THE TUBERCLE BACILLUS 303 



the decolorising action of 25-30 per cent, mineral acid ; 

 that is to say, it is strongly " acid-fast," and this property 

 is made use of for demonstrating its presence in tissues, 

 etc., and for diagnostic purposes. This " acid-fastness " 

 is due to the chemical constitution of the bacillus (see 

 p. 309). In old and particularly healing lesions red- 

 staining granules may take the place of definite bacilli : 

 these are the " splitter " forms of Spengler. 



Cultural characters. The tubercle bacillus is aerobic 

 and facultatively anaerobic, and thrives best at a tem- 

 perature of 37 C. or thereabouts, but development even 

 then is slow, four weeks at least being required for an 

 appreciable growth. Primary cultivations from the lesions 

 cannot be obtained on ordinary culture media but should 

 be made on (a) Dorset's egg medium, (6) glycerinated 

 potato in Roux's tubes (Fig. 9), the bulb being filled 

 with 5 per cent, glycerin in physiological salt solution, 

 (c) glycerin brain agar, or (d) glycerinated serum (preferably 

 dogs'). Twort 1 has successfully isolated the bacillus 

 from sputum by direct cultures in an ericolin medium. 

 Dorset's egg medium is prepared thus : the contents of 

 four eggs are well beaten up, 25 c.c. of water are added, 

 and the mixture is strained through muslin. The fluid is 

 then tubed, and the tubes are heated in the sloping position 

 to 70 C. for four hours. At the time of inoculation, a 

 drop or two of sterile water should be added. Brain agar 

 is prepared by making a 3 per cent, nutrient agar of -f 20 

 reaction, adding an equal volume of pounded ox-brain, 

 and sufficient glycerin to make 5 per cent, in the mixture, 

 and sterilising. Egg broth is also a good culture medium. 



After culture on these media for some generations, the 

 tubercle bacillus will develop on 5 per cent, glycerin agar 

 (reaction + 15 or 20), and in 5 per cent, glycerin broth 

 (veal is best) ; it will also grow, though very slowly, on 



1 Proc. Roy. Soc. Lond., B vol. Ixxxi, 1909. 



