Ever wondered how doctors know which antibiotic will work best... Show more
Biology137 views·Updated May 18, 2026·3 pagesHow Antibiotics and Antiseptics Affect Bacteria: Practical Guide
Gabriela@gabriela.my.school.journey16
1 / 3
1
of 3
Setting Up Your Bacterial Culture
Getting bacteria to grow properly in the lab requires aseptic techniques – basically keeping everything sterile so you don't accidentally grow the wrong microorganisms. You'll be testing how different antibiotics and antiseptics affect bacterial growth by measuring something called the zone of inhibition.
Your independent variable is the type of antibiotic or antiseptic disc you use, whilst your dependent variable is the size of the zone of inhibition (the clear area where bacteria can't grow). Keep everything else constant – disc size, concentration, temperature, and incubation time.
The process starts with sterilising everything: your petri dish, agar, and work surfaces using heat or disinfectant. You'll use a sterilised inoculating loop (heated in a flame) to spread bacteria evenly across the agar surface, keeping the petri dish lid barely open to prevent contamination.
Top Tip: Always tape the lid with a cross pattern – this lets oxygen in whilst preventing harmful anaerobic bacteria from taking over, and store plates upside down so condensation doesn't drip onto your bacterial colonies.
2
of 3
Safety First in the Lab
Working with bacteria and chemicals means taking proper precautions to protect yourself and get accurate results. The main hazards you'll face include Bunsen burners, strong disinfectants, glass equipment, and potentially contaminated materials.
When using Bunsen burners to sterilise equipment, tie back hair and tuck in loose clothing. Always use the safety flame when not actively heating, and place heatproof mats underneath. Turn off gas supplies and let equipment cool before handling.
Strong disinfectants can irritate skin and eyes, so wear gloves and safety goggles throughout the experiment. Wash hands thoroughly after use. Glass petri dishes can break and cause cuts – check for chips or cracks before use and wear cut-resistant gloves when handling broken glass.
Remember: Dispose of used cotton buds and other contaminated materials properly, and always disinfect work surfaces before and after the experiment to prevent spreading infections.
3
of 3
Measuring Results and Drawing Conclusions
After 48 hours of incubation at 25°C, you'll see clear circular areas around some discs where bacteria couldn't grow – these are your zones of inhibition. The larger the zone, the more effective that antibiotic or antiseptic is at killing or preventing bacterial growth.
Measure the diameter of each zone using a ruler, then calculate the area using πr². If zones are irregularly shaped, take measurements from different positions around the clear zone and calculate a mean for accuracy.
The most effective treatment will have the largest zone of inhibition because it killed or prevented growth of the most bacteria. Your control disc (soaked in sterile water) should show no clear zone, proving that any inhibition is due to the active substance, not the paper disc itself.
Exam Success: Remember that contaminated cultures give invalid results because unwanted microorganisms interfere with bacterial growth – this is why aseptic technique is absolutely crucial for reliable data.
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Biology137 views·Updated May 18, 2026·3 pagesHow Antibiotics and Antiseptics Affect Bacteria: Practical Guide
Gabriela@gabriela.my.school.journey16
Ever wondered how doctors know which antibiotic will work best against a bacterial infection? This practical investigation shows you exactly how to test the effectiveness of antibiotics and antiseptics against bacteria in the lab, using proper scientific techniques to get... Show more
1
of 3Sign up to see the content. It's free!
- Access to all documents
- Improve your grades
- Join milions of students
Setting Up Your Bacterial Culture
Getting bacteria to grow properly in the lab requires aseptic techniques – basically keeping everything sterile so you don't accidentally grow the wrong microorganisms. You'll be testing how different antibiotics and antiseptics affect bacterial growth by measuring something called the zone of inhibition.
Your independent variable is the type of antibiotic or antiseptic disc you use, whilst your dependent variable is the size of the zone of inhibition (the clear area where bacteria can't grow). Keep everything else constant – disc size, concentration, temperature, and incubation time.
The process starts with sterilising everything: your petri dish, agar, and work surfaces using heat or disinfectant. You'll use a sterilised inoculating loop (heated in a flame) to spread bacteria evenly across the agar surface, keeping the petri dish lid barely open to prevent contamination.
Top Tip: Always tape the lid with a cross pattern – this lets oxygen in whilst preventing harmful anaerobic bacteria from taking over, and store plates upside down so condensation doesn't drip onto your bacterial colonies.
2
of 3Sign up to see the content. It's free!
- Access to all documents
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Safety First in the Lab
Working with bacteria and chemicals means taking proper precautions to protect yourself and get accurate results. The main hazards you'll face include Bunsen burners, strong disinfectants, glass equipment, and potentially contaminated materials.
When using Bunsen burners to sterilise equipment, tie back hair and tuck in loose clothing. Always use the safety flame when not actively heating, and place heatproof mats underneath. Turn off gas supplies and let equipment cool before handling.
Strong disinfectants can irritate skin and eyes, so wear gloves and safety goggles throughout the experiment. Wash hands thoroughly after use. Glass petri dishes can break and cause cuts – check for chips or cracks before use and wear cut-resistant gloves when handling broken glass.
Remember: Dispose of used cotton buds and other contaminated materials properly, and always disinfect work surfaces before and after the experiment to prevent spreading infections.
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of 3Sign up to see the content. It's free!
- Access to all documents
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Measuring Results and Drawing Conclusions
After 48 hours of incubation at 25°C, you'll see clear circular areas around some discs where bacteria couldn't grow – these are your zones of inhibition. The larger the zone, the more effective that antibiotic or antiseptic is at killing or preventing bacterial growth.
Measure the diameter of each zone using a ruler, then calculate the area using πr². If zones are irregularly shaped, take measurements from different positions around the clear zone and calculate a mean for accuracy.
The most effective treatment will have the largest zone of inhibition because it killed or prevented growth of the most bacteria. Your control disc (soaked in sterile water) should show no clear zone, proving that any inhibition is due to the active substance, not the paper disc itself.
Exam Success: Remember that contaminated cultures give invalid results because unwanted microorganisms interfere with bacterial growth – this is why aseptic technique is absolutely crucial for reliable data.
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