Summary
Stepwise dilution of a sample by a constant factor to obtain countable concentrations.
In depth
Each step dilutes by a factor (such as 1 in 10); counting colonies on plates gives colony-forming units: CFU/mL = colonies / (dilution × volume plated). Plates with 30 to 300 colonies are counted.
Each tenfold dilution reduces counts tenfold. Plate several replicates. Calculate original concentration by multiplying back.
Key idea: dilution gives countable colonies.
Serial dilution reduces concentration step by step by a constant factor, typically 10-fold: one part sample added to nine parts diluent, mixed and repeated with a fresh pipette tip each time. This gives a series such as 10^-1, 10^-2, 10^-3.
It is used to prepare standards for calibration curves and to bring dense cultures to a countable level. In plate counts, 0.1 mL of a suitable dilution is spread on agar; colonies are counted on plates with 30 to 300 colonies, and the original concentration (colony-forming units per mL) is calculated: CFU/mL = colonies / (volume plated x dilution).
Example: 150 colonies from 0.1 mL of 10^-5 dilution means 1.5 x 10^8 CFU per mL.
Remember: mix thoroughly between steps.
Frequently asked questions
What is Serial Dilution?
Stepwise dilution of a sample by a constant factor to obtain countable concentrations.
Which area of biology covers Serial Dilution, and how advanced is it?
Serial Dilution is one of the 51 topics in Lab Methods & Biostatistics. It is rated Intermediate, so it suits learners who already know the core vocabulary. Related topics in this area include ELISA, Standard Deviation, Reliability and Validity.
Can you give an example of Serial Dilution?
Counting bacteria in milk.
What should I learn before Serial Dilution?
Work through the Beginner topics in Lab Methods & Biostatistics first, such as Scientific Method, Hypothesis, Variables, then return to Serial Dilution. Intermediate topics assume the Beginner vocabulary.
What topics are related to Serial Dilution?
The closest topics to Serial Dilution are ELISA, Standard Deviation, Reliability and Validity, Flow Cytometry. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Serial Dilution?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Serial Dilution. To practise the whole of Lab Methods & Biostatistics, use the Lab Methods & Biostatistics quiz.
How is Serial Dilution different from ELISA?
Serial Dilution: Stepwise dilution of a sample by a constant factor to obtain countable concentrations. ELISA: An antibody-based assay that detects and quantifies proteins or antibodies in samples. Read both side by side and note what each one explains.
How should I study Serial Dilution step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with ELISA and explain the difference aloud. Finally, take the Lab Methods & Biostatistics quiz and come back to this page for any question you miss.