Introduction
The human microbiome plays an important role in maintaining physiologic balance throughout the body. In the oral cavity, diverse microbial species interact with tissues and immune pathways to support overall health. These organisms form structured communities that exist in equilibrium with host defenses, contributing to mucosal protection, immune regulation, and resistance to pathogenic colonization. Disruption of this balance can lead to dysbiosis defined as alterations in microbial composition or function associated with inflammatory and autoimmune disease processes [1,2].
Advances in sequencing technologies have improved understanding of the relationship between microbial communities and cellular defense responses. Changes in oral and gastrointestinal microbiota may influence processes involved in immune regulation and epithelial barrier integrity. Because the oral cavity serves as a key interface between the external environment and the immune system, disruption of normal oral flora may have important clinical consequences. Evidence also suggests that alterations in gut microbiota can influence host defense responses that affect oral tissues [1–3].
During clinical evaluation, dental professionals routinely assess the mouth and surrounding craniofacial structures, allowing dentists to recognize early signs of microbial imbalance. These shifts in the microbiome frequently contribute to these disease pathways and alter oral and general health. Greater understanding of these relationships may enhance recognition of pathology and facilitate appropriate referral when systemic conditions are suspected.
The Oral–Gut Microbiome Axis and Immune Regulation
Increasing research describes communication between oral and gastrointestinal microbiomes through immune pathways. Microbial metabolites and inflammatory mediators produced in the gastrointestinal tract can influence host cellular responses in distant tissues, including the oral mucosa.
This interaction is often described as the oral–gut microbiome axis. Changes in gut microbial composition have been associated with alterations in immune function, particularly affecting regulatory T-cell activity and inflammatory cytokine signaling. The gut microbiome supports homeostasis by influencing immune cell differentiation and mediating the resulting cell-mediated responses. Oral microorganisms may also influence the gut microbiome, as bacteria are continuously swallowed and can colonize the gastrointestinal tract. These interactions between oral and gut microbiota represent a bidirectional relationship that may contribute to systemic inflammation when disruptions occur [4].
Influence of the Microbiome on Oral Medicine and Orofacial Pain
In healthy individuals, epithelial cells in the oral mucosa work with innate immune mechanisms to maintain homeostasis. These cells recognize microbial components through pattern recognition receptors such as toll-like receptors. Activation of these receptors leads to the release of mediators that recruit immune cells to the affected area. Under normal circumstances, this response helps control microbial challenges. However, persistent dysbiosis may disrupt this process and contribute to chronic inflammation [2].
Microbial imbalance can influence adaptive immune responses. Certain microbial mediators can activate T lymphocytes and stimulate the release of inflammatory cytokines, including interleukin-1β, interleukin-6, and tumor necrosis factor-α. Elevated levels of these mediators may damage epithelial barriers and promote persistent mucosal inflammation. These mechanisms are particularly relevant in oral medicine conditions associated with immune dysregulation, such as oral lichen planus and autoimmune disorders involving the salivary glands [3].
Microbiome in Autoimmune and Inflammatory Oral Diseases
The relationship between microbial dysbiosis and immune regulation can be observed in several key oral diseases below:
Oral Lichen Planus: Oral lichen planus (OLP) is a chronic inflammatory condition involving immune-mediated destruction of the oral epithelium. Clinically, OLP may present as reticular white striations, erythematous mucosal lesions, or painful erosive ulcers. The precise cause remains unclear, but microbial dysbiosis has been investigated as a contributing factor.
Studies examining the oral microbiome in OLP patients have reported differences in diversity compared with healthy individuals. These alterations may influence inflammatory signaling pathways that contribute to the T-cell–mediated immune responses characteristic of the disease [5,6]. More recent sequencing studies have identified distinct microbial communities within OLP lesions when compared with adjacent healthy mucosa. These findings suggest that local microbial changes may contribute to the inflammatory environment seen in affected tissues [6,7]. Some research also indicates that gut microbiota may influence the pathophysiology involved in OLP, supporting the concept of an oral–gut microbiome relationship.
Sjögren’s Syndrome: Sjögren’s syndrome is an autoimmune condition in which microbial dysbiosis has been proposed as a contributing factor. The disorder is characterized by lymphocytic infiltration of the salivary and lacrimal glands, resulting in reduced salivary secretion. Saliva plays a critical role in maintaining microbial balance through antimicrobial components, immunoglobulins, and buffering mechanisms that regulate microbial growth. When salivary flow decreases, these protective functions become compromised, allowing opportunistic microorganisms to proliferate.
Several studies have reported differences in the oral microbial composition of individuals with primary Sjögren’s syndrome compared with healthy individuals. These alterations are hypothesized to contribute to inflammatory responses through immune activation. Clinically, patients commonly experience dry oral tissues, mucosal irritation, recurrent infections, and burning sensations [8, 9].
Oral Candidiasis: Microbial dysbiosis may also contribute to opportunistic infections of the oral mucosa. Oral candidiasis is commonly associated with Candida albicans, a fungal organism that normally exists as part of the commensal oral microbiota. Under certain conditions, however, this organism can become pathogenic. Interactions between fungal cells and bacterial communities within oral biofilms influence the development of candidiasis, as reductions in bacterial populations that normally limit fungal growth allow Candida species to proliferate, while immune dysfunction may further increase susceptibility to infection [2,10].
Clinically, oral candidiasis may present with erythematous mucosal inflammation, pseudomembranous plaques, or burning sensations affecting the tongue and oral mucosa. These findings can resemble other chronic oral lesions and occasionally mimic neuropathic pain disorders such as burning mouth syndrome.
Microbiome and Burning Mouth Syndrome: Shifts in the microbiome may also play a role in orofacial pain conditions. Burning mouth syndrome (BMS) is characterized by persistent oral burning sensations in the absence of visible mucosal lesions. Although its disease process is not fully understood, emerging research suggests that changes in the salivary microbiome may be contributory [10].
Studies investigating the oral microbiota in BMS patients have identified differences in microbial diversity and composition compared with healthy individuals. Such changes may modulate inflammatory signaling and neuroimmune pathways involved in pain perception and sensitize peripheral nerve fibers, including branches of the trigeminal nerve. This sensitization can increase neuronal responsiveness and contribute to abnormal pain regulation. Some researchers also propose that microbial interactions with neural signaling pathways, sometimes referred to as the microbiome–brain axis, have an effect on pain processing.
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Oral Manifestations of Gastrointestinal Disease
Gastrointestinal disorders may influence oral health through mechanisms involving systemic inflammation, microbiome alterations, and malabsorption-related nutritional deficiencies. Conditions such as inflammatory bowel disease, including ulcerative colitis and Crohn’s disease, as well as gastroesophageal reflux disease (GERD), have all been associated with oral manifestations that may complicate clinical evaluation.
Inflammatory Bowel Disease: Inflammatory bowel diseases (IBD), including ulcerative colitis and Crohn’s disease, are commonly associated with oral findings such as mucosal ulceration, erythema, swelling, and aphthous-like lesions.. In some patients, these manifestations may precede gastrointestinal symptoms, making recognition of these findings particularly important for clinicians involved in oral diagnosis [11].
Recent studies have also explored the role of the oral–gut microbiome axis in IBD. Alterations in gut microbial communities have been linked to inflammatory immune pathways that impact the oral mucosa. Some studies have reported similarities between oral microbial profiles and microbial communities detected in the gastrointestinal tract of patients with inflammatory bowel disease, suggesting potential interactions. These findings support the concept that dysbiosis affecting one mucosal site may influence immune responses at other mucosal surfaces [2,4].
Nutritional Deficiencies in Gastrointestinal Disease: GI disorders may also contribute to oral manifestations through malabsorption-related nutritional deficiencies. Chronic intestinal inflammation, impaired absorption, and dietary restrictions associated with IBD can lead to deficiencies in micronutrients such as iron, vitamin B12, folate, and zinc. These nutrients play important roles in epithelial maintenance, immune balance, and neural function [12].
Common intraoral findings associated with these deficiencies include mucosal atrophy, glossitis, angular cheilitis, and dysgeusia. Iron deficiency can result in atrophic changes of the oral mucosa, while vitamin B12 and folate deficiencies may affect epithelial integrity and nerve function. Zinc deficiency has also been associated with alterations in taste perception and impaired mucosal healing. Because these symptoms may resemble other oral conditions, nutritional deficiencies related to gastrointestinal disease should be considered during diagnostic evaluation [2,3].
Gastroesophageal Reflux Disease: Another gastrointestinal condition with potential oral implications is gastroesophageal reflux disease (GERD). GERD involves the retrograde flow of gastric contents into the esophagus and in some cases the oral cavity. Chronic exposure of oral tissues to these acids causes mucosal irritation and dental erosion and may cause taste disturbances. As a result, patients may experience oral discomfort or burning sensations resembling other oral conditions.
In addition to direct mucosal irritation, long-term pharmacologic management of GERD may influence nutrient absorption. For example, prolonged use of proton pump inhibitors has been associated with altered absorption of vitamin B12, magnesium, and other micronutrients, which may indirectly contribute to oral symptoms [12].
Diagnostic and Clinical Considerations
Gastrointestinal disorders may produce oral manifestations that resemble primary oral diseases through immune, microbial, and nutritional mechanisms. This overlap can complicate diagnosis, particularly when oral findings occur in the absence of obvious gastrointestinal symptoms. For clinicians involved in oral medicine and orofacial pain evaluation, recognition of potential systemic contributors is therefore essential. Consideration of GI disease, microbiome-related interactions, and nutritional deficiencies may help guide appropriate diagnostic testing and interdisciplinary management when evaluating patients with unexplained oral symptoms [4,10].
As research in this area continues to expand, consideration of microbial influences may become increasingly important in the evaluation of oral disease and may improve recognition of oral mucosal conditions and chronic oral pain disorders for dental clinicians. Emerging diagnostic strategies may also incorporate microbiome analysis as a potential biomarker for certain conditions. Identification of disease-associated microbial patterns could assist in distinguishing inflammatory mucosal disease from other disorders with similar clinical presentations. In addition, therapeutic approaches aimed at restoring microbial balance are being explored, including antimicrobial strategies and probiotic therapies [1,2].
Conclusion
The human microbiome is increasingly recognized as an important factor in inflammatory and autoimmune diseases affecting the oral cavity. Literature suggests that dysbiosis within oral and gut microbial communities may contribute to immune dysregulation, mucosal inflammation, and altered pain signaling. These mechanisms are particularly relevant in oral medicine and orofacial pain practice and may also be valuable for general dentists seeking to expand their knowledge. Continued research into microbiome–host interactions will likely provide new insights into the diagnosis and management of associated oral disease. Future studies in this area may further clarify the role of dysbiosis in oral inflammatory and pain conditions and support the development of targeted diagnostic and therapeutic strategies.
Figure 1. The oral–gut microbiome axis depicts interactions between oral and gastrointestinal (GI) microbial communities and their influence on immune regulation. Microorganisms from the oral cavity can enter the GI, where gut microbiota produce metabolites such as short-chain fatty acids (SCFAs) that influence immune signaling pathways. These interactions can affect cytokine signaling and the activity of regulatory T cells (Tregs), contributing to systemic inflammatory mediators that may impact oral tissues and promote mucosal inflammation and epithelial barrier disruption.
Figure 2 . This diagram illustrates how disruption of the healthy oral microbiome leads to microbial dysbiosis that activates epithelial toll-like receptors and stimulates pro-inflammatory cytokine signaling (IL-1β, IL-6, TNF-α). Persistent immune activation contributes to chronic mucosal inflammation and the development of oral conditions including oral lichen planus, Sjögren’s syndrome, oral candidiasis, and burning mouth syndrome.
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References
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