Isolation and preliminary characterization of four new lytic bacteriophages against Aeromonas salmonicida, the causative agent of furunculosis

Aeromonas (A.) salmonicida, a Gram-negative bacteria belonging to the Aeromonadaceae family, is a primary
fish pathogen that causes furunculosis in salmonids, carp and perch, as well as septicemia in a variety of fish.
This species is considered as one of the main bacterial pathogens responsible for important economic losses in
aquaculture industry. Large amounts of antibiotics such as oxytetracycline, quinolones and sulfonamides are used
to treat this infection, which highly contributes to the emergence of antibiotic-resistant strains. The application
of bacteriophages (phages) in aquaculture seems to be a promising solution to control pathogenic bacteria in this
field because these organisms are well adapted to aquatic environments. The aim of this work was to isolate and
characterize new lytic phages against A. salmonicida. Phage isolation was performed by the enrichment method
from water samples collected in different fish farming ponds and natural aquatic environments in southern
Belgium in early 2022. From these water samples, four new lytic phages were isolated. A preliminary host
spectrum test showed that three of these four phages were active against other wild A. salmonicida strains while
the fourth one showed a narrower host spectrum. These four phages were not active against any of the A.
hydrophila strains tested. After having determined temperature and pH stabilities, adsorption times and kinetics
of these four new phages, further studies are needed to analyse their genomes and to assess the in vivo safety
and efficacy of these phages.