Gracilicutes

Gracilicutes
Escherichia coli cells magnified 25,000 times
Scientific classification
Domain:	Bacteria
(unranked):	Gracilicutes Gibbons and Murray 1978[1]
Kingdoms/Phyla
Proteobacteria Planctobacteria Sphingobacteria Spirochaetae

Gracilicutes (Latin: gracilis, slender, and cutis, skin, referring to the cell wall) is a controversial taxon in bacterial taxonomy.

Traditionally gram staining results were most commonly used as a classification tool, consequently until the advent of molecular phylogeny, the Kingdom Prokaryotae (as the domains Bacteria and Archaea were known then) was divided into four phyla,^[2][3]

• Gracilicutes (gram-negative, it is split in many groups, but some authors stil use it in a narrower sense)
• Firmacutes [sic] (gram-positive, subsequently corrected to Firmicutes,^[4] today it includes the Mollicutes but excludes the Actinobacteria)
• Mollicutes (gram variable, posteriorly renamed as Tenericutes, 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.^[5][6]

This taxon was revived in 2006 by Cavalier-Smith as an infrakindgom containing the phyla Spirochaetae, Sphingobacteria, Planctobacteria, and Proteobacteria.^[7] However, this taxon is not generally accepted and the three-domain system is followed.^[8]

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.^[7]

Relationships

The following graph shows Cavalier-Smith's version of the tree of life, indicating the status of Gracilicutes.^[9]

[A]  Chlorobacteria  [B]  Hadobacteria  [C]   [D]  Cyanobacteria  [E]   [F] Gracilicutes Spirochaetae Sphingobacteria (FCB) Planctobacteria (PVC) Proteobacteria  [G]  Eurybacteria  [H] [I]  Endobacteria (Firmicutes)  [J]  Actinobacteria  [K] Neomura    [L]  Archaea  [M]  Eukarya

Legend: [A] Gram-negative with a peptidoglycan cell wall like Chlorosome. [B] Oxygenic Photosynthesis, Omp85 and four new catalases. [C] Glycobacterial revolution: outer membrane with insertion of lipopolysaccharides, hopanoids, diaminopimelic acid, ToIC and TonB. [D] Phycobilin chromophores. [E] Flagella. [F] Four sections: an amino acid in HSP60 and FtsZ and a domain in RNA polymerases β and σ. [G] Endospores. [H] Gram-positive Bacteria: hypertrophy of the wall peptidoglycan, sortasa enzyme and a loss of the outer membrane. [I] Glycerol 1-P dehydrogenase. [J] Proteasome and phosphatidylinositol. [K] Neomura revolution: Replacement of peptidoglycan by glycoproteins and lipoproteins. [L] Reverse DNA gyrase and ether lipid isoprenoids. [M] Phagocytosis.

References

1. Gibbons, N. E. & Murray, R. G. E. 1978. Proposals concerning the higher taxa of bacteria. Int J Syst Bacteriol 28:1–6, (PDF)
2. Gibbons, N.E. & Murray, R.G.E. (1978). Proposals concerning the higher taxa of bacteria. International Journal of Systematic Bacteriology, 28, 1-6, .
3. Krieg, N.R. & Holt, J.C. (eds., 1984). Bergey’s Manual of Systematic Bacteriology, 1st ed., vol. 1, Williams and Wilkins, Baltimore, .
4. Murray, R.G.E. (1984). The higher taxa, or, a place for everything...?. In: N.R. Krieg & J.G. Holt (ed.) Bergey's Manual of Systematic Bacteriology, vol. 1, The Williams & Wilkins Co., Baltimore, p. 31-34
5. Woese, C. R. (1987). "Bacterial evolution". Microbiological reviews 51 (2): 221–271. PMC 373105. PMID 2439888.
6. Don J. Brenner, Noel R. Krieg, James T. Staley (July 26, 2005) [1984(Williams & Wilkins)]. George M. Garrity, ed. Introductory Essays. Bergey's Manual of Systematic Bacteriology 2A (2nd ed.). New York: Springer. p. 304. ISBN 978-0-387-24143-2. British Library no. GBA561951.
7. Cavalier-Smith T (2006). "Rooting the tree of life by transition analyses". Biol. Direct 1: 19. doi:10.1186/1745-6150-1-19. PMC 1586193. PMID 16834776.
8. Krieg, N.R.; Ludwig, W.; Whitman, W.B.; Hedlund, B.P.; Paster, B.J.; Staley, J.T.; Ward, N.; Brown, D.; Parte, A. (November 24, 2010) [1984(Williams & Wilkins)]. George M. Garrity, ed. The Bacteroidetes, Spirochaetes, Tenericutes (Mollicutes), Acidobacteria, Fibrobacteres, Fusobacteria, Dictyoglomi, Gemmatimonadetes, Lentisphaerae, Verrucomicrobia, Chlamydiae, and Planctomycetes. Bergey's Manual of Systematic Bacteriology 4 (2nd ed.). New York: Springer. p. 908. ISBN 978-0-387-95042-6. British Library no. GBA561951.
9. Cavalier-Smith T (2006). "Cell evolution and Earth history: stasis and revolution" (PDF). Philos Trans R Soc Lond B Biol Sci 361 (1470): 969–1006. doi:10.1098/rstb.2006.1842. PMC 1578732. PMID 16754610.