Pseudomonadati is a kingdom containing approximately one-third of

prokaryote A prokaryote (; less commonly spelled procaryote) is a unicellular organism, single-celled organism whose cell (biology), cell lacks a cell nucleus, nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Ancient Gree ...

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

, mostly

gram-negative bacteria Gram-negative bacteria are bacteria that, unlike gram-positive bacteria, do not retain the Crystal violet, crystal violet stain used in the Gram staining method of bacterial differentiation. Their defining characteristic is that their cell envelo ...

and their relatives. It is the closest relative of an even larger kingdom of

Bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...

, the

Bacillati Bacillati, formerly known as Terrabacteria, is a kingdom containing approximately two-thirds of prokaryote species, including those in the gram positive phyla (Actinomycetota and Bacillota) as well as the phyla Cyanobacteria, Chloroflexota, and ...

, which are mostly

gram-positive bacteria In bacteriology, gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall. The Gram stain ...

Names

The

synonymous A synonym is a word, morpheme, or phrase that means precisely or nearly the same as another word, morpheme, or phrase in a given language. For example, in the English language, the words ''begin'', ''start'', ''commence'', and ''initiate'' are a ...

name "Hydrobacteria" (hydro = "water") refers to the moist environment inferred for the common ancestor of those species. In contrast, species of Bacillati possess adaptations for life on land. Since 2024, the only validly published name for this group is kingdom Pseudomonadati (there used to be none, because no levels above

phylum In biology, a phylum (; : phyla) is a level of classification, or taxonomic rank, that is below Kingdom (biology), kingdom and above Class (biology), class. Traditionally, in botany the term division (taxonomy), division has been used instead ...

could exist in earlier versions of the

Prokaryotic Code The International Code of Nomenclature of Prokaryotes (ICNP) or Prokaryotic Code, formerly the International Code of Nomenclature of Bacteria (ICNB) or Bacteriological Code (BC), governs the scientific names for Bacteria and Archaea.P. H. A. Sneath ...

). "Gracilicutes," which was described in 1978 by Gibbons and Murray, is sometimes used in place of Pseudomonadati. However, "Gracilicutes" included

Cyanobacteria Cyanobacteria ( ) are a group of autotrophic gram-negative bacteria that can obtain biological energy via oxygenic photosynthesis. The name "cyanobacteria" () refers to their bluish green (cyan) color, which forms the basis of cyanobacteri ...

(a member of Bacillati) and was not constructed under the now generally accepted

three-domain system The three-domain system is a taxonomic classification system that groups all cellular life into three domains, namely Archaea, Bacteria and Eukarya, introduced by Carl Woese, Otto Kandler and Mark Wheelis in 1990. The key difference from ea ...

. More recently, a redefinition of "Gracilicutes" was proposed but it did not include a

molecular phylogeny Molecular phylogenetics () is the branch of phylogeny that analyzes genetic, hereditary molecular differences, predominantly in DNA sequences, to gain information on an organism's evolutionary relationships. From these analyses, it is possible to ...

or statistical analyses. Also, it did not follow the

, claiming instead that the lineage of

eukaryotes The eukaryotes ( ) constitute the domain of Eukaryota or Eukarya, organisms whose cells have a membrane-bound nucleus. All animals, plants, fungi, seaweeds, and many unicellular organisms are eukaryotes. They constitute a major group of ...

Archaea Archaea ( ) is a Domain (biology), domain of organisms. Traditionally, Archaea only included its Prokaryote, prokaryotic members, but this has since been found to be paraphyletic, as eukaryotes are known to have evolved from archaea. Even thou ...

is nested within Bacteria as a close relative of

Actinomycetota The Actinomycetota (or Actinobacteria) are a diverse phylum of Gram-positive bacteria with high GC content. They can be terrestrial or aquatic. They are of great importance to land flora because of their contributions to soil systems. In soil t ...

, a tree not supported in any molecular phylogeny.

Evolution

Pseudomonadati and Bacillati were inferred to have diverged approximately 3 billion years ago, suggesting that land (continents) had been colonized by prokaryotes at that time.

Phylogeny and taxonomy

They include these superphyla and phyla:

Acidobacteriota Acidobacteriota is a phylum of Gram-negative bacteria. Its members are physiologically diverse and ubiquitous, especially in soils, but are under-represented in culture. Description Members of this phylum are physiologically diverse, and can be ...

Aquificota The ''Aquificota'' phylum (biology), phylum is a diverse collection of bacteria that live in harsh environmental settings. The name ''Aquificota'' was given to this phylum based on an early genus identified within this group, ''Aquifex'' (“wate ...

, Bdellovibrionota, Campylobacterota,

Deferribacterota The Deferribacteraceae are a family of gram-negative bacteria which make energy by anaerobic respiration.Huber, H., and Stetter, K.O. "Family I. ''Deferribacteraceae'' fam. nov." In: Bergey's Manual of Systematic Bacteriology, 2nd ed., vol. 1 (T ...

, Dependentiae,

Desulfobacterota The Thermodesulfobacteriota, or Desulfobacterota, are a phylum of anaerobic Gram-negative bacteria. Many representatives are sulfate-reducing bacteria, others can grow by disproportionation of various sulphur species, reduction or iron, or even u ...

, Desulfuromonadota, Elusimicrobiota, FCB superphylum, Myxococcota,

Nitrospirota The Nitrospirota are a phylum of bacteria. They include multiple genera such as '' Nitrospira'', the largest. History of knowledge The first member of this phylum, '' Nitrospira marina'', was discovered in 1985. The second member, '' Nitrospi ...

Proteobacteria Pseudomonadota (synonym "Proteobacteria") is a major phylum of gram-negative bacteria. Currently, they are considered the predominant phylum within the domain of bacteria. They are naturally found as pathogenic and free-living (non-parasitic) ...

, PVC superphylum, and

Spirochaetota A spirochaete () or spirochete is a member of the phylum Spirochaetota (also called Spirochaetes ), which contains distinctive diderm (double-membrane) Gram-negative bacteria, most of which have long, helically coiled (corkscrew-shaped or ...

. Some unrooted molecular phylogenetic analyses have not supported this dichotomy of Bacillati and Pseudomonadati, but the most recent genomic analyses, including those that have focused on rooting the tree, have found these two groups to be monophyletic. Together, Pseudomonadati and Bacillati form a large

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

containing 97% of

prokaryotes A prokaryote (; less commonly spelled procaryote) is a single-celled organism whose cell lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Ancient Greek (), meaning 'before', and (), meaning 'nut' ...

and 99% of all species of

known by 2009, and placed by Battistuzzi and Hedges in the proposed

taxon In biology, a taxon (back-formation from ''taxonomy''; : taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular name and ...

Selabacteria, in allusion to their phototrophic abilities (selas = light). Currently, the bacterial phyla that are outside of Pseudomonadati + Bacillati, and thus justifying the taxon Selabacteria, are debated and may or may not include Fusobacteria. The definition of two major divisions within the domain

, Pseudomonadati, and Bacillati, has come largely from rooted phylogenetic analyses of genomes. Unrooted analyses have not fully supported this division, drawing attention to the importance of rooted trees of life. The two recent analyses of bacterial phylogeny both supported the division of Pseudomonadati and Bacillati. However, they interpreted the evolution of the

cell wall A cell wall is a structural layer that surrounds some Cell type, cell types, found immediately outside the cell membrane. It can be tough, flexible, and sometimes rigid. Primarily, it provides the cell with structural support, shape, protection, ...

differently, with one concluding that the last common ancestor of Bacteria was a monoderm (

) and the other concluding that it was a diderm (

). The following tree is redrawn from one of those two recent studies, showing the phylogeny of bacterial phyla and superphyla, with the position of Fusobacteria being unresolved and DST being the closest relative of Bacillati:

References

{{Authority Control Bacteria taxa Bacteriology Kingdoms (biology) Low-importance medicine articles