18 F. M. BURNET 



of the viropexis type rather than a limited injection of the contents of the 

 virus particle, as occurs with bacterial viruses. 



As a reasonable current interpretation, it may be suggested that temporary 

 attachment of the virus particle to cell surface takes place under the com- 

 bined influence of electrostatic forces and specific union of viral enzyme with 

 a sequence of prosthetic groups of surface mucoprotein. This provides an 

 opportunity for viropexis to occur and trap the particle before enzymatic 

 action has continued long enough to release it from the primary attachment. 

 Whether the enzyme has any function after entry of the particle into the 

 cytoplasm is unknown. 



It is certain that the process of initiating infection is a complex one, even 

 in the almost ideal circumstances of the allantoic cavity. Cairns (1957) studied 

 the detail of infection in a group of embryos infected with a single infective 

 dose of influenza virus. He found a very wide scatter in the lengths of time 

 elapsing before liberation of new virus indicated that infection had been 

 successfully initiated. The longest delay observed was 28 hours. Obviously, a 

 wide variety of essentially accidental factors can increase the time needed 

 before the conditions for effective interaction are found. 



Enteroviruses. Very little information is available, except for work inci- 

 dental to the establishment of satisfactory titration methods using the 

 Dulbecco plaque technique. With standard monkey kidney cell monolayers, 

 the process of adsorption is relatively slow. Dulbecco et al. (1956) show 

 protocols indicating that exposure of cells to virus for 30 minutes at 37°C. 

 gives about two-thirds the number of plaques obtained after exposure for 90 

 minutes, and tests on the supernatant fluids after adsorption give concordant 

 results. Younger (1956) has published similar findings and showed that the 

 unabsorbed fraction on subculture produced virus unchanged in its adsorp- 

 tive qualities. 



In view of the interest in living virus vaccines against poliomyelitis, it is 

 most unfortunate that no appropriate experimental methods are available to 

 measure adsorption to significant types of human cell, intestinal or neuronal. 

 It is far from clear whether differences in virulence are related to differences 

 in adsorption to cell surface or to factors associated with multiplication within 

 the cell. No real advance in the understanding of virulence is likely to be 

 possible, however, until an experimental attack on each aspect is possible. 



The work on hemagglutination by the mouse poliovirus GD VII and in- 

 hibition of infection and hemagglutination by a polysaccharide from the 

 mouse intestine (Mandel and Racker, 1953; Mandel, 1957) is referred to in 

 Chapters III and IV. The indication from this, that there may be a specific 

 relationship between some configuration of the virus surface and a muco- 

 polysaccharide of the cell surface, seems to call for a serious study of the 

 polioviruses generally from this point of view. 



