Antibiotics are introduced to the aquatic environment through the discharge of wastewaters and runoff from catchments. Their potential presence is of concern due to the direct negative action they have on microbes and the development and transfer of antibiotic resistance. The source, distribution and fate of antibiotics and antibiotic resistant bacteria (ARB) was investigated in South- East Queensland to assess the risk of this issue to the environment and subsequently public health.
A method was established and validated for detection of up to 28 antibiotics in wastewater, marine waters and freshwaters using solid phase extraction, coupled with high-performance liquid chromatography tandem mass spectrometry. Antibiotics were identified in hospital wastewaters (3 sites) and wastewater treatment plants (WWTPs) (5 sites) in the low µg L-1 range (0.01 – 64 µg L-1). Conventional and advanced wastewater treatment processes were assessed for their ability to remove antibiotics from the water phase. Average removal of antibiotics from the water phase was found to be 92%, with concentrations of antibiotics in final effluents from ranging from 1 ng L-1 to 3.4 µg L-1. Antibiotics were ubiquitously identified in the surface waters of South- East Queensland (81 sites), with significantly (p<0.001) higher diversity and concentrations of antibiotics associated with WWTP discharges (point sources) compared to non-point sources. No antibiotics were detected above instrument detection limits in drinking water samples (20 sites) indicating sufficient barriers were in place to prevent exposure via this pathway.
The Clinical and Laboratory Standards Institute (CLSI) disk susceptibility test was used to screen for antibiotic resistance in Escherichia coli isolated from WWTPs (100 isolates), environmental surface waters (162 isolates) and oysters (200 isolates). An additional new method was developed for screening antibiotic resistance of E. coli to four commonly used antibiotics (ampicillin, tetracycline, sulfamethoxazole and ciprofloxacin) in environmental surface waters. This method was validated and calibrated against the CLSI disk susceptibility test and was successfully utilised to screen for antibiotic resistance in seven rivers (57 sites). Overall, antibiotic resistance and multiple antibiotic resistance (MAR) in E. coli was found to be significantly (p<0.05) higher amongst isolates of point-source origin compared to those of non-point source origin. A positive correlation (p<0.001, r=0.95) was demonstrated between E. coli concentration and total riverine wastewater discharge for the investigated rivers, suggesting the primary source of E. coli in these surface waters was WWTP discharge. Negligible antibiotic resistance was observed in E. coli isolated from oysters.
Toxicological risk assessments, using calculated risk quotients, would suggest that the presence of antibiotics at the identified concentrations are unlikely to pose a direct risk to aquatic organisms or human health. The implications of their role in the transfer, maintenance and development of antibiotic resistance however, including the presence of resistant organisms themselves, is largely unknown.
The information generated in this study has important implications for wastewater treatment and discharge, particularly for recycled water. Findings of this study have been utilised for the generation of recycled water (indirect potable reuse) guidelines in Australia and will provide regulators with critical information for the regulation and monitoring of these contaminants in the aquatic environment.