304 SPECIFIC MICRO-ORGANISMS 



poison is set free from some substance in the dead cells by the 

 action of the tissue cells or body fluids upon them, and it is quite 

 certain that the bacteria-free culture fluid (old tuberculin) becomes 

 toxic as a result of such an action. 



Tubercle bacilli outside the body are moderately resistant 

 to harmful influences. In dried sputum, they have been found 

 alive after eight months. Direct sunlight kills the bacilli in 

 sputum in a few minutes if this be exposed in a thin transparent 

 layer. In thicker masses the effect of light is uncertain. In 

 buried cadavers the bacilli remain alive and virulent for 2 to 6 

 months. In watery suspensions the bacilli are killed by heating 

 to 60 C. for 15 minutes. In milk, heating at 60 C. for 20 minutes 

 or at 65 C. for 15 minutes kills the tubercle bacilli, provided all 

 the fluid is heated to this temperature for the full period. The 

 bottle should be tightly stoppered and completely immersed 

 in the hot water. Dry heat at 100 C. for 30 minutes is effective. 

 Against chemical disinfectants B. tuberculosis is rather resistant, 

 doubtless because of the waxy constituent of the cells. Absolute 

 alcohol and mercuric chloride i to 500 fail to disinfect sputum 

 in 24 hours. Five per cent carbolic acid is effective in this time. 

 Formalin, 5 per cent solution, requires about 12 hours. B. 

 tuberculosis remains alive in strong antiformin solutions (a pro- 

 prietary preparation of chlorinated caustic alkali) for 30 to 60 

 minutes, whereas ordinary bacteria are rapidly disintegrated 

 by this chemical agent. 



Tuberculin is a name applied to various chemical products 

 of the tubercle bacillus. The oldest and most important tuber- 

 culin was described by Koch in 1890. It is made by growing 

 the bacillus on the surface of 4 per cent glycerin broth in shallow 

 flasks at 37 C. for eight to ten weeks, steaming the cultures 

 for one hour and filtering through porcelain, or often merely 

 through paper, to remove the dead bacilli. The filtrate is then 

 concentrated to one-tenth its original volume by evaporation at 

 90 on the water-bath. The product keeps indefinitely in sealed 

 containers and is known as Koch's old tuberculin (" alt tuber- 



