Anna Pinar-Méndez

Antimicrobial resistance (AMR) is a major global health threat, and environmental reservoirs such as wastewater (WWTPs) and drinking water treatment plants (DWTPs) may facilitate its persistence and spread despite reducing bacterial loads. We investigated 152 antibiotic-resistant Escherichia coli strains collected across treatment stages from two WWTPs and one DWTP in Barcelona, Spain. Strains were characterized through antimicrobial susceptibility testing, whole-genome sequencing, multilocus sequence typing, biofilm assays, and screening of antimicrobial resistance genes (ARGs), virulence factors (VFGs), biocide and heavy-metal tolerance genes (HMTGs). Although E. coli bacterial loads decreased along treatment, AMR remained highly prevalent: 85.5% of strains were multidrug-resistant (MDR), 5.3% extensively drug-resistant, and 11.2% carbapenemase-producers. Strains harboring integrase genes were 2.4 to 11.8 times more likely to harbor ARGs for sulfonamide, aminoglycoside, phenicol, trimethoprim, mercury and quaternary-ammonium compounds. Strains carrying bla _CTX-M genes were 3.0 to 20.3 times more likely to carry VFGs, while high-risk clones were 3.2 to 7.0 times more associated with VFGs. Some MDR and high-risk E. coli clones persisted in reclaimed water, and one MDR strain was detected at the DWTP inlet. These findings highlight environmental AMR reservoirs as a public health concern and support a One Health approach integrating antibiotic stewardship and environmental monitoring.

Background: Wastewater treatment plants (WWTPs) are hotspots of antimicrobial resistance (AMR) due to inputs from diverse anthropogenic sources. Aeromonas spp., ubiquitous in aquatic environments, often carry clinically relevant antibiotic resistance genes (ARGs) and can persist beyond fecal contamination indicators, making them promising sentinel organisms for AMR dissemination. The aim of this study was to assess the suitability of Aeromonas spp. in this role by characterizing resistance profiles, associated virulence factor genes (VFGs), genetic mobility, and persistence across wastewater and drinking water treatment processes in the Barcelona metropolitan area, Spain. Methods: Isolates were phenotypically characterized and screened for ARGs, VFGs, integrons, and heavy metal tolerance genes, followed by whole-genome sequencing (WGS). Biofilm formation was assessed in vitro. Conjugation assays with Escherichia coli evaluated horizontal gene transfer (HGT) potential. Results: A total of 428 antibiotic-resistant Aeromonas spp., the most abundant antibiotic-resistant bacteria isolated during the 2023 sampling campaigns from two WWTPs and one drinking water treatment plant (DWTP), were characterized. Trimethoprim/sulfamethoxazole (SXT) non-susceptibility was most frequent (72%), followed by cefoxitin resistance (65.4%). The sul1 (57.5%) and bla_MOX (78.6%) genes predominated among SXT- and β-lactam-resistant isolates. The merA gene was detected in 23.6%; 97.9% harbored at least one VFG (aerA, act, fla, alt, or hlyA), and 70.3% carried intI1. Half formed biofilm. Conjugation confirmed bi-directional HGT, and WGS revealed persistent ST3458 clones across treatment stages. Conclusions: WWTPs and DWTPs act as reservoirs of antibiotic-resistant Aeromonas spp., demonstrating persistence and HGT potential. Findings support their use as sentinel organisms for AMR surveillance in aquatic environments and for assessing treatment efficacy, highlighting variability across treatment types and locations, and reinforcing their relevance for urban water reclamation monitoring.

Monitoring bacterial communities in a drinking water treatment plant (DWTP) may help to understand their regular operations. Bacterial community dynamics in an advanced full-scale DWTP were analyzed by 16S rRNA metabarcoding, and microbial water quality indicators were determined at nine different stages of potabilization: river water and groundwater intake, decantation, sand filtration, ozonization, carbon filtration, reverse osmosis, mixing chamber and post-chlorination drinking water. The microbial content of large water volumes (up to 1100 L) was concentrated by hollow fiber ultrafiltration. Around 10 million reads were obtained and grouped into 10,039 amplicon sequence variants. Metabarcoding analysis showed high bacterial diversity at all treatment stages and above all in groundwater intake, followed by carbon filtration and mixing chamber samples. Shifts in bacterial communities occurred downstream of ozonization, carbon filtration, and, more drastically, chlorination. Proteobacteria and Bacteroidota predominated in river water and throughout the process, but in the final drinking water, the strong selective pressure of chlorination reduced diversity and was clearly dominated by Cyanobacteria. Significant seasonal variation in species distribution was observed in decantation and carbon filtration samples. Some amplicon sequence variants related to potentially pathogenic genera were found in the DWTP. However, they were either not detected in the final water or in very low abundance (<2%), and all EU Directive quality standards were fully met. A combination of culture and high-throughput sequencing techniques may help DWTP managers to detect shifts in microbiome, allowing for a more in-depth assessment of operational performance.