576 INFECTION AND IMMUNITY. 



Resistance. In its resistance to desiccation the tubercle ba- 

 cillus is exceeded only by spore-forming organisms ; 

 it lives approximately for three months in dried 

 sputum which appears to form a protective coat- 

 ing about it. Direct sunlight destroys it in a few 

 hours at the most, whereas diffuse light kills it 

 only after from five to seven days (Koch). It is 

 said that the guinea-pig when exposed to sunlight 

 withstands tuberculosis for a longer time than one 

 which is kept in the dark. Koentgen rays are bac- 

 tericidal for the organism, killing it in about one 

 hour (Eieder). Under moist heat a temperature 

 of 55 C. kills it in from four to six hours, 60 C. 

 in one hour, 70 C. in from ten to twenty minutes, 

 80 C. in five minutes, from 90 to 95 C. in from 

 one to two minutes. When embedded in sputum 

 it is more resistant, five minutes being required to 

 kill it at the boiling temperature. Corrosive sub- 

 limate is not a good disinfectant in this case, inas- 

 much as it produces an albuminous precipitate 

 around the organism which prevents penetration 

 of the sublimate. Five per cent, carbolic acid 

 added to equal parts of sputum kills the bacil- 

 lus in 24 hours. Formalin vapor is a good dis- 

 infectant for dry, but not for moist sputum. lodo- 

 form is not a good disinfectant, in spite of its bene- 

 ficial influence on the infectious process. The re- 

 sistance of the bacillus to gastric digestion has an 

 important bearing on the occurrence of infection 

 in the intestinal tract. The gastric juice of the 

 dog, in one instance, failed to kill the bacillus after 

 six hours' exposure, although it had the power of 

 prohibiting proliferation. 



virulence. The bacillus of human tuberculosis, although 

 fairly constant in its virulence, may be attenuated 



