Phenotypic and genomic characterizations of four bacteriophages active against Aeromonas salmonicida causing furunculosis in salmonids

The Gram-negative bacteria Aeromonas (A.) salmonicida is a primary fish pathogen causing furunculosis in salmonids as well as septicemia in a variety of fish. As this disease is responsible for significant losses in aquaculture worldwide and tend to exhibit antimicrobial (multi-)resistances, the aims of this study were to isolate bacteriophages (phages) targeting this pathogenic species and perform their in vitro characterization. Three A. salmonicida strains and a collection of sewage, fish farm and aquatic environment water samples from Belgium and France were used to isolate A. salmonicida-specific phages. The host spectrum, using a collection of wild A. salmonicida selected according to their antimicrobial resistance profile, and the stability at different temperatures and pH were determined for each newly isolated phage. Their morphologies and genomes were then investigated by electron microscopy and sequencing. Four phages, named vB-AsaM-ULASA1 (47,813bp), vB-AsaM-ULASA2 (170,823bp), vB-AsaM-ULASA3 (164,381bp), and vB-AsaM-ULASA4 (171,205bp), were isolated from fish farms and aquatic environments. The phages ULASA2, 3 and 4, affiliated to the Straboviridae family, showed a large host spectrum against antimicrobial resistant strains and high resistance to temperature and pH. The phage ULASA1, for which no known family could be attributed, presented a more restricted host spectrum and less biochemical tolerance. All 4 presented a myovirus morphotype. In light of these results, three of these phages targeting A. salmonicida could represent potential new candidates for phage therapy to combat (multi-)resistant A. salmonicida strains increasingly occurring in aquaculture.