90 VARIATIONS IN VIRULENCE [ch. vi 



which followed the elimination of the sugar (lactose) by the 

 bacteria was inhibited by its further addition. 



Glenn (1911) sought to ascertain by experiment whether 

 this inhibition in indol formation was due to the acid pro- 

 duced by the splitting up of the sugar. He found, however, 

 that even when the acid was neutralised indol was not formed 

 until all the sugar had been eliminated. 



It is interesting to note that, as long ago as 1889, Cart- 

 wright Wood explained the fact that indol formation did not 

 take place in the presence of glycerine, on the ground — not 

 that the glycerine interfered with the activity of the bacteria 

 — but that the glycerine offered them a pabulum which they 

 preferred. 



(b) The second observation is that in the case of some 

 organisms — for example B. diphtheriae (Theobald Smith 

 1899, Fisher 1909) — toxins are formed in a culture only if the 

 amount of the sugar in the medium is very small — ^not more 

 than a "trace." 



In one case, therefore, we find that organisms do not split 

 up proteid material as long as they can subsist on other food- 

 stuffs such as carbohydrates, and in the^ second case we find 

 that some organisms, at any rate, do not elaborate toxins in 

 the presence of much carbohydrate material. Both these 

 observations lend support to the theory that the development 

 of toxicity may result from an alteration in metabolism brought 

 about by a change in the kind of food-stuffs available. 



If once this altered metabolism is established the step is 

 a short one from a saprophytic existence to a parasitic one. 

 The organism now trained to feed on "vital" material has 

 Only to cross the border line between the dead and living 

 tissues to become a virulent parasite. 



The invasion of the living tissues, however, on the part of 

 an organism, although it may necessitate the altered assimi- 

 lation to which we have been attributing toxicity, does not 

 always confer on the organism the property of virulence. For 

 example, in a case recorded by Pansini, staphylococci were 

 repeatedly subcultured from the blood over a period of years 

 without there being any evidence of toxaemia. 



