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Aseptic techniques are the steps used to prepare uncontaminated bacterial cultures. Free GCSE AQA Biology notes with key terms and 15 questions answered.
Setting up sterile cultures and bacterial growth is part of Cell structure in GCSE AQA Higher Biology.
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Aseptic techniques are the steps used to prepare uncontaminated bacterial cultures. Free GCSE AQA Biology notes with key terms and 15 questions answered.
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Use this lesson to stay inside Cell structure while connecting the detail back to the wider Cell biology topic.
Read the core explanations from Setting up sterile cultures and bacterial growth before testing yourself from memory.
Imagine trying to test whether a new antibiotic works against a specific bacterium. You set up your experiment, add the antibiotic, and find it doesn't work. But here's the problem - your culture was contaminated with three different types of bacteria, and you don't know which one you were actually testing.
This is why uncontaminated cultures are essential for investigating disinfectants and antibiotics. Without proper sterile technique, your results are meaningless. You need to know exactly which microorganism you're working with.
Trap: Many students think 'sterile' and 'clean' mean the same thing. Clean means visible dirt is removed. Sterile means ALL microorganisms are killed - even the invisible ones.
Before you can grow bacteria safely, everything that touches your culture must be sterile. This means three key components need sterilising: Petri dishes, culture media, and inoculating loops.
Petri dishes and culture media are sterilised using an autoclave - a pressure cooker that reaches 121°C under pressure. This temperature kills all microorganisms, including heat-resistant bacterial spores that normal boiling water can't destroy.
Inoculating loops need different treatment because they're used repeatedly during the procedure. You sterilise them by passing through a Bunsen burner flame until they glow red-hot. This burns off any microorganisms from previous use.
Now you understand why sterilisation matters, let's walk through the complete procedure. This catches many students out in exams because they miss crucial safety steps.
Worked example: Setting Up a Sterile Bacterial Culture
Describe how to prepare an uncontaminated culture using aseptic technique
Answer: Sterile culture ready for investigation of antibiotics or disinfectants
Critical detail: The Petri dish must be stored upside down. If stored right-way up, condensation forms on the lid and drips onto the agar, potentially washing bacteria around and ruining your results.
Once your sterile culture is set up, the bacteria start multiplying rapidly. Understanding this process helps explain why contamination is such a problem - even one unwanted bacterial cell can become millions overnight.
This creates exponential growth: 1 becomes 2, 2 becomes 4, 4 becomes 8, and so on. After just 3 hours (9 divisions), one bacterium becomes 512 bacteria. After 6 hours, over 250,000 bacteria.
Worked example: Calculating Bacterial Population Growth
A culture starts with 100 bacteria. If they divide every 20 minutes, how many bacteria after 2 hours?
Answer: 6,400 bacteria after 2 hours
Bacteria can be grown in two main ways: as colonies on agar gel plates (solid medium) or dispersed in nutrient broth solution (liquid medium). Agar plates let you see individual colonies and count them. Broth cultures grow faster but you can't easily count individual bacteria.
School microbiology has strict safety rules that frequently appear in exam questions. The 25°C temperature limit is crucial - it prevents growth of dangerous pathogens that thrive at body temperature.
Exam depth extension: In setting up sterile cultures and bacterial growth, connect why sterile cultures matter to the sterilisation essentials using clear cause-and-effect statements. When you explain biological processes, include what changes, why it changes, and the consequence for cells, organisms, or populations. If calculations appear, show each step, include units, and end with a sentence that interprets the result in biological context.
In exam questions about aseptic technique, marks are often awarded for specific details. Saying 'sterilise the equipment' gets no marks. Saying 'autoclave Petri dishes and agar at 121°C under pressure' gets the mark.
Common exam mistake: Students often write that we incubate at 25°C 'because it's room temperature'. This gets no marks. The correct reason is safety - preventing pathogen growth.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Aseptic techniques are the steps used to prepare uncontaminated bacterial cultures. Petri dishes and culture media are sterilised in an autoclave at 121°C under pressure, inoculating loops are passed through a Bunsen burner flame until red-hot, and the Petri dish is stored upside down.
Binary fission is the division process by which bacteria multiply, creating exponential growth: 1 becomes 2, 2 becomes 4, 4 becomes 8, and so on. After 9 divisions one bacterium becomes 512 bacteria, which is why even one unwanted bacterial cell can become millions overnight.
An autoclave reaches 121°C under pressure, working like a pressure cooker. This temperature kills all microorganisms, including heat-resistant bacterial spores that normal boiling water cannot destroy, so it is used to sterilise Petri dishes and culture media.
An uncontaminated culture is prepared by autoclaving Petri dishes and agar at 121°C under pressure, flaming the inoculating loop until red-hot and letting it cool, touching it to the bacterial sample and streaking it across the agar, then taping the lid on and storing the dish upside down at a maximum of 25°C.
Sterile technique in microbiology means making sure everything that touches a culture is sterile, so that all microorganisms are killed and only the wanted microbe grows. Petri dishes and culture media are sterilised in an autoclave, and inoculating loops are passed through a Bunsen burner flame until red-hot.
School cultures are incubated at no more than 25°C for safety reasons. The 25°C temperature limit prevents the growth of dangerous pathogens that thrive at body temperature. Saying it is 'because it's room temperature' gets no marks in exams.
An uncontaminated culture is a bacterial culture containing only one type of microorganism, with no unwanted bacteria, fungi or other microbes present. Uncontaminated cultures are essential for investigating disinfectants and antibiotics, because without them the results are meaningless: it must be known exactly which microorganism is being used.
Sterile and clean do not mean the same thing. Clean means visible dirt is removed, while sterile means all microorganisms are killed, even the invisible ones. Microbiology work needs sterile equipment and media, not just clean ones.
The inoculating loop is cooled in air after flaming because a red-hot loop would kill the bacteria on it. Flaming sterilises the loop, and cooling it prevents thermal shock to the bacteria before it is dipped into the sample and streaked across the agar.
The Petri dish lid is secured with adhesive tape to prevent airborne contamination while still allowing some gas exchange. The lid is replaced immediately after the bacteria are streaked across the agar, and the dish is then stored upside down.
Agar plates are a solid medium on which bacteria grow as colonies, while nutrient broth is a liquid medium in which bacteria are dispersed. Agar plates let individual colonies be seen and counted, whereas broth cultures grow faster but individual bacteria cannot easily be counted.
After 3 hours of binary fission, dividing every 20 minutes, one bacterium becomes 512 bacteria after 9 divisions. With this exponential growth, after 6 hours there are over 250,000 bacteria, which is why even one unwanted cell can ruin a culture overnight.
The correct reason for incubating school cultures at a maximum of 25°C is safety: it prevents growth of dangerous pathogens that thrive at body temperature. Saying it is because 25°C is room temperature is a common mistake that gets no marks.
An inoculating loop is sterilised by passing it through a Bunsen burner flame until it glows red-hot. This burns off any microorganisms from previous use. Loops need this treatment, rather than an autoclave, because they are used repeatedly during the procedure.
Petri dishes are stored upside down so that condensation does not form on the lid and drip onto the agar. If stored right-way up, condensation drips onto the agar, potentially washing bacteria around and ruining the results.