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FIND OUT MORE. Biology 72 • Cells 73 • Life on Earth 70–71

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MICROORGANISMS

Any living thing that is invisible to the

naked eye and can be seen only under a

microscope is called a microorganism.

They include. BACTERIA,

protists, and some fungi, such

as yeasts.. VIRUSES are usually

included, but they are

not really living organisms.

WHAT IS A PROTIST?

This is a single-celled organism found in the

sea, fresh water, soil, and in or on other living

things. Animal-like protists, called protozoa, get

their energy by eating food. They include ciliates –

protists that move by beating hair-like fibres called

cilia. Some protozoa cause diseases such as malaria.

Plant-like protists, called algae, make their food by

photosynthesis, and include ocean phytoplankton and

green pond algae.

HOW DO VIRUSES REPRODUCE?

A virus invades a living cell, hijacks its genes, and

forces it to produce many new virus particles. These

new particles then break out of the cell. Viruses are

not regarded as living things because they cannot

reproduce alone.

VIRUSES

Only a very powerful microscope can show

the minute chemical package known as a

virus. Many viruses cause disease. Viruses

are active only once they have infected a

living animal, plant, or bacterial cell.

BACTERIA

The most abundant organisms on Earth,

bacteria are found on land, in water, and in

the air. Bacteria consist of one tiny cell.

They have a protective cell wall, but, unlike

other cells, lack a nucleus.

ARE ALL BACTERIA HARMFUL?

While some bacteria are harmful, such as those that

cause diseases, others are useful. These include

bacteria in our intestines that supply us with vitamins,

those that are used to make foods, and soil bacteria

that recycle nutrients from dead animals and plants.

4 YELLOW FEVER MOSQUITO

This tropical mosquito pierces

human skin to feed on warm

blood. It carries a virus that

causes yellow fever, which

is sometimes passed on as

a mosquito feeds and can

be fatal.

1 DIDINIUM

Fresh water ciliate protist

Didinium can kill and eat other

protists bigger than itself. Here, it

attacks Paramecium, another ciliate,

immobilizing it with an explosive dart.

BACTERIA ON NEEDLE 3

This scanning electron

micrograph (SEM) shows the

tiny size of these rod-

shaped bacteria (orange).

They are clustered on the

tip of a syringe needle,

normally used for

giving injections.

1 COMMON COLD VIRUS

This is an adenovirus, one of

the viruses that cause the

common cold. Like all viruses,

it consists of a thread, DNA or

RNA, which carries the infection

instructions. This is surrounded by

a protective protein coat that can

be seen here.

Paramecium is

covered with cilia

that beat rhythmically

to make it move

Needle-like mouthpart

sucks up blood into this

yellow fever mosquito

Bacteria grouped

together at the tip

of a needle

Mouth of Didinium opens wide to

swallow prey whole

Didinium moves by

beating two rows of cilia

LOUIS PASTEUR FRENCH

(1822–1895)

Louis Pasteur founded the

science of microbiology. He

proved that microorganisms

cause infectious diseases,

developed vaccines, and

discovered pasteurization (the

heat-killing of bacteria in food).

microbes