Microorganisms

Microorganisms (microbes) are living things too small to see without a microscope. The main groups are bacteria, viruses and fungi. Some are useful, some cause disease, and we can stop harmful ones from spreading.

Key facts

  • Bacteria are single cells with no nucleus (the microbes in yoghurt, Salmonella). Viruses are much smaller particles that can only reproduce inside living cells (colds, flu, measles). Fungi include yeasts, moulds and mushrooms.
  • A virus is much smaller than a bacterium, and a bacterium is smaller than a yeast cell.
  • Useful microbes: yeast makes bread rise and is used to make beer and wine; bacteria make yoghurt and cheese; the mould Penicillium makes penicillin; decomposers recycle nutrients.
  • Harmful microbes (pathogens) cause disease and make food go off. They spread through the air, water, food, touch and animals such as mosquitoes.
  • Microbes grow fastest where it is warm and damp, with food. A fridge slows them down; cooking kills them.
  • To stop the spread: wash your hands, catch coughs and sneezes in a tissue, cook food thoroughly, keep food cool, drink clean water and be vaccinated.
  • Antibiotics kill bacteria but not viruses.

Worked examples

Example 1

Yeast, sugar and warm water are put in a bottle with a balloon over the top. What happens, and why?

  1. Yeast is a living fungus. In warm water it feeds on the sugar.
  2. As it uses the sugar, it gives off carbon dioxide gas: the bubbles.
  3. The gas fills the balloon, so it blows up. In cold water this is very slow; in boiling water the yeast is killed.

Answer: the balloon blows up, because the yeast feeds on the sugar and gives off carbon dioxide gas

Example 2

Sort these into bacteria, viruses and fungi: yeast, flu, the microbes in yoghurt, athlete’s foot, measles, tuberculosis.

  1. Bacteria: the microbes that make yoghurt, and the one that causes tuberculosis.
  2. Viruses: flu and measles. Antibiotics do not work on them.
  3. Fungi: yeast, and the fungus that causes athlete’s foot.

Answer: bacteria: the microbes in yoghurt, tuberculosis; viruses: flu, measles; fungi: yeast, athlete’s foot

Example 3

A bacterium divides into two every 20 minutes. Starting with one, how many are there after 2 hours, if they all survive?

  1. Work out how many times it divides. 120 ÷ 20 = 6
  2. Each division doubles the number: 1, 2, 4, 8, 16, 32, 64.
  3. So after 6 doublings: 2^6 = 64

Answer: 64

Example 4

Explain how washing your hands and keeping food in a fridge help to stop food poisoning.

  1. Bacteria such as Salmonella can be on hands, raw meat and work surfaces. Washing with soap removes them.
  2. Bacteria grow fastest in warm conditions. In a fridge (about 4°C) they grow very slowly, so they don’t reach harmful numbers.
  3. Cooking food thoroughly kills any bacteria left.

Answer: washing removes microbes before they reach food or your mouth; the cold in a fridge slows the growth of bacteria in food