Culture Club
Grow bacteria the safe way: culture media, aseptic technique, and measuring how well an antibiotic works from its zone of inhibition.
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Free interactive practice on the steps that lose marks under exam pressure.
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Culture Club
To study microorganisms, or to test whether an antibiotic actually works, you first have to **grow** them safely. Microorganisms are grown on a **culture medium** that supplies everything they need. Usually this is **nutrient agar**: a jelly poured into a Petri dish to set as an **agar plate**. The medium provides a **carbohydrate** as an energy source, plus **mineral ions** and sometimes proteins and vitamins. Ahead: the **aseptic technique** that keeps the experiment clean and you safe, and how to measure how well an antibiotic kills them.
Why the carbohydrate?
A nutrient agar contains carbohydrate. What is it for?
- An energy source for the growing microorganisms
- To make the agar set solid
- To kill unwanted bacteria
- To colour the colonies
Keep it clean: aseptic technique
If stray microorganisms from the air, your skin or the equipment get in, they **contaminate** the culture and ruin the result. **Aseptic technique** prevents that. Every step has a reason, and the reason is what the marks are for:
Order the steps
Put these steps of preparing an agar plate into the correct order.
- Sterilise the Petri dish and nutrient agar
- Pass the inoculating loop through a flame
- Let the loop cool, then spread the bacteria on the agar
- Secure the lid with tape (not fully sealed)
- Incubate the plate upside down at 25 °C
Why each step?
- Flame the inoculating loop
- Work near a lit Bunsen burner
- Tape the lid, but don't seal it
- Incubate at 25 °C, not 37 °C
- Kills microorganisms on the loop
- The rising hot air keeps airborne microbes off the agar
- Lets air in so harmful anaerobic microbes don't grow
- Avoids growing pathogens that thrive at body temperature
Say the purpose
The whole point of aseptic technique is to prevent _____ of the culture by unwanted _____.
Testing antibiotics (RP2)
To test antibiotics or antiseptics, spread bacteria over an agar plate, then place paper discs soaked in different substances on top. Where a substance stops the bacteria growing, a **clear ring** appears around the disc. A **zone of inhibition**. A disc soaked in **sterile water** is used as a **control**, to show any effect is due to the substance, not the disc itself.
Measure the effect
A zone of inhibition has a diameter of 20 mm. Its cross-sectional area = π r². Taking π = 3.14, what is the area in mm²? (remember r is half the diameter)
Which antibiotic wins?
Antibiotic A leaves a large, completely clear zone; antibiotic B leaves a slightly smaller zone that is cloudy with some growth. Which is more effective, and why?
- A. Its zone is both larger AND clearer
- A, but only because the ring looks bigger
- B. A cloudy zone shows it is still working
- They are equally effective
Run the plate
You are setting up RP2 to compare two antibiotics. Make three decisions.
- You have just passed the inoculating loop through the flame. What do you do next?
- Alongside the two antibiotic discs you add a third, soaked only in sterile water. Why?
- After incubation, antibiotic A has a clear zone of 12 mm across and antibiotic B one of 20 mm. What do you conclude?
In the exam
Cultured and measured. Grade-9 habits for culturing microorganisms: • **Aseptic technique** always needs the **WHY**: it prevents contamination by unwanted microorganisms. Not just the steps. • School plates are incubated at **25 °C** and taped-but-not-sealed for safety (no body-temp pathogens, no anaerobes). • Zone of inhibition: **area = π r²** from the diameter across the clear ring; "more effective" means **larger AND clearer**.