The relevance of actinobacteria as sources of antioxidant compounds: Evaluation of Streptomyces isolates from rhizosphere collected at Brazilian Caatinga

Abstract Actinobacteria constitute a group of Gram-positive bacteria found in different ecosystems, such as soil, seawater, human skin as well as lung and gastrointestinal tracts. Secondary metabolites produced by these microorganisms have important biological activities related to beneficial effects on human health and actinobacteria are the prokaryotes of greatest economic and biotechnological values. Actinobacteria from different genera have been reported as antioxidant-producing microorganisms. These antioxidant-producing strains can be isolated from different environments and polysaccharides and secondary metabolites have been related with this property. The long periods of drought, high insolation, and temperatures of the Caatinga (a seasonally dry tropical forest found in the semiarid region of Northeastern Brazil) are characteristics that favor the development of organisms with specific adaptations to survive this environment and the oxidative stress to which they are subjected daily. Thus, Caatinga microorganisms are promising sources of compounds with antioxidant action. Culture filtrates of Streptomyces sp. isolates (CA-02, CA-06, CA-07, and CA-17) from the rhizosphere soil of a Caatinga plant showed high phenolic and flavonoid contents. The filtrates of CA-02 and CA-06 contained vitexin and rutin while those from CA-07 and CA-17 contained rutin. All the filtrates showed antioxidant activity by the phosphomolybdenum method (total antioxidant activity), DPPH and ABTS+ assays, and NO inhibition assay, with best results found for CA-02. In conclusion, actinobacteria are promising sources for obtaining antioxidant molecules, whose production can be optimized under controlled conditions, aiming at a range of biotechnological applications.