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.”
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