From Isolation to Plant Growth Evaluation: Development of a Streptomyces-Based Bioinput Using Spent Yeast and Composting Leachate

This study aimed to develop a sustainable bioinput using Streptomyces sp. BEI-18A cultivated in an alternative culture medium (ACM) formulated with winery spent yeast and composting leachate. Actinomycetes were initially isolated from grape waste composting piles and screened for agricultural potential in vitro. Streptomyces sp. BEI-18A was selected for further bioinput development based on its high siderophore production. The ACM formulation was optimized in three steps: (I) determining the optimal concentration of winery spent yeast through mixture design; (II) assessing the effect of composting leachate addition on microbial growth; and (III) establishing the final composition of ACM components. The optimized ACM consisted of 3 g/L spent yeast, 2 g/L sucrose, 1 g/L soybean extract, and 10% (v/v) composting leachate. Cultivation of Streptomyces sp. BEI-18A in this medium resulted in a bioinput containing 7.80 × 107 CFU/mL. Its agricultural potential was validated in pot experiments with wheat and soybean, where application of the bioinput promoted significant improvements in early plant growth, enhancing several phytometric parameters. The results highlight the feasibility of valorizing agro-industrial residues as low-cost substrates for microbial bioinput production. This approach represents a promising strategy to foster sustainability in agriculture while reducing environmental impacts.