Gram-positive, aerobic, filamentous bacteria of the order Actinomycetales, abundant in soil, compost and water, growing in branching threads that resemble fungal mycelium. They synthesise a remarkable share of known antibiotics and produce geosmin and 2-methylisoborneol — the compounds behind earthy-musty taints in drinking water, farmed fish and beetroot. Food scientists meet them as allies supplying natural antimicrobials such as natamycin for cheese rinds, and as adversaries causing off-flavours in raw materials. Some are plant or animal pathogens, and several are exploited industrially for lipase and cellulase production.