This protocol describes how to prepare serial dilutions in order to enumerate phage from a stock solution. Before you begin you should also prepare for the double agar plaque assay onto which fresh dilutions should be plated.
The stock solution has a titer = x PFU/ml (PFU = plaque forming units)
Alternatively = y PFU in z ul
Serial Dilution Procedure
1. Prepare a set of labelled tubes from #1 to #8. Fill each tube with 900ul of HFB (or PBS will suffice and we sometimes add DMSO as a cryoprotectant for immediate freezing).
2. Add 100ul (10-1x) stock phage to 900ul HFB and mix -> tube #1
3. Transfer 100ul (10-2x) #1 to 900ul HFB and mix -> tube #2
4. Transfer 100ul (10-3x) #2 to 900ul HFB and mix -> tube #3
And so on... Do not forget to change tips and mix between transfers.
Tube #1 -> plate 100ul (1/10) -> 10-2x
Tube #2 -> plate 100ul (1/10) -> 10-3x
Tube #3 -> plate 100ul (1/10) -> 10-4x
and so on...
The number of plaques on any given plate can be used to back-calculate the titer of the original stock. You should choose plates with plaque counts between 30 and 300.
E.g. count n plaques on plate from 10-7x plate:
x = n / 10-7
x = n * 10+7
OR, if z = 200ul (e.g., mutagenesis)
y = (n * 10+7) * 200/1000
For a diagrammatic version of these calculations see: