Isolation and characterization of virulent phages infecting Shewanella baltica and Shewanella putrefaciens, and their application for biopreservation of chilled channel catfish (Ictalurus punctatus).

The growth of Shewanella spp., mainly S. baltica and S. putrefaciens, is responsible for the spoilage of chilled fresh fish. Phages are an alternative tool to control bacterial growth. In this study, virulent phages infecting 4 S. baltica and 6 S. putrefaciens strains were isolated and characterized. Transmission electron microscopy revealed that 6 out of 10 phages (3 phages infecting S. baltica and 3 phages infecting S. putrefaciens) belonged to Myoviridae, while the other 4 phages (1 phage infecting S. baltica and 3 phages infecting S. putrefaciens) belonged to Siphoviridae. Phage SppYZU01 and SppYZU05 showed the broadest host range, being lytic towards all the 4 S. baltica strains and 5 out of the 6 S. putrefaciens strains, respectively. The genome sequence of SppYZU01 had no similarity with known genome sequences, while that of SppYZU05 was 88.5% similar to the genome of a virulent S. putrefaciens-infecting phage (Spp001). According to the host range and lytic activity, 3 phages, including SppYZU01, SppYZU05, and SppYZU06, were combined into a cocktail (designated as SPMIX3-156). SPMIX3-156 showed potential as an antimicrobial agent to control S. baltica and S. putrefaciens strain growth in catfish matrices. Bacterial growth in the catfish muscle juice inoculated with 104 colony-forming units (CFU)/mL of Shewanella strains was partially inhibited by 105 plaque-forming units (PFU)/mL of SPMIX3-156 at both 25 °C and 4 °C. The catfish fillets inoculated with Shewanella strains were used as a model to evaluate the biopreservative effects of SPMIX3-156. Total viable counts of fillet samples treated with 107 PFU/mL of SPMIX3-156 were reduced by 3.21 and 2.75 log units after 1 day at 25 °C and 10 day at 4 °C, respectively, compared to those of untreated samples. Fillet quality indices, including pH, total volatile basic nitrogen, and sensory value of the SPMIX3-156-treated samples, considerably improved compared to those of the control samples at both 4 °C and 25 °C. Our results suggest that SPMIX3-156 is a promising biological agent against S. baltica and S. putrefaciens, and may have a potential use in chilled fish fillet biopreservation.