Wastewater treatment configuration shapes ESBL- and KPC-producing gram-negative bacteria in effluents from a Colombian city: a one health perspective.

Wastewater-based surveillance is increasingly used to track antimicrobial resistance (AMR), yet evidence from treatment configurations common in small and mid-sized cities, particularly upflow anaerobic sludge blanket (UASB) reactors and stabilization ponds, remains limited. We conducted culture-based surveillance in two full-scale municipal wastewater treatment systems in Colombia: a UASB-trickling filter line and a stabilization pond system. Influent, biological-stage, and effluent samples were analyzed for extended-spectrum β-lactamase (ESBL)-associated and carbapenem-resistant Gram-negative bacilli, antimicrobial susceptibility profiles, and β-lactamase genes, including blaTEM, blaSHV, blaCTX-M groups, AmpC-related determinants, and carbapenemase genes. Total heterotrophic bacteria declined in both systems, but attenuation of resistant Gram-negative bacilli differed by configuration. The UASB-trickling filter system did not reduce carbapenem-resistant Gram-negative bacilli and showed higher ESBL and multidrug-resistant (MDR) frequencies in effluent than in influent. In contrast, stabilization ponds achieved >1-log reductions in carbapenem-resistant Gram-negative bacilli and lower ESBL and MDR frequencies. blaCTX-M group 1 predominated across isolates, and blaKPC was detected in both systems in Enterobacterales and Aeromonas spp. Conventional organic-load indicators did not predict AMR-related outcomes, whereas treatment configuration was more closely associated with resistant-bacteria persistence and attenuation.