A Sphingomonadales-associated phenylalanine catabolic pathway is implicated in ARDS recovery through the modulation of alveolar macrophages

Background Acute respiratory distress syndrome (ARDS) carries a high mortality despite supportive care, and broad host-targeted therapies have failed to improve survival. Emerging evidence suggests that respiratory microbiota and their metabolic activities may modulate ARDS pathogenesis, but the underlying mechanisms remain unclear. Results ARDS survivors demonstrated significantly greater beta-diversity and enrichment of the order Sphingomonadales relative to non-survivors, a finding successfully validated in the external Kitsios cohort. Functional profiling identified phenylalanine metabolism as the key discriminatory pathway, with phenylalanine 4-monooxygenase (PhhA; EC 1.14.16.1) significantly enriched in survivors and mild ARDS. Sphingomonadales abundance correlated inversely with sputum L-phenylalanine levels and whole-genome sequencing confirmed a conserved phhA module in N. resinovorum . In the murine injury model, intratracheal N. resinovorum significantly reduced lung injury indicators. Critically, heterologous expression of phhA in a non-pathogenic E. coli chassis fully recapitulated the protective effects of native N. resinovorum , establishing PhhA as both necessary and sufficient for microbial phenylalanine catabolism-mediated lung protection. Mechanistically, we identified alveolar macrophages (AMs) as the cellular target through which this protective axis operates, demonstrating that N. resinovorum selectively preserves the AM compartment and that AM depletion largely abolishes its protective effects. Conclusion These findings establish a functional Sphingomonadales -PhhA-phenylalanine catabolic axis in ARDS. Respiratory Sphingomonadales abundance correlates with favorable outcomes and enhanced phenylalanine-catabolic potential. N. resinovorum degrades local phenylalanine via PhhA, and heterologous phhA expression alone is sufficient to recapitulate lung protection. This protection requires alveolar macrophages, as their depletion abolishes the effect. Thus, microbial phenylalanine catabolism acts as a spatially localized enzymatic buffer that compensates for deficient host metabolism in the injured lung, representing a mechanistically defined target for microbiota-informed ARDS therapy. Trial registration ClinicalTrials.gov, NCT07380997. Registered January 24, 2026.