues and from 

 other organisms which may be present. Such decolorising 

 agents are sulphuric or nitric acid in 20 per cent solution. 

 Preparations can thus be obtained in which the tubercle 

 bacilli alone are coloured by the stain first used, and the 

 tissues can then be coloured by a contrast stain. Leprosy 

 bacilli, however, retain the stain in the same way, though 

 not so firmly, as tubercle bacilli, and thus constitute an 

 exception to this reaction being peculiar to the latter. The 

 .smegma bacillus also may retain the colour in the above 

 method of staining (vide p. 255). The spores of many 

 bacilli become decolorised more readily than tubercle 



