Bacillati

The unique properties of the cell wall in gram-positive taxa, which likely evolved in response to terrestrial conditions, have also contributed toward pathogenicity in many species. This suggests that terrestrial adaptations may have played a larger role in prokaryote evolution than currently understood.

The closest relatives of the Bacillati are the Pseudomonadati ("Hydrobacteria"), originally named in reference to the moist environment inferred for the common ancestor of those species. 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 containing 97% of prokaryotes and 99% of all species of Bacteria known by 2009.

Taxonomic history

"Terrabacteria" (syn. Bacillati) was proposed in 2004 for Actinomycetota, Cyanobacteria, and Deinococcota and was expanded later to include Bacillota 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". 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.

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.

Other related proposals

The clade formed by Pseudomonadati and Bacillati has been named "Selabacteria", in allusion to their phototrophic abilities (σέλας "light" in Greek). 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.

Cavalier-Smith's "Glidobacteria" included some members of Bacillati but excluded the large gram positive groups, Bacillota 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 accepted three-domain system. For example, it claimed that eukaryotes split from Archaea 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.