Development and Organization of the Aerial Mycelium in Streptomyces coelicolor

SUMMARY: The anatomy of individual colonies of Streptomyces coelicolor was studied at various developmental stages in situ by means of surface-impressions and thin sections. Young colonies consisted of substrate mycelium composed of a loose network of hyphae with uniform appearance. The uppermost cells produced the very closely packed hyphae of the aerial mycelium. Subsequently the surface layer of the aerial mycelium began to sporulate but became overgrown by young hyphae which formed a new sporulating zone above the first. The process was repeated several times. Many of the early produced spores germinated immediately. When the climax of sporulation was reached the aerial mycelium showed two trends of development: one towards spore formation (in the surface layer) and one towards lysis of non-sporulating hyphae (below the sporulating zone). Sporulation was initiated by coiling of hyphal tips which were then divided by cross walls into chains of spore-sized compartments. The basal non-sporulating parts of the hyphae disintegrated later. The most conspicuous cytological change during lysis was the appearance of large dense granules.