BACTERIUM ANTHRACIS 173 



at 54 C. or 130 F. in ten minutes; the spores are 

 killed by boiling ten minutes or in dry heat at 140 C. 

 or 285 F. for ten minutes. The resistance to chemical 

 agents has been considered on page 57. It is best 

 not to rely on any chemical killing of anthrax spores, 

 as different cultures vary in resistance and the environ- 

 ment plays an important part. Anthrax bacilli grow 

 well and characteristically on laboratory culture media. 

 It is not possible to produce a passive immunity to 

 anthrax, but among the great achievements of Pasteur 



FIG. 47. Threads of Bacterium anthracis containing spores. 

 X about 1200 diameters. (Abbott.) 



was the discovery of a method of rendering sheep 

 actively immune to anthrax. He discovered that by 

 growing anthrax bacilli at a temperature of 42 C. 

 or 106 F. instead of 37 C. or 98 F. he was able to 

 reduce their virulence considerably. By varying the 

 length of time of cultivation at this temperature two 

 different strengths were obtained. He now injected 

 the weaker, and followed a few days later with the 

 more virulent. The resistance of the animal can thus 

 be raised to a high level for about a year. The method 

 is not practicable for human beings. 



