140 THE MORE CHRONIC INFECTIOUS DISEASES 



adults who are really not suffering from their slight latent 

 infection, and it is therefore not reliable. It should only be 

 used in children. The supposed cause of the tuberculin test 

 .either under or upon the skin, is the stimulation of the tuber- 

 culous disease by the introduced toxin, and the outpouring 

 from the tubercles of more of their own poison. No reaction 

 of any sort follows the administration of tuberculin to persons 

 free from tuberculosis. 



Morphology and General Characteristics. The tubercle 

 bacillus is a true parasite, that is, it does not multiply in nature 

 outside the animal body. It is a rather large organism, about 



70000 incn wide and fr m 20000 to 5~oVo incn lon g- Jt 



may be straight or slightly bent, usually single, but also in 

 pairs. It is non-motile, and produces no spores. It stains 

 with considerable difficulty, owing to its thick cell wall. 

 There is much fatty and waxy matter in the tubercle bacillus 

 which gives it its resistant power. It grows upon laboratory 

 culture media very slowly. For this reason it must be 

 obtained in as pure a condition as possible. Cultures are best 

 made from the lesions in guinea-pigs. For its growth this 

 organism requires the addition of glycerin, blood serum, or 

 egg to the ordinary nutrient broths and jellies. It will grow 

 only at body temperature, and not at room temperature. 



It is killed by an exposure to 60 C. or 142 F. in thirty 

 minutes, to 70 C. or 160 F. in ten minutes, and at 95 C. or 

 200 F. in one minute in watery suspension. Dry heat at 

 100 C. or 212 F. requires about one hour. The organisms 

 resist drying in the dark for considerable periods. Direct 

 sunlight kills them if in thin layer or small clumps within four 

 hours. Diffused light requires two weeks for their destruc- 

 tion. Sputum protected from direct sunlight may contain 

 living bacilli possibly for one year. Five per cent, carbolic 

 acid should certainly kill them in sputum in twelve hours; 

 in watery suspension in thirty minutes. Bichloride of mercury 

 is not of value for sputum disinfection, but in strength of 1 to 

 1000 in watery suspension is fatal in one hour. No kind of 



