Health-based risk assessment and surveillance benchmarks for antibiotic-resistant E. coli in treated hospital...

Hospital effluents are a potential urban source of antimicrobial-resistant bacteria (ARB), yet no discharge criteria exist. We combined national surveillance data from Thailand with forward and reverse quantitative microbial risk assessment (QMRA) to evaluate risks from extended-spectrum β-lactamase (ESBL)-producing Escherichia coli. Forward QMRA considered swimmer exposure by age group and three dose-response models (non-resistant, resistant, and highly virulent proxies). Median risks for non-resistant and resistant surrogates were very low (∼10^-9 pppe), but under the virulent proxy, the 95th percentile exceeded the U.S. EPA benchmark of 0.036 pppe for children (0.0486), men (0.0391), and closely in women (0.026). Reverse QMRA translated this benchmark into effluent thresholds: with dilution, values exceeded 10⁸ CFU/100 mL and were not useful for monitoring, while no-dilution scenarios yielded thresholds near 10⁵ CFU/100 mL for resistant surrogates and ∼10¹ CFU/100 mL for the virulent model, with a worst-case of 20 CFU/100 mL. Based on these results, 100 CFU/100 mL was identified as a feasible surveillance criterion, protective for children and measurable with culture-based methods, with 1000 CFU/100 mL suggested as an action level. This study provides new health-based benchmarks for resistant bacteria in treated hospital effluents and offers an urban-relevant framework for linking microbial risk modeling with AMR surveillance and future effluent standard development.