INFECTION AND INFECTIOUS AGENTS 37 



Factors Altering Virulence. — Virulence may be diminished 

 hy frequent transfers in certain artificial media, culturinj? at 

 temperatures above the optimum, or in some instances by desicca- 

 tion. It may be increased, within limits, by growing the organisms 

 on media containing blood or body fluids that favor capsule forma- 

 lion and also by animal passage. Pasteur showed that the passage 

 of rabies virus through rabbits increased its virulence for the 

 latter but diminished it for certain other animals. He found that 

 exaltation of virulence, by animal ])assage, has a limit beyond 

 wliicli no increase could be obtained. 



Relation of Capsule Formation to Virulence. — It seems that 

 all pathogenic bacteria produce capsules when growing within the 

 tissues of a host. It has been observed that when cultured outside 

 the body the pneumococcus loses its virulence for white mice 

 simultaneously with its ability to form capsules. Hence capsule 

 formation is regarded as a factor in virulence. 



Hypothesis of Welch. — Welch offered an interesting hypothesis 

 which bears his name. In this he postulates that the bacterial cell 

 possesses a defensive mechanism which enables it to oppose the 

 defenses of the body. 



Correlation of Virulence and Colony Type. — Arkwright 

 (1921) called attention to two important colony variations in a 

 member of the colon-typhoid-dysentery group. These were the R 

 or rough, granular colonies, and S or smooth, moist types. Both 

 Arkwright and Baerthlein noted that S colonies might give rise 

 to R colonies but that it was apparently more difficult for the 

 process of dissociation to extend from R to S types. There is a 

 tendency for colony types to remain stable but dissociation may 

 occur spontaneously or may be induced. Intermediate between 

 the R and the S is the colony which has potentialities of both. 



In the same year that Arkwright called attention to the exist- 

 ence of R, S, and colony types, de Kruif (1921), working with 

 colony variants of Bact. lepisepticwm, found that cultures obtained 

 from S colonies were virulent for rabbits while those obtained 

 from R colonies were not. 



The phenomenon of colony variation just described is called 

 bacterial dissociation. It is now known that practically aU 

 bacteria may undergo dissociation and that there is a definite 

 correlation between colony type and other biologic characteristics. 



