Saccharomyces boulardii CNCM I-745 Stimulates Intracellular Antimicrobial Activity Against Salmonella Typhimurium in Murine Macrophages

Salmonella enterica serovar Typhimurium (ST) is an intracellular pathogen that survives within macrophages and disseminates to systemic organs, thereby evading host immune defenses. Previous studies have shown that the probiotic yeast Saccharomyces boulardii CNCM I-745 improves survival in ST-infected mice, reduces bacterial translocation, and modulates cytokine expression, including the upregulation of interferon-γ and the downregulation of interleukin-10, both of which are involved in the regulation of inducible nitric oxide synthase (iNOS), a key mediator of macrophage antimicrobial activity. The present study was designed to investigate the transcriptional regulation of iNOS and associated antimicrobial responses in ST-infected RAW264.7 murine macrophages pretreated or cotreated with S. boulardii. Gene expression levels of iNOS and selected cytokines were analyzed in RT-qPCR assays. Bacterial adhesion was quantified by colony-forming unit (CFU) counting, and intracellular survival was assessed using a gentamicin protection assay. S. boulardii did not affect bacterial adhesion, but it significantly reduced intracellular ST survival, particularly under pretreatment conditions (p < 0.05). This effect was associated with increased iNOS gene expression. Interferon-γ expression was mainly induced by pretreatment, whereas tumor necrosis factor-α and interleukin-10 were modulated under cotreatment conditions. These findings indicate that S. boulardii modulates macrophage antimicrobial gene expression and suggest that probiotic pretreatment enhances innate immune responses against intracellular bacterial infections.