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

This core practical is part of Pearson's practical-learning programme. The method below is one defensible implementation, not a confidential instruction sheet. Centres may adapt equipment while preserving the scientific purpose, controls, measurements and safety requirements.

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 safe teaching culture, sterile nutrient broth, flasks, colorimeter or spectrophotometer, sterile pipettes, incubator and disinfectant. Select every measuring instrument by range and resolution, not by 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: incubation time or one controlled growth factor.
• Dependent variable: optical density or another validated population proxy.