Types of Micro-organisms

Section 1 of 6What is a micro-organism?

Too small to see

• Too small to see Micro-organisms are living organisms that are too small to be seen with the naked eye, so a microscope is needed to observe them.

Mostly harmless

• Mostly harmless The great majority of micro-organisms do no harm and many are useful.
• They take part in nutrient recycling, decomposition and partnerships with other organisms, which keep ecosystems working.

A balance

• A balance Public health depends on the balance between helpful and harmful micro-organisms.
• Hygiene, sanitation and disease control measures aim to reduce harmful ones while leaving the helpful ones in place.

Section 2 of 6Beneficial micro-organisms

Useful micro-organisms

Food fermentation

• Food fermentation Yeast and bacteria are used to make bread, yoghurt, cheese and alcoholic drinks.

Genetic engineering

• Genetic engineering Bacteria can be given human genes so that they make useful products such as insulin.

Antibiotic production

• Antibiotic production Some fungi and bacteria naturally make chemicals that kill other bacteria, and these are harvested as medicines.

In nature

• In nature Decomposers break down dead material and return nutrients to the soil, where plants can use them again.

Section 3 of 6Pathogens

Definition

• Definition A pathogen is a micro-organism that causes disease.
• Pathogens can infect plants, animals and humans, and some cause serious illness.

Bacteria

• Bacteria Single-celled organisms with no nucleus.
  • They can reproduce quickly inside the body and many release toxins that make us ill.

Viruses

• Viruses Much smaller than bacteria.
  • They can only reproduce by invading living cells and taking over the cell's machinery.

Fungi

• Fungi Some fungi infect the skin or other tissues of animals and plants.

Protists

• Protists Single-celled organisms with a nucleus.
  • Some are pathogens that are carried from person to person by other animals.

Section 4 of 6Comparing the four groups

Group	Key feature	Type of effect
Bacteria	Single-celled, no nucleus	Reproduce rapidly and release toxins
Viruses	Tiny, reproduce only inside host cells	Damage the cells they invade
Fungi	Cells with a nucleus; many are multicellular	Can infect skin and other tissues
Protists	Single-celled with a nucleus	Can cause disease when they infect a host

Section 5 of 6Key terms

Term	Meaning
Micro-organism	A living organism too small to be seen without a microscope.
Pathogen	A micro-organism that causes disease.
Beneficial micro-organism	A micro-organism that has a helpful role in nature, industry or medicine.
Fermentation	Use of micro-organisms such as yeast to produce foods and drinks.
Decomposer	A micro-organism that breaks down dead material and recycles nutrients.
Hygiene	Practices such as hand washing that reduce the spread of pathogens.

Section 6 of 6Exam technique

1

Saying that all micro-organisms are harmful. Most are harmless or useful.

2

Using the words 'germ' or 'bug' in an answer instead of the correct term pathogen, or bacteria, virus, fungus or protist.

3

Describing a virus as a living cell. Viruses are not cells and can only reproduce inside host cells.

4

Listing only bacteria as pathogens; viruses, fungi and protists cause disease as well.

5

Forgetting that micro-organisms can infect plants as well as animals.