The Actinobacteria or Actinomycetes are a group of Gram-positive bacteria. Most are found in the soil, and they include some of the most common soil life, playing an important role in decomposition of organic materials, such as cellulose and chitin. 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.

The ability of Actinobacteria to thrive in low nutrient evironments and to consume lignocellulose (a component of decaying plant matter not normally consumed by most soil bacteria) have allowed the Actinobacteria to become the predominant bacteria in karst cave systems, where they survive on a seasonally fluctuating supply of carbon washed in from the surface. Actinobacteria colonies can be seen as small (1 - 10 mm) yellow or white round colonies, usually on upper cave walls above the main streamways.

Some Actinobacteria form braching 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.

Representative genera include:

• Actinomyces
• Arthrobacter
• Bifidobacterium
• Corynebacterium
• Frankia
• Micrococcus
• Micromonospora
• Mycobacterium
• Nocardia
• Propionibacterium
• Streptomyces

Actinobacteria are unsurpassed in their ability to produce many compounds that have pharmaceutically useful properties. 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.

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. Abstract

• Groth I, and Saiz-Jimenez C (1999). Actinomycetes in hypogean environments. Geomicrobiology Journal 16: 1-8.

Bacteria | Actinobacteria | Soil biology

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