Bacteria

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Characteristics of Bacteria

Bacteria are so small that they can be seen only under a microscope that magnifies them at least 500 times their actual size. Some become visible only at magnifications of 1,000 times. They are measured in micrometers (µm) and average about 1 to 2 µm in length. One micrometer equals one-millionth of a meter (0.0000001 m or about 0.000039 in).

Bacteria not only have many uses, they also occur in diverse shapes and types. As a group they carry out a broad range of activities and have different nutritional needs. They thrive in a variety of environments.

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Types of Bacteria

Scientists use various systems for classifying bacteria into different types. One of the simplest systems is by shape. Other systems depend on oxygen use, source of carbon, and response to a particular dye.

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Classification by shape

Most bacteria come in one of three shapes: rod, sphere, or spiral. Rod-shaped bacteria are called bacilli. Spherical bacteria are called cocci, and spiral or corkscrew-shaped bacteria are called spirilla. Some bacteria come in more complex shapes. A hairlike form of spiral bacteria is called spirochete (see Spirochetes). Streptococci and staphylococci are well-known disease-causing bacteria among the cocci.

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Aerobic and Anaerobic Bacteria

Scientists also classify bacteria according to whether they need oxygen to survive or not. Aerobic bacteria require oxygen. Anaerobic bacteria cannot tolerate oxygen. Bacteria that live in deep ocean vents or within Earth are anaerobic. So are many of the bacteria that cause food poisoning.

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Autotrophic and Heterotrophic Bacteria

All bacteria require carbon for growth and reproduction. Bacteria called autotrophs (“self-feeders”) get their carbon from CO₂. Most bacteria, however, are heterotrophs (“other feeders”) and derive carbon from organic nutrients such as sugar. Some heterotrophic bacteria survive as parasites, growing within another living cell and using the nutrients and cell machinery of their host cells. Some autotrophic bacteria, such as cyanobacteria, use sunlight to produce sugars from CO₂. Others depend instead on energy from the breakdown of inorganic chemical compounds, such as nitrates and forms of sulfur.