Methods for Cultivating Viruses

Unlike bacteria that can thrive on agar plates, viruses present a unique challenge in the laboratory. These microscopic pathogens are obligate intracellular parasites, meaning they can only replicate inside living cells. This fundamental requirement has shaped how scientists approach virus cultivation, leading to three main methods that provide the necessary living environment for viral growth and replication.

Why viruses require living cells for cultivation 🔗

Viruses lack the cellular machinery needed for independent reproduction. They cannot synthesize proteins, generate energy, or replicate their genetic material without hijacking a host cell’s biological systems. This dependence on living cells makes virus cultivation essential for diagnosis, research, and vaccine production. Scientists have developed specific techniques to meet these requirements, each serving different purposes in virology.

Animal inoculation method 🔗

Animal inoculation involves introducing viral samples into laboratory animals to support viral replication. Common laboratory animals include mice, rabbits, hamsters, and guinea pigs, with suckling mice less than 48 hours old being most commonly used. The choice of animal depends on the specific virus being studied and its host range requirements.

How animal inoculation works 🔗

Scientists inoculate animals through various routes including intracerebral, subcutaneous, intraperitoneal, or intranasal injection. After inoculation, researchers observe the animals for disease symptoms or death. The virus is then isolated and purified from infected tissues for further analysis. Infant mice are particularly useful for isolating coxsackievirus and rabies virus, as these viruses replicate effectively in young animal hosts.

Advantages and limitations 🔗

This method allows researchers to study pathogenesis, clinical symptoms, and antibody production in a living system. Animal models remain useful for pathogenesis studies and immunology experiments, particularly when in vitro methods are unavailable. However, animal inoculation has significant drawbacks. It is expensive, time-consuming, and raises ethical concerns about animal welfare. Additionally, not all viruses can be propagated in available animal models, and some may require specific animal species.

Embryonated egg cultivation 🔗

The embryonated egg method, developed in the 1930s, uses fertilized chicken eggs as a cultivation system. Eggs aged 8 to 11 days are commonly used, as they provide multiple sites for viral inoculation while maintaining sterile conditions and various tissue types.

Inoculation sites within the egg 🔗

Different viruses replicate in specific locations within the embryonated egg. The yolk sac is used for cultivating Japanese encephalitis and West Nile virus, while the amniotic cavity is primarily used for influenza virus isolation. The allantoic cavity yields large quantities of virus for vaccine production, particularly for influenza and rabies vaccines. The chorioallantoic membrane is used to identify poxviruses through characteristic pock formation.

Benefits of egg-based cultivation 🔗

Embryonated eggs offer several practical advantages. They are readily available, cost-effective, and require less labor than other methods. The eggs are naturally sterile and free from contaminating bacteria and many latent viruses. Most influenza vaccine manufactured for annual flu vaccination programs is cultured in hens’ eggs. This method remains important for vaccine production despite the availability of cell culture techniques.

Current applications 🔗

While cell cultures have largely replaced embryonated eggs for routine virus isolation, this method continues to be valuable for growing certain avian viruses and for large-scale vaccine production. The egg system produces high viral yields, making it economically viable for pharmaceutical manufacturing.

Tissue culture methods 🔗

Tissue culture has become the most widely used method for virus cultivation in diagnostic virology. This technique involves growing viruses in cultured cells maintained under controlled laboratory conditions. Cell cultures provide a versatile platform for virus isolation, identification, and research.

Primary cell cultures 🔗

Primary cell cultures consist of normal cells freshly obtained from tissues that have been cultivated in vitro for the first time. These cells have the normal diploid chromosomal number and are capable of only limited growth, typically 5 to 10 divisions. Common examples include monkey kidney cell cultures, human embryonic kidney cells, and chick embryo cells.

Primary cultures are prepared by dissociating tissues using proteolytic enzymes like trypsin or collagenase, followed by mechanical shaking. The cells are then washed, counted, and suspended in a growth medium containing essential amino acids, vitamins, glucose, and salts, supplemented with fetal calf serum and antibiotics. These cells attach to glass or plastic surfaces and form a confluent monolayer within a week.

Primary monkey kidney cell cultures are particularly useful for isolating myxoviruses, paramyxoviruses, enteroviruses, and some adenoviruses. However, these cultures cannot be maintained in serial culture for extended periods, limiting their long-term use.

Diploid cell strains 🔗

Diploid cell strains represent a middle ground between primary cultures and continuous cell lines. These cells retain their original diploid chromosome number and can be subcultured up to 50 times before undergoing senescence. They are usually fibroblasts derived from fetal lung tissue.

Diploid cells are used for cultivating viruses from patients and for producing some live virus vaccines. For example, WI-38 human embryonic lung cells are used for rabies virus cultivation, while human fetal diploid cells effectively isolate adenovirus, picornaviruses, herpes simplex virus, cytomegalovirus, and varicella-zoster virus.

To maximize their utility, diploid cells from early passages are stored frozen in liquid nitrogen at negative 196 degrees Celsius with dimethyl sulfoxide to prevent freezing damage. These frozen cells remain viable for decades, providing a consistent supply for research and diagnostic purposes.

Continuous cell lines 🔗

Continuous or immortal cell lines are derived from cancerous tissue and can be subcultured indefinitely without senescing. These cell lines have altered and irregular chromosome numbers, distinguishing them from normal diploid cells. Immortalization may occur spontaneously or can be induced by chemical mutagens, tumorigenic viruses, or oncogenes.

Common continuous cell lines include HeLa cells from cervical carcinoma, Hep-2 cells from laryngeal epithelioma, and Vero cells from monkey kidney tissue. Each cell line has specific susceptibilities to different viruses. For instance, Hep-2 cells excel at recovering respiratory syncytial viruses, adenoviruses, and herpes simplex virus.

These cell lines grow rapidly and can be maintained easily through serial subculture. They can reach much higher cell densities than diploid cells because they have reduced contact inhibition, allowing them to grow in multiple layers.

Detecting viral growth in cell cultures 🔗

Scientists observe several indicators to confirm viral replication in cell cultures. Cytopathic effects are the most common signs, including cell rounding, detachment from surfaces, syncytia formation, and complete cell lysis. Different viruses produce characteristic cytopathic effects that aid in initial identification.

Other detection methods include hemadsorption tests for viruses that express hemagglutinins on infected cell surfaces, immunofluorescence using specific antibodies, and electron microscopy for direct viral visualization. These techniques allow researchers to identify and quantify viruses with high precision.

Choosing the right cultivation method 🔗

The selection of cultivation method depends on several factors including the virus type, purpose of cultivation, available resources, and required viral yield. Animal inoculation is reserved for viruses that cannot grow in other systems or when studying disease pathogenesis. Embryonated eggs remain valuable for influenza virus isolation and vaccine production. Tissue culture offers the most versatility and is preferred for routine diagnostic work and research applications.

Modern virology laboratories typically maintain multiple cell culture types to maximize their ability to isolate different viruses from clinical specimens. The combination of primary cells, diploid cell strains, and continuous cell lines ensures comprehensive coverage of most human viruses encountered in diagnostic settings.

What do you think? How might advances in cell culture technology further reduce the need for animal-based virus cultivation methods? What challenges remain in cultivating viruses that currently cannot be grown in laboratory systems?

References

1. https://pmc.ncbi.nlm.nih.gov/articles/PMC7173560/
2. https://bio.libretexts.org/Bookshelves/Microbiology/Microbiology_(OpenStax)/06%3A_Acellular_Pathogens/6.03%3A_Isolation_Culture_and_Identification_of_Viruses
3. https://microbenotes.com/virus-cultivation-purposes-and-methods/
4. https://pmc.ncbi.nlm.nih.gov/articles/PMC7173454/