They should be able to explain why: • Petri dishes and culture media must…

The idea

Culturing microorganisms requires aseptic technique to avoid contamination. Petri dishes and culture media are sterilised before use to kill unwanted microorganisms that would otherwise grow and compete with, or contaminate, the intended culture. Inoculating loops are sterilised by passing them through a flame, which kills microorganisms on the loop so only the intended species is transferred. The lid is secured with adhesive tape to prevent airborne contamination and to stop the culture escaping, and the dish is stored upside down so condensation runs onto the lid rather than onto the culture, preventing colonies from merging. In school laboratories, incubation is generally at 25°C because this reduces the risk of growing harmful human pathogens while still allowing microbial growth.

What earns the marks

• Explain that sterilising Petri dishes and culture media kills unwanted microorganisms, preventing contamination and competition with the intended culture.
• Explain that passing an inoculating loop through a flame sterilises it, so only the intended microorganism is transferred and false results are avoided.
• Explain that securing the lid with adhesive tape prevents airborne microorganisms entering and reduces the risk of the culture escaping.
• Explain that storing the dish upside down allows condensation to collect on the lid, keeping the agar surface clear so colonies remain separate and identifiable.
• Explain that incubation at 25°C in school laboratories reduces the likelihood of growing microorganisms harmful to humans, while still permitting growth for study.
• Recognise that these measures together form aseptic technique, which keeps cultures pure and protects the worker and the environment.

Common mistakes and how to put them right

• Thinking the lid should be completely sealed.

  Put it right: Taping all around prevents oxygen entry and can encourage anaerobic growth; the lid is secured but not fully airtight.

• Believing 25°C is chosen because microorganisms cannot grow at higher temperatures.

  Put it right: Many grow well above 25°C, but higher temperatures increase the risk of culturing human pathogens.

• Assuming sterilising the loop is only about cleaning it.

  Put it right: The purpose is to kill microorganisms so the transfer is pure and results are valid.

Exam tips

• Link each precaution to a clear consequence, such as contamination, competition, condensation or pathogen risk, rather than just naming the step.
• Use the term aseptic technique when explaining the set of precautions as a whole.
• When explaining the 25°C rule, refer to the reduced risk of harmful microorganisms rather than claiming growth is impossible at other temperatures.