CH. vi] VARIATIONS IN VIRULENCE 79 



Substances which increase the virulence of an organism 

 may be formed in the tissues as a result of infection by an 

 organism of a different species. For example, the staphylo- 

 coccus aureus produces more extensive local lesions if, with 

 the organism, is injected a small quantity of the serum from 

 a case of spreading cellulitis; if the local exudate from a 

 cellulitis is substituted for the serum no such effect is produced, 

 showing that the phenomenon is due to bodies formed not by 

 the bacteria which cause the cellulitis but by the tissues. 

 Moreover the serum from a case of spreading cellulitis has 

 the same effect in the case of other kinds of infection due 

 to the pneumococcus, B. typhosus, the tubercle bacillus and 

 cholera (Hektoen). 



(d) Yet a fourth contributing factor to the loss of virulence 

 on artificial media is the fact that other organisms are, as far 

 as possible, excluded. The object of the investigator is to 

 obtain a pure culture, by the use of selective media or by 

 subculturing. In pathological secretions the primary infecting 

 organism grows in the presence of many other saprophytic 

 or parasitic bacteria. A typhoid stool contains a multitude 

 of organisms in addition to Eberth's bacillus and pneumonic 

 sputum often supplies evidence of a mixed infection. The 

 influence of symbiosis on the virulence of organisms will be 

 referred to later, but it is noteworthy, in this connection, that 

 many organisms which are more prone than others to lose 

 virulence when grown on artificial media are likewise more 

 often found in pathological conditions associated with other 

 organisms. 



Whatever the explanation may be, the fact remains that 

 cultures on artificial media tend to lose virulence. On the 

 other hand if bacteria grow in pathological secretions, and 

 particularly if they successfully invade the body of an animal 

 and multiply in that animal's tissues, their virulence often 

 becomes greatly increased. 



8. Pathological secretions possess the power not only of 

 preserving virulence but of actually developing that property 

 on the part of organisms growing in them. For example, the 

 comparatively harmless B. coli communis, present in large 



