Antibiotic resistance profile of E. coli isolates in 17 municipal wastewater utilities across Oregon

Wastewater treatment utilities are considered one of the main sources and reservoirs of antimicrobial resistance. The objective of this study was to determine the diversity and prevalence of antibiotic-resistant Escherichia coli in wastewater treatment systems across the state of Oregon. Influent, secondary effluent, final effluent, and biosolids were collected from 17 wastewater treatment utilities across Oregon during the winter and summer seasons of 2019 and 2020 (n = 246). E. coli strains were recovered from samples by culturing on mTEC, followed by confirmation with MacConkey with MUG agar plates. Antibiotic susceptibility of 1143 E. coli isolates against 8 antibiotics were determined, and resistance profiles and indices were analyzed between utilities, seasons, and flows. Antibiotic resistance phenotypes were detected in 31.6% of the collected E. coli isolates. Among those antibiotic-resistant E. coli isolates, multi-drug resistance (i.e., resistance to three or more classes of antibiotics) was harbored by 27.7% with some strains showing resistance to up to six classes of antibiotics. The most prevalent resistance was to ampicillin (n = 207) and the most common combinations of multi-drug resistance included simultaneous resistances to ampicillin, streptomycin, and tetracycline (n = 49), followed by ampicillin, streptomycin, and sulfamethoxazole/trimethoprim (n = 46). Significant correlations were observed between resistance to sulfamethoxazole/trimethoprim and resistances to ampicillin, ciprofloxacin, and tetracycline (p < 0.001). A small percentage (1.1%) of the E. coli isolates displayed extended-spectrum beta lactamase (ESBL) activity and a single isolate carried resistance to imipenem. Compared to wastewater influent, ciprofloxacin resistance was significantly more prevalent in biosolids (p <0.05) and tetracycline resistance was significantly lower in effluent (p <0.05). Seasonal impact on antibiotic-resistant E. coli in wastewater influent was observed through significantly higher multiple antibiotic resistance (MAR) index, ampicillin resistance prevalence, and ciprofloxacin resistance prevalence in summer compared to winter (p < 0.05). This state-wide study confirms the widespread distribution of antibiotic-resistant, multi-drug resistant, and extended-spectrum beta lactamase-producing E. coli in wastewater systems across different flows and seasonal variations, making them the recipients, reservoirs, and sources of antimicrobial resistance.