Polystyrene-degrading bacteria modulate host stress and toxicity responses to microplastic exposure in <i>Caenorhabditis elegans</i>

Microplastic exposure is an emerging health risk. Host-associated plastic-degrading commensal bacteria can directly interact with microplastic particles and alter them physically and chemically, thereby potentially modulating microplastic toxicity. Despite numerous reports of plastic-degrading bacteria isolated from host intestines, how these interactions affect host physiology remains unclear. Here, we compared two polystyrene-degrading bacteria-Enterobacter hormaechei LG3 and Bacillus amyloliquefaciens SCGB1-in Caenorhabditis elegans exposed to laboratory-manufactured 1-μm polystyrene microspheres (Mi-PS). LG3-fed worms showed dose-dependent physiological impairment in response to Mi-PS, whereas SCGB1-fed worms exhibited attenuated or negligible impairment. The strains interacted with Mi-PS via distinct physiological and metabolic responses, reflected by differences in biofilm formation, particle attachment, and metabolite profiles. These strain-specific differences were confirmed to directly influence host outcomes. Under identical exposure conditions (10 mg/L, 50 h), LG3-fed worms accumulated more Mi-PS particles in the gut than SCGB1-fed worms (n = 48; mean ± SD, 3.28 ± 4.22 vs 0.63 ± 1.03 particles per worm). A transcriptome-guided validation framework provided mechanistic clues to strain-specific microplastic interactions. LG3-associated impairment coincided with the formation of oxidized Mi-PS particles, production of oxidized styrene intermediates, and microparticle-driven changes in bacterial cell properties, including activation of the lipopolysaccharide biosynthesis pathway. In contrast, SCGB1-associated attenuation was consistent with isobutyrate/isovalerate-mediated modulation of host DAF signaling. Collectively, these results link bacteria-microplastic interactions to host outcomes and offer actionable insight for assessing and mitigating microplastic-related health risks.