Co-occurrence of microplastics, heavy metals, and Pseudomonas aeruginosa in sewage sludge: One matrix, multiple threats to agricultural soils

• Microplastic concentrations by mass and count were determined in sewage sludge • Annually, 586 billion of microplastics (817 kg) enter Hungarian soils via sludge • Co-occurrence of microplastics and heavy metals may foster antimicrobial resistance Sewage sludge is widely used in agriculture due to its nutrient content; however, it also contains emerging pollutants such as heavy metals, microplastics (MPs), and antibiotic-resistant bacteria. This study assessed their co-occurrence in sludges from eight wastewater treatment plants in Hungary. For the first time, we quantified the mass concentration of MPs in sewage sludge alongside particle number-based concentrations. The average MP concentration in sewage sludge was 54.1 particles per gram of dry weight (dw) and 75.5 µg/g dw, corresponding to an annual number of 5.86 × 10 11 MP particles, or 817 kg of MPs introduced into agricultural soils in Hungary. Additionally, heavy metals were present in all samples, including Zn (630 mg/kg dw.) and Cu (197 mg/kg dw.) representing the highest average concentrations. The presence of opportunistic human pathogen Pseudomonas aeruginosa was determined in 7 out of 8 samples. Their antibiotic-resistant pheno- and genotypes were investigated revealing that the majority of the strains were resistant to at least one antibiotic including aminoglycosides and β-lactams. The isolated strains also demonstrated biofilm formation capacity. Heavy metal concentrations in the sludge often approached the sublethal tolerance thresholds of the isolated strains, creating selective pressure that can promote the development of antimicrobial resistance.