How are microorganisms grown safely in the lab, and how is aseptic technique used?
Culturing microorganisms on agar, the use of aseptic technique to avoid contamination, and the related practical.
A focused answer to the WJEC GCSE Science Double Award Unit 4 topic on culturing microorganisms, covering how microorganisms are grown on agar, the use of aseptic technique to avoid contamination, and the practical investigating the effect of antiseptics or antibiotics.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
WJEC Double Award Unit 4 wants you to describe how microorganisms are cultured on agar, explain the use of aseptic technique, and describe the related practical.
Culturing microorganisms
The agar provides the nutrients, water and warmth that microorganisms need to grow. A culture lets scientists study microorganisms and test substances such as antibiotics on them.
Aseptic technique
These steps keep the culture pure and keep the experimenter safe.
Safe storage in schools
In schools, cultures are stored at no more than 25 degrees Celsius. This is below human body temperature, so it reduces the growth of harmful pathogens that grow best at 37 degrees Celsius. The Petri dish lid is taped but not completely sealed, which stops dangerous anaerobic bacteria from growing. These safety rules are commonly tested, so it is important to know that 25 degrees Celsius is the maximum and why.
The antibiotic/antiseptic practical
A common practical tests how well antibiotics or antiseptics kill bacteria. Bacteria are spread evenly over an agar plate, and small discs soaked in different antibiotics (or antiseptics) are placed on the surface. After incubation, a clear zone (zone of inhibition) appears around each disc where the bacteria have been killed. The larger the clear zone, the more effective the substance. Measuring the diameter or area of each clear zone allows the substances to be compared fairly.
Making the test fair
For the antiseptic or antibiotic test to be fair, several variables must be kept the same. The same type and amount of bacteria should be spread evenly over each plate, the discs should be the same size, the plates should be incubated at the same temperature for the same time, and a control disc soaked only in water (or sterile solution) should be included to show that any clear zone is due to the substance, not the disc itself. Keeping these constant means the size of the clear zone is a fair measure of how effective each substance is, which examiners reward.
Why microorganisms are useful
Although this topic is about disease, it is worth remembering that many microorganisms are useful, and culturing them is important in industry. Yeast is used to make bread and alcohol; bacteria are used to make yoghurt and cheese; and microorganisms are grown to produce antibiotics (such as penicillin from a mould) and other medicines. They are cultured in large containers under controlled conditions of temperature, food and oxygen. This shows that culturing microorganisms is not just for studying disease but also for making products we rely on.
Try this
Q1. What jelly is used to grow microorganisms in a Petri dish? [1 mark]
- Cue. Agar.
Q2. What does a larger clear zone around an antibiotic disc show? [1 mark]
- Cue. The antibiotic is more effective at killing the bacteria.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style4 marksDescribe the aseptic technique used when culturing bacteria on an agar plate, and explain why each step is needed.Show worked answer →
A Unit 4 practical question worth 4 marks. Reward: sterilise the equipment and the inoculating loop (pass it through a flame) to kill unwanted microbes (1, +1 for why); pass the neck of the bottle through a flame and lift the lid only slightly to stop airborne microbes getting in (1); tape the lid of the Petri dish (but not seal it fully) and store at no more than 25 degrees Celsius to prevent growth of harmful pathogens (1). Markers credit sterilising, working without contamination and safe storage. A common error is to fully seal the dish or to incubate at body temperature.
WJEC style3 marksAn antibiotic disc is placed on a plate of bacteria. Describe what you would see and how you would measure its effect.Show worked answer →
A Unit 4 practical question. Reward: a clear zone (zone of inhibition) forms around the disc where the bacteria have been killed (1); the larger the clear zone, the more effective the antibiotic (1); measure the diameter (or area) of the clear zone to compare antibiotics (1). Markers credit the clear zone, larger zone meaning more effective, and measuring the zone. A common error is to say the bacteria grow towards the disc.
Related dot points
- Pathogens as disease-causing microorganisms, the types of pathogen, how communicable diseases spread, and how their spread is reduced.
A focused answer to the WJEC GCSE Science Double Award Unit 4 topic on disease, covering pathogens and the types (bacteria, viruses, fungi, protists), how communicable diseases are spread, and how their spread can be reduced.
- The body's defences against pathogens, including physical and chemical barriers and the role of white blood cells.
A focused answer to the WJEC GCSE Science Double Award Unit 4 topic on body defences, covering the physical and chemical barriers that keep pathogens out and how white blood cells defend the body by engulfing pathogens and producing antibodies.
- How vaccination produces immunity, how antibiotics treat bacterial infections, and the problem of antibiotic resistance.
A focused answer to the WJEC GCSE Science Double Award Unit 4 topic on vaccines and antibiotics, covering how vaccination produces immunity, how antibiotics treat bacterial infections but not viruses, and the problem of antibiotic resistance.
- The action of carbohydrase, protease and lipase enzymes, the products of digestion, and the food tests for starch, reducing sugar, protein and fat.
A focused answer to the WJEC GCSE Science Double Award Unit 1 topic on digestive enzymes and food tests, covering carbohydrase, protease and lipase and their products, and the iodine, Benedict's, biuret and emulsion tests.