The Invaders

The Invaders

We often say that 'germs' cause diseases. What do we mean by this?

We mean that microbes invade our bodies and cause damage. The commonest types of microbes are bacteria and viruses.

Other microbes that rarely cause damage include some microscopic parasites and fungi.

Bacteria are fairly small cells. They have a variety of different shapes including:

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1. Spheres - known as 'cocci'.

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2. Rods - known as 'bacilli'.

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3. Spirals - known as 'spirilli'.

No matter what shape they have, or the weird and wonderful names, they have some basic common features.

Here you can see a typical example of a bacterial cell:

bacterial cell

Unlike other cells, bacterial cells have no nucleus. Their genetic material (DNA) is free within the cytoplasm. They replicate themselves by dividing into two.

A cell wall surrounds bacterial cells but is not made of cellulose like plant cell walls. This cell wall gives protection to the bacterial cell membrane and shape to the cell.

Some bacteria have a small tail called a flagellum which is thrashed about to propel them. Others have multiple smaller versions of this called cilia.

Harmful bacteria make us ill by either damaging our cells or producing poisonous toxins.

But we are full of nice, friendly bacteria too which help to keep us healthy, for example in our digestive system.

Viruses are strange!

They are not cells, they are much tinier and cannot replicate themselves. Are they even really alive?

Like bacteria, viruses come in all sorts of different shapes.

Viruses can be round with pointy bits sticking out or have wonderful geometrical shapes. Others look like something from space - many are quite beautiful.

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No matter what shape they are viruses share some common features. They have no nucleus. Instead they have a surrounding protein coat that gives them their unique shape. Inside this is a string of DNA.

This diagram shows the common features of a virus:

common features of a virus:

The only way viruses can make us ill is to get themselves into our cells.

They have various ways to do this, the animation below shows how this happens:

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So viruses can get into our cells in a variety of ways including landing on our cells and injecting their DNA into them. Others break down the cell membranes then sneak inside. Once inside our cells they hijack them and make millions of copies of themselves. Each of these can go off and invade other cells.

No wonder they can spread so quickly!

Now for a quick test.

Which diseases are caused by bacteria and which by viruses, click on the appropriate box for each disease:

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