Actinomycetes
Japanese: 放線菌（アクチノマイセテス）
A group of Gram-positive bacteria with branching, filamentous morphology. Major producers of antibiotics, they play important roles in decomposing organic matter in soil.
Actinomycetes are bacteria that branch in filamentous patterns like mold. Though they resemble molds in appearance, they taxonomically belong to bacteria.
The smell of soil after rain—that distinctive scent comes from geosmin, a compound produced by actinomycetes. They contribute significantly to organic matter decomposition in soil.
More importantly, actinomycetes are the greatest source of antibiotics. Streptomycin, tetracycline, erythromycin—roughly 70% of antibiotics in use today derive from actinomycetes. A tiny organism in the soil fundamentally transformed human medicine. This insight deepens fermentation understanding further.