38 CLINICAL BACTERIOLOGY. 



production. Artificial means for reducing the virulence of 

 a culture include heat (p. 40), light, electricity, as well as 

 various chemic influences for instance, iodin trichlorid with 

 diphtheria and tetanus. 



When the virulence of a bacterial culture has once been 

 attenuated, it may again be intensified by passage of the 

 bacteria through the bodies of susceptible animals. The 

 reverse condition that is, attenuation of the virulence 

 may be effected by passing the microbes through relatively 

 insusceptible organisms. Pasteur in this way attenuated 

 the bacillus of hog-erysipelas, by inoculating rabbits with 

 successive generations of the organism. In the same way 

 he procured a mitigated virus of hydrophobia by continued 

 inoculation of monkeys. 



The more virulent a bacterium, the more readily does it 

 give rise to infection, and the more severe is the course of the 

 latter. With a small amount of a pneumococcus-culture, 

 prepared a day or two days previously from an infiltrated 

 pneumonic lung, it is possible to destroy rabbits with cer- 

 tainty, in from twenty-four to forty-eight hours after the 

 development of symptoms of septicemia. With exactly a 

 like amount of the same culture it is possible, two or three 

 days later, to induce only local suppuration, which, after 

 evacuation of the abscess, progresses to recovery ; or very 

 slowly and insidiously, though without septicemia, to death. 

 After a further interval of two or three days, inoculation 

 practised in the same way with the same culture is unat- 

 tended with result the virulence is now entirely lost, and 

 infection no longer takes place. If, under the conditions 

 named, the explanation of the diminution and loss of viru- 

 lence is to be found in the age of the culture, it remains 

 completely concealed under other circumstances. Thus, 

 there may be present in the same membrane in a given 

 case of diphtheria, intensely virulent diphtheria-bacilli, and, 

 besides, others free from all virulence, and which, therefore, 

 are designated pseudodiphtheria-bacilli. Inoculation of the 

 former is capable of causing spread of the diphtheria, while 

 the nonvirulent bacilli are incapable of causing the spread 

 of the disease. 



Finally, a further illustration from human pathology may 

 be given to illustrate the relations between virulence and 

 infection. For the acquisition of pneumonia a predispos- 

 ing cause frequently is necessary a lesion of the lungs, 



