Actinobacteria

Actinobacteria or actinomycetes are a group of Gram-positive bacteria with high G+C ratio.

Contents
⇒ Characteristics
⇒ Genera
⇒ References

Characteristics
They include some of the most common soil life, playing an important role in decomposition of organic materials, such as cellulose and chitin and thereby playing a vital part in organic matter turnover and carbon cycle. This replenishes the supply of nutrients in the soil and is an important part of humus formation. Other Actinobacteria inhabit plants and animals, including a few pathogens, such as Mycobacterium, Corynebacterium, Nocardia, Rhodococcus and a few species of Streptomyces.

Actinobacteria are well known as secondary metabolite producers and hence of high pharmacological and commercial interest. In 1940 Selman Waksman discovered that the soil bacteria he was studying made actinomycin, a discovery which granted him a Nobel Prize. Since then hundreds of 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 the unrelated 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 are responsible for the peculiar odor emanating from the soil after rain, mainly on warmer climates.

Actinobacteria

Scanning electron micrograph of Actinomyces israelii.

Scientific classification

Kingdom:    Bacteria Phylum:      Actinobacteria                    Margulis Class:         Actinobacteria

Subclasses/Orders

⇒ Acidimicrobidae   ⇒ Acidimicrobiales     Contains one genus Acidimicrobium ⇒ Actinobacteridae   ⇒ Actinomycetales   ⇒ Bifidobacteriales ⇒ Coriobacteridae   ⇒ Coriobacterales     Contains one family Coriobacteriaceae ⇒ Rubrobacteridae   ⇒ Rubrobacterales     Contains one genus Rubrobacter ⇒ Sphaerobacteridae   ⇒ Sphaerobacterales     Contains one genus Sphaerobacter

Genera
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. Examples of organisms in this order include:

⇒ Suborder Actinomycineae   ⇒ Family Actinomycetaceae       Actinomyces   ⇒ Family Propionibacteriaceae       Propionibacterium   ⇒ Family Frankiaceae       Frankia   ⇒ Family Micrococcaceae       Arthrobacter       Micrococcus   ⇒ Family Micromonosporaceae       Micromonospora   ⇒ Family Streptomycetaceae       Streptomyces ⇒ Suborder Corynebacterineae   ⇒ Family Mycobacteriaceae       Mycobacterium   ⇒ Family Corynebacteriaceae       Corynebacterium   ⇒ Family Nocardiaceae       Nocardia

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.

Genomes of 44 different strains of Actinobacteria from different genera are either already sequenced or underway right now.
References
- Stackebrandt E, Rainey FA, and Ward-Rainey NL (1997). Proposal for a new hierarchic classification system, Actinobacteria classis nov. Int J Syst Bacteriol 47:479-491.