Bacillati, formerly known as Terrabacteria, is a kingdom containing approximately two-thirds 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, including those in the gram positive

phyla Phyla, the plural of ''phylum'', may refer to: * Phylum, a biological taxon between Kingdom and Class * by analogy, in linguistics, a large division of possibly related languages, or a major language family which is not subordinate to another Phy ...

( Actinomycetota and

Bacillota The Bacillota (synonym Firmicutes) are a phylum of bacteria, most of which have Gram-positive cell wall structure. They have round cells, called cocci (singular coccus), or rod-like forms (bacillus). A few Bacillota, such as '' Megasphaera'', ...

) as well as the phyla

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 ...

, Chloroflexota, and Deinococcota. It derives its name (''terra'' = "land") from the evolutionary pressures of life on land. Bacillati possess important adaptations such as resistance to environmental hazards (e.g., desiccation, ultraviolet radiation, and high salinity) and oxygenic

photosynthesis Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...

. Also, the unique properties of the cell wall in gram-positive

taxa 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 ...

, which likely evolved in response to terrestrial conditions, have contributed toward pathogenicity in many species. These results now leave open the possibility that terrestrial adaptations may have played a larger role in prokaryote evolution than currently understood. "Terrabacteria" ( syn. Bacillati) was proposed in 2004 for Actinomycetota, Cyanobacteria, and Deinococcota and was expanded later to include

and Chloroflexota. Other phylogenetic analyses have supported the close relationships of these phyla. Most species of prokaryotes not placed in "Terrabacteria" were assigned to the taxon " Hydrobacteria", (which is also known as kingdom Pseudomonadati) in reference to the moist environment inferred for the common ancestor of those species. Some 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. Bacillati and Pseudomonadati were inferred to have diverged approximately 3 billion years ago, suggesting that land (continents) had been colonized by prokaryotes at that time. Together, Bacillati and Pseudomonadati 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 and 99% of all species of Bacteria known by 2009, and placed in the taxon Selabacteria, in allusion to their phototrophic abilities (''selas'' = light). Currently, the bacterial phyla that are outside of Bacillati + Pseudomonadati, and thus justifying the taxon Selabacteria, are debated and may or may not include Fusobacteria. The name “Glidobacteria” included some members of Bacillati but excluded the large gram positive groups,

and Actinomycetota, and is not supported by molecular phylogenetic data. Moreover, the article naming Glidobacteria did not include a molecular phylogeny or statistical analyses and did not follow the widely used three-domain system. For example, it claimed that eukaryotes split from

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 ...

very recently (~900 Mya), which is contradicted by the fossil record, and that lineage of eukaryotes + Archaea was nested within Bacteria as a close relative of Actinomycetota. In 2022, new rules were introduced for kingdom-level taxa of prokaryotes, and the same two authors who proposed those new rules, proposed new names in 2024. They concluded that “the taxonomically preferable solution for bacterial kingdoms seems to be to accept the subdivision apparent in the study by Battistuzzi and Hedges,” with refinement. The new (and only valid) name is kingdom Bacillati.

Phylogeny

The phylogenetic tree according to the phylogenetic analyses of Battistuzzi and Hedges (2009) is the following and with a molecular clock calibration. Timeline of life

Recent molecular analyses have found roughly the following relationships including other phyla, whose relationships were uncertain. On the other hand, Coleman et al. named the clade composed of Thermotogota, Deinococcota, Synergistota and related as DST and furthermore the analysis suggests that ultra-small bacteria (CPR group) may belong to Bacillati being more closely related to Chloroflexota. According to this study the phylum

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 ...

sometimes included belongs to Pseudomonadati and that the phylum Fusobacteriota can belong to both Bacillati and Pseudomonadati. The result was the following:

References

{{Taxonbar, from=Q873914 Bacteria taxa Bacterial nomenclature Kingdoms (biology) Low-importance medicine articles