190 BACTERIOLOGY FOR NURSES 



pure culture is obtained on media inoculated with such tissue. 

 Sputum, on the contrary, usually contains many other varieties 

 which grow with much greater facility than the tubercle bacillus 

 and so completely inhibit the growth of the latter. Isolation from 

 such material is best accomplished by injecting it into guinea 

 pigs ; the animal will die in from four to six weeks and the bacilli 

 may be obtained in pure culture from the lymph nodes near the 

 point of injection and frequently from tubercles in the various 

 organs. The optimum temperature for growth is 37 to 38 C. ; 

 below 30 and above 42 C. development rarely occurs. 



Resistance. Tubercle bacilli show a greater degree of resist- 

 ance to external influences than most non-spore-bearing organisms. 

 When completely dried they can withstand a temperature of 

 100 C. for forty-five minutes ; separated in fluids such as milk 

 they are destroyed by exposures to 60 C. in twenty minutes. 

 Cold has little effect upon them. In sputum exposed to direct 

 sunlight the organisms are killed in a few hours ; in diffuse daylight 

 in a few days. Dried in rooms that have little light, they have been 

 found alive after ten months. It is not probable that the tubercle 

 bacilli ever multiply outside of the body save in freshly expecto- 

 rated sputum and on artificial culture media. In sputum they are 

 destroyed in six hours by the addition of an equal quantity of 5 

 per cent carbolic acid. Bichloride of mercury is unsatisfactory 

 as a disinfectant because it combines with the mucus present and 

 has little effect upon the bacteria. 



Pathogenesis. Tubercle bacilli do not produce true toxins, 

 but their bodies contain poisonous substances, probably of the 

 nature of endotoxins. In the animal body the local lesion pro- 

 duced is usually in the form of a tubercle or nodule which varies 

 somewhat in the different tissues. If the bacilli gain entrance 

 to the connective tissue their first action appears to be on the con- 

 nective tissue cells which soon begin to show that some irritant 

 is acting upon them. The cells become swollen and mitotic divi- 

 sion occurs, the resulting so-called epitheloid cells being much 

 larger than the parent cell and possessing paler nuclei. Very soon 

 small foci of epitheloid cells are formed about the bacilli and at 



