ATTENUATION OF BACTERIA 31 



were able in each case to cope with or limit the action of a few mil- 

 lion to a certain locality, but could not withstand the injection of 

 overwhelming numbers, which caused the animal's death. 



Bacteria to be successfully infectious must be virulent. Viru- 

 lence is best described as the power of a parasite to invade and grow 

 within the body by resisting its natural defenses, and gradations 

 depend upon the ratio of these two forces (Wolff Eisner). Pfeiffer's 

 explanation of virulence assumes that bacteria have binding posts 

 or receptors and the more of these a germ has, the more of the 

 natural defenses it can anchor and remove from the field. Their 

 virulence can be lessened by cultivation at a higher temperature 

 than the body, 42.5 C. to 47 C.; by drying; the exposure to light; 

 the action of chemicals; compressed oxygen; and by passing the or- 

 ganism through the bodies of non-susceptible animals. The atten- 

 uation or weakening of the pathogenic powers of bacteria is useful 

 for the production of various vaccines which are valuable in preven- 

 tive medicine. 



By growing the anthrax bacillus at a high temperature, 42.5 C., 

 it becomes so avirulent that it is incapable of destroying sheep or 

 rabbits. It is then used as a vaccine to prevent infection with 

 virulent bacilli. By exposing the spinal cords of animals dead from 

 hydrophobia to the action of drying for various periods, Pasteur was 

 able to attenuate the virus, so that it would not produce hydro- 

 phobia, but on the contrary, it, by repeated inoculation, caused 

 immunity. The inoculation of monkeys (which are non-suscept- 

 ible) with hydrophobia virus attenuates it. The growth of the small- 

 pox organism in the cow, causing cow-pox, so reduces the virulence 

 of the germ that it is incapable of producing small-pox in man, but 

 only vaccinia; infection with this gives immunity against small-pox. 

 The flesh of animals that have died from quarter-evil is so changed 

 by heat and desiccation that if it is injected into susceptible animals, 

 they do not succumb but are vaccinated against infection with the 

 virulent organism. 



When we speak of attenuation of virulence we usually refer to the 



