Viral cultivation unlike the cultivation or culture of other microbial forms including bacteria and fungi is quite unique and different from the usual routine cultivation techniques undertaken in the conventional microbiology laboratory (both in the hospital and in an academic institution). One of the major reasons for this is because viruses unlike other microbes exist as a living thing ‘only’ inside a living host cell. Viruses only replicate inside a living host cell including those of humans, plants, and animals, other mammals and microbes as well. They are obligate intracellular parasites that engage in a close relationship with their host; and this allows viruses to take over the cellular and replication machinery of their host cell to their own advantage.
Outside a suitable host cell, viruses are usually inert, and incapable of autonomous replication without a suitable host cell. Virtually all the materials required for viral replication including cellular energy are provided by the viral-infected host cell. Viruses rarely replicate independent of a living host cell. Thus, the culture of viruses requires the culture of living host cells as hosts for the unperturbed growth and replication of the virus being propagated. Viruses cannot grow and replicate in non-living culture media or agar plates as is applicable to bacteria and fungi that can readily be propagated in such growth medium. These aforementioned facts are some of the factors that distinguish viral replication from the propagation or cultivation of prokaryotic and eukaryotic cells in the laboratory. Several reasons exist for the cultivation of viruses in the laboratory.
Some of the major reasons for viral cultivation are highlighted in this section. Viruses are cultivated for the following reasons:
- Viruses are cultivated in order to prepare viruses used for the production of vaccines. Vaccines are used as prophylaxis and preventive measures for some infectious diseases including those caused by pathogenic viruses and bacteria.
- Viruses are cultivated in order to isolate pathogenic viruses from clinical samples as an aid in the diagnosis of viral infections caused by pathogenic viruses.
- Viruses are cultivated in order to identify and classify viruses from clinical samples and other viral-laden samples.
- Viruses are cultivated in order to study their host-parasite relationship. This will help to develop strategies geared towards containing the negative effect of pathogenic viruses in humans, plants and animals.
- Viruses are cultivated in order to study the genetics of viruses inclusive of their structures and replication patterns. This will help in the taxonomy of both old and newer viruses.
- Viruses are cultivated for other research purposes especially in studying the efficacy of novel antiviral drugs.
Viruses can be cultivated in vitro and in vivo – depending on the discretion of the researcher and the type of viral cultivation to be undertaken. The in vivo viral cultivation techniques include: the use of a natural viral host, experimental animals, transgenic animals and embryonated eggs. In vivo viral cultivation techniques are generally undertaken in living host cells which include embryonated eggs, experimental animals and transgenic animals as aforementioned. However, this is not the case for the in vitro viral cultivation technique – which is usually carried out in cell culture or tissue culture plates or flasks that contain living cells of animals that support the growth of the virus(s) being propagated.
Living cells from plants and other microbes can also serve as the supporting cells for the growth or propagation of the viruses being isolated – since viruses can ‘only’ replicate inside a living host cell. Organ culture (performed for viruses that attack specialized organs of the body) and explant culture (which are rarely performed) are two other methods or techniques of cell culture that can be used to cultivate viruses in the laboratory. Generally, viral cultivation can be divided into three (3) main groups including animal inoculation systems, cell or tissue culture systems and primary cell culture system.
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