TUBERCLE BACILLUS 193 



Morphology and Stains. Slender rods, generally unbranched, 

 i. .5/ long, and .4/1 thick, usually slightly bent; are non-motile, and 

 have no spores or flagella. In old cultures, and sometimes in 

 sputum, branching forms are seen, and, rarely, some that are club- 

 shape. On acid potato, thread forms are found. In the continuity 

 of most of the bacilli, unstained spaces are seen; in others dense 

 deep red granules are found by fuchsin. As this bacillus is difficult 

 to stain, special methods have been devised to demonstrate it, as 

 the sheathing capsule renders it extremely unsusceptible to the 

 ordinary methods of staining. The cause of 

 this resistance is supposed to be a fatty or 

 waxy substance in the capsule which is more 

 than probable, because of the fact that stains 

 that are fat selective, such as Sudan III, color 

 it very well. Boiling hot carbol-fuchsin gives 

 it the best stain. It keeps the color in spite 



of the action of strong solutions of mineral acids FIG. 68. Tubercle 



j.i , i i i o i ,. bacilli in sputum; 



in water, or dilute alcohol. So when tissues, sta } ne d w i t jf f uc hsin 



or secretions, are stained with hot carbol- and methylene blue. 

 fuchsin for a short time, or cold carbol-fuchsin M 



for a long time, and then treated with a 25 

 percent solution of HNO 3 , or H 2 SO 4 , in water, everything is 

 deprived of the red color, except the tubercle bacilli. All such 

 organisms that are acid proof, are called "acid-fast." There 

 are many other bacilli that have this property. Aniline water 

 and gentian violet solution also stain it. Gram's method dyes 

 the organism violet. Sometimes very young bacilli do not stain 

 at all. 



Vital Requirements. This bacillus thrives best at 37.5 C. It 

 grows slowly, is a strict parasite, and an obligate aerobe. In cul- 

 tures it dies quickly in sunlight, and in diffuse daylight it dies in a 

 few days. It resists drying and light in sputum for months. Its 

 thermal death-point (moist) is 80 C. for ten minutes; can resist 

 60 C. for one hour, but succumbs to 95 C. in one minute. It is 

 13 



