Viruses are obligate intracellular parasites so they depend on host for their survival. They cannot be grown in non-living culture media or on agar plates alone, they must require living cells to support their replication.
The primary purpose of virus cultivation is:
- To isolate and identify viruses in clinical samples.
- To do research on viral structure, replication, genetics and effects on host cell.
- To prepare viruses for vaccine production.
Cultivation of viruses can be discussed under following headings:
- Animal Inoculation
- Inoculation into embryonated egg
- Cell Culture
1. Animal Inoculation
- Viruses which are not cultivated in embryonated egg and tissue culture are cultivated in laboratory animals such as mice, guinea pig, hamster, rabbits and primates are used.
- The selected animals should be healthy and free from any communicable diseases.
- Suckling mice(less than 48 hours old) are most commonly used.
- Suckling mice are susceptible to togavirus and coxsackie virues, which are inoculated by intracerebral and intranasal route.
- Viruses can also be inoculated by intraperitoneal and subcutaneous route.
- After inoculation, virus multiply in host and develops disease. The animals are observed for symptoms of disease and death.
- Then the virus is isolated and purified from the tissue of these animals.
- Live inoculation was first used on human volunteers for the study of yellow fever virus.
Advantages of Animal Inoculation
- Diagnosis, Pathogenesis and clinical symptoms are determined.
- Production of antibodies can be identified.
- Primary isolation of certain viruses.
- Mice provide a reliable model for studying viral replication.
- Used for the study of immune responses, epidemiology and oncogenesis.
Disadvantages of Animal Inoculation
- Expensive and difficulties in maintenance of animals.
- Difficulty in choosing of animals for particular virus
- Some human viruses cannot be grown in animals,or can be grown but do not cause disease.
- Mice do not provide models for vaccine development.
- It will lead to generation of escape mutants
- Issues related to animal welfare systems.
2. Inoculation into embryonated egg
- Good pasture in 1931 first used the embryonated hen’s egg for the cultivation of virus.
- The process of cultivation of viruses in embryonated eggs depends on the type of egg which is used.
- Viruses are inoculated into chick embryo of 7-12 days old.
- For inoculation, eggs are first prepared for cultivation, the shell surface is first disinfected with iodine and penetrated with a small sterile drill.
- After inoculation, the opening is sealed with gelatin or paraffin and incubated at 36°c for 2-3 days.
- After incubation, the egg is broken and virus is isolated from tissue of egg.
- Viral growth and multiplication in the egg embryo is indicated by the death of the embryo, by embryo cell damage, or by the formation of typical pocks or lesions on the egg membranes
- Viruses can be cultivated in various parts of egg like chorioallantoic membrane, allantoic cavity, amniotic sac and yolk sac.
Chorioallantoic Membrane (CAM):
- Inoculation is mainly for growing poxvirus.
- After incubation and incubation, visible lesions called pocks are observed, which is grey white area in transparent CAM.
- Herpes simplex virus is also grown.
- Single virus gives single pocks
- This method is suitable for plaque studies.
- Inoculation is mainly done for production of vaccine of influenza virus, yellow fever, rabies.
- Most of avian viruses can be isolated using this method.
- Inoculation is mainly done for primary isolation of influenza virus and the mumps virus.
- Growth and replication of virus in egg embryo can be detected by haemagglutination assay.
Yolk sac inoculation:
- It is also a simplest method for growth and multiplication of virus.
- It is inoculated for cultivation of some viruses and some bacteria (Chlamydia, Rickettsiae)
- Immune interference mechanism can be detected in most of avian viruses.
Advantages of Inoculation into embryonated egg
- Widely used method for the isolation of virus and growth.
- Ideal substrate for the viral growth and replication.
- Isolation and cultivation of many avian and few mammalian viruses.
- Cost effective and maintenance is much easier.
- Less labor is needed.
- The embryonated eggs are readily available.
- Sterile and wide range of tissues and fluids
- They are free from contaminating bacteria and many latent viruses.
- Specific and non specific factors of defense are not involved in embryonated eggs.
- Widely used method to grow virus for some vaccine production.
Disadvantages of Inoculation into embryonated egg
- The site of inoculation for varies with different virus. That is, each virus have different sites for their growth and replication.
3. Cell Culture (Tissue Culture)
There are three types of tissue culture; organ culture, explant culture and cell culture.
Organ cultures are mainly done for highly specialized parasites of certain organs e.g. tracheal ring culture is done for isolation of coronavirus.
Explant culture is rarely done.
Cell culture is mostly used for identification and cultivation of viruses.
- Cell culture is the process by which cells are grown under controlled conditions.
- Cells are grown in vitro on glass or a treated plastic surface in a suitable growth medium.
- At first growth medium, usually balanced salt solution containing 13 amino acids, sugar, proteins, salts, calf serum, buffer, antibiotics and phenol red are taken and the host tissue or cell is inoculated.
- On incubation the cell divide and spread out on the glass surface to form a confluent monolayer.
Types of cell culture
1. Primary cell culture:
- These are normal cells derived from animal or human cells.
- They are able to grow only for limited time and cannot be maintained in serial culture.
- They are used for the primary isolation of viruses and production of vaccine.
- Examples: Monkey kidney cell culture, Human amnion cell culture
2. Diploid cell culture (Semi-continuous cell lines):
- They are diploid and contain the same number of chromosomes as the parent cells.
- They can be sub-cultured up to 50 times by serial transfer following senescence and the cell strain is lost.
- They are used for the isolation of some fastidious viruses and production of viral vaccines.
- Examples: Human embryonic lung strain, Rhesus embryo cell strain
3. Heteroploid cultures (Continuous cell lines):
- They are derived from cancer cells.
- They can be serially cultured indefinitely so named as continuous cell lines
- They can be maintained either by serial subculture or by storing in deep freeze at -70°c.
- Due to derivation from cancer cells they are not useful for vaccine production.
- Examples: HeLa (Human Carcinoma of cervix cell line), HEP-2 (Humman Epithelioma of larynx cell line), Vero (Vervet monkey) kidney cell lines, BHK-21 (Baby Hamster Kidney cell line).
Susceptible Cell Lines
- Herpes Simplex Vero Hep-2, human diploid (HEK and HEL),human amnion
- VZV human diploid (HEL, HEK)
- CMV human diploid fibroblasts
- Adenovirus Hep2, HEK,
- Poliovirus MK, BGM, LLC-MK2, human diploid, Vero, Hep-2,Rhadomyosarcoma
- Coxsackie B MK, BGM, LLC-MK2, vero, hep-2
- Echo MK, BGM, LLC-MK2, human diploid, Rd
- Influenza A MK, LLC-MK2, MDCK
- Influenza B MK, LLC-MK2, MDCK
- Parainfluenza MK, LLC-MK2
- Mumps MK, LLC-MK2, HEK, Vero
- RSV Hep-2, Vero
- Rhinovirus human diploid (HEK, HEL)
- Measles MK, HEK
- Rubella Vero, RK13
Advantages of cell culture
- Relative ease, broad spectrum, cheaper and sensitivity
Disadvantage of cell culture
- The process requires trained technicians with experience in working on a full time basis.
- State health laboratories and hospital laboratories do not isolate and identify viruses in clinical work.
- Tissue or serum for analysis is sent to central laboratories to identify virus.
Cultivation of plant viruses and bacteriophages
Cultivation of plant viruses
There are some methods of Cultivation of plant viruses such as plant tissue cultures, cultures of separated cells, or cultures of protoplasts, etc. viruses can be grown in whole plants.
Leaves are mechanically inoculated by rubbing with a mixture of viruses and an abrasive. When the cell wall is broken by the abrasive, the viruses directly contact the plasma membrane and infect the exposed host cells. A localized necrotic lesion often develops due to the rapid death of cells in the infected area. Some plant viruses can be transmitted only if a diseased part is grafted onto a healthy plant.
Cultivation of bacteriophages
Bacteriophages are cultivated in either broth or agar cultures of young, actively growing bacterial cells.