|Scanning electron micrograph of Actinomyces israelii.|
|Class:||Actinobacteria Stackebrandt et al. 1997|
|Subclasses & Orders|
Actinobacteria are a group of Gram-positive bacteria. They were all believed to have high guanine and cytosine content in their DNA. However, recently it has been shown that several freshwater Actinobacteria actually have low G+C content.  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. Although understood primarily as soil bacteria, they might be more abundant in freshwaters. Actinobacteria is one of the dominant bacterial phyla and contains one of the largest of bacterial genera, Streptomyces. Analysis of glutamine synthetase sequence has been suggested for phylogenetic analysis of Actinobacteria.
Actinobacteria include some of the most common soil life, freshwater life, and marine life, playing an important role in the decomposition of organic materials, such as cellulose and chitin, and thereby playing a vital part in organic matter turnover and the carbon cycle. In the soil, this replenishes the supply of nutrients and is 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 causing diphtheria; Nocardia which has several pathogenic species commonly causing nocardiosis and Rhodococcus which has two pathogenic species a major one affecting the tobacco plant and one which is largely an equine pathogen affecting 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, the Actinomycetes are responsible for the peculiar odor emanating from the soil after rain (petrichor), mainly in warmer climates. The chemical that produces this odor is known as geosmin.
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.
The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature (LPSN)  and National Center for Biotechnology Information (NCBI) and the phylogeny is based on 16S rRNA-based LTP release 106 by The All-Species Living Tree Project 
♪ 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)
- 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.
- "MB451 Actinobacteria lecture". Retrieved 2008-11-21.[dead link]
- 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.
- 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). "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.
- C.Michael Hogan. 2010. Bacteria. Encyclopedia of Earth. eds. Sidney Draggan and C.J.Cleveland, National Council for Science and the Environment, Washington DC
- 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.
- "HowStuffWorks "What causes the smell after rain?"". Science.howstuffworks.com. 2000-09-29. Retrieved 2012-10-16.
- Gardnerella at the US National Library of Medicine Medical Subject Headings (MeSH)
- J.P. Euzéby. "Actinobacteria". List of Prokaryotic names with Standing in Nomenclature (LPSN) . Retrieved 2011-11-17.
- Sayers et al. "Actinobacteria". National Center for Biotechnology Information (NCBI) taxonomy database . Retrieved 2011-06-05.
- All-Species Living Tree Project."16S rRNA-based LTP release 106 (full tree)". Silva Comprehensive Ribosomal RNA Database . Retrieved 2011-11-17.
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