76 VARIATIONS IN VIRULENCE [ch. vi 



Adarai, 1892) observed that its virulence was heightened by 

 anaerobic growth. 



Pasteur, when investigating cultures of chicken cholera, 

 found that their virulence gradually disappeared, but he 

 discovered that it was maintained if he grew the organisms 

 in sealed tubes so that oxygen was excluded. By this 

 procedure loss of moisture was likewise prevented and this 

 fact may possibly have been of no less importance than the 

 exclusion of air. 



Other organisms which, normally, do not readily lose viru- 

 lence, do so rapidly if grown in an atmosphere of compressed 

 air (Muir and Ritchie). 



Harass (1906) succeeded in growing certain "anaerobic" 

 bacteria in the presence of air and he found that under these 

 conditions the bacillus of malignant oedema lost virulence 

 though the bacillus botulinus retained it. It is well known 

 that the bacillus of diphtheria produces toxins more plentifully 

 when there is an abundant supply of air (Clark, 1910). 



{d) In other cases the loss of virulence on artificial 

 cultivation is to be attributed to the influence of sunlight, 

 which is known to have this eflfect on B. anthracis and other 

 pathogenic bacteria (Marshall Ward and Blackman, 1910). 



(e) The reaction of the medivm, may also influence viru- 

 lence. Miss Peckham (1897) found that the addition of an 

 alkali to the^nedium increased the virulence of a strain of 

 B. coli. Undue acidity of the medium may result from the 

 action of the organisms themselves in splitting up the 

 carbohydrate present. 



7. Practically every organism becomes less virulent when 

 cultivated for any length of time outside the body, that is to 

 say, on artificial media, even under the most favourable 

 conditions. The common pus cocci and the pneumococcus 

 afford good illustrations of this. The loss of virulence occurs 

 even when the growth is abundant and it persists on sub- 

 culture. 



Possibly some of the factors responsible for this change 

 are those just mentioned, namely differences in temperature, 

 the presence of oxygen, exposure to sunlight, the increased 



