Employing quantitative microbial risk assessment (QMRA) to derive aquatic thresholds for antibiotic-resistant strains of Escherichia coli, Staphylococcus aureus and Salmonella typhi: a case study in the Yangtze Estuary

Antimicrobial resistance poses a global public health crisis, yet environmental thresholds for antimicrobial-resistant bacteria remain underdeveloped. In this study, a quantitative microbial risk assessment (QMRA) approach was applied to derive environmental thresholds and evaluate the health risks of (i) third-generation cephalosporin-resistant Escherichia coli (3CEC); (ii) carbapenem-resistant Escherichia coli (CREC); (iii) methicillin-resistant Staphylococcus aureus (MRSA); (iv) fluoroquinolone-resistant Salmonella typhi (FRST), along with the corresponding general pathogens, i.e., Escherichia coli, Staphylococcus aureus and Salmonella typhi, in recreational waters at bathing beaches in the Yangtze Estuary. Using an acceptable disease burden of 1 × 10^-4 disability-adjusted life years (DALYs) per person per case, the derived environmental thresholds for 3CEC, CREC, MRSA and FRST were 6.95 × 10^-2, 3.95 × 10^-2, 4.10 × 10^-3, and 2.85 × 10^-1 copies/mL, respectively. Seasonal variations in health risks were evident: risk levels for 3CEC, CREC and FRST were higher in the low-flow season than in the high-flow season [median hazard quotients (HQs) = 3.80 × 10^-3, 6.70 × 10^-3, 0.629 in high-flow season and 1.00, 1.77, 4.38 in low-flow season], whereas the opposite trend was observed for MRSA (median HQ = 37.5 in high-flow season and 1.01 in low-flow season, P < 0.001). These patterns were consistently supported by infection probabilities and disease burden (DALYs). Sensitivity analysis revealed that pathogen abundance was the most influential part in the QMRA models, while other parameters showed low sensitivity. These results validated the thresholds derived and highlighted the necessity of frequent surveillance and accurate detection in practice.