MYCOBACTERIACE^: THE TXJBERCLE BACILLUS 315 



forms are often somewhat bent, and they frequently contain 

 refractile granules. When stained these forms have a beaded 

 or banded appearance. Spores have not been observed. Branch- 

 ing forms occur sometimes in cultures, suggesting a close relation 

 to actinomyces and streptothrix. There is a considerable amount 

 of a waxy substance in the body of the bacillus, which makes it 

 difficult to stain and also difficult to decolorize after it has been 

 stained. Hot carbol-fuchsin is generally employed, applying 

 it for one to two minutes. The preparation is then washed and 

 decolorized in dilute mineral acid (2 to 20 per cent) and in alcohol. 



Pig. 129. — Bacillus tuberculosis, fiom a, pure cultute. X 1000. 



Tissue elements and most other materials may be completely 

 bleached by this treatment, leaving the tubercle baciUi still 

 colored. B. tuberculosis is Gram-positive. 



Cultures are most readily obtained by transferring bits of 

 tuberculous tissue, free from other micro-organisms, to moist 

 slants of inspissated blood serum or Dorset's egg medium. If 

 the available material is already contaminated, the extraneous 

 organisms may usually be eliminated by inoculating ^it into 

 guinea-pigs and making the cultures from the tuberculous guinea- 



