Bacteria

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Actinobacteria

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Aquificae

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Bacteroidetes/Chlorobi

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Chlamydiae/Verrucomicrobia

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Chloroflexi

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Chrysiogenetes

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Cyanobacteria

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Deferribacteres

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Deinococcus-Thermus

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Dictyoglomi

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Fibrobacteres/Acidobacteria

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Firmicutes

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Fusobacteria

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Gemmatimonadetes

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Nitrospirae

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Planctomycetes

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Proteobacteria

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Spirochaetes

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Thermodesulfobacteria

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Thermomicrobia

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Thermotogae

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Bacteria (singular, bacterium) are a major group of living organisms. Most are microscopic and unicellular, with a relatively simple cell structure lacking a cell nucleus, and organelles such as mitochondria and chloroplasts. Their cell structure is further described in the article about prokaryotes, because bacteria are prokaryotes, in contrast to organisms with more complex cells, called eukaryotes. The term "bacteria" has variously applied to all prokaryotes or to a major group of them, depending on ideas about their relationships.

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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 with a very different composition (peptidoglycans). Many move around using flagella, which are different in structure from the flagella of other groups.

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Bacteroidetes: Class Bacteroides...

Chlorobi: REDIRECT Green sulfur bacteria...

Chlamydiae: Chlamydia...