VARIATION IN VIRULENCE. 95 



several hours, and at 50° to 53° in a few minutes. By a proper regu- 

 lation of the reduction through the action of heat the anthrax bacillus 

 can be attenuated to such a degree that it will kill only mice, or mice 

 and guinea-pigs, or, besides these, also rabbits. Also spores (symp- 

 tomatic anthra.x) may bo attenuated by means of dry heat or short 

 careful steam disinfection (Kitt). 



(b) By growth upon unfavorable nutrient media. An addition of 

 phenol(5jij)andof bichromate of potassium (0.04% to 0.02;?^) to nutri- 

 ent media is employed to attenuate the anthrax bacillus, and iodin 

 trichlorid the diphtheria bacillus. Growth on media containing sugar 

 always, in time, lowers the virulence (Lev}'), 



(c) By the action of sunlight, compressed oxygen, etc. 



(d) By repeated passage tlu-ough unsuitable animals. Swine erysip- 

 elas becomes much less virulent after repeated passage through rabbits, 

 the variola organism (although not a bacterium) after passing through 

 the cow. 



It is much more difficult to restore a heightened viru= 

 lence to attenuated bacteria. In general it is true that 

 the more rapidly the attenuation is accomplished, the 

 more rapidly Avill the virulence be spontaneously restored. 



Varieties which have gradually (spontaneously, i. e., 

 from the action of their metabolic products) suffered loss 

 in virulence may have their virulence again increased 

 in most cases, but not always, in one of the following 

 ways : 



1. Growing in bouillon to which has been added ascitic fluid 

 (streptococci, diphtheria bacilli). (Compare von Dungereu, C. B. xix, 

 137, and special part. ) 



2. By first inoculating especially susceptible animals, — i. e., very 

 young animals of the susceptible variety, — and if these succumb, trans- 

 ferring the causative agent directly with the blood of this animal to 

 an older, more resisting example of the susceptible species, and later to 

 still more resisting species of animals. Each passage through the ani- 

 mal strengthens the virulence, until a certain maximum is reached. 

 Compare also Knorr's experience with the Streptococcus pyogenes. 



3. By first infecting susceptible animals with large quantities of 

 fresh bouillon cultures of the variety concerned, the simultaneously 

 introduced metabolic products cooperate to heighten the disposition 

 of the animal for the injected organisms. 



4. Together with the bacteria (especially staphylococci and strep- 

 tococci) there is injected a large quantity of the metabolic products of 

 Bad. rmlgare. The action is explained as under 3. 



5. By the injection, together with the weakened bacterium, also of 

 another which in itself is entirely harmless ; for example, with the Ba- 

 cillus cedematis maligni or Bacillus anthracis, the Bact. prodigiosum. 



6. The injection of the culture mixed with an injurious substance 

 not of bacterial origin ; for example, lactic acid. "With the Bac. 

 cedematis maligni increased pathogenic properties have thus been ob- 



