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Updated Apr 2, 2026
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3.4 Microbiology Study Guide - WJEC A Level Biology
katie ♡
@katielcnn
1 / 3
Bacterial Classification and Cell Wall Structure
Bacteria come in three main shapes that you'll need to recognise: bacillus , coccus (spherical), and spiral . Think of them as nature's building blocks with different architectural designs.
The cell wall is what gives bacteria their strength and shape, whilst protecting them from bursting when water enters by osmosis. It's like a protective jacket that keeps everything intact.
Gram staining is a brilliant technique that sorts bacteria into two groups based on their cell wall structure. The process involves four steps: crystal violet stains both types purple, iodine locks in the stain, alcohol removes colour from some bacteria, and safranin gives a red counter-stain.
Gram-positive bacteria have thick peptidoglycan walls and stay purple after staining. They're vulnerable to penicillin, which weakens their cell walls, and lysozyme, which breaks down their peptidoglycan. Gram-negative bacteria turn red because they have an extra lipopolysaccharide layer that protects them from penicillin, requiring different antibiotics that target protein synthesis instead.
Quick Tip: Remember "Positive = Purple" to nail gram staining questions in your exams!
Microbial Growth Conditions and Aseptic Techniques
Microorganisms are surprisingly picky about their living conditions. They need nutrients like glucose for energy and nitrogen for making amino acids, plus growth factors such as vitamins. Temperature matters too - most prefer 25-45°C, with disease-causing bacteria loving our body temperature of 37°C.
When it comes to oxygen, bacteria fall into three camps: obligate aerobes must have oxygen, facultative anaerobes prefer oxygen but can survive without it, and obligate anaerobes actually die in oxygen's presence.
Aseptic techniques are your best friend in the lab - they prevent contamination and keep everyone safe. Key practices include sterilising work surfaces, flaming bottle necks, never putting lids on surfaces, and only lifting Petri dish lids slightly. Always incubate student cultures at 25°C, not 37°C, to avoid growing dangerous pathogens.
Sterilisation happens in an autoclave - essentially a high-pressure steam cooker that heats equipment to 121°C for 15 minutes. This kills absolutely everything, making your apparatus completely sterile.
You can measure bacterial populations directly by counting colonies (viable counts) or using a haemocytometer (total counts), or indirectly by measuring how cloudy the culture looks using light absorption.
Lab Safety: Never seal Petri dishes completely - partial sealing prevents anaerobic conditions that could encourage pathogenic growth!
Culture Media and Population Counting Techniques
Different culture media serve specific purposes in microbiology. Defined media contain only known ingredients, whilst undefined media include components you can't fully identify. Selective media only allow certain bacteria to grow, and complete media provide everything needed for healthy growth.
Serial dilution is your go-to method for counting bacterial populations accurately. You create a series of 10-fold dilutions (10⁻¹, 10⁻², 10⁻³, etc.) until you get countable colonies on plates. The sweet spot is usually plates with 30-300 colonies - fewer isn't statistically reliable, and more becomes impossible to count.
A haemocytometer gives more accurate counts than colony counting and uses a special microscope slide to count both living and dead cells. It's brilliant for getting precise total counts.
Turbidimeters measure how cloudy your bacterial culture looks using a colorimeter. The cloudier it is, the more bacteria you have. You compare your reading to a standard graph plotting light absorbance against bacterial numbers. Like haemocytometers, this gives total counts since dead cells also contribute to cloudiness.
The population size equation is straightforward: Number of colonies × Dilution factor ÷ Volume of sample. Master this formula and you'll breeze through practical calculations.
Counting Tip: Always choose plates where you can count distinct, separate colonies - merged colonies or overcrowded plates will ruin your accuracy!
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Thomas R
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This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE Knowunity AI. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE Knowunity AI. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user
3.4 Microbiology Study Guide - WJEC A Level Biology
katie ♡
@katielcnn
Ever wondered how scientists identify bacteria or grow microorganisms in labs? Understanding bacterial classification, growth requirements, and laboratory techniques is crucial for A-level Biology and opens doors to careers in medicine, research, and biotechnology.
Bacterial Classification and Cell Wall Structure
Bacteria come in three main shapes that you'll need to recognise: bacillus , coccus (spherical), and spiral . Think of them as nature's building blocks with different architectural designs.
The cell wall is what gives bacteria their strength and shape, whilst protecting them from bursting when water enters by osmosis. It's like a protective jacket that keeps everything intact.
Gram staining is a brilliant technique that sorts bacteria into two groups based on their cell wall structure. The process involves four steps: crystal violet stains both types purple, iodine locks in the stain, alcohol removes colour from some bacteria, and safranin gives a red counter-stain.
Gram-positive bacteria have thick peptidoglycan walls and stay purple after staining. They're vulnerable to penicillin, which weakens their cell walls, and lysozyme, which breaks down their peptidoglycan. Gram-negative bacteria turn red because they have an extra lipopolysaccharide layer that protects them from penicillin, requiring different antibiotics that target protein synthesis instead.
Quick Tip: Remember "Positive = Purple" to nail gram staining questions in your exams!
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Microbial Growth Conditions and Aseptic Techniques
Microorganisms are surprisingly picky about their living conditions. They need nutrients like glucose for energy and nitrogen for making amino acids, plus growth factors such as vitamins. Temperature matters too - most prefer 25-45°C, with disease-causing bacteria loving our body temperature of 37°C.
When it comes to oxygen, bacteria fall into three camps: obligate aerobes must have oxygen, facultative anaerobes prefer oxygen but can survive without it, and obligate anaerobes actually die in oxygen's presence.
Aseptic techniques are your best friend in the lab - they prevent contamination and keep everyone safe. Key practices include sterilising work surfaces, flaming bottle necks, never putting lids on surfaces, and only lifting Petri dish lids slightly. Always incubate student cultures at 25°C, not 37°C, to avoid growing dangerous pathogens.
Sterilisation happens in an autoclave - essentially a high-pressure steam cooker that heats equipment to 121°C for 15 minutes. This kills absolutely everything, making your apparatus completely sterile.
You can measure bacterial populations directly by counting colonies (viable counts) or using a haemocytometer (total counts), or indirectly by measuring how cloudy the culture looks using light absorption.
Lab Safety: Never seal Petri dishes completely - partial sealing prevents anaerobic conditions that could encourage pathogenic growth!
Sign up to see the contentIt's free!
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Improve your grades
Join milions of students
Culture Media and Population Counting Techniques
Different culture media serve specific purposes in microbiology. Defined media contain only known ingredients, whilst undefined media include components you can't fully identify. Selective media only allow certain bacteria to grow, and complete media provide everything needed for healthy growth.
Serial dilution is your go-to method for counting bacterial populations accurately. You create a series of 10-fold dilutions (10⁻¹, 10⁻², 10⁻³, etc.) until you get countable colonies on plates. The sweet spot is usually plates with 30-300 colonies - fewer isn't statistically reliable, and more becomes impossible to count.
A haemocytometer gives more accurate counts than colony counting and uses a special microscope slide to count both living and dead cells. It's brilliant for getting precise total counts.
Turbidimeters measure how cloudy your bacterial culture looks using a colorimeter. The cloudier it is, the more bacteria you have. You compare your reading to a standard graph plotting light absorbance against bacterial numbers. Like haemocytometers, this gives total counts since dead cells also contribute to cloudiness.
The population size equation is straightforward: Number of colonies × Dilution factor ÷ Volume of sample. Master this formula and you'll breeze through practical calculations.
Counting Tip: Always choose plates where you can count distinct, separate colonies - merged colonies or overcrowded plates will ruin your accuracy!
We thought you’d never ask...
What is the Knowunity AI companion?
Our AI Companion is a student-focused AI tool that offers more than just answers. Built on millions of Knowunity resources, it provides relevant information, personalised study plans, quizzes, and content directly in the chat, adapting to your individual learning journey.
Where can I download the Knowunity app?
You can download the app from Google Play Store and Apple App Store.
Is Knowunity really free of charge?
That's right! Enjoy free access to study content, connect with fellow students, and get instant help – all at your fingertips.
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Most popular content: Bacteria
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Students love us — and so will you.
4.6/5
App Store
4.7/5
Google Play
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE Knowunity AI. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE Knowunity AI. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user