Adaptive laboratory evolution enables microbial access to untreated, virgin high-density polyethylene under environmentally relevant conditions.

Synthetic plastics, particularly polyethylene, are persistent environmental contaminants due to their chemical inertness and resistance to natural degradation. Among them, high-density polyethylene (HDPE) presents a critical challenge because its highly crystalline and hydrophobic structure severely limits microbial colonization and carbon utilization. Most reported HDPE biodegradation studies rely on physicochemically pretreated materials, leaving microbial mineralization of untreated, virgin HDPE largely unexplored. In this study, adaptive laboratory evolution (ALE) was employed to isolate microorganisms capable of degrading untreated HDPE under environmentally relevant selective pressures. Activated sludge and municipal landfill samples, wherein plastics and diverse microbial communities coexist for extended periods, were used as inoculum sources. Long-term ALE was conducted in a mineral salt medium that contains virgin HDPE as the sole carbon source, imposing a stringent directional selective pressure. Through this approach, Pandoraea sp. B8 was isolated and showed reproducible surface deterioration of HDPE films together with sustained growth. Importantly, HDPE degradation was accompanied by measurable carbon dioxide evolution, indicating biological mineralization rather than surface modification alone. Overall, these results suggest that evolutionary adaptation can enable microbial access to untreated HDPE and influence the environmental persistence and transformation of this polymer under environmentally plausible conditions without physicochemical pretreatment.