|Scanning electron micrograph of Actinomyces israelii.|
Stackebrandt et al. 1997
|Subclasses & Orders|
Actinobacteria is a phylum of 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 behave much like fungi, helping to decompose the organic matter of dead organisms so that the molecules can be taken up anew by plants. 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 actinobacteria (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.
Beyond the great interest in actinobacteria 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 freshwaters. Actinobacteria is one of the dominant bacterial phyla and contains one of the largest of bacterial genera, Streptomyces. Streptomyces and other actinobacteria are major contributors to biological buffering of soils. They are also the source of many antibiotics.
Although some of the largest and most complex bacterial cells belong to the Actinobacteria, the group of marine Actinomarinales has been described as possessing the smallest free-living prokaryotic cells.
Most Actinobacteria of medical or economic significance are in subclass Actinobacteridae, and belong to the 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.
Actinobacteria, especially Streptomyces sp., are recognized as the producers of many bioactive metabolites that are useful to humans in medicine, such as antibacterials, antifungals, antivirals, antithrombotics, immunomodifiers, anti-tumor drugs and enzyme inhibitors; and in agriculture, including insecticides, herbicides, fungicides and growth promoting substances for plants and animals. Actinobacteria-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)  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)
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- Gardnerella at the US National Library of Medicine Medical Subject Headings (MeSH)
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