From gut to mouth: salivary signatures of carcinogenic microbiota

Recent oncological research has repositioned saliva as a non-invasive, dynamic diagnostic medium that reflects the molecular and microbial perturbations driving gastrointestinal malignancies. Growing evidence highlights characteristic salivary microbial fluctuations, such as the depletion of commensal species (Neisseria elongata, Streptococcus mitis) as well as enrichment of pathogenic taxa (Granulicatella adiacens, Leptotrichia, Fusobacterium nucleatum), which are distinctly associated with initiation of pancreatic, gastric, and colorectal cancers. Gastrointestinal cancers, on the other hand, alter salivary gland function and oral microbiota, increasing the risk of caries and periodontitis. Bacterial translocation via the oral-gut axis, a more common process than previously believed, appears to mediate this bidirectional relationship. Salivary circulating tumor DNA (ctDNA), microRNAs (miRNAs), extracellular vesicles (EVs), and proteomic signatures have established connections between oral and systemic health, and could be exploited for the detection and monitoring of oncogenic events. Challenges such as the low abundance of salivary biomarkers and the lack of standardized collection methods limit currently the application of saliva as a diagnostic tool, but it is expected next-generation sequencing, digital PCR, and nanoscale biosensors to pave the way toward precision salivary diagnostics.