78 VARIATIONS IN VIRULENCE [CH. vi 



injected into the peritoneal cavity, after being cultivated on 

 agar for periods of two weeks and three weeks respectively. 



A striking example of the influence exerted by the culture 

 medium is afforded by the observation that vaccination with 

 a plague strain grown on agar will protect rats against itself 

 but not against the same strain grown on serum (quoted 

 Penfold, 1914). 



(b) The artificial or "unnatural" physical conditions 

 under which laboratory cultures grow are no less important. 

 The difference between a medium of solidified blood serum 

 and the blood circulating in the living body is not only one 

 of chemical composition. When grown on solid media the 

 organisms are crowded together and there must necessarily 

 be an accumulation and concentration of acids formed from 

 the medium, and also of excreted toxins in their immediate 

 vicinity, which may inhibit their power to produce more of 

 the latter substances. 



If in this case the metabolism of the organisms could be 

 " damped down," the possibly inhibitory influence of such an 

 accumulation might be prevented. Shiga (quoted by Bahr, 

 191 2) found that the virulence of his dysentery bacillus could 

 be maintained for as long as 12 months by keeping the 

 cultures at freezing temperature. Whether decreased meta- 

 bolism is the true explanation in this case or not, it is 

 impossible to say without further investigation. 



In the living body, on the other hand, the products of the 

 metabolism of the organism are dissolved and carried away 

 and absorbed and excreted. 



(c) Moreover, in the living tissues the presence of toxins 

 provokes the formation or liberation of " immune bodies " as 

 a protective measure on the part of the body, and it has been 

 shown by Ainley Walker (1903) that the influence of these 

 " immune bodies " whatever their exact nature may be is 

 to heighten the virulence of the organism which led to their 

 production. This observer states, with regard to B. typhosus, 

 that " the result of growing the bacillus in its immune serum 

 was a diminution in its agglutinability, a heightening of its 

 virulence and an increase in its resistance to serum protection." 



