Antimicrobial efficacy of CS-SeNPs against pathogens in aquaculture: A potential trade-off with fish gut health?

Aquatic pathogens pose a major challenge to the aquaculture industry, leading to significant economic losses. Selenium-based nanoparticles (SeNPs) are emerging as effective antimicrobials, but their impact on beneficial fish microbiota remains underexplored. This study evaluates the antimicrobial efficacy and cytotoxicity of chitosan–selenium nanoparticles (CS-SeNPs) against aquatic pathogens and fish gut bacteria. CS-SeNPs were synthesized by heat-assisted synthesis and characterized using various spectroscopic techniques (UV–Vis, FTIR, DLS, XRD, and Raman Spectroscopy), Antimicrobial and cytotoxic effects were tested against Vibrio cholerae and Shewanella algae (aquaculture wastewater isolates) and Brachybacterium sp. and Mammalicoccus sciuri (fish gut isolates). Cytotoxicity, lipid peroxidation and ROS generation assays were performed across a concentration gradient (up to 1000 μg/mL) to evaluate dose-dependent responses. CS-SeNPs showed Amax at 250 nm, indicating the formation of CS-SeNPs. XRD spectrum revealed amorphous nature of CS-SeNPs and a Raman analysis peak at 251 cm⁻¹, indicated presence of localized monoclinic Se and Se₈. The average particle size was 164 nm. CS-SeNPs showed maximum cytotoxicity of 73.65 % (V. cholerae), 99.06 % (S. algae), 75.88 % (Brachybacterium sp.), and 89.0 % (M. sciuri). Corresponding lipid peroxidation values were 73.7 %, 99.06 %, 69.68 %, and 74.90 %, respectively, suggesting ROS-mediated cell damage. ROS production was estimated by DCFH-DA assay and confirmed elevated ROS levels. CS-SeNPs exhibited potent antimicrobial activity via oxidative stress mechanisms but also significantly affected beneficial gut bacteria. These findings highlight the need for cautious application of CS-SeNPs in aquaculture to prevent unintended microbiome disruption and ensure sustainable disease management.