|Scanning electron micrograph of Actinomyces israelii.|
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, it is recommended that the use of the epithet high G+C Gram positive organisms to refer to Actinobacteria 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.
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.
Streptomyces and other actinobacteria are major contributors to biological buffering of soils and have roles in organic matter decomposition conductive to crop production.
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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