Microbiological techniques — OCR GCSE Biology
Test yourself on Microbiological techniques with OCR GCSE practice questions.
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Microbiological techniques explained
Quantitative microbiology requires accurate measurement of many parameters.
Read the full explanation
Liquid culture volumes are transferred with sterile graduated micropipettes or syringes (e.g. 0.1 cm³) to give uniform lawns. Clear zones of inhibition are measured with a transparent millimetre ruler through the base of the inverted, unopened petri dish; two perpendicular diameters are averaged and area found using Area = πr². Mass is recorded with a balance, for example when weighing agar powder or a filter-paper disc. Time is timed with a digital clock over 24 to 48 hours, and temperature is checked with a calibrated digital thermometer at 25 °C. Gas volumes may be collected in a gas syringe, and pH is measured with a pH meter or indicator paper to check media or buffer solutions.
Your focus
- Measure liquid culture volumes accurately using sterile pipettes to establish uniform bacterial lawns.
- Measure clear zone diameters safely through the petri dish base and calculate inhibition zone areas (Area = πr²).
- Select appropriate apparatus to measure length, area, mass, time, temperature, volume of liquids and gases, and pH.
Show all 4 objectives
- Monitor and record incubator temperature and incubation duration to ensure reproducible microbiological data.
Microbiological techniques exam tips
Marking Points
- Dispense precise volumes of bacterial culture (e.g. 0.1 cm³) using sterile micropipettes or graduated syringes to produce uniform lawns.
- Measure zone of inhibition diameters through the bottom of the unopened petri dish to the nearest millimetre using a transparent ruler.
- Calculate mean diameter from perpendicular measurements and determine zone area using the equation Area = πr².
- Record mass with a balance, for example when weighing agar or filter-paper discs, and measure pH with a pH meter or indicator paper.
- Monitor and record incubation temperature with a digital thermometer (25 °C) and time the incubation period over 24 to 48 hours.
Examiner Tips
- 💡Always note that measurements of inhibition zones are taken without opening the lid, ensuring safety and containment of cultured microorganisms.
- 💡Show all calculation steps clearly: state diameter, divide by 2 to find radius, then calculate πr² with correct units (mm²).
- 💡Match each measurement to its apparatus: ruler for length, balance for mass, clock for time, thermometer for temperature, syringe for gas volume, pH meter for pH.
Common Mistakes
- Opening the lid of the petri dish after incubation to measure inhibition zones, releasing potentially hazardous cultured bacteria into the room.
- Using the diameter instead of the radius in the area formula Area = πr², overestimating the clear zone area by a factor of four.
- Dispensing unmeasured volumes of bacterial suspension onto agar plates, resulting in uneven lawn thickness and irregular diffusion zones.
- Recording pH with a meter that has not been calibrated, or reading a meniscus at eye level incorrectly when measuring liquid volume.