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How do you stop unwanted microbes from ruining a culture, and keep yourself safe while handling them?

Describe aseptic technique and methods of sterilisation, and explain why they are essential in biotechnology

A focused answer to the O-Level outcome on aseptic technique. What contamination is, how to work aseptically, the main sterilisation methods, and why sterility matters.

Generated by Claude Opus 4.88 min answerUpdated 2026-06-06

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking
The answer
Examples in context
Try this

What this dot point is asking

This outcome asks you to describe aseptic technique and sterilisation and to explain why they are essential. The core idea is purity: a culture is only useful if it contains the organism you want and nothing else, and the people handling it must stay safe.

The answer

What contamination is

Contamination is the presence of unwanted microorganisms in a culture, coming from the air, equipment, surfaces or the worker. Contaminants compete with the desired organism, can spoil the product, and make results meaningless because you cannot tell which organism caused an effect.

Aseptic technique

Aseptic technique is the set of careful working practices that keep contaminants out. The key steps are:

Disinfect the work surface before and after work.
Work near a lit Bunsen burner, whose rising hot air carries airborne microbes away from the work.
Flame the necks of bottles when opening and closing them.
Sterilise the inoculating loop by heating it red hot, then cool it briefly before touching the culture.
Open lids as little and as briefly as possible, lifting a plate lid only slightly.
Flame the loop again after use to kill any remaining microbes.
Wash hands and tape and label plates after use.

Methods of sterilisation

Sterilisation kills or removes all microorganisms from equipment and media before use:

Heat in an autoclave. Steam under pressure, typically about $121$ degrees Celsius, kills microbes and their spores. Used for media, glassware and instruments.
Dry heat or flaming. A flame sterilises inoculating loops and bottle necks.
Chemical disinfectants. Used on surfaces and skin.
Filtration. Heat-sensitive liquids are passed through a fine filter that removes microbes.

Why it all matters

Sterility keeps the culture pure so the product and results are reliable, protects workers from disease-causing microbes, and stops cultured organisms escaping into the environment.

Spotting and fixing breaks in aseptic technique

A student transfers bacteria but their plate later grows several different unexpected colonies. They describe their method: they left the agar lid fully off on the bench, used a loop straight from the drawer, and did not work near a flame. Identify the errors and the correct practice.

Step 1: The open lid

Leaving the lid fully off lets airborne microbes settle on the agar. Correct practice is to lift the lid only slightly and replace it quickly.

Step 2: The unsterilised loop

Using a loop straight from the drawer transfers whatever was on it. Correct practice is to flame the loop red hot, then cool it before use.

Step 3: No flame nearby

Working away from a Bunsen burner removes the protective rising air current. Correct practice is to work close to a lit flame.

Step 4: Conclude

All three breaks let contaminants reach the agar, which is why several unexpected colonies grew. Identifying each error and stating the correct aseptic step is exactly what the question rewards.

Examples in context

Example 1. Autoclaving media. Before bacteria are grown, the nutrient agar and glassware are autoclaved with steam under pressure to kill any microbes and their spores. Starting from a sterile base is what makes a pure culture possible.

Example 2. Industrial scale-up. In a pharmaceutical fermenter making insulin, even one contaminating microbe could ruin a whole batch worth a great deal of money. The entire vessel and its contents are sterilised first, showing aseptic principles applied at industrial scale.

Try this

Q1. State two ways a culture could become contaminated. [2 marks]

Cue. Airborne microbes settling on the culture, and microbes carried on an unsterilised loop or unflamed bottle neck (the worker's hands are also acceptable).

Q2. Explain why an inoculating loop is flamed before and after transferring bacteria. [2 marks]

Cue. Before, to sterilise it so it carries no unwanted microbes; after, to kill the bacteria on it so they are not spread.

Q3. Name the method used to sterilise nutrient media and glassware, and state how it works. [2 marks]

Cue. An autoclave, which uses steam under pressure at about $121$ degrees Celsius to kill microorganisms and their spores.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original6 marksDescribe the steps you would take to transfer bacteria from a culture to a fresh agar plate using aseptic technique.

Show worked answer →

Examiners want an ordered procedure with the reason for the key aseptic steps.

First, clean the work surface with disinfectant and work near a lit Bunsen burner, whose upward air current helps keep airborne microbes away. Flame the neck of the culture bottle when it is opened and closed to kill microbes at the opening.

Sterilise the inoculating loop by heating it in the flame until red hot, then let it cool briefly so it does not kill the bacteria. Remove the lid of the culture bottle, take a loopful of culture, and replace the lid quickly. Lift the lid of the agar plate only slightly, like a clam shell, to limit exposure to the air, streak the loop across the agar, and close the lid.

Finally, flame the loop again to kill any remaining bacteria, and tape and label the plate. Throughout, keep lids off for as short a time as possible.

What markers reward: disinfecting the surface, working near a flame, flaming the bottle neck and the loop, cooling the loop before use, opening lids minimally, and flaming the loop afterwards, with the reasons for the main steps.

Original4 marksExplain why aseptic technique and sterilisation are essential when culturing microorganisms in biotechnology.

Show worked answer →

The answer should explain both the scientific and the safety reasons.

Aseptic technique and sterilisation prevent contamination of the culture by unwanted microorganisms from the air, equipment or the worker. Contaminants would compete with the desired organism, spoil the product, and make any results unreliable because you could not be sure which organism caused an effect.

They also protect the worker and others from harmful microorganisms, since some can cause disease, and they stop the cultured organism escaping into the environment.

What markers reward: preventing contamination that would spoil the culture or product and make results unreliable, and protecting people and the environment from harmful or escaped microorganisms.