Question

How do we dilute a bacterial culture 50-fold, 100-fold, and 200-fold?

Answer 1

Serial dilution is basically a series of dilutions done sequentially. To dilute a bacterial culture, the serial dilution method is most commonly used. This method is performed to convert a dense solution into a more usable concentration.

Complete step by step answer:
- It is found that the dilution method is used to estimate the concentration of an unknown sample.
- Basically, a serial dilution involves the process of taking a sample diluting it by a series of volumes of sterile diluent, that can be saline or distilled water.
- The formula for serial dilution is shown below

$$\begin{aligned} DF=\dfrac{V_{f}}{V_{i}} \end{aligned}$$

Where ${V_{i}}$ is the aliquot volume
${V_{f}}$ is the final volume = aliquot volume + diluent volume
DF = 50
To obtain a dilution factor of 50, we will carry out a single dilution.
Add 0.2mL of sample to 9.8mL of diluent
${V_{f}}$ = 0.2mL + 9.8mL = 10.0mL

$$\begin{aligned} DF=\dfrac{V_{f}}{V_{i}}=\dfrac{10.0}{0.2} =50 \end{aligned}$$

DF = 100
To obtain a dilution factor of 100, we will carry out single dilution.
Add 0.1mL of sample to 9.9mL of diluent
${V_{f}}$ = 0.1mL + 9.9mL = 10.0mL

$$\begin{aligned} DF=\dfrac{V_{f}}{V_{i}}=\dfrac{10.0}{0.1}=100 \end{aligned}$$

DF = 200
To obtain a dilution factor of 200, we will carry out a serial dilution.

$$\begin{aligned} DF=DF_{1}\times DF_{2}\times n \end{aligned}$$

Add 0.1mL of sample to 9.9mL of diluent.
DF_{1}= 100
Add 5mL of solution 1 to 5ml of saline.
${V_{i}}$ = 5mL
${V_{f}}$ = 5mL + 5mL = 10mL

$$\begin{aligned} DF_{2}=\dfrac{V_{f}}{V_{i}} =\dfrac{10}{5} =2 \end{aligned}$$ $$\begin{aligned} DF=DF_{1}\times DF_{2}=100\times 2 = 200 \end{aligned}$$

Note: It is found that serial dilution is mainly performed to avoid having to pipette very small volumes to make a dilution of a solution. Furthermore, 10-fold dilutions are commonly used to prepare the diluted analytes in the laboratory.