POWERS OF RESISTANCE 241 



gresses more rapidly than on serum, but this medium is not 

 suitable for obtaining cultures from the tissues, inoculations 

 with tubercular material usually yielding a negative result. The 

 growth has practically the same characters as on serum, but is 

 more luxuriant. In glycerin broth, especially when the layer is 

 not deep, tubercle bacilli grow readily in the form of little white 

 masses which fall to the bottom and form a powdery layer. If, 

 however, the growth be started on the surface it spreads super- 

 ficially as a dull whitish, wrinkled pellicle which may reach the 

 walls of the flask ; this mode of growth is specially suitable for 

 the production of tuberculin (vide infra). The culture has a 

 peculiar fruity and not unpleasant odour. On ordinary agar 

 and on gelatin media no growth takes place. 



It was at one time believed that the tubercle bacillus would only grow 

 on media containing animal fluids, but of late years it has been found that 

 growth takes place also on a purely vegetable medium, as was first shown 

 by Pawlowsky in the case of potatoes. Sander has shown that the 

 bacillus grows readily on potato, carrot, macaroni, and on infusion of 

 these substances, especially when glycerin is added. He also found 

 that cultures from tubercular lesions could be obtained on glycerin potato 

 (p. 46). 



The optimum temperature for growth is 37 to 38 C. 

 Growth ceases above 42 and usually below 28, but on long- 

 continued cultivation outside the body and in special circum- 

 stances, growth may take place at a lower temperature, e.g. 

 Sander found that growth took place in glycerin-potato broth 

 even at 22 to 23 C. 



Powers of Eesistance. Tubercle bacilli have considerable 

 powers of resistance to external influences, and can retain their 

 vitality for a long time outside the body in various conditions ; 

 in fact, in this respect they may be said to occupy an inter- 

 mediate position between spores and spore-free bacilli. Dried 

 phthisical sputum has been found to contain still virulent bacilli 

 (or their spores) after two months, and similar results are obtained 

 when the bacilli are kept in distilled water for several weeks. 

 So also they resist for a long time the action of putrefaction, 

 which is rapidly fatal to many pathogenic organisms. Sputum 

 has been found to contain living tubercle bacilli even after being 

 allowed to putrefy for several weeks (Fraenkel, Baumgarten), and 

 the bacilli have been found to be alive in tubercular organs which 

 have been buried in the ground for a similar period. They are 

 not killed by being exposed to the action of the gastric juice for 

 six hours, or to a temperature of - 3 C. for three hours, even 

 when this is repeated several times. It has been found that 

 16 



