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ORIGINAL RESEARCH article

Front. Microbiol.

Sec. Antimicrobials, Resistance and Chemotherapy

Volume 16 - 2025 | doi: 10.3389/fmicb.2025.1532748

Susceptibility of Aeromonas salmonicida subsp. salmonicida bacteria from French farmed trout to antibiotics commonly used in fish farming, and attempt to set epidemiological cut-off values

Provisionally accepted
Antoine ROSTANG Antoine ROSTANG 1,2*Florine Bachelet Florine Bachelet 1Catherine Fournel Catherine Fournel 1Titouan Carabin Titouan Carabin 1Nora Navarro-Gonzalez Nora Navarro-Gonzalez 1,2Segolene Calvez Segolene Calvez 1,2
  • 1 INRA Biologie, Épidémiologie et Analyse de Risque en santé animale (BIOEPAR), Nantes, France
  • 2 INRA Ecole Nationale Vétérinaire, Agroalimentaire et de l'alimentation de Nantes-Atlantique (Oniris), Nantes, Pays de la Loire, France

The final, formatted version of the article will be published soon.

    Aeromonas salmonicida subsp. salmonicida (ASS) is a bacterium that can cause opportunistic infections in humans and animals. In farmed rainbow trout it causes furunculosis, with more frequent outbreaks when water temperatures are higher, an additional consequence of global warming. When infections occur, antibiotics are sometimes required. However, data on ASS susceptibility is limited. The aim of this study was to determine the distribution of Minimum Inhibitory Concentrations (MICs) of eight antibiotics commonly used in fish veterinary medicine in a population of pathogenic ASS, and to calculate provisional epidemiological cut-off values (called COEpid). To date, only four epidemiological cut-off values (ECV) have been established by CLSI, and none by EUCAST.In this study, 406 bacterial strains of ASS were collected exclusively from diseased French farmed trout over a 10-year period (2012-2021). A combination of PCR, MALDI-TOF and specific characteristics of the bacterial culture was used to identify each isolate to species level. All MIC data were obtained by the broth dilution method according to CLSI recommendations. Our COEpid meets the CLSI defined ECV for florfenicol (4 mg/L) and oxytetracycline (1 mg/L). In the absence of a defined ECV, we proposed a COEpid of 1 mg/L for doxycycline. For sulfadiazine alone, all strains tested were non-wild-type (NWT) with very high MICs. The COEpid was calculated as 4.8/0.25 mg/L for sulfadiazine + trimethoprim (one two-fold dilution difference from the ECV established by CLSI for ormetoprim + sulfadimethoxine). For quinolones, COEpid were 4 mg/L, 4 mg/L and 0.5 mg/L for oxolinic acid, flumequine and enrofloxacin, respectively, with a cross-resistance mechanism. This COEpid for oxolinic acid far exceeds the CLSI defined ECV (0.125 mg/L).A total of 12 strains (3%) were classified as NWT for all antibiotics tested. Over the period studied, the proportion of bacteria susceptible to the different molecules remained stable, except for the tetracycline family. These data will be available to establish internationally agreed epidemiological cut-off values, which are lacking for some antibiotics. These cut-offs are essential to assess and monitor the emergence of bacterial populations with resistance traits, and to establish clinical breakpoints for better use of antimicrobials in fish.

    Keywords: MIC, Aeromonas, Trout, antibiotics, Furunculosis, COWT, ECV, Epidemiologic

    Received: 22 Nov 2024; Accepted: 11 Feb 2025.

    Copyright: © 2025 ROSTANG, Bachelet, Fournel, Carabin, Navarro-Gonzalez and Calvez. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

    * Correspondence: Antoine ROSTANG, INRA Biologie, Épidémiologie et Analyse de Risque en santé animale (BIOEPAR), Nantes, France

    Disclaimer: All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.

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