Which of the following statements regarding a viral plaque assay is incorrect?

Question

BlackTom AI · Accepted Answer

The question asks which statement about a viral plaque assay is incorrect, so I will evaluate each option in turn and explain why it is not correct or is potentially misleading.
Option 1: 'In a plaque assay the amount of virus per well remains constant, but the number of cells varies.' In a standard plaque assay, you inoculate each well with a fixed amount of virus per well and then apply a uniform layer of susceptible cells. The goal is to compare how different dilutions of the virus form plaques under similar cell conditions. The assertion that the number of cells varies while the virus amount per well stays constant contradicts the typical experimental setup, where the cell layer is kept constant across wells to allow accurate comparison of plaque formation. Therefore, this statement is incorrect because it misrepresents how both virus input and cell numbers are controlled in the assay.
Option 2: 'Crystal violet stains all the cells remaining in the well allowing you to count the number of plaques.' This describes the standard readout: after an incubation period, living cells in the monolayer are stained with crystal violet, and plaques appear as clear zones where cells have been destroyed by viral infection. The plaques are counted against the stained, intact cell background. Thus, this statement is correct and aligns with common plaque assay practice, making it a true description rather than an incorrect one.
Option 3: 'To ensure accuracy, wells with between 10-100 plaques should be counted.' In practice, plaque counting protocols typically target a count range of roughly 20-200 plaques per plate (or per well, depending on format) to balance counting accuracy and statistical reliability. A range of 10-100 is narrower on the higher end and excludes plates that exceed 100 plaques, which could still be counted accurately, while the lower bound of 10 is close to the lower limit where counting variance increases. Because the widely accepted guideline emphasizes a higher upper limit (often 200) for accuracy, this statement is misleading and not aligned with standard recommendations. Therefore, this option is incorrect due to not reflecting the conventional counting range.
Option 4: 'You cannot perform a typical plaque assay with a virus that does not cause a CPE.' Classic plaque assays rely on cytopathic effect (CPE) to create clear plaques in a cell monolayer; if a virus does not produce visible CPE under the assay conditions, plaques are not formed and cannot be counted using the conventional readout. However, there are alternative approaches (such as staining infected cells with antibodies or using reporter systems) to visualize infection and quantify, so the prohibition stated in this option is too absolute. Consequently, this statement is incorrect because it overlooks these alternative methods and the broader conceptual adaptations that allow plaque-based quantification for non-CPE viruses.
In summary, options 1, 3, and 4 each contain statements that are not aligned with standard plaque assay practice or common caveats, whereas option 2 accurately describes the usual readout method. The incorrect statements are the ones that mischaracterize the experimental setup or the accepted counting guidelines and limitations of the plaque assay.

Which of the following statements regarding a viral plaque assay is incorrect?Single choice

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