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 ac


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 fl
loats 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 so
ort 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
with us today. Most have changed and would no longer be able to survive the
harshness of Earth’s early environment. Yet o
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 are truly remarkable in
terms of their adaptations to extreme environments and their abilities t
survive and thrive in parts of Earth that are inhospitable to other forms
of life. Anywhere there is life, it includes bacterial life.

Bacterial Killers

Some dramatic infectious diseases result from exposure to bacteria
that are not part of our normal bacterial community. Cholera, one of the
world’s deadliest diseases today, is caused by the bacterium Vibrio
cholerae. Cholera is spread in water and food contaminated with the
bacteria, and by people who have the disease. After entering the body, the
cholera bacteria grow in the intestines, often along the surface of the
intestinal wall, where they secrete a toxin (poison). This toxin causes
massive loss of fluid from the gut, and an infected person can die of
dehydration (fluid loss) unless the lost fluids, and the salts they
contain, are replaced. Cholera is common in developing regions of the world
that lack adequate medical care.

Another major bacterial killer is Mycobacterium tuberculosis, which
causes tuberculosis (TB), a disease of the lungs. Tuberculosis is
responsible for more than 2 million deaths per year worldwide. Although
antibiotics such as penicillin fight many bacterial diseases, the TB
bacterium is highly resistant to most antibiotics. In addition, the TB-
causing bacteria readily spread from person to person.