Adjuvant commensal therapy enhances antibiotic efficacy against Staphylococcus aureus

Abstract Background Staphylococcus aureus is a common human pathobiont whose nasal colonization represents a major risk factor for subsequent infection. The prevalence of antimicrobial resistance, coupled with antibiotic-induced microbiota perturbation, has rendered S. aureus -associated infections increasingly difficult to treat. Commensal bacteria have emerged as potential modulators of both host immune responses and pathogen pathogenesis; however, whether they can act as therapeutic adjuvants to enhance treatment outcomes in S. aureus- associated infections remains unclear. Here, we assess the efficacy of commensal bacteria as an adjuvant therapeutic strategy. Results Using A549 Dual™ reporter cells, we demonstrate that commensals and pathobionts elicit distinct activation profiles of NF-κB and interferon regulatory factor (IRF) signaling pathways and differential IL-8 production. In a 3D humanized airway tissue model, D. pigrum significantly enhanced antibiotic-mediated reduction of S. aureus survival relative to antibiotic treatment alone ( D. pigrum KPL1914: 37.3% reduction, 95% CI 2.0–72.6%, P adj = 0.0340; D. pigrum AMBR11: 48.1% reduction, 95% CI 2.3–93.9%, P adj = 0.0356), while preserving barrier integrity and augmenting pro-inflammatory cytokine responses. However, the loss of efficacy with heat-killed D. pigrum indicates that viable bacteria and active host- and/or pathobiont-directed interactions are required to mediate its anti- S. aureus effects. Dual RNA-seq transcriptomic analysis revealed enrichment of host inflammatory pathways and reduction of S. aureus pathogenic potential during D. pigrum antibiotic adjuvant treatment in the airway tissue model. Furthermore, co-culture experiments revealed that D. pigrum constrains core metabolic pathways in S. aureus . Conclusions These findings demonstrate that D. pigrum limits S. aureus through both host immune modulation and direct suppression of bacterial pathogenesis and adaptive fitness, supporting its potential as a microbiota-based therapeutic adjuvant to enhance antibiotic efficacy against S. aureus -associated infections.