THE BACILLUS OF TUBERCULOSIS 289 



appear which stain with more difficulty than the rest of the bacillus 

 and also retain the stain with greater tenacity. The bacilli, however, 

 containing these bodies are not appreciably more resistant than those 

 not having them; although, therefore, these bodies have some of the 

 characteristics of a spore, they lack the quality of resistance to dele- 

 terious influences and cannot be considered true spores. 



The bacilli have a thin capsule, shown in one way by the fact that 

 they appear thicker when stained with fuchsin than with methylene 

 blue. The capsule is believed to contain the 

 greater portion of the wax-like substance pecu- Fl - 96 



liar to the bacillus. 



Staining Peculiarities. These are very im- 

 portant, for by them its differentiation and 

 recognition in microscopic preparations of 

 sputum, etc., are rendered possible. Owing to 

 the waxy substance in its envelope it does not 

 readily take up the ordinary aniline colors, but 

 when once stained it is very difficult to decol- 

 orize, even by the use of strong acids. The 

 more recently formed bacilli are much more 

 easilv stained and decolorized than the older 



f TM i- i i i ,110 Branched forms. (From 



forms. Ehrlich devised a method of staining c. F. Craig.) 



which proved to be satisfactory viz., the use of 



a solution of an aniline color fuchsin or methyl violet in a saturated 

 aqueous solution of aniline oil and decolorization of other bacteria with 

 a solution of a mineral acid, to be followed by a contrast stain, such as 

 methylene blue. (Plate I., Figs. 1 and 2.) Various modifications of 

 Ehrlich's method are now commonly used. The tubercle bacilli can be 

 demonstrated also by Gram's method of staining. 



Biology. The bacillus tuberculosis is a parasitic, aerobic, non-motile 

 bacillus, and grows only at a temperature of about 37 C., limits 30 

 to 42 C. It does not form true spores. 



RESISTANCE. The bacilli, possibly on account of the nature of their 

 capsule, have a somewhat greater resisting power than most other 

 pathogenic bacteria, since frequently the bacilli resist desiccation at 

 the ordinary temperatures for months; most bacilli die, however, soon 

 after drying. Upon serum cultures the bacilli seldom live longer than 

 six to eight months. They frequently retain their vitality for several 

 weeks in putrefying material, such as sputum. Cold has little effect 

 upon them. When dry the more resistant organisms stand dry heat 

 at 100 C. for hours; but when moist, as in milk, they are quickly 

 killed viz., at 55 C. in four hours, at 60 C. in thirty minutes, at 

 65 C. in fifteen minutes, at 70 C. in ten minutes, at 80 C. in five 

 minutes, and at 95 C. in one minute. One reason why in some 

 experiments they appear to withstand high temperatures is, as pointed 

 out by Theobald Smith, that when heated in a test-tube in the usual 

 way the cream which rises on heating is exposed on its surface to a 

 lower temperature than the rest of the milk, and as this contains the 



19 



