128 MODIFICATIONS OF BIOLOGICAL CHARACTERS. 



could be modified by special treatment in such a manner that it no 

 longer produced a fatal form of the disease. He found that the viru- 

 lence was greatest when cultures were made from fowls which had 

 died from a chronic form of the disease, and that this virulence was 

 not lost by successive cultivations in chicken bouillon, repeated at 

 short intervals. But when an interval of more than two months 

 was allowed to elapse without renewing the cultures, the virulence 

 was diminished and fewer deaths occurred in fowls inoculated with 

 such cultures. This diminution of virulence became more marked 

 in proportion to the length of time during which a culture solution 

 containing the microbe remained exposed to the action of the atmo- 

 sphere, and at last all virulence was lost as a result of the death of 

 the pathogenic microorganism. When the virus was preserved in 

 hermetically sealed tubes it did not undergo this modification, but re- 

 tained its full virulence for many months. According to Pasteur, 

 the various degrees of modification of virulence resulting from pro- 

 longed exposure to the air may be preserved in successive cultures 

 made at short intervals. Subsequent experiments with cultures of 

 the anthrax bacillus gave similar results and enabled him to produce 

 an ' ' attenuated virus " for his protective inoculations. 



In the case of the anthrax bacillus it was found that the spores 

 retain their full virulence for years, and that the production of an at- 

 tenuated virus required the exclusion of these reproductive elements. 

 Cultivations were consequently made at a temperature of 43 to 43 

 C., at which point this bacillus is incapable of producing spores. 

 Cultivation at this temperature for eight days gave an attenuated 

 virus suitable for use in protective inoculations. 



Attenuation by Heat. Toussaint has shown that a similar modi- 

 fication of virulence may be produced by exposure for a short time 

 to a temperature a little below that which destroys the vitality of the 

 pathogenic organism. This is best accomplished, according to Chau- 

 veau, in the case of the bacillus of anthrax, by exposure for eighteen 

 minutes to a temperature of 50 C. Exposure to this temperature for 

 twenty minutes is said to completely destroy the vitality of the bacillus. 



Attenuation by Antiseptic Agents. The writer, in 1S80, ob- 

 tained evidence that attenuation of virulence may result from ex- 

 IM .sure to the action of antiseptic agents. In a series of experiments 

 made to determine the comparative value of disinfectants, the blood 

 of a rabbit recently dead from a form of septicaemia induced by the 

 subcutaneous injection of my own saliva, and due to the presence of 

 a micrococcus (Micrococcus pneumonue crouposie), was subjected to 

 the action of various chemical agents, and subsequently injected 

 into a rabbit to test the destruction of virulence. In the published 

 report of these experiments the following statement is made : 



