Actinobacteria are Gram-positive bacteria with a high mol% G+C composition, widely distributed in soil where they play an important part in the decomposition of organic matter. These bacteria are responsible for the distinctive scent of freshly exposed, moist soil. Actinobacteria are morphologically diverse ranging from coccoid, fragmenting hyphal forms to those with a highly differentiated branched mycelium. Many of these bacteria also produce external spores. Several actinobacteria are important human, animal and plant pathogens. However, they are major producers of medically important antibiotics, especially members of the genus Streptomyces, the most abundant group.