Lytic anti- Staphylococcus aureus phage M963 resensitizes its host to methicillin by a tarS -dependent mechanism

Bacteriophage M963 targeting a clinical strain of methicillin-resistant Staphylococcus aureus (MRSA) was isolated from wastewater. Purification was optimized and the phage was subsequently characterized as a lytic, dsDNA phage within the family Roundtreeviridae and genus Rosenblumvirus . Host range testing against a small library of S. aureus isolates from patients with bacteremia and periprosthetic joint infections showed lytic activity against >60% of tested strains. Evaluation of phage-resistant mutant derivatives of the cognate host showed disruptive mutations within tarS , a gene responsible for modifying wall teichoic acid. tarS products have been proposed as a target of other members of Roundtreeviridae. Synergy testing in MRSA demonstrated phage-antibiotic synergy between M963 and vancomycin, tetracycline, and methicillin. The M963-methicillin synergy prompted testing of the phage-resistant mutant derivatives, showing that phage resistance led to a transient methicillin-susceptible phenotype at clinically relevant minimum inhibitory concentrations in some mutants. Phage M963 is a candidate for compassionate use therapy in appropriate patients. Author Summary Bacteriophages are viruses that kill bacteria. Because of antibiotic resistance, bacteriophages are being used in the treatment of some infections that are resistant to antibiotics or are not improving despite antibiotics. This manuscript describes the discovery of a bacteriophage, named M963, that kills antibiotic resistant Staphylococcus aureus (MRSA). This bacteriophage can kill MRSA from patients with prosthetic joint infections by binding to structures, called wall teichoic acids, on the outside of MRSA. Although the MRSA can evolve to avoid the bacteriophage, this leaves it vulnerable to new antibiotics. By working together with antibiotics, bacteriophage M963 may be a good option for patients with MRSA infections not responding appropriately to standard treatments.