THE TUBERCLE BACILLUS 275 



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

 Recent observations, however, go to establish the latter view, 

 and this is now generally accepted by authorities. It has also 

 been found that under certain circiimstances tubercle bacilli in 

 the tissues produce a radiating structure^closely similar to that 

 of the actinomyces. This was found by Babes and also by 

 Lubarsch to be the case when the bacilli were injected under 

 the dura mater and directly into certain solid organs, such as 

 the kidneys in the rabbit. Club-like structures may be present 

 at the periphery ; these are usually not acid-fast, but they retain 

 the stain in the Weigert-Gram method. Similar results obtained 

 with other acid-fast bacilli will be mentioned below, and these 

 organisms would appear to form a group closely allied to the 

 streptothricese, the bacillary parasitic form being one stage of 

 the life-history of the organism. This group is often spoken of 

 as the mycobacteria. 



Staining Reactions. The tubercle bacillus takes up the 

 ordinary stains very slowly and faintly, and for successful stain- 

 ing 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. One of the best and most convenient methods is the 

 Ziehl-Neelsen method (see p. 109). 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. 

 Within recent years certain other bacilli have been discovered 

 which present the same staining reactions as tubercle bacilli; 

 they are therefore called " acid-fast " (vide infra). The spores 

 of many bacilli become decolorised more readily than tubercle 

 bacilli, though some retain the colour with equal tenacity. 



Bulloch and Macleod, by treating tubercle bacilli with hot alcohol 

 and ether, extracted a wax which gave the characteristic staining 

 reactions of the bacilli themselves. The remains of the bacilli, further, 

 when extracted with caustic potash, yielded a body which was probably 

 a chitin, and which was acid-fast when stained for twenty-four hours 

 with carbol-fuchsin. . Benians considers that a waxy material in some 



