Genomic characterization and virulence assessment of three Aeromonas salmonicida subsp. salmonicida strains isolated from salmonids in western Europe

Objectives Aeromonas (A.) salmonicida subsp. salmonicida is the etiological agent of furunculosis, a fish disease highly aggressive for salmonids and responsible for significant economic losses in aquaculture industry worldwide. The aims of this work were to study the antimicrobial resistance profile, perform an in-depth genomic characterization, and assess the bacterial virulence in a preliminary in vivo model of three European isolates of this subspecies originating from salmonids displaying symptoms of furunculosis.

Methods A. salmonicida isolates were tested against 24 antibiotics before being sequenced through Illumina technology to explore their phylogenetic relationships and their composition in mobile genetic elements (MGEs), as well as in virulence and antibiotic resistance genes. Bacterial virulence was finally assessed in an alternative in vivo model using Galleria (G.) mellonella larvae.

Results Two of these A. salmonicida isolates exhibited a multi-drug resistance profile to commonly used antibiotics for treating furunculosis. Genomic analyses revealed that the chromosomes of the three strains were closely related to the European reference strain A449 of A. salmonicida and they harbored multiple plasmids identified to display antibiotic resistance genes, such as the pRAS3.5, pAB5S9b and the pAsa4b. The three strains showed a marked virulence from a dose of 102 CFU/10µl in the G. mellonella model.

Conclusion Overall, these findings contribute to enrich the European collection of characterized genomes of A. salmonicida subsp. salmonicida and support the use of G. mellonella larvae as a new infection model for the study of this pathogenic subspecies causing furunculosis in salmonids.