Antidepressant desvenlafaxine identified in wastewater promotes transformation and antibiotic resistance risk in Acinetobacter baylyi ADP1 via metabolic adaptations

Wastewater treatment plants (WWTPs) are known reservoirs of antibiotic resistance genes (ARGs). Non-antibiotic compounds, such as antidepressants, may further promote ARG acquisition through horizontal gene transfer (HGT). Desvenlafaxine, a serotonin-norepinephrine reuptake inhibitor (SNRI) listed on the EU Surface Water Watch List, is among the most frequently detected antidepressants in WWTP effluents, yet its role in HGT has not been examined. Here, we detected desvenlafaxine at the highest concentrations among four antidepressants monitored across three municipal WWTPs in western New York. Using Acinetobacter baylyi ADP1 as a model recipient in natural transformation assays ( n = 6), we found that desvenlafaxine significantly increased transformation frequency at 10 mg/L (1.74 ± 0.33-fold) and 50 mg/L (1.49 ± 0.19-fold; P _adj < 0.05). Effects occurred without induction of reactive oxygen species or loss of membrane integrity, consistent with its very low toxicity (IC ₂₀ ~1,300 mg/L). Instead, desvenlafaxine induced dose-dependent increases in membrane fluidity and shifts to less negative zeta potentials, suggesting that electrostatic interactions between its cationic amine group and the negatively charged membrane reduce surface repulsion and facilitate plasmid proximity during uptake. Non-targeted proteomics revealed a biphasic response: at 10 mg/L, competence-associated proteins (PilB, ComM) were upregulated, and STRING analysis identified networks linked to membrane transport, transcriptional regulation, and envelope remodeling, while no connected network was recovered at 50 mg/L. Electron microscopy confirmed higher pilus frequency at both doses. Together, these findings reveal a non-oxidative, envelope-mediated route by which a widely detected antidepressant increases bacterial competence.