Bacteria, one-celled organisms visible only through a microscope. Bacterialive all around us and within us. The air is filled with bacteria, and theyhave even entered outer space in spacecraft. Bacteria live in the deepestparts of the ocean and deep within Earth. They are in the soil, in ourfood, and on plants and animals. Even our bodies are home to many differentkinds of bacteria. Our lives are closely intertwined with theirs, and thehealth of our planet depends very much on their activities. Bacterial cells are so small that scientists measure them in unitscalled micrometers (µm). One micrometer equals a millionth of a meter(0.0000001 m or about 0.000039 in), and an average bacterium is about onemicrometer long. Hundreds of thousands of bacteria would fit on a roundeddot made by a pencil. Bacteria lack a true nucleus, a feature that distinguishes them fromplant and animal cells. In plants and animals the saclike nucleus carriesgenetic material in the form of deoxyribonucleic acid (DNA). Bacteria alsohave DNA but it floats within the cell, usually in a loop or coil. A toughbut resilient protective shell surrounds the bacterial cell. Biologists classify all life forms as either prokaryotes oreukaryotes. 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 havea nucleus, belong to the group called eukaryotes. The word prokaryote comesfrom Greek words meaning “before nucleus”; eukaryote comes from Greek wordsfor “true nucleus.” Bacteria inhabited Earth long before human beings or other livingthings appeared. The earliest bacteria that scientists have discovered, infossil remains in rocks, probably lived about 3.5 billion years ago. Theseearly 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 stillwith us today. Most have changed and would no longer be able to survive theharshness of Earth’s early environment. Yet others have not changed somuch. Some bacteria today are able to grow at temperatures higher than theboiling point of water, 100oC (212oF). These bacteria live deep in theocean or within Earth. Other bacteria cannot stand contact with oxygen gasand can live only in oxygen-free environments—in our intestines, forexample, or in the ooze at the bottom of swamps, bogs, or other wetlands.Still others are resistant to radiation. Bacteria are truly remarkable interms of their adaptations to extreme environments and their abilities tosurvive and thrive in parts of Earth that are inhospitable to other formsof life. Anywhere there is life, it includes bacterial life. Bacterial Killers Some dramatic infectious diseases result from exposure to bacteriathat are not part of our normal bacterial community. Cholera, one of theworld’s deadliest diseases today, is caused by the bacterium Vibriocholerae. Cholera is spread in water and food contaminated with thebacteria, and by people who have the disease. After entering the body, thecholera bacteria grow in the intestines, often along the surface of theintestinal wall, where they secrete a toxin (poison). This toxin causesmassive loss of fluid from the gut, and an infected person can die ofdehydration (fluid loss) unless the lost fluids, and the salts theycontain, are replaced. Cholera is common in developing regions of the worldthat lack adequate medical care. Another major bacterial killer is Mycobacterium tuberculosis, whichcauses tuberculosis (TB), a disease of the lungs. Tuberculosis isresponsible for more than 2 million deaths per year worldwide. Althoughantibiotics such as penicillin fight many bacterial diseases, the TBbacterium is highly resistant to most antibiotics. In addition, the TB-causing bacteria readily spread from person to person.