Bacteria

I

Introduction

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.

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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, 100^oC (212^oF). 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.

II

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.

A

Bacteria and Human Health

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. Dental plaque, the sticky film on our teeth, consists primarily of masses of bacteria. These bacteria ferment (break down) the sugar we eat to produce acids, which over time can dissolve the enamel of the teeth and create cavities (holes) in the teeth.

Tooth decay provides a good example of how multiple factors contribute to bacterial disease. The human body hosts the bacteria, the diet supplies the sugars, and the bacteria produce the acid that damages the teeth.

A

1

Bacteria That Inhabit the Body

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. Bacteria that inhabit the surface of the stomach, for example, must deal with extremely strong acid in the digestive juices.

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. Animals raised in a completely germ-free environment, without any contact with bacteria, are highly susceptible to infectious diseases if they are exposed to the outside world. 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.

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