Teacher sheet · Ages 15–16 · about 45–50 minutes
Microbial growth and fermentation
Lesson page: echoesof.science/school/microbes/ages-15-16/ · Pupil worksheet: echoesof.science/school/microbes/ages-15-16/print/
Learning aims
- Compare prokaryotes, eukaryotic fungi and viruses.
- Use N = N₀ × 2ⁿ and explain the phases of a growth curve.
- Measure a rate of fermentation and evaluate the method.
- Explain why antibiotics do not treat viral infections and how resistance arises.
Lesson plan
| Time | Activity |
|---|---|
| 5 min | Starter: one bacterium doubles every 20 minutes. How many after 3 hours? (512) |
| 10 min | Read the text; sketch a growth curve with the four phases. |
| 25 min | Fermentation rate practical. |
| 5 min | Calculations and evaluation. |
| 5 min | Quiz or doubling game. |
Materials for the experiment
- Dried baker's yeast and sugar (sucrose) solution
- Boiling tubes with bungs and delivery tubes
- Water baths or beakers of water at about 20, 30 and 40 °C (from warm tap water)
- Thermometers
- Measuring cylinders or upturned graduated tubes in a trough of water
- Stopwatch
Safety: Wear eye protection. Use warm tap water baths no hotter than about 40 °C (no boiling water near pupils). Check delivery tubes are not blocked so pressure cannot build up. Do not taste anything. Use only baker's yeast: do not culture bacteria or moulds from the environment. Wash hands afterwards.
Common misconceptions
| Pupils may think… | Actually… |
|---|---|
| Bacteria keep doubling for ever. | In a closed culture food runs out and waste builds up, giving stationary and death phases. |
| Viruses are tiny bacteria. | Viruses are acellular and replicate only inside host cells. |
| Bacteria become resistant because the patient's body gets used to antibiotics. | It is the bacteria that become resistant; overuse selects for resistant strains. |
Worksheet answers
- Bacterium: prokaryotic cell, no membrane-bound nucleus; yeast: eukaryotic cell with a nucleus and a rigid cell wall; virus: not a cell, DNA or RNA in a protein capsid, sometimes an envelope.
- 2 hours = 6 generations; N = 500 × 2⁶ = 32 000 cells.
- Example: rate rose from about 0.3 cm³/min at 20 °C to about 1.5 cm³/min at 40 °C; any answer that matches their own data and quotes cm³/min.
- Gas dissolving in water, temperature drift, reading the meniscus, unequal mixing of yeast. Improvements: repeat and average, use a gas syringe, stir before dividing, keep tubes in the bath throughout.
Quiz answers
- A bacterium is a prokaryote. This means it… has no membrane-bound nucleus. Prokaryotes are cells without a membrane-bound nucleus.[5]
- Which statement about viruses is correct? They can only replicate inside a host cell. Viruses are obligate intracellular parasites.[6]
- 100 bacteria double every 20 minutes. How many after 1 hour? 800. 3 generations: 100 × 2³ = 800.[7]
- In which phase of the growth curve do numbers rise exponentially? Log phase. Cells divide actively in the log phase.[7]
- Which gas is collected in the yeast experiment? Carbon dioxide. Yeast fermentation releases carbon dioxide.[5]
- Why does overusing antibiotics cause a problem? It selects for resistant bacteria. Resistant 'superbugs' become more common.[9]
Going further
Plot log₂(N) against time for an ideal culture. Why does the log phase become a straight line, and how do you read the generation time from its gradient?
Sources used in this lesson
- [2] KS2: Useful Microbes (e-Bug, UK Health Security Agency): reliability High
- [4] What Counts as a Microbe? (American Society for Microbiology): reliability High
- [5] Microbiology: Types of microorganisms (Encyclopaedia Britannica): reliability Medium-high
- [6] Microbiology, 6.1 Viruses (OpenStax, Rice University): reliability High
- [7] Microbiology, 9.1 How Microbes Grow (OpenStax, Rice University): reliability High
- [9] Antibiotic and antimicrobial resistance (NHS (UK)): reliability High
Full list with links and a note on each source's reliability: echoesof.science/school/microbes/sources/
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