Microplastics and pathogen risk across ecosystems: From biofilm to antimicrobial resistance and host susceptibility

Microplastics (MPs) are ubiquitous in terrestrial and aquatic ecosystems, where they rapidly acquire organic coatings and biofilms (the plastisphere) and interact with co-occurring chemical pollutants. However, the conditions under which MPs become ecologically relevant in increasing disease risk remain underexplored. A key controversy is that microbial detection or enrichment on MPs is often treated as evidence of pathogen "vectoring," yet most studies do not quantify viability/infectivity, detachment, or delivered dose to hosts under environmentally realistic conditions. This review synthesizes evidence on MP-pathogen interactions and dispersal across ecosystems and reframes "MPs as vectors" through a vectorial-capacity lens that distinguishes association from transmission relevance and links MP-mediated risk to measurable dose delivery at host-relevant interfaces. Across ecosystems, evidence supports biofilm-driven persistence and enrichment of opportunistic taxa, but direct demonstrations of MP-mediated infection remain limited. We further highlight an unresolved issue, whether MPs confer unique transmission advantages compared with size-matched natural particulates that also sorb microbes and contaminants but are rarely used as comparators. We examine host susceptibility as a risk multiplier: MP exposure can compromise epithelial barriers via oxidative stress, modulate innate immunity, and disrupt microbiome-mediated colonization resistance. Plastisphere biofilms may also function as eco-evolutionary microhabitats that enrich antibiotic-resistant bacteria (ARB) and antibiotic resistance genes (ARGs), with plausible enhancement of horizontal gene transfer, although field-scale attribution is still scarce. Finally, we outline priorities for standardized evidence grading, comparator-based study designs, and quantitative metrics (loading, viability decay, detachment kinetics) to enable risk attribution and guide monitoring and mitigation.