A level Biology B · Edexcel 9BI0 · Topic 6A
Culturing microorganisms
Key points
- Microorganisms need nutrients, water, a suitable temperature and pH, and (for aerobes) oxygen.
- Aseptic technique: work near a Bunsen flame, flame the inoculating loop until red hot, briefly open lids, and sterilise media and equipment in an autoclave.
- Tape the Petri dish lid on with two small strips (not sealed all round) so anaerobic pathogens are not encouraged, and incubate below 30 °C in schools.
- Serial dilution: dilute the culture step by step (e.g. 1 in 10 each time), spread a known volume on agar, count colonies and multiply back by the dilution factor.
- Count plates with roughly 30–300 colonies; each colony is assumed to come from one cell.
- Turbidity (cloudiness) measured with a colorimeter estimates population size, but cannot tell live cells from dead ones.
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Most microorganisms are too small to see, so we grow them into visible colonies or cloudy cultures. Doing this safely and without contamination needs careful aseptic technique, and counting them needs serial dilution.
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