• Bacteria are prokaryotes (no nucleus, no membrane-bound organelles) and need nutrients, oxygen (if aerobic), and the right temperature/pH to grow.
• Plants can't run away from microbes, so they've evolved chemical weapons — antiseptics and antibiotics — to kill or stop them.
• Humans have exploited these plant chemicals to make real drugs: aspirin from willow bark, quinine from cinchona bark.
• Core Practical 9 tests how good a plant extract is at killing bacteria, using the disc diffusion method — you measure the size of the clear zone (zone of inhibition) around each disc.
• Aseptic technique is essential throughout — it stops your results being ruined by contamination from unwanted microbes.
• Before any new drug reaches the pharmacy shelf, it goes through rigorous testing: computer models → human tissue → animals → three phases of clinical trials on humans, often using placebos and double-blind design.
• Drug testing wasn't always this careful — William Withering's "digitalis soup" in the 1700s shows just how risky trial-and-error medicine used to be.

Bacteria are simple, but picky, organisms

Bacteria are prokaryotes — meaning their cells have no nucleus and no membrane-bound organelles (no mitochondria, no chloroplasts, nothing wrapped in its own little membrane "room"). Everything just floats loose in the cytoplasm. Despite being so structurally simple, bacteria are incredibly good at surviving and reproducing fast — as long as the environment gives them what they need.

Think of setting up conditions for bacterial growth a bit like setting up a cosy Airbnb for a guest who reproduces every 20 minutes if they're happy:

• Nutrients — the "food" that provides raw materials for growth and respiration (this is why we grow bacteria in a nutrient broth before the experiment, or on nutrient agar during it).
• Oxygen — needed by bacteria that respire aerobically (not all bacteria do — some are anaerobic — but this chapter focuses on aerobic ones, which is why we don't need to seal the plate airtight).
• Correct temperature and pH — not too hot, not too cold, not too acidic, not too alkaline. This matters because bacterial metabolism runs on enzymes, and enzymes only work efficiently within a narrow temperature/pH range. Too extreme in either direction and the enzymes denature — the bacteria's whole metabolic machinery grinds to a halt.