Microorganisms are living things too small to be seen with the naked eye; a microscope is needed to observe them. They are found almost everywhere — in soil, water, air and inside other organisms.
Main groups of microorganisms
- Bacteria — prokaryotic, single-celled; some cause disease, many are harmless or useful (e.g. in yoghurt).
- Fungi — eukaryotic; include yeasts and moulds. Yeast is used in bread and alcohol production.
- Protozoa — eukaryotic, single-celled, animal-like (e.g. Amoeba, Paramecium).
- Algae — eukaryotic, photosynthetic (e.g. Chlorella).
Viruses — living or non-living?
Viruses are much smaller than bacteria and sit on the border between living and non-living things. A virus consists only of genetic material (DNA or RNA) surrounded by a protein coat. Outside a host it is inert, but inside a living host cell it takes over the cell's machinery to make copies of itself, then destroys the cell.
Example
The influenza virus and the virus causing COVID-19 can only reproduce inside a living host cell; they cannot grow on their own in food or water.
Useful and harmful roles
- Useful: decomposition of dead matter, nitrogen fixation, food production (yoghurt, cheese, tapai), medicine (antibiotics) and sewage treatment.
- Harmful: cause diseases such as tuberculosis (bacteria), athlete's foot (fungi), malaria (protozoa) and dengue or measles (viruses); also spoil food.
Because a virus is not a cell and has no metabolism of its own, ordinary antibiotics do not work against viruses — they act only on bacteria. Viral diseases are instead prevented by vaccines, which train the immune system to recognise the virus. Good hygiene, clean water and proper food storage help to control the spread of harmful microorganisms in daily life.
Key idea
A virus = genetic material + protein coat. It reproduces only inside a living host cell, so it is not fully living.
Remember
Not all microorganisms are harmful — many are essential decomposers and are used in food and medicine.