CH. vi] VARIATIONS IN VIRULENCE 77 



acidity of the medium. Other contributing factors are found, 

 no doubt, in the nature of the medium itself both as regards 

 its chemical composition and its physical properties. 



(a) The difference in chemical composition between the 

 body fluids and laboratory media must necessarily profoundly 

 influence the metabolism of organisms transferred from one 

 to the other. The more closely the artificial medium used 

 resembles chemically the body fluids, the less influence will 

 this factor exert. For example, on solidified blood serum, or 

 media to which blood has been added, or ascitic fluid, viru- 

 lence is maintained for a greater length of time than on other 

 media. Eyre and Washbourn (1899) found that the parasitic 

 pneumococcus kept its virulence, undiminished for a couple 

 of months on blood agar but not on ordinary media. Anne 

 Williams (1902) found many strains of diphtheria bacillus, 

 which were quite non-pathogenic when inoculated from broth, 

 were highly toxic when inoculated from serum culture or 

 ascitic broth. 



In the case of pathological exudations the question of 

 chemical composition is of even greater significance, for the 

 composition of these fluids is dependent on processes of great 

 complexity and differs widely from that of healthy excretions, 

 and it is apparently by virtue of this very difference that 

 pathological exudations possess the power of maintaining 

 the virulence of organisms growing in them. It is found in 

 practice that the best fluid in which to preserve organisms 

 from a pathological source unchanged for subsequent examina- 

 tion is normal saline to which a considerable quantity of the 

 infected material itself has been added, whether it be blood 

 or pus or fluid from a serous cavity or some other secretion. 

 Horrocks took the urine of a "typhoid carrier" which was 

 loaded with virulent bacilli and kept it for 12 months in 

 flasks exposed to the light and frequently opened to the air. 

 At the end of that time he found that the bacilli were as 

 virulent as when first examined, though, when subcultured 

 on to agar or into broth, virulence was rapidly lost. Thus, 

 two strains of virulent typhoid bacilli from the urine of 

 " Carrier /" and " Carrier S" failed to kill a guineapig, when 



