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.

According to standard microbiology lab manuals, there is a difference between dilution and dilution factor.

**Dilution**

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} **

**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 10 ^{1} **

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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