Characterization of a lytic Staphylococcus aureus phage JAsa55 and its biocontrol potential in milk.

Staphylococcus aureus (S. aureus) is a prominent foodborne pathogen that frequently causes food poisoning by contaminating various foods, particularly dairy products, and poses severe threats to food safety and public health. The increasing prevalence of multidrug-resistant S. aureus strains has further limited the efficacy of conventional interventions. In this study, we isolated and characterized a novel lytic phage JAsa55 using an MDR S. aureus strain as host. Transmission electron microscopy showed that JAsa55 possesses an icosahedral head and a long non-contractile tail, consistent with a siphovirus-like morphology. Notably, JAsa55 retained detectable activity after exposure to 4-60°C and remained active under pH 3.0-11.0. One-step growth analysis indicated a latent period (15 min) and an estimated burst size (∼500 PFU/cell) based on the first rise phase. Genome annotation did not identify obvious integrase genes, known virulence factors, or antimicrobial-resistance genes. In artificially contaminated milk, phage JAsa55 reduced S. aureus populations by up to ∼1.6 log CFU/mL at 4°C and ∼3.3 log CFU/mL at 25°C within 24 h. These findings suggest that JAsa55 has lytic activity against selected MDR S. aureus strains and may be further evaluated as a candidate component for phage-based control strategies in milk.