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 aactivities.

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 aa 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

with us today. Most have changed and would no longer be able tto 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 are truly remarkable in

terms of ttheir adaptations to extreme environments and their abilities to

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.