Effect of temperate bacteriophage vB_SauS_S1 on the adaptability and pathogenicity of Staphylococcus aureus ST398

Livestock-associated Staphylococcus aureus ST398 is a highly pathogenic species that causes infections in a wide variety of animals, including humans. The bacteriophage (phage) vB_SauS_S1 was isolated originally using a ST398 strain as its "isolating host", then the spot tests showed it was able to infect 73.33% (22/30) ST398 isolates. Phage S1 was assigned as a temperate phage based on genome analysis and phenotypic validation. Phylogenetic analysis showed that S1 was closely related to temperate phages tp310-2 and SA137ruMSSAST121PVL. Following infection of ST398 by phage S1, the lysogenic strain showed enhanced biofilm forming ability compared to the wildtype strain, and the invasion rate of MAC-T cells increased by 10.39%. The minimum inhibitory concentration showed that phage S1 did not change the antibiotic sensitivity of the lysogen strain, and the virulence of the lysogen strain did not change significantly in the injection models of Galleria mellonella (G. mellonella) and mice. The lysogen demonstrated superinfection immunity and reduced sensitivity to virulent phage infection. Thus, this study contributes to understanding the co-evolutionary relationships between temperate phages and the multi-host zoonotic pathogen S. aureus ST398.