104 INFECTION. 



Conditions Modifying Infection. 



THE GERM : Virulence : Much depends upon the activity 

 of the germ at the time of infection. Its activity is known 

 as its virulence. This virulence is extremely variable and 

 subject to many influences. Not only is there a difference in 

 the virulence of various pathogenic organisms, but the viru- 

 lence of any one germ is also subject to variations. 



All pathogenic bacteria have a toxin-forming and a vege- 

 tative function. When the vegetative function especially is 

 marked, the toxin-forming function is diminished, and vice 

 versa. It is apparently impossible for both functions to be 

 present in the same proportions at the same time. This varia- 

 tion is dependent on the biologic characteristics. In one 

 instance the organism will grow very luxuriantly with the 

 production of little toxin. All its energy is spent in the 

 direction of growth and propagation. At other times it 

 grows very poorly, but produces enormous quantities of toxin 

 or small quantities of an extremely powerful toxin. The 

 toxin-forming function is carried on at the expense of the 

 reproductive function. 



In the laboratory the virulence is attenuated by repeated 

 transplantation on artificial media. Those conditions which 

 are conducive to growth and multiplication are supplied, and 

 by artificial selection the vegetative function becomes the pre- 

 dominating one. If this vegetative organism is then rapidly 

 passed from one animal to another, conditions are created 

 which favor the development of the toxin-forming function, 

 with the result that the organism multiplies more slowly, but 

 elaborates enormous quantities of toxin or small quantities of 

 a very powerful toxin. 



The virulence may be increased by growing the bacterium 

 in a collodion sac placed in the peritoneal cavity of an animal, 

 so that it may become accustomed to the body-fluids. 



By associating certain organisms the virulence may either 

 be increased or diminished. The association of the Strepto- 

 coccus pyogenes with the bacillus of diphtheria increases the 

 virulence of the latter considerably. Frequently this is seen 

 clinically in cases of diphtheria in which there is a mixed 



