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Scanning electron micrograph of Actinomyces israelii.
Scientific classification
Class:Actinobacteria Stackebrandt et al. 1997
Subclasses & Orders
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Scanning electron micrograph of Actinomyces israelii.
Scientific classification
Class:Actinobacteria Stackebrandt et al. 1997
Subclasses & Orders
  • ?Nostocoida limicola I ♠
  • ?Candidatus Planktophila Jezbera et al. 2009
  • ?CathayosporangiumRunmao et al. 1995
  • ?Tonsillophilus suisAzuma and Bak 1980
  • Acidimicrobidae Stackebrandt et al. 1997 emend. Zhi et al. 2009
  • Coriobacteridae Stackebrandt et al. 1997 emend. Zhi et al. 2009
  • Nitriliruptoridae Kurahashi et al. 2010
    • Nitriliruptorales Sorokin et al. 2009
    • Euzebyales Kurahashi et al. 2010
  • Rubrobacteridae Rainey et al. 1997 emend. Zhi et al. 2009
    • Gaiellales Albuquerque et al. 2012
    • Rubrobacterales Rainey et al. 1997 emend. Zhi et al. 2009
    • Solirubrobacterales Reddy and Garcia-Pichel 2009
    • Thermoleophilales Reddy and Garcia-Pichel 2009
  • Actinobacteridae Stackebrandt et al. 1997 emend. Zhi et al. 2009

Actinobacteria are a group of Gram-positive bacteria. They were all believed to have high guanine and cytosine content in their DNA.[1][2] However, recently it has been shown that several freshwater Actinobacteria actually have low G+C content. [3] The G+C content of freshwater Actinobacteria can be as low as 42%. In view of this, use of the epithet high G+C Gram positive organisms to refer to Actinobacteria needs to be discontinued. They can be terrestrial or aquatic.[4] Although understood primarily as soil bacteria, they might be more abundant in freshwaters.[5] Actinobacteria is one of the dominant bacterial phyla and contains one of the largest of bacterial genera, Streptomyces.[6] Analysis of glutamine synthetase sequence has been suggested for phylogenetic analysis of Actinobacteria.[7]


Actinobacteria include some of the most common soil life, freshwater life, and marine life, and play an important role in the decomposition of organic materials (such as cellulose and chitin). They thereby play a vital role in organic matter turnover and the carbon cycle replenishing the supply of nutrients and becoming an important part of humus formation. Other genuses of Actinobacteria inhabit plants and animals, and include some well-known pathogens; for example, the genus Mycobacterium includes the species M. tuberculosis (which causes tuberculosis) and M. leprae (which causes leprosy). Corynebacterium includes C. diphtheriae which causes diphtheria, Nocardia has several pathogenic species which cause nocardiosis, and Rhodococcus has two pathogenic species: one affects the tobacco plant, and the other is an equine pathogen that largely affects foals.

Actinobacteria are well known as secondary metabolite producers and hence of high pharmacological and commercial interest. In 1942 Selman Waksman discovered that the soil bacteria he was studying made actinomycin, a discovery for which he received a Nobel Prize. Since then, hundreds of other naturally occurring antibiotics have been discovered in these terrestrial microorganisms, especially from the genus Streptomyces.

Some Actinobacteria form branching filaments, which somewhat resemble the mycelia of fungi, among which they were originally classified under the older name Actinomycetes. Most members are aerobic, but a few, such as Actinomyces israelii, can grow under anaerobic conditions. Unlike the Firmicutes, the other main group of Gram-positive bacteria, they have DNA with a high GC-content, and some Actinomycetes species produce external spores.

Some types of Actinobacteria are responsible for the peculiar odor emanating from the soil after rain (petrichor), mainly in warmer climates. They produce a chemical, geosmin, that forms a large part of the odor.[8]

The Actinomycetes are also known to form intracellular inclusions of polyhydroxyalkanoates under certain environmental conditions (e.g. lack of elements such as phosphorus, nitrogen, or oxygen combined with an excessive supply of carbon sources).


Most Actinobacteria of medical or economic significance are in subclass Actinobacteridae, order Actinomycetales. While many of these cause disease in humans, Streptomyces is notable as a source of antibiotics.

Of those Actinobacteria not in 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.[9]


The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature (LPSN) [10] and National Center for Biotechnology Information (NCBI)[11] and the phylogeny is based on 16S rRNA-based LTP release 106 by The All-Species Living Tree Project [12]

?Nostocoida limicola I ♠

?Candidatus Planktophila limnetica Jezbera et al. 2009

?Cathayosporangium alboflavumRunmao et al. 1995

?Tonsillophilus suisAzuma and Bak 1980

Rubrobacter Suzuki et al. 1989


Gaiella occulta Albuquerque et al. 2012

Thermoleophilum Zarilla and Perry 1986





Euzebya tangerina Kurahashi et al. 2010

Nitriliruptor alkaliphilus Sorokin et al. 2009


?Boyliae praeputialeYates et al. 2002

?Frankia alni(Woronin 1866) Von Tubeuf 1895

?Motilibacter peucedani Lee 2012

Acidothermus cellulolyticus Mohagheghi et al. 1986




Actinocatenispora Thawai et al. 2006 emend. Seo and Lee 2009



Pseudonocardiaceae [incl. Actinopolyspora]



Streptomyces Waksman and Henrici 1943 emend. Witt and Stackebrandt 1991 [incl. Kitasatospora & Streptacidiphilus]

Sporichthya Lechevalier et al. 1968





Kineococcus Yokota et al. 1993

Angustibacter luteus Tamura et al. 2010

Micrococcineae [incl. Actinomycetaceae & Bifidobacteriaceae]

♪ Prokaryotes where no pure (axenic) cultures are isolated or available, i. e. not cultivated or can not be sustained in culture for more than a few serial passages
♠ Strains found at the National Center for Biotechnology Information (NCBI) but not listed in the List of Prokaryotic names with Standing in Nomenclature (LSPN)


  1. ^ 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. PMC 2168647. PMID 17804669. 
  2. ^ "MB451 Actinobacteria lecture". Retrieved 2008-11-21. [dead link]
  3. ^ 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. 
  4. ^ 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. 
  5. ^ Ghai R, Rodriguez-Valera F, McMahon KD, et al. (2011). "Metagenomics of the water column in the pristine upper course of the Amazon river". In Lopez-Garcia, Purification. PloS ONE 6 (8): e23785. doi:10.1371/journal.pone.0023785. PMC 3158796. PMID 21915244. 
  6. ^ C.Michael Hogan. 2010. Bacteria. Encyclopedia of Earth. eds. Sidney Draggan and C.J.Cleveland, National Council for Science and the Environment, Washington DC
  7. ^ Hayward D, van Helden PD, Wiid IJ (2009). "Glutamine synthetase sequence evolution in the mycobacteria and their use as molecular markers for Actinobacteria speciation". BMC Evol. Biol. 9: 48. doi:10.1186/1471-2148-9-48. PMC 2667176. PMID 19245690. 
  8. ^ "HowStuffWorks "What causes the smell after rain?"". 2000-09-29. Retrieved 2012-10-16. 
  9. ^ Gardnerella at the US National Library of Medicine Medical Subject Headings (MeSH)
  10. ^ J.P. Euzéby. "Actinobacteria". List of Prokaryotic names with Standing in Nomenclature (LPSN) [1]. Retrieved 2011-11-17. 
  11. ^ Sayers et al. "Actinobacteria". National Center for Biotechnology Information (NCBI) taxonomy database [2]. Retrieved 2011-06-05. 
  12. ^ All-Species Living Tree Project."16S rRNA-based LTP release 106: full tree. accessdate=2011-11-17". 

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