(g) (i) how to culture microorganisms effectively, using aseptic techniques

The idea

Culturing microorganisms means providing a suitable nutrient medium, the right temperature and oxygen availability so that a population grows predictably, while excluding unwanted contaminants. Aseptic technique is the set of precautions that keeps the culture pure: sterilising apparatus and media before use, working close to a lit Bunsen flame so rising air carries contaminants away, flaming the neck of the vessel, using sterile loops and keeping lids on when not inoculating. Effectiveness also depends on choosing the medium, controlling temperature in an incubator and not opening plates unnecessarily.

What earns the marks

• Sterilise glassware, media and loops before use, for example in an autoclave or by flaming, so that contaminants are killed before inoculation.
• Work near a lit Bunsen flame because the upward convection current reduces the chance of airborne microorganisms landing in the vessel.
• Flame the neck of the culture vessel or bottle when opening and closing it to prevent entry of contaminants.
• Use a sterile inoculating loop and replace or re-sterilise it between transfers so that a pure culture is maintained.
• Keep lids or caps on vessels when not actively inoculating, and minimise the time a plate is open.
• Incubate at a controlled, appropriate temperature so that growth is effective and the culture is not exposed to unnecessary contamination.

Common mistakes and how to put them right

• Thinking aseptic technique means killing all microorganisms in the finished culture.

  Put it right: The aim is to exclude contaminants while allowing the desired microorganism to grow.

• Believing the Bunsen flame sterilises the medium.

  Put it right: The flame creates an upward air current and is used to flame loops and necks, not to treat the agar.

• Assuming any clean-looking bench is sterile.

  Put it right: Surfaces and equipment must be treated with a suitable disinfectant or sterilised, not simply wiped dry.

Exam tips

• Read each option and ask whether it directly reduces contamination or merely sounds hygienic.
• Link each aseptic step to its purpose, such as the convection current from the flame or the killing of contaminants by sterilisation.
• Eliminate options that describe leaving vessels open, reusing unsterilised loops or incubating without temperature control.