Solids retention time, influent antibiotic concentrations, and temperature as selective pressures for antibiotic resistance in activated sludge systems

This study evaluated the occurrence and potential proliferation of antibiotic resistance during biological wastewater treatment as a function of solids retention time (SRT), influent antibiotic concentrations, and temperature. Two phases of experiments were performed in laboratory-scale sequencing batch reactors (SBRs) fed with primary effluent from a full-scale wastewater treatment plant. Phase 1 evaluated SRTs of 2, 7, and 20 days with ambient antibiotic concentrations, and phase 2 evaluated a constant SRT of 7 days with influent antibiotic concentrations of 1×, 10×, and 100× relative to ambient levels. Ampicillin, sulfamethoxazole/trimethoprim, tetracycline, and vancomycin resistance were evaluated among Gram positive cocci (Staphylococcus and Streptococcus) using spread plate and minimum inhibitory concentration (MIC) assays. The laboratory-scale data demonstrated that biological treatment, in addition to longer SRTs, higher influent antibiotic concentrations, and higher temperatures, often resulted in greater relative prevalence of antibiotic resistance (up to 35% of the target population), and antibiotic resistant isolates were generally resistant to antibiotic concentrations 32 times higher than their baseline MICs. Some of these relationships were antibiotic-specific, with SRT having no significant impact on tetracycline resistance, influent antibiotic concentration having no significant impact on sulfamethoxazole/trimethoprim resistance, and temperature having no significant impact on vancomycin resistance.