Gracilicutes
Escherichia coli cells magnified 25,000 times
Scientific classification
Domain:	Bacteria
Clade:	Gracilicutes Gibbons and Murray 1978[1]
Superphyla/Phyla
Various definitions, see text

Gracilicutes (Latin:gracilis, slender, andcutis, skin, referring to the cell wall) is aclade inbacterial phylogeny.^[2]

Traditionallygram staining results were most commonly used as a classification tool, consequently until the advent ofmolecular phylogeny, the KingdomMonera (as the domainsBacteria andArchaea were known then) was divided into fourphyla,^[1][3]

• Gracilicutes (gram-negative, it is split in many groups, but some authors still use it in a narrower sense)
• Firmacutes [sic] (gram-positive, subsequently corrected toFirmicutes,^[4] today it excludes theActinomycetota)
• Mollicutes (gram variable, later renamed Tenericutes and nowMycoplasmatota, e.g.Mycoplasma)
• Mendosicutes (uneven gram stain, "methanogenic bacteria" now known asmethanogens and classed asArchaea)

This classification system was abandoned in favour of thethree-domain system based onmolecular phylogeny started by C. Woese.^[5][6]

Using hand-drawn schematics rather than standard molecular phylogenetic analysis, Gracilicutes was revived in 2006 byCavalier-Smith as an infrakindgom containing the phylaSpirochaetota,Sphingobacteria (FCB),Planctobacteria (PVC), andProteobacteria.^[7] It is a gram-negative clade that branched off from other bacteria just before the evolutionary loss of the outer membrane orcapsule, and just after the evolution offlagella.^[7] Most notably, this author assumed an unconventional tree of life placingChloroflexota near the origin of life andArchaea as a close relative ofActinomycetota. This taxon is not generally accepted and the three-domain system is followed.^[8]

A taxon called"Hydrobacteria" was defined in 2009 from a molecular phylogenetic analysis of core genes. It is in contrast to the other major group ofeubacteria calledBacillati.^[9] Some researchers have used the name Gracilicutes in place of"Hydrobacteria", but this does not agree with the original description of Gracilicutes by Gibbons and Murray, noted above, which included cyanobacteria and did not follow thethree-domain system. Also as noted above, the use of Gracilicutes by Cavalier-Smith can be rejected because it was a major alteration of an earlier taxonomic name, was not based on a statistical analysis, and did not follow thethree-domain system. The most recent genomic analyses have supported the division ofBacteria into two major superphyla, corresponding toBacillati andPseudomonadati.^[10][11]

The phylogenetic tree according to the phylogenetic analyzes of Battistuzzi and Hedges (2009) is the following and with amolecular clock calibration.^[9]

Recent phylogenetic analyzes have found that proteobacteria are a paraphyletic phylum that could encompass several recently discovered candidate phyla and other phyla such asAcidobacteriota,Chrysiogenota,Deferribacterota, and possiblyAquificota. This suggests that Gracilicutes orPseudomonadati as a clade may comprise several candidates more closely related to Proteobacteria, Spirochaetes, PVC group, and FCB group than to bacteria from the cladeBacillati. Some of these phyla were classified as part of the proteobacteria. For example, Cavalier-Smith in his proposal of the 6 kingdoms includedAcidobacteriota,Aquificota,Chrysiogenota, andDeferribacterota as part of the proteobacteria.^[7]

Phylogenetic analyzes have found roughly the following phylogeny between the major and some more closely related phyla.^{[12][13][14][15]}

According to the phylogenetic analysis of Hug (2016), the relationships could be the following.^[16]

The following graph showsCavalier-Smith's version of the tree of life, indicating the status of Gracilicutes. However, this tree is not supported by any molecular analysis so it should not be considered phylogenetic.

Cavalier-Smith's Tree of Life, 2006^{[cstol 1]}

Legend:
[A]Gram-negative with apeptidoglycancell wall likeChlorosome.
[B] OxygenicPhotosynthesis,Omp85 and four newcatalases.
[C]Glycobacterial revolution: outer membrane with insertion oflipopolysaccharides,hopanoids,diaminopimelic acid,ToIC andTonB.
[D]Phycobilinchromophores.
[E]Flagella.
[F] Four sections: anamino acid inHSP60 andFtsZ and a domain inRNA polymerases β and σ.
[G]Endospores.
[H]Gram-positive Bacteria: hypertrophy of the wallpeptidoglycan,sortase enzyme and a loss of the outer membrane.
[I]Glycerol 1-Pdehydrogenase.
[J]Proteasome andphosphatidylinositol.
[K]Neomura revolution: Replacement ofpeptidoglycan byglycoproteins andlipoproteins.
[L] ReverseDNA gyrase andether lipidisoprenoids.
[M]Phagocytosis.

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• Bacteriology
• Infrakingdoms
• Obsolete bacteria taxa

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