How fast do germs multiply?
Start with one germ on a plate of food left out in the warm. Choose how often it doubles and how many hours pass, and watch one germ become thousands, then millions, which is why cooked food should not sit out for long.
Core concept
Bacteria multiply by splitting in two; in warm food they can double every 20 minutes, so after n doublings one germ becomes 2ⁿ, which grows faster and faster
Topic: Our body and health
Bacteria are living things made of one cell: The tiny building block that every living thing is made of. All key words, far too small to see. When they have food, water and warmth, each one grows and then splits into two. Those two split into four, then eight, sixteen, and so on. After n splits, one germ has become 2 × 2 × … × 2 = 2ⁿ germs. In warm cooked food some bacteria split every 20 minutes, so in one hour they double three times. Growth like this starts slowly and then explodes: each doubling adds more germs than all the doublings before it. Cold slows the splitting down, and boiling kills most bacteria. That is why we keep food covered and cool, and eat or refrigerate cooked food within about two hours.
What you will learn
- Work out the number of doublings from the time and the doubling time.
- Find the number of germs as a power of 2.
- Explain why food left out in the warm becomes unsafe within hours.
Before you start
- Know powers: 2³ = 2 × 2 × 2.
Materials
- A notebook to make a doubling table
Equipment
- None
Procedure
- Record 4 hours with a doubling every 20 minutes.
- Change the time to 8 hours (overnight) and record.
- Change the doubling time to 60 minutes, as in a cooler room, and record.
How this simulation is simplified
- One germ to start, with plenty of food and nothing killing it; each germ splits in two once per doubling time.
- Real growth slows when the food runs out; these are simple model numbers.