Serial Dilution
Plan a dilution series step by step — or work back from colonies on a plate to the number of bacteria in the original sample (CFU/mL).
What is a Serial Dilution?
A bacterial culture can hold a billion cells per millilitre — far too many to count. A serial dilution reduces the concentration in equal steps: transfer a small volume into fresh medium, mix, and repeat. Six 1 : 10 steps dilute a million times.
Spreading a known volume of a diluted tube on an agar plate lets each living cell grow into a visible colony. Counting colonies and multiplying back by the dilution gives the colony-forming units (CFU) in the original sample.
Live Diagram
6 tubesCommon Step Sizes
for 10 mL per tube| Step | Transfer | Diluent | log reduction |
|---|---|---|---|
| 1 : 2 | 5 mL | 5 mL | 0.30 |
| 1 : 5 | 2 mL | 8 mL | 0.70 |
| 1 : 10 | 1 mL | 9 mL | 1 |
| 1 : 100 | 0.1 mL | 9.9 mL | 2 |
Worked Techniques
Dilution factors multiply. Several small steps are more accurate than one huge step, because tiny volumes are hard to pipette precisely.
Keep the same volume in every tube so each step has the same dilution factor.
Each colony grew from one viable cell (or clump), so the count per millilitre plated, times the dilution, gives the original concentration.
Sources & Verification
- M. T. Madigan et al., Brock Biology of Microorganisms, 15th ed., Pearson, 2018 — Ch. 5 (Microbial Growth and Its Control).
- S. S. Zumdahl & S. A. Zumdahl, Chemistry, 10th ed., Cengage, 2018 — Ch. 4 (Types of Chemical Reactions and Solution Stoichiometry).