Advanced microscopy of oral methanogenic archaea: insights and challenges.

Methanogenic archaea are an underexplored yet functionally significant component of the human oral microbiota, implicated in eubiosis and dysbiosis, periodontal disease, peri-implantitis, endodontic infections and other systemic diseases. Their low abundance, unique cell wall structures and limited taxonomic markers have historically hampered their detection and visualization. These methodological limitations have contributed to an incomplete understanding of their spatial organization and potential roles in oral health and disease. Recent advances in microscopy and molecular imaging have transformed our ability to study these microorganisms in situ, providing increasingly detailed spatial, structural and functional information. This review synthesizes established and emerging imaging approaches for oral methanogenic archaea, spanning rapid low-resolution detection, molecularly targeted imaging, ultrastructural and nanoscale characterization and emerging label-free and functional methods. We critically compare these approaches according to their taxonomic specificity, spatial resolution, quantitative capability, sample-preparation constraints and current level of validation for oral methanogens. Emphasis is placed on biological insights gained, including spatial organization within biofilms, interspecies interactions and potential functional roles in oral health and disease. Key gaps remain, including the need for standardized imaging protocols, real-time in situ visualization and quantitative methods suitable for clinical translation. To address these challenges, we propose an integrative framework combining tiered imaging modalities to guide experimental design and interpretation. By distinguishing established applications from translational and extrapolative approaches, this review provides a methodological roadmap for future imaging studies of oral methanogens and highlights priorities for their rigorous investigation in dental research.