Micro-organisms (the tiniest creatures)

What is a micro-organism?

A micro-organism is a living organism, so small that we can only see it when we use a microscope.

Micro-organism are made of cells, like all living organisms. Most micro-organism are made of only one cell.

There are several groups of micro-organisms.

Bacteria 

Each of these orange cylinders is a bacterium. Each bacterium is made of only one cell.

Bacteria are found everywhere. (Bacteria is a plural word. The singular of bacteria is bacterium.) The bacteria in the photograph live in the soil. Their cells are much smaller than human cells. You could line up one thousands of these bacteria, end to end, between two of the millimetre marks on your ruler.

Microscopic fungi

Many fungi, including mushrooms and toadstools, are large. However, there are also some microscopic ones. Yeast, for example, is a single-celled microscopic fungus.

Single-celled algae and protozoa

If you are able to look at some pond water under a microscope, you will see many tiny living organisms in the water. Some of them are tiny plant-like organisms, called algae. Some of them are one-celled animals, called protozoa.

Micro-organisms and decay

Micro –organisms are everywhere. They live in the air, in the soil, in water, on our skin and inside our bodies. One teaspoon of soil may contain one thousand million bacteria.

Decay

The spots on the apples are colonies of fungi.

                     

Micro-organisms are growing on this apple. They have changed the apple, and have made it go bad.

Apples come from plants. A substance that has been made from living organisms is called organic matter. 

Micro-organisms can break down organic matter. They make it decay. This is a nuisance to us if they make our food decay.

But most of the time, decay by micro-organisms is useful. 

Micro-organisms break down dead bodies and animal waste. They return the nutrients in the organic matter to the soil. Plants can use the nutrients to help them grow.

Slowing down decay

Most micro-organisms grow fastest when they have:

• A warm temperature
• Plenty of water
• Plenty of oxygen.

We can use this information to find ways of slowing down the rate at which food decays.

Micro-organisms and food

making cheese and yogurt

Micro-organisms can feed on the same things that humans eat. Sometimes, we like the changes that micro-organisms make in our food. For example, some special kinds of bacteria change milk into yogurt or cheese.

These bacteria feed on sugar in the milk. They change the sugar into a weak acid, called lactic acid. Lactic acid gives yogurt its sharp taste.

Making bread

                   
We use yeast to make some kinds of bread. They yeast feeds on nutrients in the flour. When the yeast respires, it makes carbon dioxide gas. The gas makes bubbles in the dough, which makes the dough rises.

Micro-organisms and disease

Most micro-organism are harmless. Many of them are useful to us.

However, there are some micro-organisms that can make people ill. If they get inside your body, they reproduce there. They produce harmful substances called toxins. The toxins can damage your cells, and can make you feel ill.

Diseases that are caused by micro-organisms are called infectious diseases. This means that they can be passed from one person’s body move to another person.

Some examples of harmful micro-organisms

One kind of bacterium causes a disease called tuberculosis, or TB for short. TB bacteria grow inside cells in the lungs. Over time, this makes the person very weak and they may die. Drugs called antibiotics can be used to kill bacteria that are causing disease.

Malaria is a disease caused by a protozoan. The protozoa live in the blood. The protozoa are passed from one person to another by mosquitoes. Sometimes the malaria protozoa infect the brain, which is very dangerous.

Influenza (flu) and colds are caused by a virus. A virus is even smaller than a bacterium. Viruses are so small that you can only see them using a special kind of microscope, called an electron microscope.

Viruses do not show any of the characteristics of living things, until they get inside a living cell. Then they force the cell to copy the virus, making many new viruses that burst out of the cell and infect other cells.

Louis Pasteur (first discoverer of micro-organisms)

Louis Pasteur was born in France in 1812. At that time, no-one knew that micro-organisms could cause disease.

Pasteur was asked to investigate a disease that was killing silkworms. Silkworms are caterpillars that make silk. Pasteur did experiments that showed that the disease passed from one silkworm to another through the air, or when the people handled the silkworms.

Pasteur had already discovered that microscopic organisms were involved in making wine from grapes. He thought that perhaps other microscopic organisms were making the silkworms ill.

Pasteur’s work gave ideas to other scientists. By the 1870s, people had begun to use antiseptics to prevent infections. (An antiseptic is a substance used to kill micro-organisms outside the body.)

But it was many years before all biologist and doctors really believed that micro-organisms caused infectious disease.