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 hairlike fibers called cilia. Some protozoa cause diseases such as malaria. Plantlike protists, called algae, make their food by photosynthesis, and include ocean phytoplankton and green pond algae.

BIOGRAPHY: LOUIS PASTEUR (1822–1895)

French scientist 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).

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.

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.

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.

FIND OUT MORE

Biology
Cells
Life on Earth

Copyright © 2007 Dorling Kindersley