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.
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 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. Phylogeny
The currently accepted taxonomy is based on the List of Prokaryotic names with Standing
in Nomenclature and National Center for Biotechnology Information and the phylogeny is based on
16S rRNA-based LTP release 106 by The All-Species Living Tree Project
Notes: ♪ Prokaryotes where no pure 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 but not listed in the List of Prokaryotic names with Standing
in Nomenclature References
Further reading Pandey, B.; Ghimire, P.; Agrawal, V.P.. "Studies
on the antibacterial activity of the Actinomycetes isolated from the Khumbu Region of Nepal".
International Conference on the Great Himalayas: Climate, Health, Ecology, Management and Conservation.
Kathmandu. Baltz, R.H.. "Antibiotic discovery from actinomycetes:
Will a renaissance follow the decline and fall?". SIM News 55: 186–196.
Baltz, R.H.. "Antimicrobials from Actinomycetes: Back to the Future". Microbe 2: 125–131.
External links Actinomycetes genome database