# Exploring Aeromonas veronii in Migratory Mute Swans (Cygnus olor): A Debut Report and Genetic Characterization

## TL;DR

This study reports the first isolation of Aeromonas veronii from migratory mute swans in China, showing it can cause disease and is resistant to multiple antibiotics.

## Contribution

First documentation of Aeromonas veronii in migratory mute swans, expanding its known host range and highlighting its zoonotic potential.

## Key findings

- A. veronii strain HNZZ-1/2022 was isolated from a dead migratory mute swan and shown to cause disease in goslings.
- The strain exhibited multi-drug resistance to meropenem, ampicillin, and enrofloxacin.
- It carried six virulence genes and had an LD50 of 3.48 × 10⁸ CFUs/mL in goslings.

## Abstract

Aeromonas veronii (A. veronii) can cause serious disease in humans and various animals and is ubiquitous in terrestrial and aquatic milieus. Therefore, A. veronii has recently been considered to be an emerging pathogen worldwide. However, there is no documentation of clinical symptoms and pathological changes in A. veronii causing disease in migratory mute swans. In this study, we isolated and identified an A. veronii strain from migratory mute swans in China for the first time. We further explored its pathogenicity and antimicrobial susceptibility. Our results indicated that the newly identified strain has a strong ability to cause disease and exhibits multi-drug resistance. This is the first report of A. veronii being isolated from a migratory mute swan, which expands its known host spectrum. These findings not only help us to better understand the zoonotic potential of A. veronii but are also very significant in terms of improving the understanding of its dynamic transmission among the environment, animals, and humans.

Aeromonas veronii (A. veronii) is a ubiquitous bacterium in terrestrial and aquatic environments. It has a significant impact on animal and human health, with it becoming an emerging crucial pathogen worldwide. However, there have been no reports of mute swan infections. In the present study, after an observation of pathological changes, one bacterial strain isolated from a dead migratory mute swan was identified as A. veronii HNZZ-1/2022 based on its morphology, matrix-assisted laser desorption ionization–time of flight mass spectrometry (MALDI-TOF MS), and sequence analysis of the 16S rRNA and gyrB genes. To explore its pathogenicity, virulence gene detection and a gosling infection experiment were subsequently carried out, respectively. Six virulence genes for cytotonic enterotoxins (alt), lateral elastase (ela), lipase (lip), cytotoxic enterotoxin (act), aerolysin (aerA), and polar flagellin (fla) were present in the template DNA of A. veronii HNZZ-1/2022. Experimentally infected goslings exhibited hemorrhages of various different degrees in multiple organs. The half-maximal lethal dose (LD50) value of A. veronii strain HNZZ-1/2022 was estimated to be 3.48 × 108 colony forming units (CFUs) per mL for goslings. An antimicrobial susceptibility test showed that the A. veronii HNZZ-1/2022 strain was resistant to meropenem, ampicillin, and enrofloxacin. To date, this is the first report of A. veronii in migratory mute swans, thus expanding the currently known host spectrum. These results suggest that the migratory mute swan is a new host for A. veronii and demonstrate the need for extensive surveillance and research of A. veronii to minimize its transmission between animals, the environment, and humans.