Microbial Supernatants as Green Nanofactories for Antimicrobial Silver Nanoparticles—Update from 2015

Nanomaterials are emerging as promising alternatives to conventional antibiotics in the fight against a wide variety of pathogenic bacterial species. Among metal-based nanomaterials, silver nanoparticles (AgNPs) are of particular interest because of their broad-spectrum antimicrobial activity and potential applications in the biomedical field. Biologically synthesized AgNPs (Bio-AgNPs) can be produced using plant extracts, bacteria and fungi. These approaches are eco-friendly, cost-effective, and associated with low cytotoxicity. This review provides an up-to-date summary of the recent literature, presenting a comprehensive overview of the biological synthesis of AgNPs, with particular emphasis on bacterial and fungal derivatives. This green synthesis typically follows intracellular or extracellular pathways, in which enzymes, proteins, and metabolites mediate the reduction of silver ions and stabilization of nanoparticles (NPs). Physicochemical parameters, such as pH, temperature, and precursor concentration, and various features of Gram-positive and Gram-negative bacteria strongly influence NPs’ attributes, including size, morphology, and functional properties. In addition, culture supernatants and media components may drive NPs formation, supporting the feasibility of cell-free (CF) biosynthesis of NPs. We also summarize the principal antimicrobial mechanisms proposed for AgNPs and place these findings in a broader context by considering other metal NPs (Au, Ti, Zn, and Cu). Despite substantial recent progress and growing interest in NPs, major challenges remain, including mechanistic understanding, limited reproducibility, and difficulty in scaling up production. Overall, microorganisms and their CF derivatives represent a sustainable and versatile platform for developing functional nanomaterials, such as AgNPs, with significant potential for antimicrobial applications even in combating antimicrobial resistance (AMR).