Pearson International A Level Biology Core Practical 13: Investigate microorganism growth in liquid culture

This guide practises interpreting a teaching culture's optical time course. It is not a complete microbiological culture protocol or a claim to reproduce Pearson's prescribed procedure. Use current course documents, institution-approved organisms and procedures, trained supervision and an activity-specific risk assessment.

Aim and scientific principle

Track microbial population growth in liquid culture using aseptic handling and an indirect optical or viable-count measure.

Begin by translating the title into a testable question. Identify what is deliberately changed, what is measured directly, what is calculated and what must remain controlled. Predict a relationship only after naming the mechanism. A prediction without a reason is weak; a reason without measurable variables cannot guide a practical design.

Apparatus and setup

Use institution-approved teaching cultures and materials, suitable optical equipment and the containment, handling and decontamination arrangements specified by the approved laboratory procedure. A teaching label does not establish absence of risk. Consider measuring range, resolution, uncertainty, calibration and suitability for the sample, not the instrument name alone. Draw a labelled scientific setup that shows the working geometry and exactly where readings are taken. Include a control or blank when it distinguishes the target effect from drift, contamination or background response.

Before collecting results, check zero readings, calibration, leaks, alignment or sterility as relevant. Record the instrument resolution beside the first raw-data table. If a digital device displays more digits than its underlying accuracy supports, do not treat every digit as equally certain.

Method

1. Prepare identical sterile cultures and include an uninoculated blank.
2. Inoculate aseptically, mix consistently and incubate in the approved conditions.
3. At fixed intervals, measure optical density against the blank without opening cultures unnecessarily.
4. Plot response against time and identify lag, exponential and stationary regions cautiously.

Plan the analysis before starting. This reveals whether an additional measurement is needed for a derived quantity and prevents a completed experiment from lacking a mass, length, concentration, background or calibration value. A method should be detailed enough for another trained student to reproduce, while avoiding decorative steps that do not affect validity.

Variables and controls

• Independent variable: