5.17B Explain the aseptic techniques used in culturing microorganisms in…

The idea

Aseptic technique keeps cultures pure and protects the worker from contamination. Growth medium and petri dishes are sterilised in an autoclave, which uses steam under pressure to kill microorganisms, so only the intended species grows. A sterile inoculating loop is used to transfer microorganisms; it is sterilised, for example by flaming, before and after transfer so it carries only the intended organism. Petri dishes and culture vials are kept covered to prevent airborne microbes entering and to stop the cultured organisms escaping. Together these steps ensure a valid, uncontaminated culture and reduce infection risk to the technician.

What earns the marks

• An autoclave sterilises growth medium and petri dishes using steam under pressure, killing contaminating microorganisms.
• A sterile inoculating loop is used to transfer microorganisms, preventing unwanted microbes being introduced.
• Petri dishes and culture vials are kept covered to stop airborne contamination entering and to prevent cultured organisms escaping.
• Aseptic technique keeps the culture pure and reduces the risk of infection to the worker.

Common mistakes and how to put them right

• Thinking the autoclave only cleans equipment.

  Put it right: It sterilises by killing all microorganisms using steam under pressure.

• Using a non-sterile loop to transfer organisms.

  Put it right: The loop must be sterile so only the intended microorganism is transferred.

• Leaving petri dishes uncovered during work.

  Put it right: Lids are kept on to prevent contamination and release of organisms.

Exam tips

• Name the equipment and state its purpose, for example autoclave for sterilising medium and dishes.
• Explain why each step matters, linking to preventing contamination and protecting the worker.
• Use the term sterile precisely to mean free of unwanted microorganisms.