Bacteria
Bacteria
Fossil range: Archean or earlier - Recent
Scanning electron micrograph of Escherichia coli bacilli
Scientific classification
Domain: Bacteria
Phyla[1]
•
Actinobacteria (high-G+C)
Firmicutes (low-G+C)
Tenericutes (no wall)
•
Aquificae
Bacteroidetes/Chlorobi
Chlamydiae/Verrucomicrobia
Deinococcus-Thermus
Fusobacteria
Gemmatimonadetes
Nitrospirae
Proteobacteria
Spirochaetes
Synergistetes
•
Acidobacteria
Chloroflexi
Chrysiogenetes
Cyanobacteria
Deferribacteres
Dictyoglomi
Fibrobacteres
Planctomycetes
Thermodesulfobacteria
Thermotogae
The bacteria
[bækˈtɪərɪə] (singular: bac-
terium)[α] are a large group of unicellular
microorganisms. Typically a few micrometres
in length, bacteria have a wide range of
shapes, ranging from spheres to rods and
spirals. Bacteria are ubiquitous in every hab-
itat on Earth, growing in soil, acidic hot
springs, radioactive waste,[2] water, and
deep in the Earth’s crust, as well as in organ-
ic matter and the live bodies of plants and an-
imals. There are typically 40 million bacterial
cells in a gram of soil and a million bacterial
cells in a millilitre of fresh water; in all, there
are approximately five nonillion (5×1030)
bacteria on Earth,[3] forming much of the
world’s biomass.[4] Bacteria are vital in re-
cycling nutrients, with many steps in nutrient
cycles depending on these organisms, such
as the fixation of nitrogen from the atmo-
sphere and putrefaction. However, most bac-
teria have not been characterized, and only
about half of the phyla of bacteria have spe-
cies that can be grown in the laboratory.[5]
The study of bacteria is known as bacteri-
ology, a branch of microbiology.
There are approximately ten times as
many bacterial cells in the human flora of
bacteria as there are human cells in the
body, with large numbers of bacteria on the
skin and as gut flora.[6] The vast majority of
the bacteria in the body are rendered harm-
less by the protective effects of the immune
system, and a few are beneficial. However, a
few species of bacteria are pathogenic and