# Phenotypic Antimicrobial Resistance Profiles and Provisional Epidemiological Cut-Off Values of Edwardsiella anguillarum Isolated from Farmed Nile Tilapia (Oreochromis niloticus) in Brazil, with Exploratory Data on Edwardsiella tarda

## TL;DR

This study examines antibiotic resistance in Edwardsiella bacteria from farmed Nile tilapia in Brazil and proposes new thresholds to monitor resistance in aquaculture.

## Contribution

The paper introduces provisional epidemiological cut-off values for Edwardsiella anguillarum and reports resistance patterns in aquaculture settings.

## Key findings

- Most Edwardsiella anguillarum isolates remained susceptible to several antibiotic classes.
- Non-wild type and multidrug-resistant profiles were detected with a MAR index of 0.68.
- Local, species-specific pECVs are recommended for resistance surveillance in aquaculture.

## Abstract

Antimicrobial resistance in bacteria associated with aquaculture, such as Edwardsiella spp., represents an emerging challenge because of their relevance to fish health and their potential impact on animal, environmental, and human health. In this study, we primarily investigated the antimicrobial susceptibility profiles of Edwardsiella anguillarum isolated from farmed Nile tilapia (Oreochromis niloticus) in Brazil. Based on our findings, herein, we propose provisional local epidemiological cut-off values (pECVs) using the normalized resistance interpretation method, with data for Edwardsiella tarda included as an exploratory context. Fifty isolates (31 E. anguillarum and 19 E. tarda) collected between 2017 and 2025 were tested against 28 antibacterial agents using the disk diffusion method. Based on the pECVs, isolates were classified as wild type (WT) or non-WT (NWT), and the multiple antibiotic resistance (MAR) index was calculated. Most E. anguillarum isolates remained susceptible to several classes, although NWT and multidrug-resistant profiles were detected with a MAR index of 0.68, suggesting selective pressure in intensive tilapia farming systems. These findings support the use of local, species-specific pECVs for resistance surveillance in aquaculture, highlighting the importance of continuous antimicrobial resistance monitoring in aquaculture from a One Health perspective.