Actinobacteria


The Actinobacteria are 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 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, Actinobacteria 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, reflects that they were long believed to be fungi. Some soil actinobacteria 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 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 fresh waters. 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.
Some Siberian or Antarctic Actinobacteria 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 release from permafrost samples 640 kya or younger.

General

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 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.
Actinobacteria, 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. Actinobacteria-derived antibiotics that are important in medicine include aminoglycosides, anthracyclines, chloramphenicol, macrolide, tetracyclines, etc.
Actinobacteria have high guanine and cytosine content in their DNA. The G+C content of Actinobacteria can be as high as 70%, though some may have a low G+C content.
Analysis of glutamine synthetase sequence has been suggested for phylogenetic analysis of the Actinobacteria.

Phylogeny

The phylogeny is based on 16S rRNA-based LTP release 123 by 'The All-Species Living Tree' Project.

Taxonomy

The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature and National Center for Biotechnology Information.