The Combined Antibacterial Effect of Microencapsulated Bacteriophage P-2FD and Nisin on <i>Pseudomonas psychrophila</i> in Refrigerated Grass Carp

Pseudomonas psychrophila is one of the main causes of contamination of aquatic products, which not only caused serious economic losses but also posed a threat to human health. The overuse of antibiotics in aquaculture has led to the emergence of multidrug-resistant bacteria. Although phages exhibit advantages such as precisely lysing drug-resistant bacteria and leaving no chemical residues, their limited environmental stability restricts practical applications. Microencapsulation technology has been shown to enhance phage stability; however, large-scale application studies of microencapsulated phages in food matrices remain scarce. In this study, a phage-loaded microsphere with sodium alginate as the embedding material was designed, and its characteristics and the application effect in food matrices were investigated. The particle size of bacteriophage microspheres prepared with 2.2% alginate and 2% calcium chloride was 726 µm. It showed high pH stability (3.0–11.0), heat resistance (30–60°C), and good storage stability (30 days at 4°C). When phage-loaded microspheres combined with Nisin were applied to artificially contaminated fish blocks, the total bacterial count was decreased by 2.48 log 10 CFU/g at day 6, compared with the untreated control group. The total volatile basic nitrogen (TVB-N) value of the control group reached 22.68 mg/100g on the 6th day, exceeding the national food safety limit (20 mg/100g). In contrast, the TVB-N value of the experimental group treated with phage-loaded microspheres combined with Nisin decreased by 9.79 mg/100 g .This is the first demonstration that alginate-encapsulated P-2FD phage synergizes with nisin to control P. psychrophila in fish.