Characterization of a Salmonella abortus equi phage 4FS1 and its depolymerase

The significant economic losses caused by <i>S. abortus equi</i> in donkey husbandry have increased interest in exploring the potential of phages and their enzymes as control strategies. In this study, a <i>S. abortus equi</i> phage, designated 4FS1, was isolated from sewage at a donkey farm. Transmission electron microscopy (TEM) revealed a typical icosahedral head and a long, non-contractile tail. It exhibited a short latent period of 20 min and a burst size of 160 plaque-forming units (PFU) per cell. It demonstrated a broad host range, infecting 36 out of 60 <i>salmonella</i> strains, with an optimal multiplicity of infection (MOI) of 0.01 for <i>S. abortus equi</i> S1. The phage titer remained stable at 10⁹ PFU/mL between 37°C and 50°C and exceeded 10⁸ PFU/mL at pH from 5.0 to 10.0. After 1 h of UV exposure, the titer remained at 10⁷ PFU/mL and showed no significant variation across NaCl concentrations from 2.5 to 15%. The genome of phage 4FS1 consists of a 42,485 bp linear double-stranded DNA molecule with a G + C content of 49.07%. Of the 56 predicted open reading frames (ORFs), 32 were functional annotated, with no virulence or drug resistance genes identified. ORF36 was predicted to encode a depolymerase responsible for endorhamnosidase activity. Recombinant expression of the Dpo36 protein in prokaryotes significantly reduced biofilm formation and removal. Combined with healthy donkey serum, Dpo36 inhibited bacterial growth <i>in vitro</i> and enhanced the survival rates of mice infected with <i>S. abortus equi.</i> These findings highlight the promising potential of phages and their depolymerases as novel therapeutic agents against <i>S. abortus equi</i>.