Strain-dependent polystyrene biodeterioration by Flavobacterium strains isolated from weathered EPS waste

• Weathered EPS waste harbored PS-active Flavobacterium strains. • All strains formed halos and caused measurable PS film weight loss. • BS5 caused the strongest physical deterioration of PS films. • BS2 showed the strongest oxidative surface modification. • GC-MS supported strain-dependent intermediate formation during PS decay. Polystyrene (PS) is a persistent plastic contaminant because of its hydrophobicity, chemically stable carbon backbone, and resistance to microbial attack. In this study, three PS-active bacterial strains, BS2, BS3, and BS5, were isolated from weathered expanded polystyrene (EPS) buoy waste collected from the coast of Busan, Republic of Korea, and identified as members of the genus Flavobacterium . Their ability to initiate PS biodeterioration under carbon-limited incubation conditions was evaluated using complementary physicochemical analyses, including halo formation, weight loss, SEM/EDS, water contact angle, TGA/DSC, XRD, ATR-FTIR, XPS, and GC-MS. All three strains formed clear halos on PS-containing medium and caused measurable weight loss of PS films after 30 days, with BS5 showing the largest reduction, 6.04 ± 0.98%. Surface analyses showed roughening, peeling, cracking, and increased hydrophilicity of PS films after bacterial treatment. Thermal and structural analyses indicated modest destabilization of the polymer matrix while preserving the overall amorphous PS framework. Spectroscopic analyses showed strain-dependent oxidative surface modification, with BS2 exhibiting the strongest increase in surface oxygen content. Endpoint GC-MS profiling showed that bacterial incubation with PS films was associated with altered low-molecular-weight compound profiles, including hydrocarbons, oxygen-containing compounds, and aromatic derivatives. Collectively, these results show that Flavobacterium strains recovered from weathered EPS waste can initiate complementary modes of early-stage PS biodeterioration. The study identifies weathered EPS debris as a reservoir of PS-active bacteria and provides evidence for bacterial involvement in the physicochemical transformation of persistent plastic contaminants.