Actinobacteria

Actinobacteria
Scanning electron micrograph of Actinomyces israelii.
Scientific classification
Domain:	Bacteria
Phylum:	Actinobacteria
Class:	Actinobacteria Stackebrandt et al. 1997
Subclasses & Orders
?Nostocoida limicola I ♠ ?Candidatus Planktophila Jezbera et al. 2009 ?Cathayosporangium ♠ Runmao et al. 1995 ?Tonsillophilus suis ♠ Azuma and Bak 1980 Acidimicrobidae Stackebrandt et al. 1997 emend. Zhi et al. 2009 Acidimicrobiales Stackebrandt et al. 1997 emend. Zhi et al. 2009 Coriobacteridae Stackebrandt et al. 1997 emend. Zhi et al. 2009 Coriobacteriales Stackebrandt et al. 1997 emend. Zhi et al. 2009 Nitriliruptoridae Kurahashi et al. 2010 Nitriliruptorales Sorokin et al. 2009 Euzebyales Kurahashi et al. 2010 Rubrobacteridae Rainey et al. 1997 emend. Zhi et al. 2009 Gaiellales Albuquerque et al. 2012 Rubrobacterales Rainey et al. 1997 emend. Zhi et al. 2009 Solirubrobacterales Reddy and Garcia-Pichel 2009 Thermoleophilales Reddy and Garcia-Pichel 2009 Actinobacteridae Stackebrandt et al. 1997 emend. Zhi et al. 2009 Bifidobacteriales Stackebrandt et al. 1997 emend. Zhi et al. 2009 Actinomycetales Buchanan 1917 emend. Zhi et al. 2009

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, continued use of the epithet high G+C Gram positive organisms to refer to Actinobacteria must now 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.

Characteristics

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 Actinobacteria inhabit plants and animals, and include a few pathogens, such as Mycobacterium, Corynebacterium, Nocardia and Rhodococcus.

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 odour is known as Geosmin.

General

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.

Phylogeny

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

?Nostocoida limicola I ♠ ?Candidatus Planktophila limnetica Jezbera et al. 2009 ?Cathayosporangium alboflavum ♠ Runmao et al. 1995 ?Tonsillophilus suis ♠ Azuma and Bak 1980 Rubrobacter Suzuki et al. 1989   Thermoleophilidae Gaiella occulta Albuquerque et al. 2012 Thermoleophilum Zarilla and Perry 1986 Solirubrobacterales Coriobacteriaceae Acidimicrobiales  Nitriliruptoridae Euzebya tangerina Kurahashi et al. 2010 Nitriliruptor alkaliphilus Sorokin et al. 2009   Actinomycetales ?Boyliae praeputiale ♠ Yates et al. 2002 ?Frankia alni ♪ (Woronin 1866) Von Tubeuf 1895 ?Motilibacter peucedani Lee 2012 Acidothermus cellulolyticus Mohagheghi et al. 1986 Jiangellaceae Micromonosporaceae Propionibacterineae Actinocatenispora Thawai et al. 2006 emend. Seo and Lee 2009 Glycomycetaceae Streptosporangineae Pseudonocardiaceae [incl. Actinopolyspora] Corynebacterineae Catenulisporineae Streptomyces Waksman and Henrici 1943 emend. Witt and Stackebrandt 1991 [incl. Kitasatospora & Streptacidiphilus] Sporichthya Lechevalier et al. 1968 Cryptosporangiaceae Geodermatophilaceae Nakamurellaceae Kineosporiaceae Kineococcus Yokota et al. 1993 Angustibacter luteus Tamura et al. 2010 Micrococcineae [incl. Actinomycetaceae & Bifidobacteriaceae]

Notes:
♪ 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)

External links

• Actinomycetes genome database