Virus propagation in cell culture and in vitro screening assays are fundamental topics for M.Pharm students focusing on antiviral drug development and virology research. This blog-style MCQ set reinforces practical and theoretical knowledge about virus–host interactions, cell lines used for propagation, assay design (plaque assays, TCID50, reporter assays), and parameters that influence infection (MOI, adsorption, trypsinization). Questions also cover quantitative readouts like PFU and qPCR, biosafety considerations, and assay choices for screening antivirals (cytotoxicity, EC50, selectivity index). The aim is to deepen understanding of experimental design, interpretation of results, and troubleshooting common issues encountered in in vitro virology workflows.
Q1. Which cell line is classically preferred for efficient propagation and plaque assays of influenza A viruses?
- Vero cells
- MDCK cells
- HEK293 cells
- HeLa cells
Correct Answer: MDCK cells
Q2. What does multiplicity of infection (MOI) refer to in virology experiments?
- The number of host cells per mL
- The ratio of infectious virus particles to target cells
- The total number of virus particles in a stock
- The incubation time required for adsorption
Correct Answer: The ratio of infectious virus particles to target cells
Q3. What parameter is directly measured by a plaque assay?
- Viral genome copy number
- Concentration of viral antigen
- Plaque-forming units representing infectious units
- Neutralizing antibody titers
Correct Answer: Plaque-forming units representing infectious units
Q4. What is the principle endpoint of a TCID50 (tissue culture infectious dose 50%) assay?
- Concentration of viral RNA that amplifies by 50%
- Virus dilution that infects 50% of cell cultures producing CPE
- Amount of virus that reduces cell viability by 50%
- Time required for 50% of cells to show fluorescence
Correct Answer: Virus dilution that infects 50% of cell cultures producing CPE
Q5. Which statistical method is commonly used to calculate TCID50 from endpoint dilution data?
- Reed–Muench method
- Kaplan–Meier estimator
- Student’s t-test
- ANOVA
Correct Answer: Reed–Muench method
Q6. Why is trypsin often added to cell culture media when propagating influenza viruses?
- To provide a nutrient source for the virus
- To cleave hemagglutinin precursor (HA0) into HA1 and HA2 enabling fusion
- To inhibit host proteases that degrade viral proteins
- To neutralize antiviral compounds in serum
Correct Answer: To cleave hemagglutinin precursor (HA0) into HA1 and HA2 enabling fusion
Q7. During a controlled adsorption step in many infection protocols, why might the incubation be performed at 4°C?
- To inactivate any contaminating bacteria
- To allow virus binding to receptors while preventing endocytosis/internalization
- To increase viral replication rate
- To enhance proteolytic cleavage of viral proteins
Correct Answer: To allow virus binding to receptors while preventing endocytosis/internalization
Q8. Which of the following best describes common cytopathic effects (CPE) observed during viral infection in cell culture?
- Increased cell proliferation and colony formation
- Cell rounding, detachment, and syncytium formation
- Permanent resistance to viral infection
- Enhanced mitochondrial respiration without morphological change
Correct Answer: Cell rounding, detachment, and syncytium formation
Q9. What is the main readout of a plaque reduction neutralization test (PRNT) in antiviral or immunological studies?
- Number of genome copies by qPCR
- Reduction in plaque numbers indicating neutralizing antibody activity
- Cell metabolic activity using MTT
- Viral protein expression by Western blot
Correct Answer: Reduction in plaque numbers indicating neutralizing antibody activity
Q10. How do luciferase reporter assays facilitate high-throughput antiviral screening?
- They measure viral DNA methylation status
- They provide a quantitative luminescent signal proportional to replication or promoter activity
- They directly count infectious particles under a microscope
- They identify viral proteins by mass spectrometry
Correct Answer: They provide a quantitative luminescent signal proportional to replication or promoter activity
Q11. How is the selectivity index (SI) of an antiviral compound calculated?
- EC50 divided by CC50
- CC50 divided by EC50
- IC90 multiplied by EC50
- CC50 minus EC50
Correct Answer: CC50 divided by EC50
Q12. Which statement correctly contrasts primary cells and continuous (immortalized) cell lines for virus propagation?
- Primary cells are immortal and divide indefinitely; continuous lines have limited lifespan
- Primary cells better preserve in vivo physiology but have limited passages; continuous lines are immortalized and easier to culture
- Primary cells are always more permissive to all viruses than continuous lines
- Continuous lines require no biosafety considerations while primary cells do
Correct Answer: Primary cells better preserve in vivo physiology but have limited passages; continuous lines are immortalized and easier to culture
Q13. What effect does using a high MOI have on the kinetics of viral infection in a culture?
- Leads to asynchronous multi-cycle replication over many days
- Produces a synchronous, single-cycle infection with rapid onset of replication
- Prevents any viral replication due to interference
- Only affects viral entry but not replication speed
Correct Answer: Produces a synchronous, single-cycle infection with rapid onset of replication
Q14. Why must results from qPCR-based viral genome quantification be interpreted cautiously when estimating infectivity?
- qPCR amplifies only host sequences and misses viral genomes
- qPCR detects viral genomes regardless of infectivity and may include non-infectious particles or free RNA
- qPCR always underestimates genome copies compared with plaque assays
- qPCR directly measures only protein expression not genomes
Correct Answer: qPCR detects viral genomes regardless of infectivity and may include non-infectious particles or free RNA
Q15. What is the primary advantage of using pseudotyped viruses in entry assays and neutralization tests?
- They replicate faster than wild-type viruses
- They allow safe study of entry and neutralization under lower biosafety levels using surrogate cores with heterologous envelope proteins
- They produce more robust cytopathic effects for plaque assays
- They do not require any receptors to enter cells
Correct Answer: They allow safe study of entry and neutralization under lower biosafety levels using surrogate cores with heterologous envelope proteins
Q16. Why is an overlay (e.g., agarose or methylcellulose) used in plaque assays?
- To increase oxygen diffusion to cells
- To restrict diffusion of progeny virions so that plaques arise from localized infection foci
- To neutralize antibodies present in the medium
- To enhance cell division during plaque formation
Correct Answer: To restrict diffusion of progeny virions so that plaques arise from localized infection foci
Q17. Which advantage does high-content imaging offer over simple bulk readouts in antiviral screening?
- Only measures total ATP content without spatial information
- Provides single-cell resolution, morphological phenotypes, and multiplexed markers for mode-of-action insights
- Eliminates the need for controls and replicates
- Is always faster than luciferase assays for large libraries
Correct Answer: Provides single-cell resolution, morphological phenotypes, and multiplexed markers for mode-of-action insights
Q18. Which assay is commonly used to assess compound cytotoxicity and determine CC50 during antiviral screening?
- Plaque assay
- MTT or resazurin (cell viability) assays
- TCID50 assay
- Neutralization test
Correct Answer: MTT or resazurin (cell viability) assays
Q19. What does a time-of-addition experiment primarily reveal about an antiviral compound?
- The compound’s cytotoxic concentration
- The stage of the viral life cycle (entry, replication, assembly) that the compound affects
- The compound’s solubility in culture medium
- The best temperature for virus adsorption
Correct Answer: The stage of the viral life cycle (entry, replication, assembly) that the compound affects
Q20. What is the purpose of spinoculation during virus inoculation of cells?
- To separate infected from uninfected cells by density
- To enhance virus–cell contact by centrifugal force, increasing adsorption and infection efficiency
- To remove extracellular virions from the culture
- To inactivate viral particles before assay
Correct Answer: To enhance virus–cell contact by centrifugal force, increasing adsorption and infection efficiency
