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 Helicobacter pylori, Pseudomonas aeruginosa, Klebsiella pneumoniae, Enterobacteriaceae, Staphylococcus aureus, Acinetobacter baumannii, Enterococcus faecalis and Clostridioides difficile. Traditionally associated with other body sites, these bacteria may enter the oral cavity and, under favorable ecological conditions, persist through mechanisms such as biofilm formation, stress tolerance, entry into a viable but non-cultivable (VBNC) state, coaggregation with the oral microbiota, and evasion of host immune responses. Their colonization has been reported particularly in dysbiotic oral environments, including periodontitis and immunocompromised conditions, where overgrowth may also facilitate their recovery by conventional culture methods.