Effects of legacy and emerging antimicrobial compounds to early-life stages of rainbow trout (Oncorhynchus mykiss)

Antimicrobial compounds enter freshwater systems via municipal wastewater, potentially affecting aquatic life. While the toxicity of triclosan (TCS), a legacy antimicrobial, is well-documented, less is known about emerging alternatives such as chloroxylenol (PCMX) and methylisothiazolinone (MIT). This study evaluated the developmental and molecular effects of these compounds on early-life stage rainbow trout (Oncorhynchus mykiss). Embryos were exposed to nominal concentrations of 0.39-400 µg/L from hatch to 28 days post-hatch (dph). Mortality and sublethal endpoints (edema, spinal curvature, jaw deformities, swim-up time) were assessed, and transcriptomic responses were measured at 96 h using the EcoToxChip RT-qPCR platform. TCS and PCMX reduced survivability, with 28-d LC50 values of 107 and 254 µg/L, respectively. TCS increased jaw deformities and edema, while PCMX induced spinal deformities and edema at ≥241 µg/L. MIT had no observable effects on survival or development. No significant changes were detected in swim-up time or histopathology of gill, liver, or intestine across treatments. Transcriptomic analysis revealed 55, 25, and 3 differentially expressed genes (DEGs) in response to TCS, PCMX, and MIT, respectively. TCS and PCMX shared regulation of 19 genes linked to metabolic, endocrine, and reproductive pathways, suggesting similar modes of action. These findings indicate that TCS and PCMX exert low but distinct sublethal and molecular toxicity, while MIT showed minimal effects. The EcoToxChip effectively detected early transcriptomic responses, supporting its application in rapid chemical hazard assessment of both legacy and emerging antimicrobials.