Effect of iron oxide nanoparticles on antioxidant responses and secretome-driven antimicrobial activity in Bacillus strains

Iron oxide nanoparticles (Fe₃O₄ NPs) have emerged as promising modulators of microbial metabolism, yet their biochemical effects on Bacillus strains with biocontrol potential remain poorly characterized. In this study we investigated the impact of Fe₃O₄ nanoparticles on the antioxidant response and antimicrobial activity of two Bacillus strains: B. subtilis CNMN-BB-10 and B. velezensis CNMN-BB-12. Both strains were cultivated in Mueller–Hinton and Luria–Bertani broths supplemented with Fe₃O₄ NPs at 5, 10 and 15 mg/L. Catalase specific activity, ABTS (2,2′-Azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid)) radical scavenging capacity, protein content and inhibitory activity against four phytopathogenic bacteria and two Aspergillus species were assessed in cell-free supernatants and partially purified peptide fractions. The results of the study suggest that the combination of sub-inhibitory Fe₃O₄ NPs supplementation (10 mg/L) and appropriate culture medium represent a viable strategy to modulate and potentially enhance the bioactive compound profile of Bacillus strains.