1. Stepwise Dilution Process

Serial dilution involves diluting a sample in a series of steps, where each new dilution is made from the previous one.

Procedure:

1. Label test tubes (e.g., 10⁻¹, 10⁻², 10⁻³, etc.)

2. Add equal volumes of diluent (e.g., 9 mL saline) into each tube

3. Add a measured volume of the sample (e.g., 1 mL) into the first tube

4. Mix thoroughly

5. Transfer the same volume (e.g., 1 mL) from the first tube into the second tube

6. Repeat the process for subsequent tubes

👉 Each step reduces the concentration further.

2. Common Ratios

• 1:10 dilution (10⁻¹):
  1 mL sample + 9 mL diluent

• 1:100 dilution (10⁻²):
  Two consecutive 1:10 dilutions

• 1:1000 dilution (10⁻³):
  Three consecutive 1:10 dilutions

Key Concept:
Each step multiplies the dilution factor:

• 1:10 × 1:10 = 1:100

• 1:10 × 1:10 × 1:10 = 1:1000

3. Applications in the Laboratory

Microbiology:

• Estimating bacterial count (colony-forming units, CFU)

• Preparing samples for culture plating

• Isolating microorganisms at countable levels

Clinical Diagnostics:

• Diluting samples with very high analyte concentration

• Preparing standards for assays

Research & Teaching Labs:

• Studying dose-response relationships

• Preparing calibration curves

Key Tips for Accuracy

• Mix each dilution thoroughly before transferring

• Use clean pipette tips for each step

• Avoid skipping steps to prevent errors

• Label tubes clearly

Summary

Serial dilution is a controlled, step-by-step method used to systematically reduce concentration, making it essential for accurate measurements and microbial analysis.