Bacteria

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.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.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.