Virology Lab Quiz

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Virology Lab Quiz: Cell Cultures, CPE Observation, Viral Titers, Plaque Assays, and Molecular Detection

The COVID-19 pandemic made the world more aware of virology than it had been in a generation. Suddenly, terms like PCR, viral load, and vaccine platforms were in everyday conversation. For virologists and laboratory scientists who had worked with these tools for years, what changed was not the science but the urgency. Viral detection, culture, and characterisation methods that had developed over decades suddenly had to scale to meet a global emergency.

This quiz is designed for virology students, diagnostic laboratory scientists, vaccine research technicians, and anyone working in or studying laboratory methods for detecting and characterising viruses. The questions cover cell culture basics for virology, cytopathic effect (CPE) recognition, viral titer determination including plaque assay and TCID50 calculations, and molecular viral detection methods including RT-PCR and next-generation sequencing.

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Core Topics

Cell Culture Basics for Virology

Unlike bacteria, viruses cannot be grown on agar plates. They require living host cells to replicate. In a virology laboratory, this means working with cell lines: populations of cells that have been adapted to grow in culture flasks in a liquid nutrient medium (cell culture medium). The most widely used cell lines in diagnostic and research virology include Vero cells (derived from African green monkey kidney, used for arbovirus, measles, and poliovirus propagation), HeLa cells (derived from a human cervical carcinoma, one of the most widely used cell lines in research history), MDCK cells (Madin-Darby canine kidney, used for influenza propagation), and MRC-5 and WI-38 (human diploid fibroblast lines, used for vaccine production and diagnostic virus isolation).

Cell lines must be maintained in conditions that support their growth: controlled temperature (typically 37 degrees Celsius for human-pathogenic viruses), appropriate CO2 concentration to buffer the growth medium pH, and sterile conditions to prevent bacterial or fungal contamination. Working with viruses in cell culture requires appropriate biosafety containment based on the risk group of the virus being handled.

Cytopathic Effect (CPE)

When a virus infects and replicates within a cell culture, the cells often show visible changes called cytopathic effects (CPE). Observing CPE under light microscopy is the classic method for detecting viral growth in cell culture. Different viruses produce characteristic and often distinct CPE patterns. Cell rounding and detachment from the flask surface is a common, non-specific CPE seen with many viruses. Syncytium formation, where infected cells fuse to form large multinucleated giant cells, is characteristic of paramyxoviruses (including measles, respiratory syncytial virus, and mumps) and herpesviruses. Inclusion bodies, which are abnormal structures visible within infected cells, are characteristic of certain viruses: Negri bodies in neurons infected with rabies virus, and Cowdry type A inclusions in cells infected with herpes simplex virus.

The time to first visible CPE, the pattern of CPE progression, and the cell line affected all contribute to the preliminary identification of the virus. CPE observation is then followed by molecular confirmation or serological testing.

Viral Titer Determination

The titer of a virus preparation describes the concentration of infectious virus particles. There are two primary methods for measuring viral titer. The plaque assay is the most direct and quantitative method. A serial dilution series of the virus preparation is used to infect a cell monolayer, which is then overlaid with semi-solid agar or methylcellulose to restrict viral spread. As the virus lyses infected cells and spreads to adjacent cells, a visible clear area called a plaque forms in the cell monolayer. Each plaque originated from a single infectious virus particle. After a defined incubation period, plaques are counted and the viral titer is calculated as plaque-forming units per millilitre (PFU/mL).

The TCID50 (tissue culture infective dose 50) method is used for viruses that do not produce clear plaques. Serial dilutions of the virus are inoculated into multiple replicate cell culture wells at each dilution. After incubation, the proportion of wells showing CPE at each dilution is recorded. Statistical methods (the Spearman-Karber or Reed-Muench method) calculate the dilution at which 50 per cent of inoculated wells show infection, which is the TCID50 value. Approximately 0.7 PFU is equivalent to 1 TCID50.

Molecular Viral Detection

While cell culture remains important, molecular methods have become the primary approach for clinical viral diagnosis. RT-PCR (reverse transcriptase PCR) is the most sensitive and specific method for detecting RNA viruses including influenza, SARS-CoV-2, HIV, and hepatitis C virus. The method converts viral RNA into cDNA and then amplifies a specific target sequence. In quantitative RT-PCR (RT-qPCR), the amount of starting viral RNA can be calculated, providing viral load information that is clinically important for managing infections like HIV and hepatitis B.

Multiplex PCR panels simultaneously detect multiple respiratory, gastrointestinal, or meningitis pathogens from a single patient sample in a single reaction, providing faster and more comprehensive results than testing for each pathogen separately. Next-generation sequencing (NGS) is increasingly used for complete viral genome sequencing, which enables identification of novel viruses, tracking of variant emergence, and detailed outbreak investigation.

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Viral Detection Methods Compared

Cell culture remains the traditional gold standard for virus isolation, allowing the whole infectious virus to be propagated and further characterised, but it is slow (days to weeks for some viruses), requires specialist facilities, and not all clinically important viruses can be cultured in routine labs. The plaque assay provides absolute quantification in PFU/mL and is still widely used for research-grade virus stock titration and antiviral drug studies. TCID50 is the choice when clear plaques do not form. RT-PCR provides the highest sensitivity and fastest turnaround for clinical diagnosis and is the regulatory standard for many notifiable viral pathogens. Serological tests (ELISA for antigen or antibody detection) provide evidence of immune response rather than direct virus detection and are most useful in the later stages of infection or for population-level seroprevalence studies. Electron microscopy allows direct visualisation of viral particles in clinical samples or cell culture and is particularly valuable for characterising novel or unexpected viruses.

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Frequently Asked Questions

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