Isolation and characterisation of lytic bacteriophages against atypical Aeromonas salmonicida spp. isolated from fish farmed in Chile

Aeromonas salmonicida infections within the Chilean aquaculture industry are associated with atypical A. salmonicida subspecies. In addition to the significant production losses caused by outbreaks, antimicrobial treatments of this bacterial pathogen raise environmental issues. Bacteriophages are among the proposed treatment alternatives that would be more environmentally and consumer friendly than antimicrobials. To this end, the present study isolated seven bacteriophages from the Marga-Marga River (Chile) with lytic activity for A. salmonicida subsp. achromogenes ATCC® 33659^TM. Then, twelve atypical A. salmonicida isolates associated with salmonid mortalities were also tested with the isolated phages. Electron transmission microscopy and analyses of genomic material showed that the isolated viruses presented a 65 to 96 nm icosahedral head, 65 to 130 run tail, and genomic DNA material >23 kbp. These traits classified the collected bacteriophages as members of the Myoviridae family, Caudovirales order. Protein profiling highlighted characteristic traits for each phage, with proteins distributed between 25 and 140 kDa. Analysis of phage stability established optimal ranges of pH (5 to 11) and temperature (4 °C to 20 °C). In conclusion, the studied viruses are potential candidates for in situ biotechnological applications, specifically as a result of being responsive to the environmental conditions (temperature and pH) existing at aquaculture settings. Future studies are needed to evaluate the feasibility of using these bacteriophages under in situ conditions.