Evaluating bioassays to study the occurrence and fate of antibiotics in the water cycle

Risk assessment of antibiotics in water typically relies on chemical target analysis and hazards of individual compounds. This traditional approach may underestimate the total toxic pressure of this large group of compounds, as neither all classes of antibiotics nor their biologically active transformation products are fully represented in common target analysis methods. In this study, the MicroGLO panel, the WaterSCAN panel, and the Escherichia coli FhuAT bioassay were applied to extracts of different water types to evaluate the potential of in vitro bioassays to study the occurrence and fate of antibiotics in water. All three tools detected activity, representing the mechanisms of action of antibiotics in the extracts of spiked water and no or negligible activity in those of procedural blanks. This demonstrated their ability to distinguish between extracts that contain antibiotics and those that do not. All three tools detected activity in wastewater influent and effluent from different wastewater treatment plants and in several surface water bodies. Effect-based trigger values were exceeded at least once in all wastewater effluents, indicating potential risk to the environment even after wastewater treatment. This was not observed for surface water. Comparison with chemical analysis revealed that bioassays often detected activity when antibiotics were absent and vice versa, highlighting their complementary value. The E. coli FhuAT bioassay is recommended for initial screening for the presence of antibiotics, as it detects activity across different water types. This may be followed by the use of effect-directed analysis to identify the biologically active compounds causing the bioassay response, which is particularly recommended in cases where effect-based trigger values are exceeded. Alternatively, the MicroGLO panel and the WaterSCAN panel can serve to pinpoint which classes of antibiotics may be most relevant for further monitoring, as these bioassays can provide responses linked to specific mechanisms of action.