CH. xi] THE ENZYME THEORY OF DISEASE 159 



unless it is infected simultaneously with pyogenic cocci. In 

 the same way, processes of fermentation may be brought 

 about by two different organisms growing together in a 

 certain medium which neither can accomplish by itself. For 

 example, neither B. coli nor B. dentrificans alone can reduce 

 nitrates but if allowed to act upon sodium nitrate together 

 they bring about the escape of free nitrogen. 



5. There is, furthermore, a remarkable correspondence 

 between the acquirement of virulence by "animal passage" 

 and the acquisition of fresh fermenting properties by prolonged 

 growth in a medium containing a particular sugar : 



(a) The virulence acquired by "passage" through a certain 

 animal applies to that particular species of animal ; virulence 

 towards another species may be increased at the same time 

 but towards a third species it may actually be diminished. The 

 fresh fermenting power resulting from prolonged growth in a 

 sugar concerns that particular sugar ; the capacity of the 

 organism to ferment another sugar may be increased simul- 

 taneously while in respect to a third sugar the fermenting 

 power may be diminished. 



(b) The method of "passage" is more effective in con- 

 ferring virulence if repeated inoculations are made through a 

 series of animals at short intervals. The prolonged growth in 

 a particular sugar is more successful in developing fermenting 

 power if repeated subcultures are made at frequent intervals 

 on to media containing the sugar. 



(c) If virulence is readily acquired on "passage" it is 

 easily maintained and is found to persist for a long time on 

 artificial media ; on the other hand if it is very slowly 

 developed by "passage" it is quickly lost outside the body. 

 It is so, also, as regards fermenting power. In cases where 

 the property is rapidly developed, by x growth on a particular 

 sugar, it is retained for long periods on ordinary media ; on 

 the other hand, where it is very slowly acquired it is found 

 that a return to ordinary media is soon followed by reversion 

 in character. 



(d) Where virulence has been lost only for a short time 

 by a strain of organisms it is quickly restored by " passage " ; 



