Core Practicals Paper 1 Flashcards
What is the aim of the core practical investigating antibiotics and antiseptics?
To investigate the effect of different antibiotics or antiseptics on bacterial growth.
What is the independent variable in this experiment?
The type of antibiotic or antiseptic used.
What is the dependent variable in this experiment?
The size of the inhibition zone (area with no bacterial growth).
What are the control variables in this experiment?
- Type and strain of bacteria
- Volume of antibiotic/antiseptic
- Incubation time
- Incubation temperature
- Size of paper discs
How do you carry out the antibiotic and antiseptic core practical?
- Use aseptic technique to pour agar into Petri dishes.
- Spread a known bacterium (e.g. E. coli) over the agar.
- Place paper discs soaked in different antibiotics/antiseptics onto the agar.
- Incubate at 25°C for 24–48 hours.
- Measure the diameter or area of the inhibition zones.
How do you calculate the area of the inhibition zone?
Use the formula for area of a circle: π × radius².
What is an inhibition zone?
A clear area around an antibiotic or antiseptic disc where bacteria have not grown.
Why is aseptic technique important in this experiment?
To prevent contamination from other microorganisms and to ensure reliable results.
Why is the temperature kept at 25°C in school labs?
To prevent growth of harmful pathogens that thrive at higher temperatures (like 37°C).
What safety precautions should be taken during this practical?
- Wear gloves and goggles
- Use aseptic technique
- Seal Petri dish with tape but do not seal all around
- Dispose of biological waste properly
Why should you not seal the Petri dish completely?
To allow oxygen in and prevent growth of anaerobic pathogens.
What is a 6-mark model answer for setting up the antibiotic/antiseptic investigation?
- Use aseptic technique to pour agar into Petri dish
- Spread known bacteria evenly on agar
- Place sterile paper discs soaked in antibiotics or antiseptics on the agar
- Seal dish and incubate at 25°C for 24–48 hours
- Measure inhibition zones with a ruler or calculate area using πr²
- Compare the size of inhibition zones to assess effectiveness
What is a 5-mark model answer for this practical?
- Spread bacteria on agar plate using aseptic technique
- Place discs with antibiotics/antiseptics on the agar
- Incubate the plate
- Measure clear zones
- Compare results
What is a 4-mark model answer for this practical?
- Spread bacteria on agar
- Add discs soaked in antibiotics/antiseptics
- Incubate plate
- Measure clear zones
What does a larger inhibition zone indicate?
The antibiotic or antiseptic is more effective at killing or inhibiting the bacteria.
How can you improve the reliability of this experiment?
Repeat the experiment and calculate an average inhibition zone size for each treatment.
How can you improve the accuracy of your results?
Use a ruler or digital caliper to measure inhibition zones precisely and ensure consistent disc sizes and volumes.
What could cause anomalous results in this experiment?
Contamination, uneven bacterial spreading, inaccurate disc placement, or variable incubation conditions.
Why is it important to use the same bacterial strain throughout the experiment?
Different bacterial strains may respond differently to antibiotics, affecting the fairness and reliability of the results.
Why is it important to leave the plates incubating for the same amount of time?
To allow a fair comparison between the antibiotics or antiseptics, as longer exposure could increase inhibition zones.
