Effect of osmotic potential on streptomycete growth,antibiotic production and antagonism to fungi

The growth of eighteen isolates of Streptomyces spp. on agars, the osmotic potentials of which were controlled by the addition of various solutes, showed that nine isolates were able to grow at ?100 bars and two of these grew at ?150 bars. This suggests that the water requirements of soil streptomycetes are almost as diverse as those of soil fungi. In general, linear growth and sporulation declined with decreasing osmotic potential, but one isolate grew optimally at potentials between ?5 and ?10 bars. Another isolate formed maximum amounts of a water-diffusible antibiotic per unit colony area at potentials between ?20 and ?35 bars. This appears to be the first report of osmotic potential influencing the production of antibiotic by Streptomyces. The increased antibiotic production at moderate potentials was associated with inhibition of Pythium ultimum, Rhizoctonia solani and a Curvularia sp. by the streptomycete at these potentials on osmotic agar plates. The possible ecological significance of this phenomenon in soil is discussed.