# Characterizing Aeromonas spp. as a Potential Sentinel Organism for Antimicrobial Resistance Dissemination in Wastewater and Drinking Water Treatment Systems: A Case Study in the Barcelona Metropolitan Area, Spain

## TL;DR

Aeromonas bacteria are shown to be effective indicators of antibiotic resistance spread in water treatment systems in Barcelona.

## Contribution

Demonstrates Aeromonas spp.'s potential as sentinel organisms for tracking antimicrobial resistance in wastewater and drinking water.

## Key findings

- Aeromonas isolates showed high prevalence of resistance genes like sul1 and blaMOX.
- Conjugation experiments confirmed horizontal gene transfer potential among isolates.
- Persistent clones like ST3458 were identified across treatment stages.

## Abstract

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 blaMOX (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.