Genomic Characterisation and In Vivo Therapeutic Evaluation of the Novel Phage Lp ACM616_1 Targeting <i>Lactococcus petauri</i>

Piscine lactococcosis, a major bacterial disease in aquaculture causing haemorrhagic septicaemia and economic losses, was traditionally attributed to Lactococcus garvieae but now includes emerging pathogens like Lactococcus petauri. Amid rising antibiotic resistance, bacteriophage therapy offers a sustainable alternative. This study aimed to isolate, characterise, and evaluate the therapeutic potential of a lytic phage targeting L. petauri. Phage Lp ACM616_1 was isolated from aquaculture effluents in Türkiye using L. petauri as the host. Whole-genome analysis and transmission electron microscopy were used to identify it as a member of the Caudoviricetes class. It has an icosahedral head (~31 nm) and a long tail (~107 nm). Host range testing showed specificity to 10 of 11 L. petauri strains, with no activity against other pathogens. Lp ACM616_1 exhibited robust stability (70%-90% viability at -20°C to 50°C; > 75% at pH 4-10), rapid adsorption (> 85% in 30 min), a 10-20 min latent period, and high burst size. In vitro, it suppressed bacterial growth at low MOIs and showed no cytotoxicity in EPC cells. Genomic characterisation revealed a 23,157 bp double-stranded DNA genome with 36.3% GC content and 40 predicted coding sequences, supporting its classification as a novel species within the genus Chertseyvirus lacking virulence and antimicrobial resistance genes. In rainbow trout challenge trials, intraperitoneal phage injection of the phage post-infection significantly improved survival to 77% compared to 40% in the untreated control group. In contrast, oral delivery via feed was found to be ineffective, yielding a survival rate of 42%. This first L. petauri-specific phage demonstrates promising biocontrol potential, advocating for cocktails and encapsulation to enhance aquaculture applications.