Scanning electron micrograph of Escherichia coli, grown in culture and adhered to a cover slip. Credit: NIAID
Illustration associated with the linked encyclopedia article. Refer to the file record for the subject and interpretation.
Overview
Bacteria are cells without the membrane-bound nucleus characteristic of eukaryotes. Their diversity encompasses free-living organisms, symbionts and pathogens, with very different shapes and energy sources. Decomposition, nutrient cycling, fermentation and associations with animals or plants make them fundamental to ecosystems. Treating bacteria solely as agents of disease hides most of their biological importance.
Many species reproduce rapidly when conditions permit, while others persist under severe resource limitation. Their evolutionary relationships are reconstructed from molecular comparisons and cellular characteristics. Ancient bacteria-like fossils support an extremely long microbial history, but resemblance alone rarely identifies a modern group. Bacteria should therefore appear here as a broad domain containing countless lineages, rather than one primitive species frozen in time.
Hug and colleagues' 2016 genome-based analysis illustrates how much bacterial diversity ordinary culture collections can miss. The study combined published genomes with 1,011 newly reconstructed genomes from environments including subsurface waters, desert salt crust, soil, a geyser system and dolphin mouths. Metagenomic reconstruction assigns DNA fragments from a mixed environmental sample to draft genomes, allowing investigation without first growing each organism in isolation. This supplies evidence about genetic identity and metabolic potential; it does not itself demonstrate every predicted biochemical activity in a living cell.
Their main tree compared 16 ribosomal proteins across 3,083 genome representatives. Sampling was designed to approximate one representative per genus rather than count every strain as an independent major lineage. Candidate groups without established genus names were reduced to a comparable level of divergence. Included draft genomes had to meet estimated completeness and sequence-consistency criteria. A separate small-subunit ribosomal RNA analysis used fewer organisms because suitable sequences were not always available. These design decisions make the resulting tree a defined sample of genomic diversity, rather than a complete census of all bacteria.
The analysis highlighted the Candidate Phyla Radiation, many members of which lacked isolated cultured representatives in that study. Their sampled genomes were small and often had restricted predicted biosynthetic capacities, suggesting dependence on other organisms. The authors explicitly left open whether these metabolic limitations reflected loss or inherited characteristics. Deep branching order also differed between the ribosomal-protein and ribosomal-RNA trees, and the earliest splits were not confidently resolved. Branch lengths measure modeled sequence change, not geological time or a hierarchy of progress. The great breadth of bacterial branches therefore supports their evolutionary diversity without making a living small-genome bacterium equivalent to the first cellular organism.
Origins & earliest records
Bacteria were recognised through the development of microscopy and experimental microbiology, but their evolutionary history is far older. Bacteria-like fossil evidence extends deep into the Precambrian. The first appearance of the domain cannot be read directly from a rounded or filamentous fossil shape with uncertain affinity.
Evidence & interpretation
Living isolates, microscopy and genome sequences reveal bacterial structure and metabolism. Fossils and geochemistry constrain ancient microbial activity, although simple shapes can have nonbiological lookalikes. Horizontal gene transfer further complicates a purely branching ancestry, making individual gene trees imperfect guides to the domain's earliest evolutionary relationships.
Selected bibliography
Documented works and useful reading. This is not a list of every appearance.
- 01
Introduction to the Bacteria
Selected institutional scientific reading; not a complete bibliography.
- 02
A new view of the tree of life
Nature Microbiology 1:16048, published 11 April. Complete main text, Methods and accession section; defined genome sampling and unresolved deep branches. CC BY 4.0.
Family, evolution & connections
Taxonomic classification
A hierarchy of classified groups, not a chain of direct ancestors.
- domainBacteria
Encyclopedia background
An additional attributed reference, separate from the editorial profile above.
Read the open encyclopedia overview
Bacteria are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among the first life forms to appear on Earth, and are present in most of its habitats. Bacteria inhabit the air, soil, water, acidic hot springs, radioactive waste, and the deep biosphere of Earth's crust. Bacteria play a vital role in many stages of the nutrient cycle by recycling nutrients and the fixation of nitrogen from the atmosphere. The nutrient cycle includes the decomposition of dead bodies; bacteria are responsible for the putrefaction stage in this process. In the biological communities surrounding hydrothermal vents and cold seeps, extremophile bacteria provide the nutrients needed to sustain life by converting dissolved compounds, such as hydrogen sulphide and methane, to energy. Bacteria also live in mutualistic, commensal and parasitic relationships with plants and animals. Most bacteria have not been characterised and there are many species that cannot be grown in the laboratory. The study of bacteria is known as bacteriology, a branch of microbiology.
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Text from Wikipedia contributors, “Bacteria”. CC BY-SA 4.0. Extracted introduction; formatting changed. Retrieved 5 October 2026. The source article may have changed since retrieval.
References
Sources supporting this profile. Linked pages have their own scope and editorial standards.
- MuseumIntroduction to the BacteriaResearch access: 2026-10-05
- ResearchValid publication of names of two domains and seven kingdoms of prokaryotesResearch access: 2026-10-05
- ResearchHug 2016 genome-resolved diversity study, CC BY 4.0Research access: 2026-10-05
Image credits
Reference imageScanning electron micrograph of Escherichia coli, grown in culture and adhered to a cover slip. Credit: NIAID
Illustration associated with the linked encyclopedia article. Refer to the file record for the subject and interpretation.
