Introduction.

Microorganisms colonize nearly all anatomical sites of the human body, with the oral cavity hosting one of the most diverse, accessible, and densely populated microbial ecosystems. The oral microbiome comprises a complex consortium of bacteria, fungi, viruses, archaea, and protozoa that inhabit distinct ecological niches. Each niche provides unique physicochemical conditions that shape microbial composition, structure, and function. In addition to oral and dental sites, oral biofilms frequently develop on dental materials, appliances, and prostheses, where surface characteristics such as roughness, hydrophobicity, and chemical composition further influence microbial adhesion and biofilm maturation, leading to marked differences at species and strain levels. Advances in culture-independent molecular technologies, particularly 16S rRNA gene sequencing, shotgun metagenomics, and other multi-omics approaches, have greatly enhanced understanding of oral microbial diversity, functional capacity, and host-microbe interactions beyond the limitations of conventional culture-based methods. In health, the oral microbiome exists in a state of dynamic equilibrium, or eubiosis, which contributes to local and systemic homeostasis. This balance is modulated by host factors such as saliva composition, immune responses, and oral hygiene practices, as well as environmental influences including diet, tobacco use, and alcohol consumption. Disruption of this equilibrium, termed dysbiosis, has been increasingly implicated in the pathogenesis of head and neck cancers. Emerging evidence suggests that microbial dysbiosis may promote carcinogenesis through chronic inflammation, immune modulation, production of carcinogenic metabolites, and direct interactions with epithelial cells. Understanding the microbiology of head and neck cancer therefore provides critical insights into disease initiation, progression, and potential diagnostic and therapeutic strategies.