# Wild Fishes as Reservoirs of Gut Bacteria Carrying Antimicrobial Resistance Encoding Genes in Chilean Bays

## TL;DR

Wild fish in Chilean bays carry bacteria with antibiotic resistance genes, suggesting they play a role in spreading resistance in marine environments.

## Contribution

This study identifies wild fish as reservoirs of antimicrobial resistance genes in anthropogenically impacted marine areas in Chile.

## Key findings

- Pseudomonas, Vibrio, and Shewanella were the most common bacteria carrying resistance genes.
- High resistance to streptomycin, amoxicillin, and furazolidone was observed, with 76.9% of isolates showing multi-drug resistance.
- blaCTX-M1 and carbapenemase genes were detected, indicating potential for extended-spectrum β-lactamase and carbapenem resistance.

## Abstract

Objective: The main aim of the study was to evaluate the role of wild fishes inhabiting in three anthropogenic-impacted Bays in Chile as reservoirs of antimicrobial resistance genes (ARGs). Methods: A total of 245 antimicrobial-resistant isolates were isolated from fish captured in the Coquimbo (142 isolates), Concepción (44 isolates), and Puerto Montt (59 isolates) Bays, and were identified by 16S rRNA gene sequence analysis, Antimicrobial-resistant isolates were tested for susceptibility to 12 antimicrobials by an agar disk diffusion method, and the carriage of genes encoding for resistance to main antimicrobial classes, such as β-lactams, amphenicols, tetracyclines, and sulfonamides by PCR (Polymerase Chain Reaction). Results: A predominance of the Pseudomonas (37.04%), Vibrio (14.40%), and Shewanella (13.99%) genera. Antimicrobial-resistant isolates were tested for susceptibility to 12 antimicrobials by an agar disk diffusion method, showing highest resistance to streptomycin (82.4%), amoxicillin (67.4%), and furazolidone (63.3%), and lowest resistance to ciprofloxacin (3.7%), meropenem (22.5%), and oxytetracycline (29.8%) and exhibiting a high occurrence of the multi-drug resistance phenotype (76.9%). Furthermore, an important number of isolates recovered from sampled fish species carried plasmids (53.5%), floR gene (36.7%), and tet genes (19.2%), whereas the detection of sul genes and class 1-integron was rare. As an overall result, 10.6% of isolates carried at least one bla gene, encoding an extended-spectrum-β-lactamase, with a high predominance of the blaCTX-M1 gene (23 isolates), whereas 14 out of 245 isolates (5.7%) were positive for the carriage of carbapenemases encoding genes, which both groups exhibited the β-lactam resistance phenotype. Conclusions: The wide distribution of ARG-carrying bacteria in wild fishes from all sampled Bays provides evidence that wild fish are important reservoirs and drivers of spread of ARGs in the marine environment, prompting the need of a continuous surveillance of these genes in wild fishes inhabiting anthropic impacted coastal marine environments in Chile.