The interaction of microplastics with the ruminal ecosystem in vitro

Mismanaged plastic waste contaminates marine and terrestrial environments, so farm animals are increasingly exposed to microplastics (MP) in feed. Yet their interactions with the rumen microbiome are largely unknown. We evaluated these interactions in vitro with the Hohenheim Gas Test. Five MP chemical species - polylactide, polyhydroxybutyrate, high‑density polyethylene, polyvinyl chloride and polypropylene - were tested in two particle‑size classes (<125 µm and 125-500 µm) and six doses (0-70 mg per incubation cylinder). Each MP variant was incubated with rumen fluid plus hay or barley. We recorded cumulative gas production, pH and dry‑matter (DM) disappearance, followed by volatile‑fatty‑acid profiling, metaproteomics and metabolomics. MP consistently reduced cumulative gas output independent of polymer type, particle size or dose, but enhanced total DM degradation dose-dependently. In barley incubations, metaproteomics revealed a shift from Bacteroidetes toward Firmicutes and higher abundance of protein categories "replication & repair" and "translation", while "carbohydrate metabolism & transport" and "amino‑acid metabolism" declined. The results show that MPs interact with, and are at least partly degraded by, the ruminal microbiota. Progressive size reduction in the rumen could facilitate MP translocation into animal tissues, underscoring the need for in‑vivo studies on animal health and food‑safety implications.