Isolation and In Vitro Characterization of Two Lytic Jumbo Bacteriophages Targeting Aeromonas dhakensis for Biocontrol in Aquaculture

Motile Aeromonas septicemia is a major bacterial disease in aquaculture, and the role of Aeromonas dhakensis has likely been underestimated due to frequent misidentification. Alternative control strategies for this pathogen remain limited. In this study, two lytic jumbo bacteriophages, DJC6219 and NK62157, were isolated from municipal wastewater and characterized for their potential as candidate biocontrol agents against A. dhakensis. Both phages were evaluated using host range assays, transmission electron microscopy, stability tests, multiplicity of infection (MOI) optimization, growth kinetics, and genomic analyses. Both phages showed high specificity toward A. dhakensis. They had icosahedral heads and long non-contractile tails, consistent with siphovirus-like phages. Both phages remained stable within moderate temperature and pH ranges but showed reduced viability under extreme conditions. Growth inhibition was observed at MOIs of 0.1-1, while a phage cocktail achieved enhanced bacterial suppression at an MOI of 0.01. Genomic analysis confirmed that both phages are lytic and carried no antimicrobial resistance genes, virulence factors, or lysogeny-related genes. Comparative analysis indicated that both phages are closely related to known Aeromonas phages while representing independent environmental isolates. Our findings highlight the potential of DJC6219 and NK62157 as candidate biocontrol agents against A. dhakensis in aquaculture.