Interaction between Pseudomonas fragi and Pseudomonas psychrophila isolated from the eye muscle of bighead carp (Hypophthalmichthys nobilis) affects protein degradation: In vitro screen and in situ evaluation.

Pseudomonas has been identified as the dominant spoilage genus in aerobically stored bighead carp eye muscle. However, the specific roles of individual strains and their potential interactions in protein degradation have not been fully elucidated. In this study, 56 strains of spoilage bacteria were isolated and identified from the eye muscle after 10 days of chilled storage at 4 °C. After in vitro hydrolytic activity screening, Pseudomonas fragi (PF), Pseudomonas psychrophila (PP), and their mixture (Mix) were used to inoculate sterile eye muscle in situ. Differences among the three treatment groups were evaluated using a series of indicators, including SDS-PAGE, TCA-soluble peptides, free amino acids, biogenic amines, TVB-N, peptidomics, transmission electron microscopy, and color analysis. Results showed that PF exhibited the lowest histidine level (161.14 μg/g muscle) and the highest histamine level (0.26 mg/kg) after storage. PP, PF, and Mix all induced higher degradation of the myosin heavy chain, with Mix showing the highest degradation degree of 10,231.3 × 105. PF preferentially degraded desmin, titin, nebulin, and myosin, whereas PP targeted myozenin, myomesin, MyBPC/MyBPH, and tropomodulin. Mix exhibited higher degradation of troponin, CapZ, myosin light chain, and actin. Meanwhile, Mix showed the most severe myofibrillar damage and had the lowest a⁎ value after storage. In conclusion, P. fragi and P. psychrophila exhibited distinct protein degradation behaviors, and their interaction accelerated protein degradation and quality deterioration in the eye muscle of bighead carp during storage.