LIFE S FRINGES 



bacterial species by the addition of previously autoclaved sewage to 

 the culture medium. 



Nevertheless, all these experimental results have not convinced the 

 adherents to the theory of parasitic ultramicrobes. They defend the 

 rather improbable view that in all these cases the ultramicrobe al- 

 ready exists in a latent form in the healthy experimental animal or in 

 the normal bacterial culture. The corresponding non-specific proce- 

 dure would simply reduce the resistance of the host against infection, 

 and thus enable the parasite to multiply. 



For a while it seemed as if it would be possible to unnerve this 

 counter-argument. This was when our fellow-countryman, Den Doo- 

 ren de Jong, managed to liberate bacteriophage from cultures of a 

 sporeforming bacterium that had been started with bacterial spores 

 previously heated at ioo °C. Since it has been the common experience 

 that the most divergent bacteriophages are inactivated by a brief ex- 

 posure to a temperature of 75 °C, this presumably provided the ideal 

 situation in which a living organism, previously and with certainty 

 rendered free of phage, even of latent phage, could nonetheless be 

 induced to produce the virus as a result of the application of certain 

 measures. Unfortunately this experiment, too, signified no more than 

 a local break-through in the trench warfare between the adherents 

 and the opponents to the theory of the autonomous parasite. The 

 proponents invented the rather unlikely 'theory of the safety-deposit 

 box', according to which the bacterium is kind enough to permit the 

 ultramicrobe-parasite to share the benefit of the thermoresistent prop- 

 erties of the bacterial spore, and it must be admitted that a number of 

 experimental data do indeed favour this view, at least at first sight. 

 Thus the equilibrium between the two opposing forces has again been 

 restored. 



I do not wish to tire out my audience by discussing in detail the 

 various other arguments that have been brought to bear on the prob- 

 lem by both contending parties, and the attempts that have been made 

 to refute them. Rather shall I try to circumscribe the present position 

 as follows. In favour of the theory of the autonomous ultramicrobe 

 must be considered the great similarity in behaviour of filtrable agents 

 and of microscopically discernable single-celled parasitic organisms ; 

 good examples of this are found in the adaptive phenomena, changes 

 in virulence, and the immunological properties. On the opposite side 



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