6.1.2Unit 6: Practical Skills in Biology II
Measuring the Growth of Microorganisms — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 6 parts Lesson 3 of 48 Unit quiz
Section 1 of 6Counting Cells with a Haemocytometer
1
Dilute the culture, often with an equal volume of trypan blue, which stains dead cells so that only living cells are counted.
2
Fill the counting chamber of the haemocytometer, a slide marked with a grid, with the stained culture.
3
Count living cells in the four corner squares under the microscope, using a consistent rule for cells that touch the borders.
4
Calculate the mean number of cells in one corner square.
5
Convert to cells per cm³: mean count × dilution factor × 10⁴, because each corner square holds 0.1 mm³ of liquid.
Section 2 of 6Dilution Plating
Dilution plating counts only cells that are alive and able to divide, unlike a haemocytometer count.
Principle
- Each colony on an agar plate grows from one viable cell, so counting colonies estimates the number of living cells.
Why dilute
- Without dilution, colonies merge into one mass and cannot be counted individually.
Calculation
- Total viable count = number of colonies × dilution factor, divided by the volume of sample spread on the plate.
Replicates
- Several plates at each dilution are counted and the mean is used to improve reliability.
Section 3 of 6Fungi and Optical Methods
Fungi and Optical Methods
Colony diameter
- Fungi form a mycelium, so the diameter or area of growth on agar is measured and compared under different conditions.
Dry mass
- Samples of broth are filtered, the mycelium is dried in an oven, and a greater dry mass indicates more growth.
Turbidity
- A colorimeter measures how much light passes through a cloudy culture, and more cells absorb or scatter more light.
Calibration curve
- Turbidity is plotted against cell counts from known cultures so that unknown samples can be read from the graph.
Section 4 of 6Choosing a Method
Haemocytometer
- Gives a direct count but cannot easily distinguish all dead cells unless a stain is used.
Dilution plating
- Counts viable cells only, but takes time because colonies must grow.
Turbidity
- Quick and repeatable, but dead cells and other particles also make the culture cloudy.
Dry mass or area
- Suitable for fungi that do not exist as separate single cells.
| Method | Measures | Weakness |
|---|---|---|
| Haemocytometer | Cells in a known volume | Dead cells may be counted without stain |
| Dilution plating | Viable cells | Slow, colonies must be countable |
| Colorimetry | Cloudiness | Includes dead cells and debris |
Section 5 of 6Worked Example
Question
- A culture was diluted 1 in 2 with trypan blue.
- The four corner squares of a haemocytometer contained 18, 22, 20 and 24 living cells.
- Each square holds 0.1 mm³.
- Calculate the number of cells per cm³ in the original culture.
- Mean per square = (18 + 22 + 20 + 24) ÷ 4 = 84 ÷ 4 = 21.
- 1 cm³ = 1000 mm³, which is 10 000 times 0.1 mm³, so multiply by 10⁴.
- Cells per cm³ = 21 × 2 × 10⁴ = 420 000.
Answer 4.2 × 10⁵ cells per cm³.
Section 6 of 6Exam Checklist & Tips
6.1.2 – Key Points for the Exam
- Haemocytometer A slide with a grid and a chamber of known volume used to count cells.
- Dilution factor The number of times a sample has been diluted, used to calculate the original concentration.
- Viable Alive and able to reproduce.
- Turbidity How cloudy a liquid is, which increases with the number of cells in suspension.
Common Mistakes
- Forgetting to multiply by the dilution factor, which gives the count in the diluted sample only.
- Counting dead cells and living cells together when a stain is available.
- Using colony counts from a plate on which colonies have merged.
- Stating that a colorimeter counts cells directly, when it measures light transmission or absorbance.
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