THE SURFACE OF VIRUSES 127 



not change very greatly. Part of the change is certainly due to the 

 viscosity change of water. The virus is not firmly bound at 

 first, and changes in the salt concentration can cause elution 

 of the virus. This was shown by Garen and Puck (1951) who 

 found that after a 10-min adsorption process, which gave a 

 98% attachment at 3° C, 43% could be eluted by raising the 

 salt concentration. At 37° the amount that could be eluted was 

 very much less. 



By studying the reversible attachment at low temperatures 

 and the irreversible attachment which occurs as the temperature 

 is increased, the following picture of virus attachment emerges. 

 The first process of attachment is electrostatic in character. 

 It arises because a certain specific, charged grouping on the 

 virus matches a specific, similar grouping on the bacterium sur- 

 face. Such charges on the surface of colloids are well known. The 

 charges in this case are controlled by the heredity of the virus 

 and the bacterium. In the absence of any ions in the solution, 

 we can suppose these charges to be alike, so that there is repul- 

 sion. Figure 5.2, taken from Puck, Garen, and Cline (1951), 

 shows the kind of process. If, now, divalent ions are added, they 

 can attach to the surface charges, and will do so in a preferential 

 order. Suppose they first attach to the virus and not to the 

 bacterium. Then the result will be a set of opposite charges, 

 in the correct 'pattern, on the surface of the virus. The resulting 

 attraction to the bacterium is then very strong. Quite simple 

 considerations indicate that four charges which were accurately 

 matched by four equal opposite charges can produce, at 10 A, 

 a force of 10~^ dyne/virus. At 50,000 g in an ultracentrifuge, the 

 force per virus is only 10~^'- dyne. The effect is thus 10-million 

 times more potent than quite strong sedimentation. 



iVs the ionic concentration increases, the ions continue to 

 fasten to surfaces until the less favored groups on the bacterium 

 are covered. The result is now much the same as at first, and a 

 net rei)ulsion exists. The curves of Fig. 5.1 are thus nicely ex- 

 plained. The divalent ion causes attachment at a low molarity, 

 and also covers all surfaces to give repulsion at a high molarity. 

 When only one type of surface (e.g., virus) is covered, the attrac- 



