Characterization and biocontrol potential of two novel lytic phages vB_EtaS_2 and vB_EtaS_6 against Edwardsiella tarda in aquaculture

Edwardsiella tarda poses a significant challenge to aquaculture industry owing to its pathogenicity and emerging antibiotic resistance. These factors have led to substantial economic losses and growing concerns about food safety. Given the urgent need for sustainable alternatives to antibiotics, bacteriophage therapy has emerged as a promising strategy for targeted pathogen control. This study isolated two bacteriophages exhibiting high lytic activity against E. tarda from wastewater samples collected at a crocodile farm in Hainan, China. These phages, designated as vB_EtaS_2 and vB_EtaS_6, were taxonomically classified within the genus Tlsvirus of the family Drexlerviridae, based on morphological characterization and genomic sequence analysis. In vitro antibacterial assays showed that a multiplicity of infection value of 0.1 exhibited optimal antibacterial efficacy and phage propagation efficiency. Both phages maintained substantial stability under thermal stress (−20–50 °C), across a broad pH range (5–10), and in the presence of low-concentration chloroform. Whole-genome analysis confirmed the absence of antibiotic resistance genes and virulence factors in both phages, ensuring their biosafety profile. Furthermore, in vitro assays demonstrated potent bactericidal activity. In Danio rerio (zebrafish) infection models, a phage cocktail of vB_EtaS_2 and vB_EtaS_6 significantly reduced mortality rates by 40 % (p < 0.05). Collectively, these findings suggest that vB_EtaS_2 and vB_EtaS_6 demonstrate favorable therapeutic potential are promising biocontrol agents against E. tarda. They offer a sustainable, targeted alternative to antibiotics, with translational value in improving pathogen management in crocodile aquaculture.