|Scanning electron micrograph 14 of Actinomyces israelii.|
Harz 1877 (Approved Lists 1980)
The Actinomycetota (synonym Actinobacteria) are a phylum of mostly Gram-positive bacteria. They can be terrestrial or aquatic. They are of great economic importance to humans because agriculture and forests depend on their contributions to soil systems. In soil they help to decompose the organic matter of dead organisms so the molecules can be taken up anew by plants. While this role is also played by fungi, Actinomycetota are much smaller and likely do not occupy the same ecological niche. In this role the colonies often grow extensive mycelia, like a fungus would, and the name of an important order of the phylum, Actinomycetales (the actinomycetes), reflects that they were long believed to be fungi. Some soil actinomycetota (such as Frankia) live symbiotically with the plants whose roots pervade the soil, fixing nitrogen for the plants in exchange for access to some of the plant's saccharides. Other species, such as many members of the genus Mycobacterium, are important pathogens.
Beyond the great interest in Actinomycetota for their soil role, much is yet to be learned about them. Although currently understood primarily as soil bacteria, they might be more abundant in fresh waters. Actinomycetota is one of the dominant bacterial phyla and contains one of the largest of bacterial genera, Streptomyces. Streptomyces and other actinomycetota are major contributors to biological buffering of soils. They are also the source of many antibiotics.
The Actinomycetota genus Bifidobacterium is the most common bacteria in the microbiome of human infants. Although adults have fewer bifidobacteria, intestinal bifidobacteria help maintain the mucosal barrier and reduce lipopolysaccharide in the intestine.
Although some of the largest and most complex bacterial cells belong to the Actinomycetota, the group of marine Actinomarinales has been described as possessing the smallest free-living prokaryotic cells.
Some Siberian or Antarctic Actinomycetota is said to be the oldest living organism on Earth, frozen in permafrost at around half a million years ago. The symptoms of life were detected by CO2 release from permafrost samples 640 kya or younger. 
Most Actinomycetota of medical or economic significance are in class Actinomycetia, and belong to the order Actinomycetales. While many of these cause disease in humans, Streptomyces is notable as a source of antibiotics.
Of those Actinomycetota not in the Actinomycetales, Gardnerella is one of the most researched. Classification of Gardnerella is controversial, and MeSH catalogues it as both a Gram-positive and Gram-negative organism.
Actinomycetota, especially Streptomyces spp., are recognized as the producers of many bioactive metabolites that are useful to humans in medicine, such as antibacterials, antifungals, antivirals, antithrombotics, immunomodifiers, antitumor drugs, and enzyme inhibitors; and in agriculture, including insecticides, herbicides, fungicides, and growth-promoting substances for plants and animals. Actinomycetota-derived antibiotics that are important in medicine include aminoglycosides, anthracyclines, chloramphenicol, macrolide, tetracyclines, etc.
The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature (LPSN). The phylogeny is based on whole-genome analysis.
- Oren A, Garrity GM (2021). "Valid publication of the names of forty-two phyla of prokaryotes". Int J Syst Evol Microbiol. 71 (10): 5056. doi:10.1099/ijsem.0.005056. PMID 34694987.
- Euzéby JP, Parte AC. "Actinobacteria". List of Prokaryotic names with Standing in Nomenclature (LPSN). Retrieved June 7, 2021.
- Goodfellow M (2012). "Phylum XXVI. Actinobacteria phyl. nov.". In Goodfellow M, Kämpfer P, Trujillo ME, Suzuki K, Ludwig W, Whitman WB (eds.). Bergey's Manual of Systematic Bacteriology. Vol. 5 (2nd ed.). New York, NY: Springer. pp. 33–34.
- Servin JA, Herbold CW, Skophammer RG, Lake JA (January 2008). "Evidence excluding the root of the tree of life from the actinobacteria". Mol. Biol. Evol. 25 (1): 1–4. doi:10.1093/molbev/msm249. PMID 18003601.
- Ghai R, Rodriguez-Valera F, McMahon KD, et al. (2011). Lopez-Garcia P (ed.). "Metagenomics of the water column in the pristine upper course of the Amazon river". PLOS ONE. 6 (8): e23785. Bibcode:2011PLoSO...623785G. doi:10.1371/journal.pone.0023785. PMC 3158796. PMID 21915244.
- C.Michael Hogan.. 2010. Bacteria. Encyclopedia of Earth. eds. Sidney Draggan and C.J.Cleveland, National Council for Science and the Environment, Washington DC Archived 2011-05-11 at the Wayback Machine
- Ningthoujam, Debananda S.; Sanasam, Suchitra; Tamreihao, K; Nimaichand, Salam (November 2009). "Antagonistic activities of local actinomycete isolates against rice fungal pathogens". African Journal of Microbiology Research. 3 (11): 737–742.
- Turroni F, Peano C, Pass DA, Foroni E, Severgnini M, Claesson MJ, Kerr C, Hourihane J, Murray D, Fuligni F, Gueimonde M, Margolles A, De Bellis G, O'Toole PW, van Sinderen D, Marchesi JR, Ventura M (2012-05-11). "Diversity of bifidobacteria within the infant gut microbiota". PLOS ONE. 7 (5): e36957. Bibcode:2012PLoSO...736957T. doi:10.1371/journal.pone.0036957. PMC 3350489. PMID 22606315.
- Pinzone MR, Celesia BM, Di Rosa M, Cacopardo B, Nunnari G (2012). "Microbial translocation in chronic liver diseases". International Journal of Microbiology. 2012: 694629. doi:10.1155/2012/694629. PMC 3405644. PMID 22848224.
- Ghai R, Mizuno CM, Picazo A, Camacho A, Rodriguez-Valera F (2013). "Metagenomics uncovers a new group of low GC and ultra-small marine Actinobacteria". Scientific Reports. 3: 2471. Bibcode:2013NatSR...3E2471G. doi:10.1038/srep02471. PMC 3747508. PMID 23959135.
- Sussman: Oldest Plants, The Guardian, 2 May 2010
- "It's Okay to be Smart • the oldest living thing in the world: These". Archived from the original on 2018-07-13. Retrieved 2018-07-13.
- Johnson, S. S.; Hebsgaard, M. B.; Christensen, T. R.; Mastepanov, M.; Nielsen, R.; Munch, K.; Brand, T.; Gilbert, M. T. P.; Zuber, M. T.; Bunce, M.; Ronn, R.; Gilichinsky, D.; Froese, D.; Willerslev, E. (2007). "Ancient bacteria show evidence of DNA repair". Proceedings of the National Academy of Sciences. 104 (36): 14401–14405. Bibcode:2007PNAS..10414401J. doi:10.1073/pnas.0706787104. PMC 1958816. PMID 17728401.
- Gardnerella at the US National Library of Medicine Medical Subject Headings (MeSH)
- Mahajan, GB (2012). "Antibacterial agents from actinomycetes - a review". Frontiers in Bioscience. 4: 240–53. doi:10.2741/e373.
- Gupte, M.; Kulkarni, P.; Ganguli, B.N. (2002). "Antifungal Antibiotics". Appl. Microbiol. Biotechnol. 58 (1): 46–57. doi:10.1007/s002530100822. PMID 11831475. S2CID 8015426.
- Bressan, W (2003). "Biological control of maize seed pathogenic fungi by use of actinomycetes". Biocontrol. 48 (2): 233–240. doi:10.1023/a:1022673226324. S2CID 29320215.
- Atta, M.A (2009). "Antimycin-A Antibiotic Biosynthesis Produced by Streptomyces Sp. AZ-AR-262: Taxonomy, Fermentation, Purification and Biological Activities". Austral. J. Basic and Appl. Sci. 3: 126–135.
- Ventura, M.; Canchaya, C.; Tauch, A.; Chandra, G.; Fitzgerald, G. F.; Chater, K. F.; van Sinderen, D. (5 September 2007). "Genomics of Actinobacteria: Tracing the Evolutionary History of an Ancient Phylum". Microbiology and Molecular Biology Reviews. 71 (3): 495–548. doi:10.1128/MMBR.00005-07. hdl:11381/1721088. PMC 2168647. PMID 17804669.
- Ghai R, McMahon KD, Rodriguez-Valera F (2012). "Breaking a paradigm: Cosmopolitan and abundant freshwater actinobacteria are low GC". Environmental Microbiology Reports. 4 (1): 29–35. doi:10.1111/j.1758-2229.2011.00274.x. PMID 23757226.
- Hayward D, van Helden PD, Wiid IJ (2009). "Glutamine synthetase sequence evolution in the mycobacteria and their use as molecular markers for Actinomycetota speciation". BMC Evol. Biol. 9: 48. doi:10.1186/1471-2148-9-48. PMC 2667176. PMID 19245690.
- Nouioui I, Carro L, García-López M, Meier-Kolthoff JP, Woyke T, Kyrpides NC, Pukall R, Klenk HP, Goodfellow M, Markus Göker M (2018). "Genome-Based Taxonomic Classification of the Phylum Actinobacteria". Front. Microbiol. 9: 2007. doi:10.3389/fmicb.2018.02007. PMC 6113628. PMID 30186281.
- Pandey, B.; Ghimire, P.; Agrawal, V.P. (January 12–15, 2004). Studies on the antibacterial activity of the Actinomycetes isolated from the Khumbu Region of Nepal (PDF). International Conference on the Great Himalayas: Climate, Health, Ecology, Management and Conservation. Kathmandu. Archived from the original (PDF) on 2013-08-10.
- Baltz, R.H. (2005). "Antibiotic discovery from Actinomycetes: Will a renaissance follow the decline and fall?". SIM News. 55: 186–196.
- Baltz, R.H. (2007). "Antimicrobials from Actinomycetes: Back to the Future". Microbe. 2 (3): 125–131. Archived from the original on 2013-12-31.
- Actinomycetes genome database