Green synthesis, characterization, and antimicrobial activity of silver nanoparticles from water-soluble fractions of Brazilian Kefir.

Nanotechnology offers promising new avenues for combating drug-resistant pathogens. Given its antioxidant capacity, the water-soluble fraction of Brazilian kefir was hypothesized to serve as an effective reducing agent for the green synthesis of silver nanoparticles (AgNPs). It was further hypothesized that both the entire fraction (WSF) and the < 10 kDa fraction AgNPs would augment the therapeutic effects of kefir, particularly its antimicrobial activity. The successful synthesis was confirmed through the use of UV-Visible spectroscopy and Fourier-transform infrared analyses. WSF-AgNPs demonstrated potent antimicrobial activity, with minimum inhibitory concentrations of 25 µg/mL against A. baumannii (p < 0.0001) and 50 µg/mL against K. pneumoniae (p < 0.0001). Although no toxicity was observed in long-term tests on adult Drosophila melanogaster, AgNPs intake impaired larvae development. Oxidative stress analysis showed mild oxidative imbalance on advanced oxidation protein products (AOPP), sulfhydryl, and reduced glutathione (GSH) contents, with no alterations observed in reactive oxygen species (ROS) quantities, ferric reducing antioxidant power (FRAP), and catalase (CAT) activity. These findings suggest that kefir-derived AgNPs may have potential for combating drug-resistant infections. Future studies should focus on enhancing specificity through compound conjugation and investigating broader applications, including disinfectants, wound healing, and antibiotic development.