Aeromonas spp. in Urban Water Systems: Environmental AMR Surveillance Findings

A case study from the Barcelona metropolitan area describes environmental surveillance characterizing antibiotic-resistant Aeromonas spp. in urban water systems across two wastewater treatment plants (WWTPs) and one drinking water treatment plant (DWTP). Investigators report phenotypic antimicrobial susceptibility testing with targeted screening for antibiotic-resistance genes (ARGs), integron markers, and selected virulence factor genes, alongside in vitro biofilm assessment and conjugation experiments examining horizontal gene transfer (HGT). A subset of isolates also underwent short-read whole-genome sequencing (WGS) to evaluate genomic signals described as consistent with persistence across sampling campaigns and treatment stages. The report presents these findings as coordinated phenotypic, genetic, experimental, and genomic observations.

Within the sampling frame, investigators characterized 428 antibiotic-resistant Aeromonas spp. isolates recovered from the treatment train at two WWTPs and from the inlet of a DWTP. Of these, 230 isolates came from the Baix Llobregat WWTP and 180 from the Gavà-Viladecans WWTP, with an additional 18 isolates recovered at the inlet of the Sant Joan Despí DWTP. The report notes that no Aeromonas isolates were detected at the DWTP outlet. Across the WWTPs, recovery was described at multiple treatment points from primary inlet through later stages, reflecting an influent-to-effluent sampling view while also capturing the DWTP inlet but not the finished drinking water.

Susceptibility testing results were reported as broadly similar across plants, with the most frequent non-susceptibility phenotype being trimethoprim/sulfamethoxazole non-susceptibility (72%). Additional high non-susceptibility proportions were described for cefoxitin (65.4%), piperacillin–tazobactam (60.7%), cefotaxime (54.9%), and ciprofloxacin (53.5%). Using the study’s resistance-category framework, 76.4% of isolates were classified as multidrug resistant (MDR) and 1.6% as extensively drug-resistant (XDR). The authors present these phenotypes as a descriptive baseline that they relate to gene detection, biofilm findings, conjugation results, and WGS analyses in the same isolate collection.

On the genotypic side, the authors report blaMOX in 78.6% of β-lactam-resistant isolates and sul1 in 57.5% of trimethoprim/sulfamethoxazole (SXT)-non-susceptible isolates, alongside an approximately 70% prevalence of the class 1 integrase gene intI1. Virulence factor gene carriage was also common, with 97.9% of isolates harboring at least one of aerA, act, fla, alt, or hlyA (with act and alt noted as most prevalent). In biofilm assays, isolates were categorized as strong (8.7%), moderate (16.8%), weak (25.5%), or non-biofilm formers (49%). Biofilm-forming isolates were reported as more abundant than non-biofilm formers at specific WWTP stages (including Baix Llobregat primary inlet and secondary outlet, and across Gavà-Viladecans stages). Overall, the study co-reports resistance determinants, integron markers, virulence genes, and biofilm capacity among these environmental isolates.

Experimental HGT was assessed with conjugation experiments reported as bidirectional between Aeromonas and Escherichia coli. Transfer from an A. caviae donor to E. coli CV601 was described for blaPER, sul1, intI1, and act at an estimated frequency of 6 × 10−8, while transfer from an E. coli donor to an A. caviae recipient was described for blaKPC at 4.67 × 10−8, with FrepB, ColE, and IncU replicon markers identified in transconjugants. For WGS, 84 selected isolates were sequenced, yielding 62 sequence types (46 novel), and the report highlights ST3458 as detected across sampling campaigns, treatment stages, and both WWTPs, with identical AMR profiles described among ST3458 isolates from Baix Llobregat. The authors interpret the combined observations as supporting Aeromonas spp. as candidate sentinel organisms for aquatic AMR monitoring, and they note limitations including the single-metropolitan-area case-study design, reliance on culture-based methods, lack of quantitative environmental measurements, and limited plasmid resolution with short-read sequencing and PCR-based replicon typing.

Key Takeaways:

• Investigators reported recovery of 428 antibiotic-resistant Aeromonas isolates across two WWTPs and at a DWTP inlet, with no detection at the DWTP outlet.
• Reported phenotypes included trimethoprim/sulfamethoxazole non-susceptibility as most frequent (72%), with MDR in 76.4% and XDR in 1.6% of isolates.
• The authors concluded that these findings support Aeromonas spp. as candidate sentinel organisms for AMR surveillance in aquatic environments.