Actinomycetales

Actinomycetales is an order of Gram-positive, filamentous bacteria within the phylum Actinobacteria. Members grow as branching mycelia that can resemble fungal hyphae and often form aerial structures and spores. They typically have high guanine-plus-cytosine (G+C) content in their DNA. The order encompasses many genera, including Streptomyces, Actinomyces, Nocardia, Micromonospora, and Nocardiopsis. Metabolic lifestyles vary, with aerobic, microaerophilic, and some anaerobic or facultatively anaerobic representatives, and ecological niches ranging from soil to plant and animal-associated habitats.

Ecology and physiology: Actinomycetales are major soil decomposers, capable of breaking down complex organic matter through a broad array of extracellular enzymes. They contribute to nutrient cycling and soil structure, and several members produce secondary metabolites that influence microbial communities. A notable ecological trait is the production of geosmin, the earthy smell often associated with wet soil.

Industrial and medical relevance: The group is renowned for natural product biosynthesis. Streptomyces species, in particular, are prolific producers of antibiotics, antifungals, immunosuppressants, and other bioactive compounds, making Actinomycetales a cornerstone of pharmaceutical discovery.

Clinical aspects: A subset of Actinomycetales can cause disease in humans and animals. Actinomyces israelii causes actinomycosis, a chronic granulomatous infection. Nocardia species cause nocardiosis, especially in immunocompromised individuals. Other members may be opportunistic pathogens or environmental species with limited pathogenic potential. Overall, most actinomycetes contribute positively to ecosystems and biotechnology.