ABSTRACT

Actinomycetales are gram-positive, mycelium-forming, soil bacteria that play an important role in mineralization processes in nature. Streptomyces is an extremely important bacterium in biotechnology because approximately twothirds of industrial antibiotics are synthesized by members of this genus.1 Streptomycetes present a complex development cycle that includes programmed cell-death processes, as well as differentiation phenomena that lead to sporulation2-4 (Figure 16.1). After spore germination, a fully compartmentalized mycelium starts to grow and early on suffers an orderly death process in which live and dead segments alternate in the same hyphae.2-4 Subsequently, the viable segments start to enlarge as a multinucleated mycelium that grows in successive waves that determine the characteristic complex growth curves of this microorganism. Two stages of secondary multinucleated mycelium have been deŠned: early secondary mycelium that does not yet have the hydrophobic layers characteristic of aerial hyphae and late secondary mycelium that starts to express the genes encoding for the chaplin and rodlin proteins, which are mainly responsible for the formation of the hydrophobic layer characteristic of aerial structures.5 The traditionally denominated substratevegetative mycelium corresponds to the early secondary multinucleated mycelium, which still lacking the hydrophobic layers characteristic of the aerial mycelium and would match therefore to the late secondary mycelium which has acquired those hydrophobic covers.6 A vegetative role has

been postulated to these compartmentalized hyphae by contrast to the secondary multinucleated mycelium that should be considered the reproductive structure, since it is destined to sporulate.7 The secondary mycelium has been described also as the antibiotic producing.7