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Viral one-step growth

As with bacterial colonies, it is easy to screen for presence of a virus. When overlaid onto a bacterial lawn, a single virion will create a visible clearing in the lawn. These clearings are called viral plaques, and the virion responsible is called a plaque forming unit (pfu). A large percentage of virions produced will not be capable of infection. Therefore, is important to determine particles per pfu. For example, if for every pfu there are 100 ineffective virions, then particles per pfu = 100.

The following experiment shows the growth pattern of a virus. The methodology is as follows:

  • Prepare a bacterial culture in exponential growth.
  • Add 104/mL virions.
  • Incubate, removing 100µL every 2 minutes.
  • Dilute each aliquot 106χ
  • Overlay each aliquot onto a bacterial lawn.
  • Incubate overnight
  • Repeat this experiment, except add chloroform (CH3Cl) to each aliquot before dilution.

The chloroform will dissolve cytoplasmic membranes, lysing the cells, but the virions will remain unharmed. The following graph shows the results. The y-axis represents pfu/mL, and the x-axis represents time. Solid is without chloroform, dotted is with chloroform.


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| ....__.__.__.__.__.__.__
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| . /
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| . /
|______________.___/
| . .
| . .
| . .
| . .
| . .
| ...
|_____________________________________________

Notice there are two differences between pfu/mL with and without chloroform:

  • With chloroform, there is a dip. This is because the viable viruses infect cells and degenerate, causing a temporary decrease in pfu.
  • With chloroform, the graph peaks sooner. While usually it takes a while for the viruses to lyse the host cell, that phase is eliminated because the lipid bilayer is dissolved by chloroform.
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