Experimental Evolution of Phage K enhances Antibacterial Activity against USA300 MRSA in Lung Infection Models

Hypervirulent community-associated MRSA clones such as Staphylococcus aureus (S. aureus) USA300 drive rapidly progressive necrotizing pneumonia with high morbidity and limited therapeutic options. Bacteriophage K (phage K) is a well-characterized lytic phage active against S. aureus, but its efficacy is limited by restricted host range and the emergence of bacterial resistance. Here, we subjected phage K to experimental evolution on S. aureus USA300 to select an adapted variant with enhanced bactericidal properties. Wild-type phage K and the evolved derivative, designated phage KJ25, were compared using growth inhibition assays, time-kill kinetics, genomic differences and transcriptomic analyses of the bacterial response to infection. Efficacy was evaluated in an in vitro A549 lung epithelial cell infection model and ex vivo murine precision-cut lung slices (PCLS).Phage KJ25 exhibited significantly improved killing of USA300, achieving faster bacterial reduction and sustained suppression of regrowth. Genomic analysis identified a function-impairing mutation in gene gp102, encoding a predicted DNA-binding protein implicated in transcriptional regulation. RNA sequencing revealed that KJ25 infection of USA300 induced a slower and less disruptive host transcriptional takeover than wild-type phage K. Importantly, in both A549 cells and PCLS model, phage KJ25 markedly reduced bacterial burden while preserving lung tissue integrity, supporting its therapeutic potential.Collectively, these findings highlight the value of experimental evolution for tailoring therapeutic phages and support phage adaptation as a promising strategy for developing interventions against multidrug-resistant S. aureus.