Green Synthesis and Characterization of Different Metal Oxide Microparticles by Means of Probiotic Microorganisms

Microparticles (MPs) are delivery systems for bioactive compounds with particle sizes in the micrometer range (1–1000 μm). This study reports a green protocol for the biosynthesis of ZnO-, MgO-, and CaO-MPs using the probiotic strains Lactobacillus delbrueckii subsp. bulgaricus, Streptococcus thermophilus, and Leuconostoc mesenteroides. Ultraviolet–visible (UV-Vis) spectroscopy, scanning electron microscopy (SEM), and dynamic light scattering (DLS) were used for the preliminary characterization of the metal oxide MPs. Antimicrobial activity was evaluated against pathogenic and phytopathogenic microorganisms, including Salmonella typhimurium, Staphylococcus aureus, Escherichia coli, and Ralstonia solanacearum. UV-Vis analysis revealed previously reported blue shifts in the ZnO- and CaO-MPs. DLS measurements showed particle sizes larger than 1000 nm in 95% of the cases, while smaller sizes were observed by SEM. The stability of the MPs, based on their zeta potential values, ranged from relatively to moderately stable. This study demonstrates that the six probiotic lactic acid bacteria strains are capable of synthesizing ZnO-MPs, CaO-MPs, and MgO-MPs. All MPs exhibited antimicrobial activity against pathogens and phytopathogens at different concentrations. Although similar antimicrobial effects have been reported for metal oxide nanoparticles produced by probiotic bacteria, considering the potential environmental and human health impacts of nanoparticles, the use of safer materials obtained through green synthesis—such as metal oxide MPs—may represent a more suitable alternative.