Quantifying Antibiotic-Resistant Escherichia coli, Pseudomonas aeruginosa, and Enterococcus in Wastewater Effluent and Receiving River Waters in Northeast Tennessee

Wastewater treatment plants are important interfaces for the persistence and environmental dissemination of antibiotic-resistant bacteria, yet quantitative culture-based studies comparing treated effluent with paired upstream and downstream receiving waters across multiple sentinel organisms remain limited. This study quantified antibiotic-resistant Escherichia coli, Enterococcus, and Pseudomonas aeruginosa in treated municipal effluent and receiving river waters in northeast Tennessee. Grab samples were collected monthly from March through November 2025 at the following three locations: final chlorinated effluent, upstream water, and downstream water. Target organisms included cefotaxime-resistant E. coli, erythromycin- and vancomycin-resistant Enterococcus, and imipenem- and ceftazidime-resistant P. aeruginosa, measured on selective media with paired no-antibiotic plates. Antibiotic-resistant colony counts were analyzed using negative binomial regression with adjustment for sampling month and offsets for plated volume and confirmation-adjusted paired no-antibiotic counts. Among 114 samples, cefotaxime-resistant E. coli counts were significantly higher upstream than downstream, whereas effluent did not differ significantly from downstream. Vancomycin-resistant Enterococcus counts were significantly higher upstream and in effluent than downstream. Imipenem-resistant P. aeruginosa counts were highest in effluent, whereas P. aeruginosa ceftazidime-resistant counts remained low across sites. These findings indicate that antibiotic-resistant bacterial burdens were organism- and antibiotic-specific and did not show a uniform downstream enrichment pattern attributable to treated effluent alone.