Isolation, Characterization, and Evaluation of a Lytic Jumbo Phage Z90 Against Aeromonas hydrophila in American Eels (Anguilla rostrata)

<b>Background:</b><i>Aeromonas hydrophila</i> is a common bacterial pathogen that causes hemorrhagic septicaemia in several farmed aquaculture species. Phage therapy is considered a promising and feasible alternative to antibiotic treatment. <b>Methods:</b> In this study, an <i>A. hydrophila</i>-infecting jumbo phage Z90 was isolated from an aquaculture pond. The biological characteristics, genomic features, and in vitro and in vivo experiments were investigated to evaluate its application potential. <b>Results:</b> Phage Z90 was a myovirus with distinctive curled tail fibers. Additionally, phylogenetic and genomic analyses found that the phage Z90 was a novel virus belonging to the genus <i>Ferozepurvirus</i> of the family <i>Chimalliviridae</i>. One-step growth curve analysis revealed that the phage Z90 was a lytic phage, exhibiting a short latency period of 20 min and a relatively large burst size of 270 ± 42 PFU/cell. The phage Z90 particles were stable at psychrotrophic and mesophilic temperatures (10-50 °C) and a wide range of pH (pH 3-12). Genomic analysis revealed that the phage Z90 did not contain any genes encoding toxins, virulence factors, or antibiotic resistance factors. In vivo analysis demonstrated that the phage Z90 protected American eels from <i>A. hydrophila</i> infection, greatly increasing eel survival rates and alleviating symptoms caused by bacterial infections. The comparison of different phage administration methods suggested that phage Z90 was better administered through intraperitoneal injection than immersion in aquaculture water. Moreover, the combination of phage Z90 and ampicillin improved the bactericidal effect and reduced the treatment dosage compared to antibiotics or phage alone. <b>Conclusions:</b> Altogether, the findings of this study indicate that the phage Z90 can serve as a promising biocontrol agent for the treatment of <i>A. hydrophila</i> infection in aquaculture.