Gracilicutes

{{Short description|Infrakingdom of bacteria}}

{{automatic taxobox

| image = E. coli Bacteria (7316101966).jpg

| image_caption = Escherichia coli cells magnified 25,000 times

| taxon = Gracilicutes

| authority = Gibbons and Murray 1978{{cite journal | vauthors = Gibbons NE, Murray RG | title = Proposals concerning the higher taxa of bacteria. | journal = International Journal of Systematic and Evolutionary Microbiology | date = January 1978 | volume = 28 | issue = 1 | pages = 1–6 | doi = 10.1099/00207713-28-1-1 | doi-access = free }}

| subdivision_ranks = Superphyla/Phyla

| subdivision = Various definitions, see text

}}

Gracilicutes (Latin: gracilis, slender, and cutis, skin, referring to the cell wall) is a clade in bacterial phylogeny.{{cite journal | vauthors = Boussau B, Guéguen L, Gouy M | title = Accounting for horizontal gene transfers explains conflicting hypotheses regarding the position of aquificales in the phylogeny of Bacteria | journal = BMC Evolutionary Biology | volume = 8 | pages = 272 | date = October 2008 | pmid = 18834516 | pmc = 2584045 | doi = 10.1186/1471-2148-8-272 | quote = Accounting for horizontal gene transfers explains conflicting hypotheses regarding the position of Aquificales in the phylogeny of Bacteria | doi-access = free }}

Traditionally gram staining results were most commonly used as a classification tool, consequently until the advent of molecular phylogeny, the Kingdom Monera (as the domains Bacteria and Archaea were known then) was divided into four phyla,{{cite book | veditors = Krieg NR, Holt JC | date = 1984 | title = Bergey's Manual of Systematic Bacteriology | edition = 1st | volume = 1 | publisher = Williams and Wilkins | location = Baltimore }}

• 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 to Firmicutes,{{cite book | vauthors = Murray RG | chapter = The higher taxa, or, a place for everything...? | veditors = Krieg NR, Holt JC | date = 1984 | title = Bergey's Manual of Systematic Bacteriology | edition = 1st | volume = 1 | publisher = Williams and Wilkins | location = Baltimore | pages = 31–34 }} today it excludes the Actinomycetota)
• Mollicutes (gram variable, later renamed Tenericutes and now Mycoplasmatota, e.g. Mycoplasma)
• Mendosicutes (uneven gram stain, "methanogenic bacteria" now known as methanogens and classed as Archaea)

This classification system was abandoned in favour of the three-domain system based on molecular phylogeny started by C. Woese.{{cite journal | vauthors = Woese CR | title = Bacterial evolution | journal = Microbiological Reviews | volume = 51 | issue = 2 | pages = 221–271 | date = June 1987 | pmid = 2439888 | pmc = 373105 | doi = 10.1128/MMBR.51.2.221-271.1987 }}{{cite book | title = Bergey's Manual of Systematic Bacteriology|volume=2A| chapter = Introductory Essays| veditors = Garrity GM | vauthors = Brenner DJ, Krieg NA, Staley JT |publisher=Springer|location= New York |edition=2nd|isbn=978-0-387-24143-2 |pages=304 |url=https://www.springer.com/life+sciences/book/978-0-387-24143-2 |date=July 26, 2005| orig-year =1984(Williams & Wilkins)|id=British Library no. GBA561951}}

Using hand-drawn schematics rather than standard molecular phylogenetic analysis, Gracilicutes was revived in 2006 by Cavalier-Smith as an infrakindgom containing the phyla Spirochaetota, Sphingobacteria (FCB), Planctobacteria (PVC), and Proteobacteria.{{cite journal | vauthors = Cavalier-Smith T | title = Rooting the tree of life by transition analyses | journal = Biology Direct | volume = 1 | pages = 19 | date = July 2006 | pmid = 16834776 | pmc = 1586193 | doi = 10.1186/1745-6150-1-19 | doi-access = free }} It is a gram-negative clade that branched off from other bacteria just before the evolutionary loss of the outer membrane or capsule, and just after the evolution of flagella. Most notably, this author assumed an unconventional tree of life placing Chloroflexota near the origin of life and Archaea as a close relative of Actinomycetota. This taxon is not generally accepted and the three-domain system is followed.{{cite book | vauthors = Krieg NR, Ludwig W, Whitman WB, Hedlund BP, Paster BJ, Staley JT, Ward N, Brown D, Parte A | display-authors = 6 | chapter = The Bacteroidetes, Spirochaetes, Tenericutes (Mollicutes), Acidobacteria, Fibrobacteres, Fusobacteria, Dictyoglomi, Gemmatimonadetes, Lentisphaerae, Verrucomicrobia, Chlamydiae, and Planctomycetes | title = Bergey's Manual of Systematic Bacteriology|volume=4| veditors = Garrity GM |publisher= Springer |location= New York |edition= 2nd|isbn=978-0-387-95042-6 |page=908 |url=https://www.springer.com/life+sciences/book/978-0-387-95042-6 |date=November 24, 2010| orig-year =1984(Williams & Wilkins)|id=British Library no. GBA561951}}

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 of eubacteria called Bacillati.{{cite journal | vauthors = Battistuzzi FU, Hedges SB | title = A major clade of prokaryotes with ancient adaptations to life on land | journal = Molecular Biology and Evolution | volume = 26 | issue = 2 | pages = 335–343 | date = February 2009 | pmid = 18988685 | doi = 10.1093/molbev/msn247 | doi-access = free }} 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 the three-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 the three-domain system. The most recent genomic analyses have supported the division of Bacteria into two major superphyla, corresponding to Bacillati and Pseudomonadati.{{cite journal | vauthors = Coleman GA, Davín AA, Mahendrarajah TA, Szánthó LL, Spang A, Hugenholtz P, Szöllősi GJ, Williams TA | display-authors = 6 | title = A rooted phylogeny resolves early bacterial evolution | journal = Science | volume = 372 | issue = 6542 | pages = eabe0511 | date = May 2021 | pmid = 33958449 | doi = 10.1126/science.abe0511 | s2cid = 233872903 | url = https://research-information.bris.ac.uk/en/publications/51e9e402-36b7-47a6-91de-32b8cf7320d2 | hdl = 1983/51e9e402-36b7-47a6-91de-32b8cf7320d2 | hdl-access = free }}{{cite journal | vauthors = Léonard RR, Sauvage E, Lupo V, Perrin A, Sirjacobs D, Charlier P, Kerff F, Baurain D | display-authors = 6 | title = Was the Last Bacterial Common Ancestor a Monoderm after All? | journal = Genes | volume = 13 | issue = 2 | pages = 376 | date = February 2022 | pmid = 35205421 | pmc = 8871954 | doi = 10.3390/genes13020376 | doi-access = free }}