| Summary: | <p>Abstract</p> <p>Background</p> <p>The current model for the developmental cycle of <it>Streptomyces </it>confluent cultures on agar surface is based on the assumption that the only differentiation takes place along the transverse axis (bottom-up): a vegetative (substrate) mycelium grows completely live and viable on the surface and inside the agar until it undergoes a death process and differentiates to a reproductive (aerial) mycelium which grows into the air. Hence, this vertical description assumes that the development in the pre-sporulating phases is more or less homogeneous in all zones of the plate surface.</p> <p>Results</p> <p>The work presents a detailed analysis of the differentiation cycle in <it>Streptomyces antibioticus </it>ATCC11891 considering a different spatial dimension: the longitudinal axes, represented by the plate surface. A previously unsuspected complexity during the substrate mycelial phase was detected. We have demonstrated that the young substrate hyphae suffer an early death round that has not been previously described. Subsequently, the remaining mycelium grows in successive waves which vary according to the density of the spore inoculum. In the presence of dense inocula (1.5 × 10<sup>6 </sup>spores per plate), the hyphae develop in regular circles, approximately 0.5 cm in diameter. By contrast, with highly diluted inocula (6 × 10<sup>3 </sup>spores per plate), aerial mycelium develops initially in the form of islands measuring 0.9 mm in diameter. Further mycelial development occurs between the circles or islands until the plate surface is totally covered. This pattern persists throughout the entire developmental cycle including the sporulation phases.</p> <p>Conclusion</p> <p>An early death round during the substrate mycelial phase of <it>Streptomyces antibioticus </it>ATCC11891 takes place prior to successive growth periods in surface cultures. These developmental periods in turn, determine the shape of the complex multiphase growth curves observed. As shown here, these results also apply to other <it>Streptomyces </it>strains and species. Understanding these peculiarities of the <it>Streptomyces </it>developmental cycle is essential in order to properly interpret the morphological/biochemical data obtained from solid cultures and will expand the number of potential phenotypes subject to study.</p>
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