School › Microbes › Ages 15–16
Microbial growth and fermentation
Prokaryotes, viruses and fungi compared, binary fission and exponential growth (N = N₀ × 2ⁿ), growth-curve phases and antibiotic resistance. With a yeast fermentation rate investigation.
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Bacteria are prokaryotes: single cells without a membrane-bound nucleus, usually 0.5–5 μm wide. Fungi such as yeasts and moulds are eukaryotes with rigid cell walls and no chlorophyll.[5]
Viruses are acellular. A virus is DNA or RNA (never both) inside a protein coat called a capsid, sometimes with a lipid envelope. Most are about 20–900 nm across, too small for a light microscope, and they can only replicate inside a host cell.[6]
Bacteria usually reproduce by binary fission: one cell divides into two. The generation time is the time for the population to double; E. coli can double in as little as 20 minutes in ideal laboratory conditions.[7]
After n generations, N = N₀ × 2ⁿ. For example, starting with 100 cells, 6 doublings (2 hours at 20 minutes each) give 100 × 2⁶ = 6400 cells.[7]
In a closed culture, growth follows four phases: lag (cells adjust), log (exponential growth), stationary (numbers level off) and death (numbers fall).[7]
Yeast ferments sugars to carbon dioxide and alcohol, which is used in bread and other foods.[5][2]
Antibiotics act on bacteria and are not effective against viruses. Overuse selects for resistant bacteria ('superbugs') such as MRSA.[4][9]
Try it: Rate of yeast fermentation and temperature
You need
- 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.
What to do
- Mix a set mass of yeast with a set volume of sugar solution. Divide equally into three boiling tubes.
- Place each tube in a water bath at a different temperature and let it settle for 5 minutes. Record the actual temperatures.
- Fit the bung and delivery tube and collect the gas over water in an upturned measuring cylinder (or count bubbles per minute).
- Record the volume of gas every 2 minutes for 16 minutes.
- Plot volume against time for each temperature. Calculate the mean rate (cm³/min) from the steepest straight part. Repeat if time allows and estimate the uncertainty from the spread.
What happens, and why?
Yeast cells ferment sugar and release carbon dioxide, so the gas volume measures the rate of fermentation. Microbes grow and work at different rates depending on their environment, so temperature changes the rate. Early readings may show a lag before the rate becomes steady.[5][2][7]
Play: Doubling calculator
Use N = N₀ × 2ⁿ. Work out each answer, then pick the right box.
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Quiz
Other ages: Ages 6–7 · Ages 10–11