40 MASS. EXPERIMENT STATION BULLETIN 319 



VIRULENCE STUDIES 



The virulence of pathogenic bacteria has been known to fluctuate since the 

 discovery of human and animal pathogens. A decrease or loss of virulence is 

 frequently observed among laboratory strains that are cultivated on artificial 

 media. Also, it has been claimed that fluctuations in epidemics and epizootics 

 are in part due to a variation in the virulence of the causative agent. However, 

 direct and conclusive evidence to support such a statement is wanting at the 

 present time. It is accepted that avirulent strains may be isolated simultaneously 

 with highly virulent types from the same natural outbreak. The virulence of 

 pathogens may be attenuated by various means such as the application of anti- 

 septics, drugs, heat, body fluids, etc. Furthermore, diminished or complete loss of 

 virulence is frequently associated with a variation of the other properties of a 

 microorganism, such as colony and cellular form, antigenicity, and agglutinability. 

 Arkwright (1921), De Kruif (1921), and others have demonstrated that certain 

 types may possess different degrees of pathogenicity. In order to determine the 

 virulence possessed by the different variants described in this investigation, some 

 of the types were studied in the natural host and in an alien host (guinea pig). 



