Streptomyces is the largest genus of Actinomycetota and the type genus of the family Streptomycetaceae. Over 500 species of Streptomyces bacteria have been described. As with the other Actinomycetota, streptomycetes are gram-positive, and have genomes with high GC content. Found predominantly in soil and decaying vegetation, most streptomycetes produce spores, and are noted for their distinct "earthy" odor that results from production of a volatile metabolite, geosmin. Streptomycetes are characterised by a complex secondary metabolism. They produce over two-thirds of the clinically useful antibiotics of natural origin (e.g., neomycin, streptomycin, cypemycin, grisemycin, bottromycins and chloramphenicol). The antibiotic streptomycin takes its name directly from Streptomyces. Streptomycetes are infrequent pathogens, though infections in humans, such as mycetoma, can be caused by S. somaliensis and S. sudanensis, and in plants can be caused by S. caviscabies, S. acidiscabies, S. turgidiscabies and S. scabies. Taxonomy Streptomyces is the type genus of the family Streptomycetaceae and currently covers close to 576 species with the number increasing every year. Acidophilic and acid-tolerant strains that were initially classified under this genus have later been moved to Kitasatospora (1997) and Streptacidiphilus (2003). Species nomenclature are usually based on their color of hyphae and spores. Saccharopolyspora erythraea was formerly placed in this genus (as Streptomyces erythraeus). Morphology The genus Streptomyces includes aerobic, Gram-positive, multicellular, filamentous bacteria that produce well-developed vegetative hyphae (between 0.5-2.0 µm in diameter) with branches. They form a complex substrate mycelium that aids in scavenging organic compounds from their substrates. Although the mycelia and the aerial hyphae that arise from them are amotile, mobility is achieved by dispersion of spores. Spore surfaces may be hairy, rugose, smooth, spiny or warty. In some species, aerial hyphae consist of long, straight filaments, which bear 50 or more spores at more or less regular intervals, arranged in whorls (verticils). Each branch of a verticil produces, at its apex, an umbel, which carries from two to several chains of spherical to ellipsoidal, smooth or rugose spores. Some strains form short chains of spores on substrate hyphae. Sclerotia-, pycnidia-, sporangia-, and synnemata-like structures are produced by some strains. Genomics The complete genome of "S. coelicolor strain A3(2)" was published in 2002. At the time, the "S. coelicolor" genome was thought to contain the largest number of genes of any bacterium. The chromosome is 8,667,507 bp long with a GC-content of 72.1%, and is predicted to contain 7,825 protein-encoding genes. In terms