Viruses are submicroscopic particles that require living cells for their growth. So they cannot be cultivated on non-living media like bacteria. They require living tissues or living cells for their cultivation.
Embryonated chicken eggs
Since viruses can grow only in living cells, one of the most important and convenient methods for cultivation of a wide variety of viruses is chicken embryo technique.
Fertile chicken eggs incubated for 5 to 12 days can be inoculated through the shell aseptically. The opening may be sealed with paraffin wax and the egg incubated at 36 C for the time required for the growth of virus.
Chicken embryos contain several different types of cells in which various viruses will replicate. By using embryos of various ages and different methods of Inoculation it is possible to grow the type of desired virus.
For example, vaccinia virus can be grown on the chorioallantoic membrane and produce lesions or pocks. The yolk sac and embryo can also be used to grow viruses. The chick-embryo technique has been used in the production of vaccines against smallpox, yellow fever, influenza and other diseases and in immunological tests.
Cell culture is the method of interest for cultivation of viruses because of many reasons. Among them are convenience, relative economy of maintenance compared to animals, observable cytopathic effects, and choice of cells for their susceptibility to particular viruses.
On the bases of their origin and characteristics, cell cultures are of three type: primary cell culture, diploma cell strains and continuous cell lines. Primary cell cultures are derived from the normal tissues of an animal (such as mouse, hamster, chicken or monkey tissues) or a human (e.g., gingivitis tissues). When cells from these tissues are processed and cultured, the first monolayer is referred to as a primary culture. A monolayer is a confluent layer of cells covering the surface of the culture. The cells from the subcultures are called secondary cultures. Cell culture prepared from fresh tissue resembles the cells in the whole animal more closely then do the cells in continuous cell lines. Cells derived from animal tissue can be subculture for a number of times only before dying. For some type of cells only a few decisions can be possible. For others, 50 to 100 divisions can occur. Cell culture derived from embryonic tissue are generally capable of a greater number of divisions in vitro then those derived form adult tissue.
Some viruses cannot be cultivated on cell culture or in embryonated chicken eggs and must be propagated in living animals. Mice, guinea pigs, rabbits and primates are used for this purpose. Animal Inoculation is also a good domestic tool because animals can show typical disease symptoms and histology all sections of infected tissues can be examined microscopically.
Microbiology by pelczar