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8.8.4: Actinobacteria (High G + C Gram-Positive Bacteria)

Actinobacteria are a group of Gram-positive bacteria with high guanine and cytosine content in their DNA.

 

LEARNING OBJECTIVES

 

Outline the characteristics associated with Actinobacteria

 

KEY TAKEAWAYS

Key Points

 

  • Actinobacteria is one of the dominant phyla of the bacteria.
  • Actinobacteria include some of the most common soil life, freshwater life, and marine 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.
  • Actinobacteria are well known as secondary metabolite producers and hence of high pharmacological and commercial interest.
  • Some types of Actinobacteria are responsible for the peculiar odor emanating from the soil after rain (Petrichor), mainly in warmer climates.

 

Key Terms

 

  • actinobacteria: A group of Gram-positive bacteria with high guanine and cytosine content in their DNA
  • petrichor: The distinctive scent which accompanies the first rain after a long warm dry spell.
  • actinomycin: Any of a class of toxic polypeptide antibiotics found in soil bacteria of genus Streptomyces.

Actinobacteria are a group of Gram-positive bacteria with high guanine and cytosine content in their DNA. They can be terrestrial or aquatic. Actinobacteria is one of the dominant phyla of the bacteria. Analysis of glutamine synthetase sequence has been suggested for phylogenetic analysis of Actinobacteria.

Actinobacteria include some of the most common soil life, freshwater life, and marine 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 for which he received 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 (Petrichor), mainly in warmer climates. The chemical that produces this odour is known as Geosmin. 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.

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Actinomyces israelii: Scanning electron micrograph of Actinomyces israelii.

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