A novel phage phaST4: Characterization and its protective potential against Aeromonas hydrophila infection in swamp eels

The rise in antibiotic resistance of Aeromonas hydrophila has attracted increased concern and attention. In the present study, we isolated and characterized a novel phage designated phaST4 from aquaculture sewage. A morphological analysis revealed that phaST4 exhibited a symmetrical icosahedral head (85.5 ± 2.8 nm) and a long contractile tail (166 ± 3.3 nm × 22 ± 1.3 nm). Whole genome sequence analysis revealed that phaST4 is a double-stranded DNA phage of 90,646 bp in length. Its genome contains 113 putative open reading frames (ORFs) and 4 tRNAs. Moreover, no known virulence genes or drug-resistant genes were identified. Combined with a gene-sharing networks analysis, the results of sequence comparison and phylogenetic analysis indicated that phaST4 is the first representative of a new species within a previously undefined genus. The optimal multiplicity of infection (MOI) for phaST4 was 0.001, and the latent period and burst size were 20 min and 146 plaque-forming units (PFU)/infected cell, respectively. PhaST4 exhibited stable activity at 30°C–50°C and a pH range of 4–11. In addition, it showed a strong tolerance to chloroform. PhaST4 exhibited efficient biofilm inhibition and clearance capacity to its indicator host, as well as time- and MOI-dependent inhibition toward Aeromonas hydrophila in vitro. In addition, intraperitoneal injection of phaST4 significantly reduced the expression of inflammation-related genes, including NF-kB, IL-1β, and IL-6, in the liver and intestine of swamp eels challenged with A. hydrophila. A histopathological analysis revealed that phaST4 administration ameliorates liver and intestinal injury compared with the bacterial infection groups. Overall, the results indicate that phaST4 has potential use for the treatment of A. hydrophila infection.