Characterization of actinobacteria from the semiarid region, and their antagonistic effect on strains of rhizobia

Abstract

The actinobacteria are the most abundant bacterial group in the soil, where they present different characteristics and antagonistic effects on other microorganisms. The objectives of this study were to characterize and evaluate the antagonistic effect of actinobacteria from the semiarid region on rhizobia from the same climatic region. Tests were performed to observe chromogenic and micro-morphological characteristics, tolerance to pH levels of melanin production, and use of carbon sources. This occurred due to the use of modified CDA culture media and basal medium supplemented with specific carbon sources, as well as the use of Ral color chat to observe the colonies. The in vitro antagonism on rhizobia was evaluated in yeast malt agar (YMA) culture medium for the observation of inhibition halo by actinobacteria. We observed different characteristics in relation to the color of aerial mycelium and reverse pigments; tolerance to media with acid and alkaline pH; and production of melanoid pigment occurred only in three strains. The morphological characteristics were described for genera Streptomyces and Saccharothrix, later confirmed by molecular sequencing. These genera were generalists in use of carbon sources, showing their physiological versatility. Regarding antagonism, nine strains of the genus Streptomyces were able to inhibit in vitro strains of Rhizobium tropici and Bradyrhizobium yuanmingense also coming from semiarid soils. The antagonism between actinobacteria and rhizobia can directly affect the symbiosis between nitrogen-fixing bacteria and leguminous plants. These results are pioneering in observing antagonism of these species of rhizobia under the climate condition of the semiarid region, and may contribute to agricultural biotechnology.