BIOLOGICAL ASPECTS OF INTRACELLULAR STAGES OF VIRUS GROWTH 133 



tint derived from studies of virus-infected cells stained with fluorescin- 

 labeled antibody. Liu (1955) showed that fluorescence was first detectable in 

 the nuclei of infected cells and that this nuclear fluorescence was caused by 

 the presence of the soluble antigen; a similar observation has recently been 

 made for fowl plague virus by Franklin (1957). Also, early nuclear fluorescence 

 was found for herpes simplex virus (Lebrun, 1956), but it is not known 

 whether the early fluorescence is due to elementary bodies or to the soluble 

 antigen. If this point were investigated for a number of viruses it might give 

 an important hint about the development and function of soluble antigens 

 (see also Section VII). 



IV. Dynamics of the Development of Infective Virus 



In this section an attempt will be made to summarize studies on dynamic 

 aspects of the development of fully infective virus in infected cells. Ideally 

 such studies are carried out along the lines of the "one-step growth curves" 

 of bacteriophages, i.e., an attempt is made to limit experimentally the growth 

 of virus to a single cycle by preventing spread of the newly produced virus to 

 neighboring cells. In practice, studies with many viruses have not yet reached 

 this ideal. Nevertheless, an attempt will be made to analyze for a number of 

 animal viruses the rate at which infective virus appears, the yield of virus 

 (measured as infective doses) per cell and the duration of the growth cycle. 



A. Poliomyelitis Virus 



Dulbecco and Vogt (1955) carried out one-step growth curves of polio- 

 myelitis virus of all three types in suspensions of isolated monkey kidney cells. 

 The technique was the same as that which they used with western equine 

 encephalitis virus (see Section IV, B). The cells are incubated with virus in 

 buffer to allow adsorption of virus. The cells are then washed and diluted 

 greatly in nutrient medium so as to minimize readsorption of released virus. 

 They found that after a lag period of about 4 hours there was a sharp rise in 

 the titer of extracellular infective virus, which increased for 2-3 hours and then 

 tailed off. During the period of rapid rise in infectivity the increase occurred 

 at a nearly exponential rate. The yield of virus was roughly 100 infective 

 doses per infected cell for all three types. Lwoff et al. (1955) studied the 

 kinetics of virus release from single cells. They used type 1 poliomyelitis virus 

 grown in monkey kidney cells and deposited in drops of paraffin oil after they 

 were infected. The lag period was 5|-7 hours, a longer period than found for 

 suspended cells cultures, perhaps because of the difficulties of the technique 

 of culturing individual cells. Once virus liberation started, the virus was re- 

 leased extremely rapidly and 100-200 infective doses per cell were liberated 



