Baloxavir Exhibits Antibacterial Activity Against Staphylococcus aureus by Inhibiting De Novo Purine Biosynthesis.

Staphylococcus aureus remains a leading cause of morbidity and mortality worldwide, with persistent and relapsing infections posing a major global health threat. Here, we report that baloxavir, an FDA-approved influenza antiviral, exhibits antibacterial activity against S. aureus. Baloxavir demonstrated potent activity against both MSSA and MRSA clinical isolates with MICs of 2-4 μg/mL and exhibited concentration-dependent antibacterial activity in time-kill assays. Notably, baloxavir effectively eliminated intracellular S. aureus in both A549 alveolar epithelial cells and RAW264.7 macrophages at 10 μg/mL and achieved complete eradication in A549 cells at 50 μg/mL. In vivo, baloxavir (20-40 mg/kg) significantly improved survival in MRSA-infected mice from 12.5% to 75-87.5%. Transcriptomic analysis revealed significant downregulation of purine de novo biosynthesis genes, including purF and purK, which was validated by RT-qPCR (r = 0.862, p = 0.027). This study demonstrates for the first time that baloxavir possesses significant antibacterial activity against S. aureus including MRSA, positioning it as a promising repurposed candidate for treating persistent intracellular infections and post-viral superinfections.