Question

The dilution used for plating into spread plate was 10^-2

What is the dilution factor to be used in the formula (CFU x Dilution factor)/plating factor

Answer 1

Dilution (process of lowering the concentration of any compound /cells) is a very common practise in experimental biology.

In the present question

The culture has been diluted 100 times (10-2). Hence, the dilution factor will be 100.

So, while calculating the colony forming unit, we will multiply number of colonies with 100 (dilution factor).

If the solution used would have been 10-3 then the multiplying dilution factor would have been 1000 (Dilution factor)

Let us understand it by an example.

Suppose We have been given a master E.coli solution and we are asked to calculate the colony forming units.

Now, we have diluted it 100 times by performing serial dilution (10-2).

Now after plating and incubating the culture If we counted number of colonies to be (assume) =43

Then, to know the CFU in master culture, we need to multiply it with 100 because we have diluted it 100 times 10-2) . Therefore, We will multiply 43 with 100 (dilution factor)= 4300 colony forming unit.

Another factor in this calculation is plating factor (amount of volume taken in pipette at the time of plating from the diluted culture.)

It we have taken only 10 microlitre from the diluted 1000 microliter. Then we again multiply with 100 (or divide with 10-2 because we have not used the whole diluted amount for the plating.

then to put it in the formula

43 (CFU) X 100 (dilution factor)/ plating factor.