Actinobacteria are Gram‐stain‐positive microorganisms with high guanine plus cytosine content in their genome (>50%). They are quite abundant in soil and aquatic sediments where they participate in the decomposition of organic matter. These bacteria are responsible for the distinctive scent of freshly exposed, moist soil. Actinobacteria are morphologically diverse ranging from coccoid, fragmenting hyphal forms to those with a highly differentiated branched mycelium. Many of these bacteria also produce external spores resistant to UV (ultraviolet) light and dehydration. Most actinobacteria are saprophytic microorganisms but several genera are important human, animal and plant pathogens. They are major producers of medically important antibiotics, especially members of the genus Streptomyces, the most abundant group. On the basis of 16S rRNA (ribosomal ribonucleic acid) gene sequences, the phylum actinobacteria contains almost 300 genera but new taxa continue to be discovered.

Key Concepts

  • The phylum actinobacteria is one of the largest groups in the domain Bacteria.
  • Actinobacteria are Gram‐positive bacteria with a high guanine plus cytosine content in their genome.
  • Morphologically, actinobacteria are quite diverse ranging from cocci to complex filaments which form a mycelium and produce specialised reproductive structures called spores.
  • Actinobacteria are chemoorganotrophic and play an important role in the biodegradation and recycling of organic matter.
  • Actinobacteria are mainly aerobes with an oxidative metabolism, but several species are facultative anaerobes or even strict anaerobes.
  • Actinobacteria are widespread in soil but they are also present in marine and freshwater sediments, mangrove ecosystems and inside plant tissues.
  • Most actinobacteria are free‐living microorganisms, but several species are important plant and animal pathogens.
  • Actinobacteria are unsurpassed in their ability to produce many useful compounds with application in medicine, agriculture and industry.
  • Genomic sequence data has revealed that actinobacteria have the potential to produce far more secondary metabolites than previously thought.

Keywords: actinobacteria; classification; Streptomyces; secondary metabolites; phylogeny

Figure 1. Morphological diversity found in the phylum actinobacteria. (a) Nocardia sp.; (b) Streptomyces sp. producing a secondary metabolite (droplets); (c) Micromonospora sp.; (d and e) Streptomyces sp.; (f) Actinomadura sp.; (g) edge of a streptomycete colony producing an antibiotic and (h) Micromonospora sp. (b, e, g) Courtesy of Dr Ramón Santamaria.
Figure 2. Steps involved in the life cycle of a filamentous actinobacterium. Based on information in Kieser et al. ().
Figure 3. Classification of the phylum actinobacteria based on Bergey's Manual of Systematic Bacteriology.


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Trujillo, Martha E(Apr 2016) Actinobacteria. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0020366.pub2]