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Bacteria (singular: bacterium) are a major group of living organisms. The term "bacteria" (singular: bacterium) has variously applied to all prokaryotes or to a major group of them, otherwise called the eubacteria, depending on ideas about their relationships. Here, bacteria is used specifically to refer to the eubacteria. Another major group of bacteria (in the broadest, non-taxonomic sense) are the Archaea. The study of bacteria is known as bacteriology, a subfield of microbiology.
Photo: The Escherichia coli (a.k.a. e-coli) bacterium
Bacteria are the most abundant of all organisms. They are ubiquitous in soil, water, and as symbionts of other organisms. Many pathogens are bacteria. Most are minute, usually only 0.5-5.0 μm in their longest dimension, although giant bacteria like Thiomargarita namibiensis and Epulopiscium fishelsoni may grow past 0.5 mm in size. They generally have cell walls, like plant and fungal cells, but bacterial cell walls are normally made out of peptidoglycan instead of cellulose (as in plants) or chitin (as in fungi), and are not homologous with eukaryotic cell walls. Many move around using flagella, which are different in structure from the flagella of eukaryotes.
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