Concentration-Dependent and Strain-Specific Effects of Fe3O4 and SN-CNPs on Bacterial Growth, Biofilm Formation, and Electrochemical Activity

Abstract

Regulation of bacterial physiology and bioelectrochemical performance can be achieved through a promising strategy of combining nanoparticle treatment and electro-fermentation (EF). This study evaluated the effects of Fe3O4 nanoparticles and sulfur–nitrogen co-doped carbon nanoparticles (SN-CNPs) on growth, biofilm formation, and electrochemical activity in Escherichia coli BL21, E. coli DH5α, and Lactiplantibacillus plantarum. Growth kinetics assays, crystal violet biofilm quantification, and potentiostat-controlled electro-fermentation on screen-printed carbon electrodes were employed. Both nanoparticle types inhibited bacterial growth in a concentration-dependent manner across all three strains. Biofilm responses depended on the strain type and amount of concentration of nanoparticles. Fe3O4 enhanced biofilm in both E. coli strains but not in L. plantarum, while SN-CNPs enhanced biofilm only in E. coli BL21, indicating the contrasting response between the two E. coli strains attributed to potential differences in their stress response. In electro-fermentation, neither nanoparticle type generated sustained biologically derived current in E. coli under aerobic conditions, whereas L. plantarum produced stable 4 anodic current and substantially higher electrode biofilm coverage, consistent with its known capacity for flavin-mediated extracellular electron transfer. These findings demonstrate that nanoparticle effects on growth and biofilm are highly dependent on concentration and bacterial strain, and that SN-CNPs exhibit intrinsic electrochemical activity that must be distinguished from biologically derived signals in bioelectrochemical applications. The observed strain-specific biofilm responses suggest that bacterial genetic background may play an important role in determining how nanoparticles are perceived and responded to, warranting further mechanistic investigation. The results provide a comparative framework for the rational integration of nanoparticle treatment with electro-fermentation.