Actinomycetes

Actinomycetes are a group of Gram-positive, predominantly aerobic bacteria within the phylum Actinobacteria. They are characterized by high GC content and a filamentous, mold-like growth form. Many species form networks of hyphae that differentiate into aerial hyphae and, in genera such as Streptomyces, produce chains of conidial spores. They inhabit soil and freshwater, where they play a major role in decomposing complex organic materials and contributing to nutrient cycling. Their soil-dwelling activity is associated with the distinctive earthy odor geosmin.

Most actinomycetes are saprotrophs, but several genera include pathogenic species, notably Actinomyces israelii (actinomycosis) and Nocardia spp. (nocardiosis). Mycobacteria are related actinobacteria but are classified in separate lineages; the term actinomycete is therefore historical and taxonomic usage varies.

A defining feature is their ability to produce a wide array of secondary metabolites. Actinomycetes, especially Streptomyces, are renowned as antibiotic producers and provide many clinically important drugs such as streptomycin, chloramphenicol, erythromycin, and tetracycline, as well as antifungals and immunosuppressive agents. They also yield industrial enzymes and have applications in bioremediation and soil health.

Taxonomically, actinomycetes are not a formal rank but refer to filamentous, high-GC Gram-positive bacteria within Actinobacteria, including genera such as Streptomyces, Actinomyces, Nocardia, Micromonospora, and Frankia.