Fluoroquinolone resistance mediated by mutations in gyrA and grlA does not facilitate phage integration in Staphylococcus aureus

Prophages of the ΦSa3int family are commonly found in human-associated strains of Staphylococcus aureus where they encode factors for evading the human innate immune system. In contrast, they are usually absent in livestock-associated methicillin-resistant S. aureus (LA-MRSA) strains where the phage attachment site is mutated compared to the human strains. However, ΦSa3int phages have been found in a small subset of LA-MRSA strains belonging to clonal complex 398 (CC398), including a lineage that is widespread in pig farms in Northern Jutland, Denmark. This lineage contains amino acid changes in the DNA topoisomerase V and the DNA gyrase encoded by grlA and gyrA, respecively, which have been associated with fluoroquinolone (FQ) resistance. As both of these enzymes are involved in DNA supercoiling, we speculated that the mutations might impact recombination between the ΦSa3int phage and the bacterial chromosome to enhance phage integration. To examine this, we introduced the FQ resistance mutations into S. aureus 8325-4attBLA that carry the mutated CC398-like bacterial attachment site for ΦSa3int phages. Our results failed to reveal significant differences between the FQ-resistant mutant and the wildtype strain with respect to phage integration or phage release. Thus, we conclude that mutations in grlA and gyrA do not contribute to the presence of the ΦSa3int phage in LA-MRSA CC398.