Culturing and measuring microorganisms — Edexcel International A Level Biology
Aseptic technique, culture media, ways of measuring microbial growth, the bacterial growth curve and core practical 13 on growth in liquid culture.
Revision notes
4 short notes, in the order of the specification. Each one in short:
Aseptic technique is a set of procedures that prevents unwanted microorganisms contaminating a culture and stops the cultured organisms escaping. Equipment and media are sterilised, a flame creates an updraught away from the work, and plates are taped and incubated inverted at a temperature below 30 °C.
The growth of a microbial population can be measured by counting cells directly, by dilution plating to count viable colonies, by finding the mass of cells, or by measuring the turbidity of a liquid culture with a colorimeter. Methods differ in whether they distinguish living from dead cells.
A bacterial growth curve in a closed culture has four phases: lag, when cells adjust and little division occurs; exponential, when the population doubles each generation; stationary, when divisions equal deaths; and death, when deaths exceed divisions. The exponential growth rate constant is generations per unit time.
Core practical 13 investigates growth in a liquid culture by inoculating broth, keeping one variable different, taking samples or readings at regular intervals and plotting cell number or absorbance against time. Only safe, non-pathogenic strains are used, with aseptic technique and proper disposal.
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Quick check questions
Why are Petri dishes incubated upside down?
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So condensation collects in the lid and does not drip on to the agar and spread colonies.
What does a viable count measure that a total cell count does not?
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Only living cells that can divide to form colonies.
Name the four phases of the bacterial growth curve in order.
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Lag, exponential (log), stationary, death (decline).
What is the formula for the number of generations from the start and end numbers of cells?
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n = (log₁₀ Nₜ − log₁₀ N₀) ÷ 0.301.
Why is a colorimeter blank of sterile broth used in core practical 13?
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To set zero absorbance for the broth alone, so readings show only the cloudiness caused by the cells.
Frequently asked questions
What is aseptic technique in microbiology?
Aseptic technique is a set of procedures that keeps unwanted microorganisms out of a culture and stops the cultured microorganisms escaping. It includes sterilising equipment, flaming loops and bottle necks, keeping lids on, taping and inverting plates, and disinfecting benches. It protects both the experiment and the worker.
How does dilution plating measure bacterial growth?
Dilution plating counts living bacteria. A culture is serially diluted, a measured volume is spread on agar and incubated, and the colonies are counted. Each colony grew from one colony-forming unit, so colonies divided by the volume plated and multiplied by the dilution factor gives cfu per cm³ of the original culture.
Why does a bacterial growth curve level off?
The curve levels off in the stationary phase because the rate of cell division equals the rate of cell death. Nutrients run short, toxic waste such as acid builds up and oxygen may fall. Cells still divide, but deaths match divisions, so the number of living cells stays constant.
How do you calculate the exponential growth rate constant?
Work out the number of generations, n = (log₁₀ final number − log₁₀ starting number) ÷ 0.301. Then divide by the time in hours. The result, k, is the number of generations per hour in the exponential phase. The generation time is 1 ÷ k.
Why is turbidity a useful way to measure microbial growth?
Turbidity is quick and does not destroy the sample. A colorimeter records how much light the cloudy culture absorbs or scatters, and absorbance rises as cells multiply. Repeated readings can be taken from one culture. It needs a calibration curve and counts dead cells as well as living ones.
Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026