Edexcel A Level (IAL) Biology -6.2 Measuring the Growth of Microorganisms- Study Notes- New Syllabus

Methods for Measuring the Growth of Microorganisms

🌿 Introduction

Microorganisms multiply quickly, so we need reliable techniques to measure their growth. Different methods measure different things: total cells, living cells, biomass or turbidity. These techniques are widely used in research labs, food microbiology, medicine and industry.

1. Cell Counts (Direct Microscopic Counting)

What this measures

• Total number of cells, including both living and dead.

How it is done

• A sample is placed on a haemocytometer, a special slide with a ruled grid.
• Cells within selected squares are counted using a microscope.
• Calculations convert these counts into cells per unit volume.

Advantages

• Quick results.
• No incubation needed.
• Useful when immediate estimates are required.

Limitations

• Cannot distinguish live from dead cells.
• Difficult if cells clump together.
• Accuracy depends on even distribution of cells on the slide.

2. Dilution Plating (Viable Count Method)

What this measures

• Number of living cells that are capable of forming colonies.

How it is done

• The sample is diluted in steps (e.g., 1 in 10 serial dilutions).
• A known volume from each dilution is spread on nutrient agar.
• After incubation, colonies are counted.
• Each colony arises from a single viable cell.
• Colony count × dilution factor gives the number of living cells in the original culture.

Advantages

• Measures only viable cells.
• Very accurate for assessing living population size.

Limitations

• Time-consuming because of incubation.
• Some microbes do not grow well on agar.
• Requires clean, aseptic handling.

3. Mass Measurement (Biomass Estimation)

What this measures

• Total mass of microbial material, either wet or dry.

Wet Mass

• Cells are collected and weighed without removing water.
• Quick but less accurate due to variable water content.

Dry Mass

• Cells are filtered and dried completely in an oven.
• The remaining solid mass is weighed.
• More accurate but takes longer.

Advantages

• Suitable for organisms that grow as clumps or filaments (e.g., fungi).
• Good for large-scale cultures.

Limitations

• Destructive method.
• Wet mass is unreliable.
• Dry mass requires extra equipment and time.

4. Optical / Turbidity Method

What this measures

• Cloudiness (turbidity) of a culture, which increases as cell numbers increase.

How it is done

• A colorimeter or spectrophotometer passes light through the culture tube.
• Microbial cells scatter the light.
• Higher turbidity means more cells.
• Turbidity values can be converted to estimated cell numbers using a calibration graph.

Advantages

• Very fast and non-destructive.
• Allows continuous monitoring of growth over time.
• Easy to perform.

Limitations

• Measures both living and dead cells.
• Needs a calibration curve for accuracy.
• Not reliable with organisms that form clumps or biofilms.

Summary Table

Quick Recap
Cell counts → microscope → total cells.
Dilution plating → colonies → living cells only.
Mass measurement → weigh biomass → best for fungi.
Turbidity → light scattering → quick but includes dead cells.
Choose the method based on speed, accuracy, and organism type.