252 Structure of Viruses / 14 : 3 



from the liquid to the gas by going around the critical point. No inter- 

 phase forces distort the specimen. Phage particles attached by their tails 

 to E. coli, as shown in Figure 1, were prepared by this method. The most 

 common method of avoiding surface-tension effects is by freeze-drying. 

 Another difficulty in using the electron microscope arises from the fact 

 that structures such as bacteria are so dense that it is not possible to 

 observe the phage developing within the bacteria. This problem has 



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Figure 4. Electron micrograph of an E. coli bacterium infected 

 with T2 bacteriophages. After E. R. Kellenberger, Labora- 

 toire de Biophysique, University de Geneve, Switzerland. 



been solved by imbedding the bacteria in a suitable plastic and then 

 cutting sufficiently thin sections. These show the phage particles 

 developing during the latter parts of the induction period. A stained 

 section through an E. coli bacterium is shown in Figure 4. 



As can be observed in Figure 1, the phage particles are all very 

 uniform. This is characteristic of many different types of viruses. The 

 extreme uniformity makes them similar to large molecules. Molecules 

 can be crystallized, and so can many types of viruses. The historical 

 example is a plant virus, tobacco mosaic virus, which infects tobacco 

 leaves. Its crystallization led to an appreciation of the similarity of 



