Invasive guests: the role of non-oral bacteria in oral dysbiosis and systemic health.

Background: The human oral cavity harbors a diverse microbiota essential for maintaining oral and systemic health. While most resident microorganisms are commensals adapted to specific oral niches, increasing evidence highlights the presence of non-oral bacteria, including Objective: This review aims to summarize the current evidence on the presence, persistence mechanisms, clinical significance, and detection challenges of non-oral bacteria in the oral cavity, and to highlight the need for integrated diagnostic approaches. Design: A comprehensive literature review was conducted focusing on studies reporting the isolation, detection, and clinical relevance of non-oral opportunistic bacteria in the oral cavity, with emphasis on culture-based and molecular diagnostic methods. Results: Clinically, non-oral bacteria presence has been associated with dental caries, periodontal disease, endodontic infections, systemic dissemination, and their potential role as reservoirs of antibiotic resistance genes. Although several non-oral opportunistic bacteria can be readily isolated by culture when present at sufficient abundance, particularly in immunocompromised patients, detection may remain challenging in healthy individuals or low-biomass samples because of their transient or low-abundance occurrence, increasing the risk of misinterpretation. Conclusions: Recognizing the oral cavity as a dynamic niche for non-oral bacteria underscores the need for integrated culture- and molecular-based diagnostic approaches, improved infection control strategies, and further research into their pathogenic potential.