Thesynonymous 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.[5][3] Since 2024, the onlyvalidly published name for this group is kingdom Pseudomonadati (there used to be none, because no levels abovephylum could exist in earlier versions of theProkaryotic Code).[6]
"Gracilicutes," which was described in 1978 by Gibbons and Murray,[7] is sometimes used in place of Pseudomonadati. However, "Gracilicutes" includedCyanobacteria (a member of Bacillati) and was not constructed under the now generally acceptedthree-domain system.[7] More recently, a redefinition of "Gracilicutes" was proposed[8] but it did not include amolecular phylogeny or statistical analyses. Also, it did not follow thethree-domain system, claiming instead that the lineage ofeukaryotes +Archaea is nested within Bacteria as a close relative ofActinomycetota, a tree not supported in any molecular phylogeny.
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.[3]
Some unrooted molecular phylogenetic analyses[11][12] have not supported this dichotomy of Bacillati and Pseudomonadati, but the most recent genomic analyses,[9][10] including those that have focused on rooting the tree,[9] have found these two groups to be monophyletic.
Together, Pseudomonadati and Bacillati form a largeclade containing 97% ofprokaryotes and 99% of all species ofBacteria known by 2009, and placed by Battistuzzi and Hedges in the proposedtaxonSelabacteria, in allusion to their phototrophic abilities (Greekσέλας = light).[13] Currently, the bacterial phyla that are outside of Pseudomonadati + Bacillati, and thus justifying the taxon Selabacteria, are debated and may or may not includeFusobacteria.[9][10]
The definition of two major divisions within the domainBacteria, Pseudomonadati, and Bacillati, has come largely from rooted phylogenetic analyses of genomes.[5][3][9][10] Unrooted analyses have not fully supported this division,[12][11] drawing attention to the importance of rooted trees of life.
A timetree redrawn from Battistuzzi and Hedges (2009) shows the division of Pseudomonadati (syn. "Hydrobacteria") and Bacillati (syn. "Terrabacteria").[5][3]
The two recent analyses of bacterial phylogeny both supported the division of Pseudomonadati and Bacillati.[9][10] However, they interpreted the evolution of thecell wall differently, with one concluding that the last common ancestor of Bacteria was a monoderm (gram-positive bacteria[9]) and the other concluding that it was a diderm (gram-negative bacteria[10]). The following tree is redrawn from one of those two recent studies,[9] showing the phylogeny of bacterial phyla and superphyla, with the position of Fusobacteria being unresolved and DST being the closest relative of Bacillati:
A phylogeny of bacterial phyla and superphyla according to Coleman et al. (2021).[9] Pseudomonadati was referred to as "Gracilicutes" in that study.
^abcdefgBattistuzzi, F. U.; Hedges, S. B. (1 February 2009). "A Major Clade of Prokaryotes with Ancient Adaptations to Life on Land".Molecular Biology and Evolution.26 (2):335–343.doi:10.1093/molbev/msn247.PMID18988685.
^abHug, Laura A.; Baker, Brett J.; Anantharaman, Karthik; Brown, Christopher T.; Probst, Alexander J.; Castelle, Cindy J.; Butterfield, Cristina N.; Hernsdorf, Alex W.; Amano, Yuki; Ise, Kotaro; Suzuki, Yohey; Dudek, Natasha; Relman, David A.; Finstad, Kari M.; Amundson, Ronald; Thomas, Brian C.; Banfield, Jillian F. (May 2016)."A new view of the tree of life".Nature Microbiology.1 (5): 16048.doi:10.1038/nmicrobiol.2016.48.PMID27572647.S2CID3833474.
^abZhu, Qiyun; Mai, Uyen; Pfeiffer, Wayne; Janssen, Stefan; Asnicar, Francesco; Sanders, Jon G.; Belda-Ferre, Pedro; Al-Ghalith, Gabriel A.; Kopylova, Evguenia; McDonald, Daniel; Kosciolek, Tomasz; Yin, John B.; Huang, Shi; Salam, Nimaichand; Jiao, Jian-Yu; Wu, Zijun; Xu, Zhenjiang Z.; Cantrell, Kalen; Yang, Yimeng; Sayyari, Erfan; Rabiee, Maryam; Morton, James T.; Podell, Sheila; Knights, Dan; Li, Wen-Jun; Huttenhower, Curtis; Segata, Nicola; Smarr, Larry; Mirarab, Siavash; Knight, Rob (December 2019)."Phylogenomics of 10,575 genomes reveals evolutionary proximity between domains Bacteria and Archaea".Nature Communications.10 (1): 5477.Bibcode:2019NatCo..10.5477Z.doi:10.1038/s41467-019-13443-4.PMC6889312.PMID31792218.
^Battistuzzi, FU; Hedges, SB (2009). "Eubacteria". In Hedges, SB; Kumar, S (eds.).The Timetree of Life. New York: Oxford University Press. pp. 106–115.
^Pseudomonadati inLPSN;Freese, H. M.; Meier-Kolthoff, J. P.; Sardà Carbasse, J.; Afolayan, A. O.; Göker, M. (29 October 2025). "TYGS and LPSN in 2025: a Global Core Biodata Resource for genome-based classification and nomenclature of prokaryotes within DSMZ Digital Diversity".Nucleic Acids Research.53:D1–D12.doi:10.1093/nar/gkaf1110.
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