Characterization and efficacy evaluation in the Galleria mellonella model of four novel bacteriophages targeting Aeromonas salmonicida

The Gram-negative bacterium Aeromonas salmonicida subsp. salmonicida is a major fish pathogen causing furunculosis, one of the main bacterial diseases affecting salmonids. Since this bacterium leads to significant losses in aquaculture worldwide and frequently displays antimicrobial resistance, this study aimed to isolate bacteriophages targeting A. salmonicida, characterize them in vitro, and evaluate their therapeutic potential using the Galleria mellonella larvae model. Four novel phages presenting a myovirus morphotype were isolated from fish farms and aquatic environments in Belgium and France. Genomic analyses revealed that the three lytic phages vB_AsaM_ULASA2 (170,823 bp), vB_AsaM_ULASA3 (164,381 bp), and vB_AsaM_ULASA4 (171,205 bp) belong to the Straboviridae family, whereas no known family could be assigned to the phage vB_AsaM_ULASA1 (47,813 bp). Three of these phages were active against other strains of A. salmonicida, including pathogenic salmonicida subspecies displaying a multidrug-resistant profile, and showed a high biochemical tolerance. Single administrations of these phages prolonged the survival of G. mellonella larvae previously infected with A. salmonicida, even if this did not result in a complete elimination of the bacteria. In light of these results, three of the newly isolated phages represent promising candidates for phage therapy against antimicrobial-resistant A. salmonicida subsp. salmonicida strains increasingly encountered in aquaculture.