Science – Micro-organisms

Sixty years ago this March, one of the world’s most famous scientists, Alexander Fleming, died. Ten years earlier he was credited with discovering penicillin, the first antibiotic, which went on to save millions of lives around the world.

Penicillin is a substance derived from a mould, a type of micro-organism. In the real world we are used to larger organisms killing each other but the same violent battle goes on in the microscopic world of micro-organisms.

Penicillin’s use first came to light after it was discovered that the ‘juice’ that flowed out of the mould, killed a strain of staphylococci – a member of the family of bacteria which often gives humans food poisoning.

Suitable for: Year 5 to Year 6

Learning Focus: 

  • Recognise that there is a world of organisms around us that are often too small to see with the human eye
  • Recognise that some micro-organisms are helpful to humans and others are dangerous


Activity One:

In this activity, the pupils will grow their own micro-organisms. You will need to be aware of health and safety issues including the handling of moulds and so face masks and rubber gloves are essential as well as close attention to hand washing after working with the micro-organisms.

Working in groups, ask the pupils to place a piece of white bread, an apple, milk and a piece of cheese in separate jars and place them out of reach in a dry, warm and ventilated environment. You’ll need to leave them in position for several weeks.

Eventually, mould will grow on each. When the mould is visible, screw lids tightly onto the jars and allow the pupils to observe the mould through the glass. They should record similarities and differences. If you have access to a good microscope connected to a projector you can show the pupils what the mould looks like close up.

Activity Two:

In this activity, the pupils will grow yeast organisms.

In groups, ask the pupils to place a small amount of fresh baker’s yeast in a jar with some warm water and sugar. Leave the jars in a warm place for three hours, observing the changes every fifteen minutes. Ask the pupils to note changes in appearance and smell. Eventually, the action will subside and the mix will return to looking like murky water and the smell will change back to what it was at the start.

Ask the pupils what they thought had happened during the experiment.

What happens is that the yeast organism starts to feed on the sugar in the solution and uses the food to multiply. The waste products of the consumption of sugar are carbon dioxide gas (the froth) and alcohol (the smell). Eventually, the alcohol level rises so much that it poisons the yeast organisms which die.

Activity Three:

Now, using our jars of mould, the pupils will experiment with different ways to kill the mould micro-organisms. Take a photograph of each, then add a tablespoon of the following to one jar of each kind of mould and replace the jars with the lids off, in their original positions:

  • Bleach
  • Medical alcohol
  • Antibacterial surface cleaner


Finally, have one set of moulds where you simply screw the lid on tightly and one set where you place them in the science department fridge (not where food or drink for consumption is stored!)

Observe the moulds weekly over the next three weeks and note any changes.

Which of the liquids or lack of air had the greatest effect on the moulds?

What is the main problem with adding the three liquids to foodstuffs to prevent or kill mould?

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