Running and reading a plaque assay

Count clear plaques to measure how many infectious virus particles are in a sample (titer in PFU/mL).

Builds on (2 levels back)inferred · high confidence

Serial dilution and dilution factor: Titer depends on how much the sample was diluted, so you must read the dilution factor correctly before you can count back to the original.
Rate as a quotient (per mL): PFU/mL is a 'per-volume' number, so you need to divide a count by a volume: the same idea as miles per hour.

Prerequisites are inferred: pending teacher review.

Re-learn the skill with worked practice and clear examples.

Each plaque is a hole of dead cells that one virus started, so plaques count infectious virus. Titer in PFU/mL = plaques divided by (volume plated times dilution factor).

Step 1: Define a plaque

A plaque is a clear circle in a lawn of cells where one infectious virus infected a cell, copied itself, and spread outward, killing the cells. One starting virus makes one plaque, so we report counts as plaque-forming units (PFU).

Step 2: Pick a countable plate

Scientists count a plate with a clear, separate number of plaques (often 20-100). A plate that is too crowded or has zero plaques cannot be counted accurately.

Step 3: Apply the titer formula

Titer (PFU/mL) equals the plaque count divided by the volume plated in mL, then divided again by the dilution factor (the small number like 10^-5). Carry the dilution carefully.

Practice

A student plates 0.1 mL of a 10^-5 viral dilution onto a cell lawn and counts the clear plaques shown in the dish. What is the titer of the original sample in PFU/mL?

Reviewed

Plate	Volume plated	Dilution	Clear plaques counted
A	0.1 mL	10^-5	25

A.2.5 x 10^5 PFU/mL
B.2.5 x 10^6 PFU/mL
C.2.5 x 10^7 PFU/mL
D.2.5 x 10^8 PFU/mL

Show the worked solution ▾

Answer: C. 2.5 x 10^7 PFU/mL

Step 1: Write the formula: Titer = plaques divided by (volume plated x dilution factor).
Step 2: Plug in the numbers: Titer = 25 divided by (0.1 mL x 10^-5). First, 0.1 x 10^-5 = 10^-6.
Step 3: Divide: 25 divided by 10^-6 = 25 x 10^6 = 2.5 x 10^7 PFU/mL.

Why it's right: 25 plaques divided by (0.1 mL times 10^-5) equals 25 divided by 10^-6, which is 2.5 x 10^7 PFU/mL.

Why the others miss:

A: This drops two factors of ten; check that 0.1 x 10^-5 = 10^-6, not 10^-4.
B: This is off by one factor of ten, likely from forgetting the 0.1 mL volume.
D: This adds an extra factor of ten; recheck 0.1 x 10^-5 = 10^-6.

Aligned to HBS 3.3: plaque assay titer (PFU/mL) · reading level ~grade 9

Where you'd see this

A vaccine lab reports virus stock as PFU/mL so every batch can be diluted to the same infectious dose.

Careers that use this

Virologist: Titers virus stocks by plaque assay so experiments use a known, repeatable amount of infectious virus.

Video library

Watch: Running and reading a plaque assay

How to Perform a Plaque Assay

Virology Research Services · ~4 min

Guided notes

Fill these in as you work through the lesson.

Big idea: Each clear plaque in a lawn of cells comes from one infectious virus, so counting plaques and dividing by the volume and dilution gives you the virus titer.

Key terms: write the meaning

Plaque (a clear spot):
PFU (what you count):
Titer (concentration):
Dilution factor (how much weaker you made it):

The rule

Titer (PFU/mL) = number of divided by (volume plated in mL multiplied by the ).

Check yourself

Why does one virus particle make one whole plaque instead of just infecting one cell?
If you forgot to multiply by the dilution factor, would your titer be too high or too low?
Why do scientists count a plate that has 25 plaques instead of one that is too crowded to count?

Work one example

A lab plates 0.1 mL of a 10^-5 dilution and counts 25 plaques. Work out the titer in PFU/mL, showing the volume and the dilution factor in your math.