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I understand the basic idea of viral plaque assay, but I don't understand how the virus spreads so quickly in the culture.

For example, at 1:30 in an educational video about the subject, you can see various viral plaques all growing from single infection points and then multiplying to cover a circular area as the new virions spread outwards.

However, if you think about it, we know the virus cannot move, so when the new virions bud they can only infect adjacent cells (or bacteria). So the spread is limited to whatever cells are immediately adjacent to the infected cell(s).

So, let's say a virus is 100nm in diameter and replicates every 8 hours. In 4 weeks there would be 30 days or 90 replication cycles. Let's imagine that a virus can "reach" things 5 times its body diameter when it buds, so that is things 5 x 100nm = 500nm away. If we have 90 replication cycles then the total radius of the viral plaque could be at most 500nm * 90 = 45,000nm or 45 micrometers, which is a speck invisible to the naked eye.

However, when we look at the plaques in the video we can see they are 5-7mm in diameter and are very visible to the naked eye. So, how could the virus be traveling this far?

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As Bryan pointed out in the comments, viruses infect cells, where they replicate and then spread from those cells. They do not bud off from existing virions. Cultured human cells might be roughly 200 to 500x larger in diameter (20 - 50µm) than your hypothetical virus. The virus does spread only to adjacent cells, but it does so in all directions, increasing plaque diameter by roughly 2x the cell diameter with each replication cycle.

So, to estimate plaque diameter D from cell diameter d and number of replication cycles r, in a perfecly confluent monolayer, a more accurate equation might be something like: D = d + 2dr

If you plug in your theoretical 90 replication cycles and a cell diameter of 25µm, you'd get 25 + (2 x 25 x 90) = 4,525µm or 4.5mm.

In case it's not already understood, cells in a monolayer will vary in both size and shape, and replication times will vary between different viruses, environmental conditions, and cell types. So, it's not a perfect calculation, more of a back of the envelope to show that plaques can reach visible sizes in a reasonable number of replication cycles.

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