212 



TUBERCULOSIS. 



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. Leprosy bacilli, however, 

 retain the stain in the same way, though not so firmly, as 

 tubercle bacilli, and thus constitute an exception to this 



reaction being 

 peculiar to the latter. 

 Even the spores of 

 many bacilli become 

 decolorised more 

 readily than tubercle 

 bacilli, though some 

 retain the colour with 

 equal tenacity. 



Cultivation. 

 The medium first 

 used by Koch was 

 inspissated blood 

 serum (vide p. 47). 

 If inoculations are 

 made on this 

 medium with tuber- 

 cular material free 

 from other organisms 

 there appear from 

 the tenth to four- 



A B C teenth day minute 



FIG. 58. Cultures of tubercle bacilli on points of growth of 

 glycerine agar. dull whitish colour, 



A and B. Mammalian tubercle bacilli ; A is an old rn 4-J-ipr irrAcmlnr onrl 



culture, Bone of a few weeks' growth. rather irregular, and 



C. Avian tubercle bacilli. The growth is whiter slightly raised aboVC 

 and smoother on the surface than the others. , J 



the surface. In such 



cultures they usually reach only a comparatively small size 

 and remain separate, becoming confluent only when many 



