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primitive Earth, bacterial cell, animal cells, bogs, Descendents

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Bacteria, one-celled organisms visible only through a microscope. Bacteria live all around us and within us. The air is filled with bacteria, and they have even entered outer space in spacecraft. Bacteria live in the deepest parts of the ocean and deep within Earth. They are in the soil, in our food, and on plants and animals. Even our bodies are home to many different kinds of bacteria. Our lives are closely intertwined with theirs, and the health of our planet depends very much on their activities.

Bacterial cells are so small that scientists measure them in units called micrometers (µm). One micrometer equals a millionth of a meter (0.0000001 m or about 0.000039 in), and an average bacterium is about one micrometer long. Hundreds of thousands of bacteria would fit on a rounded dot made by a pencil.

Bacteria lack a true nucleus, a feature that distinguishes them from plant and animal cells. In plants and animals the saclike nucleus carries genetic material in the form of deoxyribonucleic acid (DNA). Bacteria also have DNA but it floats within the cell, usually in a loop or coil. A tough but resilient protective shell surrounds the bacterial cell.

Biologists classify all life forms as either prokaryotes or eukaryotes. Prokaryotes are simple, single-celled organisms like bacteria. They lack a defined nucleus of the sort found in plant and animal cells. More complex organisms, including all plants and animals, whose cells have a nucleus, belong to the group called eukaryotes. The word prokaryote comes from Greek words meaning “before nucleus”; eukaryote comes from Greek words for “true nucleus.” The study of bacteria is called bacteriology, a branch of microbiology.

Bacteria inhabited Earth long before human beings or other living things appeared. The earliest bacteria that scientists have discovered, in fossil remains in rocks, probably lived about 3.5 billion years ago. These early bacteria inhabited a harsh world: It was extremely hot, with high levels of ultraviolet radiation from the sun and with no oxygen to breathe.

Descendents of the bacteria that inhabited a primitive Earth are still with us today. Most have changed and would no longer be able to survive the harshness of Earth’s early environment. Yet others have not changed so much. Some bacteria today are able to grow at temperatures higher than the boiling point of water, 100oC (212oF). These bacteria live deep in the ocean or within Earth. Other bacteria cannot stand contact with oxygen gas and can live only in oxygen-free environments—in our intestines, for example, or in the ooze at the bottom of swamps, bogs, or other wetlands. Still others are resistant to radiation. Bacteria have remarkable abilities to adapt to extreme environments and thrive in parts of Earth that are inhospitable to other forms of life. Anywhere there is life, it includes bacterial life.


Marquis, Robert E., Ph.D.

Professor of Microbiology and Immunology, University of Rochester School of Medicine and Denistry.

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