What are microorganisms?
Microorganisms are living things so small that they can only be seen with a microscope. Although a single one is invisible to the naked eye, they exist almost everywhere on Earth: in soil, water, air, food and even inside our bodies. Scientists usually place them into four main groups — bacteria, viruses, fungi and protozoa. Some are helpful, while others cause disease and food spoilage.
The four main groups
- Bacteria are single-celled organisms with three common shapes: round (cocci), rod (bacilli) and spiral (spirilla). They reproduce very quickly by binary fission, where one cell simply splits into two.
- Viruses are the smallest of all and are not made of cells. A virus can only reproduce inside a living host cell, so outside a host it behaves almost like a non-living particle.
- Fungi include yeasts, moulds and mushrooms. Yeast is single-celled, while moulds grow as fine threads. They feed on dead or living matter.
- Protozoa are single-celled and animal-like. Many can move on their own and live in water or in the body. Plasmodium, which causes malaria, is a protozoan.
Example
The green mould that grows on old bread left in a humid Malaysian kitchen is a fungus. It thrives because the kitchen provides warmth, moisture and food — the same conditions that speed up the growth of most microorganisms.
Conditions for growth
Most microorganisms grow best where there is warmth, moisture and a supply of nutrients. This is why food kept cool and dry lasts longer. Understanding these groups helps us use the useful ones and control the harmful ones.
Remember
A virus is the odd one out: it is not cellular and can only multiply inside a living host, which is why antibiotics do not work against viral infections.