POWERS OF RESISTANCE. 243 



It was at one time believed that tlie 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. 47). In sub-culture the bacillus also 

 grows well upon potato which has been sterilised in 2 per cent glucose broth 

 after the manner of glycerinated potato. 



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 Resistance. — Tubercle bacilH 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 putre- 

 faction, 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 when completely dried they can resist a 

 temperature of , 100° C. for an hour, but, on the other hand, 

 exposure in the moist condition to 70° C. for the same time is 

 usually fatal. Theobald Smith, from an interesting series of 

 thermal death-point tests, concludes that bacilU suspended in 

 distilled water, normal salt solution, bouillon and milk are 

 destroyed at 60° C. in 15 to 20 minutes, the larger ' number 

 being destroyed in 5 to 10 minutes. In milk suspensions, how- 

 ever, the pellicle which fbrms during heating at 60° C. may 



