Antimicrobial susceptibility patterns of Aeromonas populations in wastewater treatment processes

The CDC states that each year 35,000 people die in the U.S.A from antibiotic resistant infections and $1.2 billion in healthcare costs are attributed to Extended-spectrum β-lactamase (ESBL)-producing Enterobacteriaceae alone. It is important to explore urban wastewater treatment plants (UWTPs) because they contain bacteria like Escherichia coli, and other members of Enterobacteriaceae, in an environment that is rich in nutrients, antibiotics, heavy metals, and other contaminants. This rich bacterial diversity and composition encourages cell-to-cell interaction, which can facilitate horizontal gene transfer of antibiotic resistant genes (ARGs) to other bacteria. A common wastewater resident that can exchange genes with E. coli are Aeromonas spp. These bacteria have been linked to human disease and exist in many different ecosystems. Our aim was to determine the prevalence of antibiotic resistance among Aeromonas in wastewater. In fall 2020, we filtered wastewater influent, pre-chlorinated and post-chlorinated effluents from Jones Island Water Reclamation Facility in Milwaukee, WI through a 0.45µm filter. To isolate for Aeromonas spp., these filters were placed on Ampicillin-Dextrin Agar plates spiked with vancomycin and irgasan (ADAVI) with or without ceftazidime. Thirty isolates from each treatment stage were tested for antibiotic susceptibility using the Kirby-Bauer disk diffusion assay on Mueller Hinton plates using the following antibiotics: Meropenem (MEM), Chloramphenicol (C), Sulfamethoxazole-Trimethoprim (SXT), Aztreonam (ATM), Ciprofloxacin (CIP), Tetracycline (Tet), Gentamicin (GM), Ceftazidime (CAZ), Tigecycline (TGC), Piperacillin/tazobactam (TZP), and Colistin (CL). Our data showed that ceftazidime-resistance among Aeromonas populations were 0.72-1.34%, including the final effluent entering Lake Michigan. Amongst isolates, a 10% increase in antimicrobial resistance (AMR) was evident among post-chlorinated effluents compared to influent and pre-chlorinated populations. Out of the 90 isolates, resistance was seen in 7 of 11 different antibiotics. Overall, our data indicates that antibiotic resistant strains of Aeromonas survive wastewater treatment processes and are entering Lake Michigan, providing a potential health risk