The Science Fox · Oct 2023Culturing Microorganisms
Key facts
Aseptic technique — why and how
Why: to prevent contamination of the culture with unwanted microorganisms
Sterilise the inoculating loop by heating in a Bunsen burner flame until red hot — then allow to cool
Sterilise the agar medium (culture medium) by autoclaving (high-pressure steam, 121°C)
Work near a Bunsen burner — rising hot air prevents contamination from above
Seal agar plates with tape — but not completely (to allow gas exchange, prevent anaerobic growth)
Incubate at 25°C maximum in school (37°C would encourage harmful pathogens)
Calculating culture results
Area of colony = π r² (if circular)
Count colonies to compare growth in different conditions
Zone of inhibition = clear area around antibiotic disc where bacteria cannot grow
Larger zone = more effective antibiotic
Mark scheme watch-outs — fill in the table
Watch-out
What to remember
Sterilising method
Boiling vs autoclaving
Incubation temperature
Sealing plates
Exam questions — 3 questions · 8 marks total
⚡ This module includes extended questions (4–6 marks). These are where most marks are lost. Read the hint carefully. Write in linked sentences. The AI will identify exactly which mark scheme points you missed.
1 markAseptic techniqueAQA Jun 2023 P1 Q3.3
Why is the inoculating loop heated in a Bunsen burner flame before use?
A To make the bacteria grow faster
B To sterilise the loop and kill any microorganisms on it
C To make the agar melt
D To increase the temperature of the agar plate
1 markIncubation temperatureAQA 8461 style
Why are school experiments incubated at 25°C rather than 37°C?
A Bacteria grow faster at 25°C
B 25°C is the temperature of a school lab
C To prevent growth of pathogens that could be harmful to humans — 37°C could encourage dangerous microorganisms
D Agar melts at 37°C
6 marksRequired practical methodAQA Jun 2024 P1 style
A student wants to investigate the effect of different antibiotics on the growth of bacteria. Describe a method the student could use. Include aseptic technique in your answer. (6 marks)
Hint: Cover: sterilisation of equipment, spreading bacteria, placing antibiotic discs, sealing and incubating, measuring results. For Level 3 you must include how you would compare the antibiotics.