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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Culture Club 🧫
To study microorganisms — or test whether an antibiotic actually works — you first have to **grow** them safely. This module: what bacteria are grown on, the **aseptic technique** that keeps the experiment clean (and you safe), and how to measure how well an antibiotic kills them.
What bacteria grow on 🍮
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.
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 contamination by unwanted microorganisms. The essentials: • Sterilise the Petri dish and agar before use. • Pass the inoculating loop through a **flame**, then let it cool. • Work near a lit **Bunsen burner**. • Tape the lid on — but don't seal it completely. • Incubate the plate **upside down** at **25 °C** (in school).
Order the steps
An interactive activity.
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
An interactive activity.
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
Why the water disc?
Why is one disc soaked only in sterile water included on the plate?
- As a control — to show any zone is caused by the antibiotic, not the paper disc
- To give the bacteria water to grow
- To clean the agar surface
- As a spare in case a disc fails
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**.