Bacteria

Bacteria are 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 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 are truly remarkable in terms of their

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.

The importance of bacteria

Much of our experience with bacteria involves disease. Although some bacteria do cause

disease, many kinds of bacteria live on or in the human body and prevent disease. Bacteria

associated with the human body outnumber body cells by ten to one. In addition, bacteria play

important roles in the environment and in industry.

We have all had bacterial diseases. Bacteria cause many cases of gastroenteritis, sometimes

called stomach flu. Perhaps the most common bacterial disease is tooth decay.

Communities of bacteria form what are called biofilms on many body surfaces. Dental

plaque is a biofilm covering the teeth. Biofilms also cover the soft tissues of our mouths and the

inner surfaces of our nose, sinuses, throat, stomach, and intestines. Even the skin has bacterial

communities that extend into hair follicles. Bacterial communities differ in each region of the body,

reflecting the environmental conditions in their specific region.

Some regions in the interior of the body are sterile—that is, devoid of living organisms other

than the cells of the body. Sterile regions include the muscles, the blood, and the nervous system.

However, even these regions face constant invasion by bacteria. The body’s immune system is

designed to rid the body of these invaders.

A healthy, balanced community of bacteria is extremely important for our health. Some of

these organisms protect us from disease-causing organisms that would otherwise infect us. Bacteria

in our bodies also provide us with needed nutrients, such as vitamin K, which the body

itself cannot make. The communities of bacteria and other organisms that inhabit the body are

sometimes called the normal microflora or microbiota.

In most cases the bacteria that cause disease are not part of the bacteria that normally inhabit

the body. They are picked up instead from sick people, sick animals, contaminated food or water, or

other external sources. Bacterial disease also can occur after surgery, an accident, or some other

event that weakens the immune system.

When the immune system is not functioning properly, bacteria that usually are harmless can

overwhelm the body and cause disease. These organisms are called opportunistic because they

cause disease only when an opportunity is presented.

Opportunistic infections became more important in the late 20th century because of diseases

such as acquired immunodeficiency syndrome (AIDS), a viral disease that ravages the immune

system.

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.

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