Urban wastewater treatment plants (WWTPs) are increasingly recognized as critical control points in the environmental dissemination of antibiotics and antibiotic resistance genes (ARGs), playing a central role within the One Health framework. If insufficient removal of antibiotics or ARGs occurs during wastewater treatment, it may facilitate the spread of antimicrobial resistance (AMR), underscoring the need to assess treatment performance.
This study investigated the occurrence, concentration, and removal of selected ARGs and antibiotics in two urban WWTPs in southeastern Spain. The plants differed in their treatment schemes, with one conducting conventional secondary treatment and the other incorporating tertiary treatment; both included lagooning systems as additional stages. A total of nine ARGs associated with resistance to clinically relevant antibiotics were analysed, intI1, sul1, mcr-1, blaKPC-3, blaTEM, blaCTX-M, blaCTX-M-32, blaOXA-48, and blaOXA-58, together with the bacterial marker gene 16S rRNA. ARG concentrations were quantified by quantitative polymerase chain reaction (qPCR) and expressed as log₁₀ absolute gene copies. In parallel, selected antibiotics were quantified using high-performance liquid chromatography.
The results showed substantial reduction of both antibiotics and most ARGs in the final effluents, indicating that the applied treatment configurations, including lagooning, effectively reduce the environmental load of AMR. This reduction is particularly relevant from a One Health perspective, as it limits the dissemination of resistant bacteria and determinants from urban wastewater into receiving aquatic environments and water reuse systems. However, certain genes of high clinical and epidemiological relevance, such as intI1, mcr-1, and blaOXA-48, showed limited removal or, in some cases, higher concentrations in effluents than in influents. Correlation analyses revealed relationships between antibiotic concentrations and ARG abundances, providing insight into interactions driving resistance dissemination. This behaviour may reflect complex microbial and physicochemical processes during treatment and highlights the need to better understand ARG dynamics, reinforcing the role of WWTPs as key monitoring interfaces.
