Actinobacterium

Actinobacteria are a phylum of Gram-positive bacteria with high guanine-cytosine (G+C) content in their DNA. An actinobacterium is any member of this phylum. They are metabolically diverse and occupy a wide range of habitats, from soil and freshwater to the human skin and gut. Most actinobacteria are aerobic, though some are facultative or obligate anaerobes.

Morphology and cell envelopes: many actinobacteria have thick peptidoglycan-rich cell walls and lack outer membranes typical of Gram-negatives. Some genera are filamentous and form branching hyphae, resembling fungi (for example Streptomyces and Actinomyces). Other members are non-filamentous rods or cocci. A subset, including Mycobacterium and Nocardia, are acid-fast due to the presence of mycolic acids in their cell walls.

Ecology and metabolism: actinobacteria play major roles in soil nutrient cycling, including decomposition of organic matter and lignocellulose. Frankia forms nitrogen-fixing symbioses with certain plants. In the human environment and microbiome, Bifidobacterium and other actinobacteria contribute to fermentations and immunity; some actinobacteria are opportunistic pathogens.

Significance and examples: the best-known genus Streptomyces produces a large array of antibiotics. Pathogenic and clinically important genera include Mycobacterium (tuberculosis, leprosy), Corynebacterium (diphtheria), Nocardia (nocardiosis), and Actinomyces (actinomycosis). Taxonomically, actinobacteria are grouped within the phylum Actinobacteria; classification relies on phylogenetic analyses of 16S rRNA and whole-genome data.