Microbiome Profiling Reveals Prognostic Heterogeneity in Staphylococcus aureus Pneumonia

Background: Staphylococcus aureus is a leading cause of severe pneumonia in mechanically ventilated patients. Clinical cultures identify pathogen presence but may not reflect lower respiratory tract microbial ecology. Whether culture-confirmed S. aureus pneumonia encompasses compositional heterogeneity with prognostic implications remains unknown. Methods: We performed 16S rRNA gene sequencing and shotgun nanopore metagenomics on endotracheal aspirate samples from mechanically ventilated patients with culture-confirmed S. aureus pneumonia in a prospective ICU registry. We quantified Staphylococcus abundance, assessed correlations with culture characteristics and host inflammatory biomarkers, and examined associations with 60-day mortality using Kaplan-Meier and Cox hazards analyses. Results: Among 109 patients, semi-quantitative culture growth and methicillin resistance showed no associations with outcomes. 16S sequencing (n=54) revealed marked heterogeneity in Staphylococcus relative abundance (range 0-96.7%), with only 33% demonstrating dominance (>50%). Dominance was associated with worse 60-day survival (50% vs. 80%,p=0.013) and remained independently predictive after adjusting for age, sex, and methicillin resistance (adjusted HR 3.24 [95%CI 1.12-9.36],p=0.030). Patients with dominance exhibited elevated pentraxin-3 (p=0.01) and reduced fractalkine (p=0.02). Nanopore metagenomics (n=28) validated these findings, with high absolute S. aureus read counts independently predicting mortality (adjusted HR 11.23 [95%CI 2.25-55.9],p=0.003). In an exploratory analysis of virulence genes (n=19), staphylokinase detection was associated with the hyperinflammatory phenotype (p=0.003) and mortality (p=0.046). Conclusions: Metagenomic profiling reveals clinically meaningful heterogeneity within culture-confirmed S. aureus pneumonia, masked by conventional diagnostics. Staphylococcus dominance identifies a high-risk phenotype with elevated bacterial burden, dysregulated host responses, and increased mortality, challenging the assumption that culture positivity represents a uniform clinical entity.