virions. The infection can become systemic if it gets into the plant’s vascular system. The cycle starts over when the virus infects a new plant host. There are certain features of the viral growth curve. It starts with an inoculation phase, prior to the penetration of virus particles. During the eclipse phase, viruses have penetrated the cells and there are no free virions. Then there is a burst phase, when virus particles are released. The burst size becomes the number of virions released after the burst phase. The viral titer is the concentration of viral particles. The culture declines if there are no bacteria to penetrate anymore.
ISOLATION AND IDENTIFICATION OF VIRUSES Because viruses require a host, it can be difficult to culture them without a host cell. They are allowed to infect the host and are harvested after separating them from host cells after they’ve been infected and after the virus particles have been allowed to replicate. A filter is used to allow viruses but not bacteria to filter into the filtrate. Viruses can be grown within a living organism, in which it is called in vivo. If they are grown in vitro, it is done inside a test tube or on an agar plate. Bacteriophages are cultured in a culture or plate of bacteria, called a bacterial lawn. There will be a clear zone or plaque where the bacteria have died off on the agar plate. Animals will grow viruses in vivo. It can be done to identify certain pathogenic viruses, to help produce vaccines, and in research settings. An embryo from a chicken is often used for these types of settings. Remember that viruses have tissue tropism so they must be grown in the presence of specific tissues. Tissue cultures are used to grow certain viruses. A commonly used immortal cell line tissue culture is the HeLa cell line, which was isolated in the middle of the twentieth century from a woman who had cervical cancer. This cell line provides consistency in growing human cells in a culture medium. These are used to grow certain animal viruses. Samples of viruses can be prepared in an infected cell line, embryo, or whole host. There will be certain cytopathic effects from the infection, which involve changes in the
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