Macroplastics as reservoirs of antimicrobial resistance: a systematic review of evidence, variability and methodological constraints

The environmental accumulation of macroplastics has raised concerns regarding their role in the spread of antimicrobial resistance (AMR). As persistent and mobile substrates, macroplastics provide surfaces for microbial colonization and may act as reservoirs and vectors of antibiotic-resistant bacteria (ARB) and antibiotic resistance genes (ARGs). However, their ecological significance remains insufficiently resolved. Following PRISMA guidelines, we conducted a systematic review of 55 peer-reviewed studies investigating environmentally collected and experimentally exposed macroplastics across marine, estuarine, and freshwater systems. Macroplastics consistently harbor clinically relevant ARB and ARGs, supporting their role as AMR reservoirs. However, evidence for systematic enrichment relative to surrounding matrices is inconsistent, and direct evidence of AMR transfer during environmental transport remains limited. Anthropogenic pressure, ecosystem type, and plastic weathering emerged as key drivers of AMR patterns, with freshwater systems typically showing higher abundance and diversity of ARB and ARGs. Weathering enhances biofilm development and the adsorption of selective agents, whereas the influence of polymer type remains inconclusive. Methodological heterogeneity, including inconsistent quantification strategies, limited environmental controls, and lack of phenotype-genotype linkage, constrains robust inference. We propose a standardized, integrative framework combining environmental baselines, harmonized AMR quantification, and functional validation to improve comparability and support reliable evaluation of macroplastic-associated AMR.