188 BACILLUS TUBERCULOSIS. 



marked in the case of those organisms which stain irregularly 

 and present unstained areas, giving the bacterium a beaded 

 appearance. This variety is usually seen only in old cultures 

 or in the sputum of chronic cases of pulmonary tuberculosis. 

 It probably represents an involution-form or a degeneration 

 of the bacterium. The special spore-staining methods are not 

 applicable. In these cases the peculiar branching or fila- 

 mentous forms of the tubercle bacillus are also seen. These 

 forms have suggested a relationship between the tubercle 

 bacillus and the actinomyces. For this reason the tubercle 

 bacillus has been placed by some among the streptothrixes, 

 and the name mycobacterium has been suggested as being 

 more correct than the appellation bacillus. 



A most remarkable characteristic of the tubercle bacillus, 

 and one which serves to differentiate it from all similar organ- 

 isms, is its behavior to certain staining solutions. It is stained 

 with great difficulty, but once it has taken up the stain it is 

 almost impossible to decolorize it. Koch first used an anilin 

 dye to which he added potassium hydrate. Ehrlich modified 

 this method by staining with an anilin dye to which a satu- 

 rated aqueous solution of anilin oil was added, and then 

 decolorizing with a strong mineral acid, which removed the 

 stain from everything except the tubercle bacillus. This was 

 followed by a contrast-stain. This method has been modified 

 in various ways by others, but the principle of overstating 

 and decolorizing with a mineral acid is the same in all the 

 methods. The Ziehl-Neelson method is probably the best. 

 Gram's stain is also applicable, but is not very satisfactory. 



It is very difficult to obtain the tubercle bacillus in pure 

 culture. It is an obligate parasite, aerobic, and absolutely 

 requires a temperature of 37 C. for its development. It 

 grows very slowly even under conditions favorable to its 

 development. Hen's eggs are an excellent culture-medium 

 for the bacillus. Either the yolk or the white, or both, may 

 be used. Small round white colonies appear in from ten to 

 fourteen days. After the tubercle bacillus has become habit- 

 uated to being cultivated, it will grow quite readily on veal- 

 or chicken-bouillon. 



It is impossible to make a plate culture of this germ, because 



