10 THE PHYSICS OF VIRUSES 



This is interpreted, at present, as follows. The red cells are 

 supposed to have receptors on them which can attach to the 

 virus. The self-eluting virus attaches at a point which is enzy- 

 matically active on the receptor as substrate. The enzyme then 

 slowly converts the receptor into some noncombining form, 

 which, therefore, ceases to be bound by the virus, and so the 

 process of elution can take place. Such red cells may resus- 

 pend, but they can no longer be acted on by the same virus 

 to agglutinate. 



Purified Virus Preparations 



In the case of the plant viruses, notably tomato bushy stunt, 

 tobacco necrosis, and southern bean mosaic virus, purified 

 preparations which can be crystallized into crystals of definite 

 form have been prepared. The ])rocedure is elaborate, although 

 one difficulty is simply in starting from enough infected plants 

 to give a high yield. Tobacco mosaic virus forms small, visible 

 crystalline aggregates. 



No crystalline preparations of animal or bacterial viruses 

 have, at the time of writing, been prepared. The concentration 

 of bacterial viruses can be raised to about 10^^ per cm^ by 

 careful growing and fractional centrifugation. However, physical 

 studies on bacterial virus are still somewhat limited by the 

 lack of a pure preparation. 



Virus Assay 



Of predominant importance in virus assay is the use of dilu- 

 tion and some all-or-nothing effect, like producing the proper 

 symptoms in a host, or an agglutination of red blood cells. Any 

 given sample containing virus is diluted in an appropriate 

 suspension medium (buffer or broth for example) by known 

 amounts, e.g., in factors of 10, and each dilution is tested for 

 the required effect. If a dilution of lO^-fold, for examjile, will still 

 produce symptoms, this number is often used to describe the 

 virus titer. To a physicist this may seem to be a very crude meas- 

 urement. Actually, for many purposes, only the logarithm of the 

 virus titer has any significance, and when this is so, the accuracy is 



