Overview of head & neck microbiota.

The human oral cavity and upper aerodigestive tract harbor one of the most complex microbial ecosystems in the body, comprising bacteria, fungi, archaea, and viruses that coexist in a finely regulated state of eubiosis. These multi-kingdom communities play fundamental roles in maintaining mucosal homeostasis through colonization resistance, metabolic activity, immune modulation, and epithelial protection. However, disruption of this equilibrium results in dysbiosis, characterized by altered microbial composition, reduced diversity, and functional reprogramming, which collectively promote chronic inflammation, epithelial damage, and carcinogenic processes. Increasing evidence links site-specific microbial alterations in the oral cavity, oropharynx, hypopharynx, nasopharynx, larynx, sinonasal tract, and saliva with the initiation and progression of head and neck squamous cell carcinomas. Rather than single pathogens, complex microbial consortia appear to shape a pro-tumorigenic microenvironment through immune dysregulation, metabolic carcinogen production, activation of oncogenic signaling pathways, and facilitation of tumor immune evasion. This chapter provides a comprehensive overview of the composition, spatial organization, and functional roles of head and neck microbiota, with particular emphasis on their involvement in cancer-associated dysbiosis. Understanding these microbial ecosystems offers a critical framework for developing microbiome-based diagnostic biomarkers, preventive strategies, and therapeutic interventions in head and neck oncology.