Why do we dilute a sample in Microbiology?

Primarily for 2 reasons

  • To get a concentration of bacterial colony which is easier to count when plated to an agar plate (countable no. of colonies)
  • To a required concentration for a specific test method or procedure.

We use serial dilution to dilute a concentrated sample.

Why do we dilute a sample in Microbiology?
According to standard microbiology lab manuals, there is a difference between dilution and dilution factor.


Dilution is the process of making a solution less concentrated.

Dilution is also called as concentration factor in some microbiology lab manuals.

Correct Equation for dilution is

Dilution (D) =Vi/Vf

Where Vi=Volume of stock transferred

            Vf = Volume of diluent +Stock volume

In the figure test a has dilution = 1 ml /10 ml (9+1) =1/10=0.1 or 10-1

Difference between Dilution and Dilution factor in Microbiology
Dilution Factor

The dilution factor (or dilution ratio) is used to express how much of the original stock solution is present in the total solution, after dilution.

Or dilution factor is the ratio between the final volume and the initial volume of the solution.

Dilution Factor is the reciprocal or inverse of dilution

Dilution Factor (DF) =Vf/Vi

Where Vf = Volume of diluent +Stock volume

            Vi=Volume of stock transferred

In the figure test a has dilution = 10 ml /1 ml =10/1=1 or 101

Dilution factor 1:10 means (1 part or stock culture +9 part of diluent forming total 10 parts)

Dilution factor is used in the calculation of Colony forming units/ml or cfu/ml of original stock solution.

Understand more: How to Calculate cfu/ml of bacterial sample?

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