The viral replication cycle is described in this chapter in two different ways. The first approach is a growth curve, which shows the amount of virus produced at different times after infection. The second is a stepwise description of the specific events within the cell during virus growth.
The growth curve depicted in Figure 29–1 shows that when one virion (one virus particle) infects a cell, it can replicate in approximately 10 hours to produce hundreds of virions within that cell. This remarkable amplification explains how viruses spread rapidly from cell to cell. Note that the time required for the growth cycle varies; it is minutes for some bacterial viruses and hours for some human viruses.
Viral growth curve. The figure shows that one infectious virus particle (virion) entering a cell at the time of infection results in more than 100 infectious virions 10 hours later, a remarkable increase. Note the eclipse period during which no infectious virus is detectable within the infected cells. In this growth curve, the amount of infecting virus is 1 virion/cell (i.e., 1 infectious unit/cell). (Reproduced with permission from Brooks GF et al. Medical Microbiology. 20th ed. Originally published by Appleton & Lange. Copyright 1995, McGraw-Hill.)
The first event shown in Figure 29–1 is quite striking: the virus disappears, as represented by the solid line dropping to the x axis. Although the virus particle, as such, is no longer present, the viral nucleic acid continues to function and begins to accumulate within the cell, as indicated by the dotted line. The time during which no virus is found inside the cell is known as the eclipse period. The eclipse period ends with the appearance of virus (solid line). The latent period, in contrast, is defined as the time from the onset of infection to the appearance of virus extracellularly. Note that infection begins with one virus particle and ends with several hundred virus particles having been produced; this type of reproduction is unique to viruses.
Alterations of cell morphology accompanied by marked derangement of cell function begin toward the end of the latent period. This cytopathic effect (CPE) culminates in the lysis and death of cells. CPE can be seen in the light microscope and, when observed, is an important initial step in the laboratory diagnosis of viral infection. Not all viruses cause CPE; some can replicate while causing little morphologic or functional change in the cell.
SPECIFIC EVENTS DURING THE GROWTH CYCLE
An overview of the events is described in Table 29–1 and presented in diagrammatic fashion in Figure 29–2. The infecting parental virus particle attaches to the cell membrane and then penetrates the host cell. The viral genome is “uncoated” by ...