124 VETERINARY BACTERIOLOGY 



is dangerous because the minimum may vary with different 

 individuals and the animal utilized may in some instances prove 

 susceptible and succumb. Usually, however, increasing numbers 

 of the organism may be given, and the animal will eventually 

 become entirely immune. This method is of more theoretical 

 than practical importance, and is rarely used. 



As noted above, microorganisms may be attenuated, that is, 

 their ability to produce disease lessened in several ways. Several 

 species of bacteria are known gradually to lessen their virulence 

 when cultivated for a time upon artificial media; for example, 

 the Streptococcus pyogenes, which produces many suppurative in- 

 fections, when cultivated upon artificial media, will finally lose 

 so much of its virulence that it proves entirely non-pathogenic 

 when inoculated into suitable animals. Inoculation of such non- 

 virulent types will increase the resistance of the body to the viru- 

 lent forms. The converse of this is also true, for the continued 

 growth and transfer of many organisms from one animal to another 

 may greatly exalt the virulence. It is possible in a given species 

 to secure in some cases every gradation from the wholly non- 

 pathogenic type to forms that are exceptionally virulent. 



Cultivation of some bacteria at a temperature higher than 

 the growth optimum results in a diminution of virulence. The 

 anthrax bacillus, whose optimum is 38, may be cultivated at a 

 temperature of "42 for a time, when it loses much of its virulence. 

 It may then be used in the form of injections to increase resistance 

 to the disease anthrax in animals. The organism which causes 

 blackleg in cattle is heated to a temperature just below its thermal 

 death-point, and is found to lose many of its pathogenic proper- 

 ties, although it still causes the production of immune substances 

 when injected into the body. A closely related method is to grow 

 bacteria in the presence of mild antiseptics. The anthrax bacillus 

 grown in the presence of carbolic acid (1: 600) has been found to 

 lose its virulence. The growth of certain microorganisms in the 

 body of animals other than the species in which they normally 

 produce disease in some cases results in a decrease of virulence 

 for the first species. The small-pox organism, for example, is 

 grown for a time in the bodies of cattle, and is found to lose much 

 of its virulence for man by this means. The injection of dead 



