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

## TL;DR

A new lytic phage Z90 was isolated and shown to effectively combat Aeromonas hydrophila infections in American eels, offering a potential alternative to antibiotics in aquaculture.

## Contribution

The discovery and characterization of a novel lytic jumbo phage Z90 with strong biocontrol potential against Aeromonas hydrophila in aquaculture.

## Key findings

- Phage Z90 is a lytic myovirus with a short latency period and high burst size.
- Phage Z90 is stable across a wide range of temperatures and pH levels.
- Phage Z90 protected American eels from A. hydrophila infection and improved survival rates.

## Abstract

Background: Aeromonas hydrophila 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. Methods: In this study, an A. hydrophila-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. Results: 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 Ferozepurvirus of the family Chimalliviridae. 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 A. hydrophila 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. Conclusions: Altogether, the findings of this study indicate that the phage Z90 can serve as a promising biocontrol agent for the treatment of A. hydrophila infection in aquaculture.