Microbial Dynamics across Molecular Subtypes and Prognostic Significance of Lactobacillus in Gastric Cancer

PURPOSE: Recent studies have revealed a diverse gastric microbiota beyond Helicobacter pylori, suggesting a role in gastric cancer (GC). We aimed to investigate the composition and characteristics of the microbiota in GC and non-cancerous gastric mucosa (NC), with a particular focus on their relationship to molecular subtypes. MATERIALS AND METHODS: We conducted 16S rRNA sequencing and whole transcriptomic analysis on fresh-frozen GC and NC tissue samples from 192 GC patients, as well as saliva samples from 12 GC patients and 18 healthy individuals. Microsatellite instability (MSI), Epstein-Barr virus (EBV) in situ hybridization, and immunohistochemistry for p53 and E-cadherin were used to define molecular subtypes. RESULTS: GC tissues exhibited significantly higher diversity compared to matched NC tissues, with microbial profiles marked by decreased Helicobacter and increased Streptococcus, Prevotella, and Lactobacillus. Saliva samples predominantly contained oral bacteria and exhibited distinct microbial profiles from gastric tissues. In GC tissue, Helicobacter abundance was negatively correlated with key immune checkpoint genes (CTLA-4, PDCD1, CD274, and LAG3), whereas Prevotella, Streptococcus, and Fusobacterium were positively correlated. MSI-high and EBV-positive subtypes showed lower levels of Helicobacter but higher levels of Lactobacillus, Prevotella, and Streptococcus compared to the epithelial-mesenchymal transition-like subtype. Notably, within MSI-high GC, a subgroup characterized by Lactobacillus-enriched and otherwise microbiota-depleted profiles was significantly associated with poorer overall and disease-free survival. CONCLUSION: These findings underscore distinct microbial patterns across GC molecular subtypes, suggesting potential biomarkers for GC diagnosis and treatment.