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 highlevels 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.