Helpful micro-organisms     Harmful micro_organisms

There is a group of living organisms that are so small we need a powerful microscope to see them.

They are called micro_organisms.

The main kinds of micro-organism are the bacteria, the fungus and a virus

Helpful micro-organisms:

Bacteria and fungi in soil help dead leaves rot.
The rotting leaves add food to the soil for plants to use.

Bacteria are used to make yogurt and cheese.

Yeast is used to make bread rise.

Yeast is also used to make wine and beer.

Harmful micro-organisms:

Bacteria and viruses can cause disease or illness.
They spread very easily from one person to another.

Bacteria in food can cause food poisoning.

Uncovered food can go mouldy.
Mould is a kind of fungus


and now in more detail........................

How micro-organisms can be helpful (friendly micro-organisms!)

Inside the soil there are millions of bacteria and fungi.

These bacteria and fungi feed on dead leaves and dead animals causing them break down (rot) and turn into simple chemicals.

The rotting leaves mix with the soil and provide food for the plants and form a vital part of the food chain.


Fact File

Soil containing lots of rotting leaves is often called loam.
Loam is dark in colour and crumbly. It is rich in food for plants

Bacteria and fungi from the soil are sometimes called decomposers.

Yogurt is made by adding special bacteria to milk.

The milk is kept warm to help the bacteria multiply.

The bacteria feed on sugars in the milk and make acid which helps the milk thicken.


Fact File

In most yogurt the bacteria are still alive when it is eaten.
The bacteria in yogurt are good for your digestion

Bacteria (and sometimes fungi) are also used to make cheese.

When bread is made a micro-organism called yeast is added to the dough to make the bread rise.
The yeast feeds on sugars in the dough and make a gas called called carbon dioxide.

When the bread is cooked the yeast dies but the carbon dioxide gas stays in the bread and forms all the little bubbles that you see.


Fact File

Yeast is also used to make the alcohol in beer and wine. The carbon dioxide bubbles make the beer fizzy.


How micro-organisms can be harmful (nasty micro-organisms!)

Some illnesses, like a sore throat or tummy upset are caused by bacteria.

Other diseases, like chicken-pox, are spread by a tiny micro-organism called a virus.

Viruses are even smaller than a bacteria
They spread easily from one person to another.

Fact File

It is important to cover your mouth when sneezing and also to wash your hands after using the toilet to try and stop the spread of diseases.

The water from some rivers (specially in hot countries) can contain bacteria and should not be drunk until after it has been boiled (which kills the bacteria)

Raw chicken can contain a bacteria called salmonella.
Salmonella causes food poisoning if you eat it


Fact File

It is important to cook chicken well to kill the salmonella.
If you eat chicken that is not cooked properly some of the bacteria may get into your gut and make you very ill.

Do not store raw food with cooked food as the salmonella can spread.

Bread (and other foods) will go mouldy if you leave it open to the air.

The mould is spread by tiny spores.

It is best to keep the bread wrapped to keep the spores away from it.


Fact File

The mould appears as blue or white blotches.
Mould is a kind of fungus


Some bacteria

E.Coli bacteria

These live inside your gut and are normally very useful as they help you digest your food

Streptococci bacteria.

The streptococcus causes a sore throat



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