BacteriaBacteria The commonest living things are bacteria. They are too small to see without a microscope. Bacteria are all around us in their billions. They float in air and live on icy mountaintops, in the scalding water of hot springs, and on the bottom of the sea. Vast numbers of bacteria live in the human body, especially on the skin and in the intestines. Most are made harmless by the immune system. In fact, we could not survive without bacteria, as we could not digest most foods or produce vitamins. However, a few kinds of bacteria can cause infectious diseases, such as tetanus and cholera.

Salmonella bacteria (red) invading human cells Salmonella bacteria (red) invading human cells
Rod-shaped bacteria, seen through a microscopeRod-shaped bacteria, seen through a microscope

Kinds of bacteria

There are around 10,000 known kinds of bacteria, but probably many more yet to be identified. Most measure about one to five microns (0.001 to 0.005 millimetres) across. They vary in form but there are three main shapes. These are: spheres or balls known as cocci, cylinders or rods, called bacilli, and corkscrew-like spirilli.

Inside a bacterium

E. coli bacteriumE. coli bacteriumA typical bacterium has a tough outer skin, or cell membrane, which contains jelly-like cytoplasm. Tiny blobs, known as ribosomes, float in the jelly and make various substances for the bacterium’s life processes. Also floating in the cytoplasm is a long, coiled-up chemical called DNA, which, if unravelled, would be more than 1000 times longer than the bacterium itself. This is the bacterium’s genes, a “manual” containing every structural detail of the organism.

A bacterium with long, whip-like “tail”, or flagellumA bacterium with long, whip-like “tail”, or flagellumMost bacteria reproduce simply by splitting in two. Some bacteria get their energy from light, like plants. Others absorb nutrients through their cell membranes. Some kinds have a long, whip-like “tail” or flagellum to help them move. This they can do extremely quickly: a bacterium can swim at about 50–60 body lengths per second. 

Bacteria are important decomposers in the soil.Bacteria are important decomposers in the soil.

Bacteria in the soil

Many kinds live in the soil and play a vital role in nature because they cause the decay or rotting of dead plants and animals, allowing their nutrients to be recycled. These nutrients are taken up by plants, which provide food for animals. Soil-living bacteria, for example, carry out the vital task called nitrogen fixation. Nitrobacter, rhizobium and others convert nitrogen, a vital element which plants cannot absorb in its gaseous form, into nitrates, which plants are able to easily absorb.

Consultant: Chris Jarvis

A quarter of a million bacteria can fit on to a single pinhead.

There are typically 1 billion bacteria in     1 g (0.04 oz) of soil and perhaps about a million in 1 ml (0.04 fl oz) of fresh water.

All the bacteria on Earth together weigh about the same as all the plants and animals—including humans.

About 90% of cells in and on the human body are bacteria.

The word bacteria comes from the Greek word meaning "cane". The first bacteria to be discovered were rod-shaped.

The ancestors of modern bacteria were the first life-forms to appear on Earth.

Some bacteria are bioluminescent. They can make the surface waters of the ocean glow at night.

Bacteria were first observed by Dutch scientist Antonie van Leeuwenhoek (1632–1723) in 1676. He used the single-lens microscope he had invented to view what he called "animalcules".

One of the largest types of bacteria is found in the sturgeon fish. Known as Epulopiscium fishelsoni, this "giant" is over 0.5 mm (0.02 inches) long and, unlike nearly all other bacteria, is visible to the naked eye.

Bacteria can swim at about 50–60 body lengths per second. This would be the equivalent of a man running at a speed of 100 m (330 ft) per second. Cheetahs, the fastest animals on land, sprint at a maximum of about 25 body lengths per second.

Bacteria are used by humans in soil fertilisers, pickling, fermentation (in the manufacture of alcohol, vinegar and some cheeses) and for disposal of sewage and toxic waste.

Bacteria that cause disease are known as pathogens. Bacterial plant diseases include leaf spot, fire blight and wilts. Human diseases caused by bacteria include tuberculosis, cholera, syphilis, typhoid and tetanus. Some bacteria attack body tissues; others produce toxins (poisons).

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