NATURE OF VIRUSES 3 



inorganic nutrients, a virus seemingly needs a more complex 

 and accurately balanced form of medium. It seems likely that 

 the physical distribution of the material of the host cell, prox- 

 imity to membrane or cell wall, etc., may be of importance. 

 This is, as yet, unproven but it seems likely. 



Because of this concern with a host, viruses are classified 

 by their host. Thus we have tobacco mosaic virus, or an E. coli 

 bacteriophage, or rabbit papilloma virus. There is, as yet, no 

 definite and accepted classification of viruses, partly because no 

 very definite means of distinguishing similar viruses exists. 



All viruses, therefore, operate by invasion of a cell. This is a 

 process which in itself merits much study. Once inside the cell, 

 the virus is instantly concerned with the whole metabolic 

 (chemical and energy turnover) process of the cell, which appar- 

 ently shifts its function to serve the purpose of the virus, starting 

 a process which ultimately yields a highly multiplied number 

 of viruses. There is evidence from bacterial viruses that after 

 invasion the virus ceases to have its normal structure and 

 becomes something much less visible, and only later, in the 

 multiple stage, does it become apparent in its normal condition. 



Virus Multiplication 



The unique nature of the bacterial host has enabled detailed 

 studies of virus multiplication to be made. These studies prob- 

 ably apply to other viruses, but it must be remembered that 

 much of our ability to generalize about viruses comes from the 

 fact that we have, so far, made detailed studies on only a few. 

 The process which occurs in a bacterial virus is the following. 

 The virus first attaches, probably at the outset by electrostatic 

 forces, but subsequently by a chemical surface action. No 

 measurable multiplication takes place for a period known as 

 the latent period, which is 13 min for T-1 E. coli bacteriophage, 

 26 min for T-2 coliphage, and 60 min for M-5 B. megaterium 

 phage. At the end of this time, lysis, or dissolution, of the 

 bacterium occurs, and a number, varying from 50 to 1,000, of 

 new virus particles are released. This process is called a hurst, 

 and the number the hurst size. Both the latent period and burst 



