Isolation and identification of viruses from clinical specimens or materials for research purposes can be accomplished by a number of different methods. No single method or technique is satisfactory for all viruses or kind of specimen.
The first step in laboratory identification of a virus is the proper cultivation and care of specimen until susceptible animals, tissue culture, embryonated eggs or other appropriate media is inoculated. Sometime it includes making the specimen bacteria free by filtration, differential centrifugation, or treatment with bactericidal agents.
Cultivation Of Animal Viruses
Viruses can grow only on the living cells. Outside living cells they show the properties of non living entity. So to obtain a pure culture of viruses it is important to use some type of media which is having living cells or laboratories animals may also be used as the living media to cultivation of viruses.
Embryonated Chicken Eggs
Embryonated chicken eggs is one of the most economical and convenient methods for cultivating a wide variety of animal viruses.
Discovery of embryonated chicken egg technique was done in 1931, this technique worked as a mile stone in the field of virology.
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 growth of the virus.
Because chick embryo contain several different type of cells in which various viruses may grow. By using embryos of various ages and different methods of inoculation it is possible to grow different type of viruses on chick embryo.
The cells to be inoculated are found in various embryonic membrane and tissues of the egg.
The yolk sac and the embryo can also be used for cultivation of viruses.
The chick embryo technique has been used in the production of vaccine against small pox, yellow fever, influenza and other diseases and in immunological tests and other studies whenever a large amount of viruses are required.
Because of convenience, relative economy of maintenance compared to animals, observable cytopathic effects, and choice of cells for their susceptibility to particular virus. Cell culture is a technique of choice of many scientists for propagation of virus.
On the bases of their origin and characteristics, cell cultures are of three types: primary cell culture, diploid cell culture, diploid cell strains, and continuous cell lines. Primary cell cultures are derived from normal tissue of animals or a human tissue. When cells from these tissues are processed and cultured, the first monolayer is referred to as primary cell culture. Cells from subculture are called secondary cultures.
Cell culture prepared from fresh tissue resemble more closely to the cells in the whole animal then do the cells in continuous cell lines. Cells derived from this manner can be subculture for a limited number of times before dying. For some type of cells only few division can be possible. For others, 50 to 100 division occur.
Some viruses cannot be cultivated in cell culture and embryonated chicken eggs and must be propagated in living animals. Mice, guinea pigs, rabbits, and primates are used for this purpose.
Reference: Microbiology Pelczar