96 MEDICAL BACTERIOLOGY 



state for months, even when exposed to sunlight. In water it survives longer 

 than in dust. Prolonged cultivation at temperatures above the optimum is 

 said to lessen its virulence in time. 



Toxin. The tubercle bacillus produces a powerful toxin. There is prob- 

 ably both extra- and intracellular toxin formation. The more virulent the 

 bacillus, the more potent the toxin liberated. The intracellular toxin of the 

 tubercle bacillus known as tuberculin is resistant to heat, a temperature of 

 i5oC. being required to destroy it. 



Dead tubercle bacilli injected into an animal produce symptoms and lesions 

 indistinguishable from those produced by living bacilli; tuberculin, injected in 

 sufficient amounts, has the same effect. 



Tuberculous subjects are very much more sensitive to tuberculin than 

 non-tuberculous subjects, hence the careful administration of tuberculin serves 

 as a diagnostic aid. 



The employment of tuberculin in the treatment of tuberculosis is of little, 

 if any, value in the vast majority of cases. Its administration as a therapeutic 

 agent is a highly specialized art and is fraught with danger. 



Attempts to immunize healthy animals against tuberculosis by the injection 

 of tuberculin have been unsuccessful. 



Sera prepared to combat the disease have failed to do so. 



Tuberculins produced by the human tubercle bacillus, the bovine tubercle 

 bacillus and the avian tubercle bacillus are identical in nearly all particulars. 

 A man infected with the human tubercle bacillus is more sensitive to human 

 tuberculin than to bovine tuberculin; a man infected with the bovine bacillus 

 is more sensitive to bovine tuberculin than to human tuberculin. 



Agglutinins for the tubercle bacillus may be detected in the blood and serous 

 exudate of some tuberculous animals and not in others; this, in addition to the 

 difficulty of determining reactions, precludes the use of agglutination tests for 

 diagnosis. 



Complement fixation tests are not yet applicable to the problems of diag- 

 nosis and treatment in this disease. Investigations have shown that comple- 

 ment fixing bodies are present in detectable amount in the blood serum of 

 patients suffering with active tuberculosis and disappear when the disease is 

 arrested, but all efforts up to the present to produce a stable, reliable antigen 

 have been unsuccessful. 



Pathogenesis. Most fatal infections in man are caused by the tubercle 

 bacillus. 



Tuberculosis may be a purely localized disease involving only the skin, eye, 

 a bone, an articulation, a kidney or other organ; tuberculosis of the lungs with 

 or without involvement of other parts is by far the most common form of the 

 disease in man. It may be generalized with lesions in many and widely sepa- 

 rated parts of the body, with or without bacteria present in the blood. 



In man the majority of infections are caused by the human tubercle bacillus, 

 some are caused by the bovine bacillus and perhaps the avian bacillus is the 

 offending organism in rare cases. Ichthic tubercle bacilli probably never 

 infect man. 



