The Oral Microbiota: Implications in Mucosal Health and Systemic Disease—Crosstalk with Gut and Brain

The oral cavity is the most important interface between the human body and the environment. In 1890, Willoughby Dayton Miller recognized the importance of oral cavity health in relation to certain microbial species in his writing, 'The Microorganisms of the Human Mouth' [1]. He also recognized that a healthy oral cavity could benefit the entire organism by preventing the onset of numerous systemic diseases [1].

The human oral cavity is colonized by a large number of microorganisms, with more than 1000 species and approximately 20 billion resident microorganisms present [2]. This microbial community, which includes bacteria, fungi, viruses and protozoa, is one of the most important and complex in the human body [3]. Nevertheless, it is possible to distinguish substantial microbial differences at the level of the tongue, tooth surface, subgingival plaque, gingival epithelium, buccal mucosa, saliva, tonsils and the oropharyngeal region [4,5,6,7,8] (Table 1). This difference is due to the presence of various microorganisms, including aerobes, anaerobes, facultative anaerobes and strict anaerobes, which can colonize different niches depending on the substrate's superficial characteristics, oxygen gradient, nutrient availability and proximity to salivary glands. These microorganisms can also interact with mucosal cells, as well as with other bacteria or fungi [9].

In the oral cavity, the mucosa and hard tissues, made of enamel and dentin, are physiologically coated with a protective film. This film is generated by the absorption of peptides, salivary proteins and macromolecules from crevicular gingival fluid, blood, bacteria, mucosa and the host's diet. Microorganisms living in the oral cavity bind to the main components of the oral film via specific adhesion molecules, thus creating ecologically and functionally stable communities. Within the resulting biofilm, the microorganisms exhibit precise bio-geographical patterns, establishing syntrophic activities and responding to signal molecules present in the oral environment, thereby promoting microbiota homeostasis [10,11]. The oral microbiota plays a key role in maintaining the health of the oral cavity by facilitating the maintenance of an efficient protective barrier against invading pathogens; nevertheless, it is also of great importance in maintaining systemic health through significant, constant interaction with microorganisms found in mucosal compartments. The quantity of saliva secreted by an adult throughout the day, along with its microbial load, reaches the gastrointestinal tract through swallowing, thereby colonizing the gut ectopically and contributing to the maintenance of stability in the intestinal microbial community and mucosa-associated lymphoid tissue (MALT) [12].

The homeostasis of the oral microbiota is regulated by a multifactorial network of host-related, environmental and microbial determinants. One of the key regulatory components is the host immune system, which influences the composition of microbiota via innate and adaptive immune mechanisms. These mechanisms include the secretion of antimicrobial peptides and the production of secretory immunoglobulin A (IgA), as well as establishing immune tolerance at the oral mucosal surface [2,13].

Saliva plays a key role in maintaining the balance of microbes in the mouth. Its flow rate, buffering capacity and biochemical composition all strongly influence microbial adhesion, nutrient availability and biofilm stability. Salivary proteins and peptides, such as mucins, lactoferrin, lysozyme, peroxidases and histatins, play a role in controlling microbes and preventing dysbiosis [14,15]. However, alterations in salivary secretion or composition are associated with an increased risk of microbial imbalance and inflammatory disease in the mouth.

The integrity of the oral epithelial barrier, supported by intercellular junctional complexes, is a critical defence mechanism that limits microbial penetration and excessive immune activation. Disruption to these barrier mechanisms can facilitate inflammatory responses driven by dysbiosis, which may contribute to mucosal damage, including mucositis, as well as inflammation of the periodontal tissue [13,14,16,17,18].

The host's genetic background affects the composition of the oral microbiota by influencing the integrity of the epithelial barrier [2].

Variation in the genes that regulate immune and inflammatory pathways can affect how individuals respond to microbial challenges. Consequently, there may be variations in the likelihood of developing periodontal disease, even when environmental factors are similar [19,20].

Genes that increase susceptibility to severe periodontitis have been identified. These include SIGLEC5, PLG, ROBO2, ABCA1, PF4 and CTSC. These genes play a role in innate immunity and tissue maintenance and repair and are therefore crucial in regulating periodontal health. This may explain why some people develop the disease despite having normal oral hygiene practices and not smoking, unlike people who do not develop periodontitis [21]. In addition, polymorphisms affecting cytokine regulation and inflammatory signalling can alter host-microbiota interactions, resulting in a less effective host response to maintaining microbial homeostasis [19].

Age-related physiological changes, such as reduced salivary function, immunosenescence and altered mucosal integrity, can also contribute to modifications in the composition of the oral microbiota [14]. Under normal conditions, these factors help to maintain a healthy balance of bacteria in the mouth. However, disruption to one or more of these regulatory mechanisms can therefore destabilize oral microbial homeostasis, promoting the transition from eubiosis to dysbiosis and increasing the risk of inflammatory diseases of the mouth and body [2,7].

Under healthy conditions, the various microbial communities in the mouth are in balance with each other. However, when this fragile balance is destroyed due to an unhealthy lifestyle or environmental factors, such as diet, smoking, alcohol consumption, poor oral hygiene, psychological stress, inflammatory diseases or immunodeficiency, a condition known as dysbiosis can develop, which can lead to disease. Oral cavity dysbiosis leads to an inflammatory process that develops into gingivitis. The colonization of the outer gumline biofilm by pathogenic microorganisms and the inflammation generated at the gumline level, causes alterations to the oral microenvironment. Over time, this leads to clinical signs of periodontal disease. Within the oral cavity, the biofilm produced by layers of peptidoglycan, polysaccharides, fibronectin, albumin, mucin and enzymes can remain in microbial homeostasis or incur pathogenic colonization, as mentioned previously. A considerable number of persistent infectious diseases in the human body are caused by virulent biofilms, particularly in the oral cavity or gut [10]. Restorative and prosthetic materials are also quickly coated with biofilms that may harbour pathogenic microorganisms and induce secondary caries, gingivitis, mucositis and peri-implantitis [22]. Oral cavity dysbiosis is the main cause of periodontal disease, also known as periodontal gum disease. This chronic inflammatory disorder is induced by oral pathogens and leads to disruption of the entire periodontal tissue, including the gingiva, alveolar bone, root cementum and periodontal ligament. Periodontal disease is the sixth most widespread disease in the world, affecting the adult population and resulting in irreversible damage to periodontal tissue [23]. Periodontal disease is considered a risk factor for various chronic extraoral diseases, including cardiovascular diseases, respiratory diseases, diabetes, non-alcoholic steatohepatitis (NASH), obesity, inflammatory bowel disease, rheumatoid arthritis, allergic diseases and Alzheimer's disease [12,24].

Periodontal pathogens and microorganisms derived from oral biofilms can reach the lower airways, where they are associated with an increased risk of pneumonia, chronic obstructive pulmonary disease (COPD) exacerbations and other respiratory infections [25,26,27]. There is also strong and consistent evidence supporting an association between periodontitis and cardiovascular disease (CVD). Large-scale epidemiological studies, meta-analyses and international consensus reports have shown that people with periodontitis are more likely to develop atherosclerotic CVD, including coronary heart disease and stroke [28,29].

Periodontal pathogens have been linked to various infectious diseases, particularly those affecting the head and neck. Fusobacterium nucleatum, particularly Fusobacterium necrophorum, is the main cause of Lemierre's syndrome, a rare upper respiratory infection that can lead to life-threatening septic thrombophlebitis of the jugular veins [30].

Fusobacterium nucleatum has been found in a wide range of placental and fetal tissues, including amniotic fluid, fetal membranes, and cord blood. Its presence is associated with chorioamnionitis, preeclampsia, preterm birth, stillbirth, and early-onset neonatal sepsis, which indicates its ability to spread to different placental and fetal compartments [31].

The bacteria Porphyromonas gingivalis and Aggregatibacter actinomycetemcomitans can increase the activity of the enzyme peptidyl-arginine deiminase (PAD), resulting in the formation of citrullinated proteins (citrullination) in joints and other tissues. The presence of these proteins induces the production of antibodies against them and the release of pro-inflammatory cytokines. The key role of PAD in triggering autoimmune diseases such as coeliac disease and rheumatoid arthritis is well established [32].

Finally, oral microbial dysbiosis has been associated with impaired immune tolerance, increased susceptibility to allergies (including allergic rhinitis, asthma, atopic dermatitis and food allergies), and the production of Th2-type inflammatory responses [33,34]. Understanding the relationship between the presence of oral microbes and the development of allergic diseases can provide important information on risk factors for allergies and identify potential therapeutic targets [35].

Periodontal disease may promote the initiation and progression of oral squamous cell carcinoma (OSCC), resulting in significant qualitative and quantitative alterations to the host's immune response and the bacterial community. Specific periodontal pathogens, such as Porphyromonas gingivalis and Fusobacterium nucleatum, are consistently present in OSCC, whereas other microorganisms in the oral cavity are associated with different cancer types. Prevotella spp. has been linked to head and neck squamous cell carcinoma (HNSCC), oropharyngeal squamous cell carcinoma (OPSCC) and laryngeal squamous cell carcinoma (LSCC) [36].

Aggregatibacter actinomycetemcomitans, Streptococcus anginosus and Tannerella forsythia have been linked to oesophageal and gut cancers, while Capnocytophaga spp. and Veillonella spp. have been associated with lung cancer [37].

NOD-, LRR- and pyrin domain-containing protein 3 (NLRP3) is an intracellular multiprotein complex that forms part of the innate immune system. The importance of local NLRP3 inflammasome activation in periodontal disease is now well established. During the development of periodontitis, an increase in inflammatory M1 macrophages is consistently observed in periodontal tissues [38,39]. Experimental evidence indicates that periodontal pathogens and their components, including the outer membrane vesicles (OMVs) of Porphyromonas gingivalis and Fusobacterium nucleatum, promote NLRP3 inflammasome activation [33,40]. This activation contributes to M1 macrophage polarisation and is associated with enhanced alveolar bone loss [40,41,42]. Porphyromonas gingivalis promotes the activation of the NLRP3 inflammasome via both canonical and non-canonical pathways, thereby inducing the expression of IL-1β and IL-18 in gingival tissues [43,44,45]. Interestingly, Porphyromonas gingivalis can also suppress IL-1β secretion induced by Fusobacterium nucleatum by blocking caspase-1 activation [46], thereby inhibiting inflammasome activation triggered by the same bacterium. Interestingly, Aggregatibacter actinomycetemcomitans activates the NLRP3 inflammasome in human macrophages through its own leukotoxin, promoting IL-1β and IL-18 expression and amplifying the local inflammatory response [47].

Inflammation of the oral cavity caused by these and other microorganisms may lead to intestinal microbiota disorder and destruction of the intestinal epithelial barrier (IEB) through ectopic colonization. This can result in the induction of endotoxemia and a systemic inflammatory response, as well as low-grade chronic inflammation (LGCI) and the onset of degenerative diseases. This shows that the prevalence of specific microorganisms in the oral cavity is closely correlated with specific intestinal illnesses [48]. At the immune level, there is evidence of increased interleukins (IL)-1α/β, tumour necrosis factor (TNF)-α, IL-6, IL-8 and IL-17, as well as increased metalloproteinases (MMPs), particularly MMP9, and prostaglandin E2 (PGE2). The cell-mediated compartment is also activated, with a relevant increase in cytokines such as IL-2, interferon (IFN)-γ, IL-4, IL-5, IL-13 and IL-10 [49]. Recent studies have reported that other bacteria are involved in triggering periodontal disease due to their toxins and virulence factors, which can easily translocate systemically and pass through the blood–brain barrier (BBB). These bacteria are Filifactor alocis and Aggregatibacter actinomycetemcomitans; the latter has already been mentioned as being involved in oesophageal and gut cancers [50]. Well-established data have identified an important pathogenic triad in Porphyromonas gingivalis, Treponema denticola and Tannerella forsythia, which is strongly associated with serious periodontal inflammation and is called 'the red complex' [32]. However, due to their pathogenic potential, the most extensively studied microorganisms are Porphyromonas gingivalis and Fusobacterium nucleatum. Table 2 and Table 3 list the main biological alterations and pathologies associated with these microorganisms [12,51,52,53].

Porphyromonas gingivalis is a common oral pathogen and the key etiological agent of chronic periodontitis. As an opportunist of the periodontium, it has the ability to transform a healthy oral microbiota into a dysbiotic one. Studies in recent years have emphasised the pro-inflammatory potential of this pathogen, which can induce the destruction of periodontal tissue by initiating a strong pro-inflammatory response mediated by cytokines such as IL-1β, IL-6, IL-8, IL-17 and IL-18, as well as upregulating the NLRP3 inflammasome [54].

Macrophages are the first cells involved in the inflammatory process through the production of pro-inflammatory cytokines. M1 macrophages are the dominant phenotype in response to Porphyromonas gingivalis infection, whereas M2 macrophages, which have an anti-inflammatory function, are less prevalent. An increase in the M1/M2 ratio, an increase in the release of pro-inflammatory cytokines and the differentiation of monocytes/macrophages into osteoclasts induce the destruction of periodontal tissue [23].

Porphyromonas gingivalis disrupts the immune activation of host cells, thereby enabling systemic inflammation and the onset of multiple chronic diseases. It triggers autoimmune diseases by acting on the enzyme PAD [32]. Porphyromonas gingivalis may also alter mitochondrial function, reducing processes related to oxidative phosphorylation, adenosine triphosphate (ATP) production, and oxygen utilisation, and resulting in increased reactive oxygen species (ROS) [54]. Although an elevated rate of oral cavity tumours is associated with viral infections, about 90% of OSCC cases in the USA are caused by papillomavirus and Epstein–Barr virus infections [55,56]. In vitro experiments have shown that Porphyromonas gingivalis may promote tumour progression by affecting different signalling pathways and stimulating OSCC cell proliferation. This occurs through the upregulation of D1 cyclin expression, a critical regulator of cell proliferation, and the reduction in p53 tumour suppressor levels and activity [57]. In addition, the increased expression of MMPs caused by this pathogen impairs the extracellular matrix and the basal membrane, promoting the invasion and metastatic spread of oral cancer cells [58]. Porphyromonas gingivalis is not only responsible for neoplastic diseases in the mouth, but is also implicated in pancreatic and gastric carcinogenesis through effector molecules and inflammatory mediators that can travel through saliva and blood to distant sites [59]. In periodontal disease, the invasion and long-distance colonization of Porphyromonas gingivalis can lead to systemic inflammation and aggravation of cardiac pathologies, particularly coronary disorders, thereby representing a risk factor for myocardial infarction [60]. The presence of Porphyromonas gingivalis within atherosclerotic plaques provides evidence of direct microbial contribution to vascular pathology [61].

Fusobacterium nucleatum is an obligate anaerobe, a non-spore-forming Gram-negative bacterium belonging to the Fusobacteria phylum [62]. It is an important component of the oral microbiota and is a commensal of the oral cavity and gastrointestinal tract. However, under specific genetic or environmental conditions, or when it is translocated from the oral mucosa to the bloodstream, it can demonstrate its potential as an opportunistic pathogen [63]. Its ability to resist acidic environments enables it to survive the passage through the stomach.

Furthermore, the presence of Fusobacterium adhesin A (FadA) enables it to bind to other bacteria and cells by up-regulating the expression of annexin A1 and inducing Wnt/β-catenin, contributing to its pathogenicity [64].

Fusobacterium nucleatum adheres to the oral biofilm via various adhesion mechanisms, such as radiation-sensitive DNA adhesins (RadD), F. autotransporter protein 2 (Fap2) and F. outer membrane protein A (FapA), thereby facilitating the aggregation of secondary colonies [65].

Due to its high heterogeneity, four different subspecies can now be distinguished: Fusobacterium nucleatum subsp. nucleatum, Fusobacterium nucleatum subsp. polymorphum, Fusobacterium nucleatum subsp. vincentii and Fusobacterium nucleatum subsp. animalis [66].

Fusobacterium nucleatum is the main microorganism found in the periodontal tissue and is the main etiological agent of periodontal disease. It is able to create an immune microenvironment that is suitable for disease progression by reducing the defensive function and superoxide production of neutrophils at an early stage, which is essential for oxidative killing [67]. The upregulation of IL-1β, IL-6 and TNF-α expression through the activation of NF-κB induces macrophage infiltration, osteoclast recruitment and activation, and bone resorption [68,69]. Fusobacterium nucleatum can inhibit the proliferation of gingival fibroblasts by promoting apoptosis and increasing the generation of reactive oxygen species (ROS) and the expression of pro-inflammatory cytokines through the activation of the protein kinase B (PKB/AKT)/MAPK signalling pathway and the NF-κB pathway [70].

In addition, the cooperation between Fusobacterium nucleatum and Porphyromonas gingivalis has been emphasised, whereby they can increase each other's infective capacity [71]. Fusobacterium nucleatum has been identified as an important microorganism in the pathogenesis of many human tumours, and its role has been extensively studied, particularly in colorectal cancer (CRC), squamous cell carcinoma (SCC), and other cancers, including gastrointestinal and breast cancers. Several mechanisms are involved in tumourigenesis and the metastasis of various cancers, including immune response modulation, virulent factors, oncogenic microRNA, the interaction of intestinal metabolites, DNA damage and epithelial–mesenchymal transition. In addition to anti-tumour activity mediated by T cells and NK cells, Fusobacterium nucleatum may stop this process via binding to TIGIT and CEACAM receptors, thereby increasing immunosuppression [72]. Some interesting studies have demonstrated that elevated levels of Fusobacterium nucleatum within tumours have a prognostic role in predicting poor recurrence-free survival in patients with oesophageal squamous cell carcinoma (ESCC) and a reduced response to antineoplastic therapy [73]. Specific sequences of Fusobacterium nucleatum have been found in significantly higher quantities in CRC tissues compared to adjacent healthy tissues [74] and in CRC patients compared to healthy individuals. The presence of Fusobacterium nucleatum in CRC is always associated with higher levels of pro-inflammatory cytokines and an inflammatory microenvironment, which induce tumour progression [75].

In addition to Porphyromonas gingivalis and Fusobacterium nucleatum, other important oral pathogens should be considered in order to gain a more complete understanding of periodontal disease.

Aggregatibacter actinomycetemcomitans is a facultative Gram-negative anaerobic bacterium belonging to the Pasteurellaceae family [50]. It can lead to inflammation, destruction and resorption of bone and gum tissue. It produces proteolytic enzymes such as collagenase and releases various toxic factors such as the cytolethal distending toxin (Cdt), which damages host cell DNA, and leukotoxin A (LtxA), which belongs to the large repeats-in-toxins (RTX) family and leads to the death of activated leukocytes [50]. In particular, LtxA is closely associated with the onset and progression of the disease since it can activate the NLRP3 inflammasome in human macrophages and induce citrullinated proteins to interact with neutrophils and give rise to pyroptosis, a pro-inflammatory process linked to the pathogenesis of periodontitis [50,76,77]. These toxic factors are contained in OMVs of Aggregatibacter actinomycetemcomitans [78,79,80]. These vesicles can also contain Peptidoglycan-associated lipoprotein (Pal) [81], the chaperonin GroEL [82], and NOD1- and NOD2-active peptidoglycan [83]. The virulent cargo contained in these vesicles, therefore represents a powerful immune trigger. The presence of Aggregatibacter actinomycetemcomitans in the subgingival plaque is always associated with periodontal disease, or it can constitute an early sign of its onset [50].

Its prevalence varies depending on the geographic origin, age, and periodontal condition of the population under consideration. It appears that conditions caused by Aggregatibacter actinomycetemcomitans may create an anaerobic environment favourable to the colonization of another emerging Gram-positive anaerobic pathogenic bacterium, Filifactor alocis [50].

Among the virulent factors expressed by Filifactor alocis is a moonlight surface protein that binds to and inhibits complement component 3 (C3), which is a key step in the complement activation cascade. There is also an exotoxin from the RTX family called RTX ftxA. Razooqi et al. have emphasised that positivity for the RTX ftxA toxin gene in Filifactor alocis carriers is associated with a greater presence of the bacterium itself and a higher progression of clinical attachment loss [84,85]. This suggests a potential role for the toxin in the pathogenicity expression of this bacterium [84,85]. The protein RTX ftxA, which is expressed by these bacteria, is strictly correlated with the onset and progression of many systemic degenerative diseases. Moreover, it is important to highlight the potential impact of the vesicles released by these pathogens on the central nervous system (CNS), given that they can pass through the bloodstream and cross the blood–brain barrier (BBB). Both Filifactor alocis and Aggregatibacter actinomycetemcomitans have developed defence mechanisms to protect themselves from oxidative damage. The former, particularly the ATCC 35896 strain, codes for an antioxidant enzyme called superoxide reductase FA 796 [86]. The latter activates an oxygen resistance transcription regulator called oxyR, which can modulate catalase expression and consequently transform the hydrogen peroxide produced by neutrophils into water and oxygen. These two oral pathogens can evade the host's immune response, thereby preventing an efficient inflammatory response from being expressed.

Tannerella forsythia, Treponema denticola and Porphyromonas gingivalis constitute a highly pathogenic triad named 'the red complex' by Socranski et al. [87,88]. These Gram-negative bacteria are able to operate in close symbiosis and release numerous virulent factors, including lipopolysaccharides (LPS). They are also able to evade the host immune response. Their presence in the oral cavity is associated with the destruction of hard and soft tissue, as well as severe periodontitis. The 'red complex' bacteria can use the sialidase enzyme to cleave sialic acid from the end of the host cell's membrane glycoproteins [89]. Tannerella forsythia specifically codes for a sialidase, NanH, which plays a crucial role in facilitating bacterial colonization by exhibiting epitopes hidden in sialic acid on epithelial cells [90]. Another characteristic of Tannerella forsythia is its ability to survive within biofilms thanks to its auxotrophy for N-acetylmuramic acid (MurNAc), which is generated by the replacement products of the cell walls of other cohabiting bacteria [91]. The pathogenicity of Tannerella forsythia involves many enzymes, including some proteases such as the cysteine protease PrtH [92], the trypsin-like cysteine protease and the secretory protease KLIKK, which can attach to collagen and elastin [93]. Other enzymes, such as karilysin, can inhibit complement activation [94] or inactivate the human antimicrobial peptide cathelicidin [95,96]. Tannerella forsythia cells are coated with a crystalline array formed by S-layer proteins TfsA and TfsB. The S layer plays a crucial role in interacting with the host [97,98,99], adhering to human gingival epithelial cells, and inhibiting the host immune response [100,101].

The glycosylated antigen BspA on the surface of Tannerella forsythia induces the release of cytokines by macrophages and dendritic cells via the TLR2 receptor [102,103], and it causes the development of foam cells in THP-1 macrophages. This suggests that this bacterium is strongly involved in the development of atherosclerotic vascular damage and myocardial infarction [104]. Furthermore, OMVs originating from Tannerella forsythia can elicit a robust pro-inflammatory response through cytokine release in macrophages and mesenchymal stromal cells of the human periodontal ligament (hPDL-MSCs) [105].

Treponema denticola is an anaerobic bacterium and spirochete that is generally part of the normal microflora present in the biofilm. However, under dysbiotic conditions, it can interact with Porphyromonas gingivalis and Tannerella forsythia, becoming a primary cause of gingival disease [2,87,88]. Treponema denticola also exhibits virulence through proteolytic exotoxins, anaerobic amino acid fermentation, production of toxic metabolites, and the presence of OMVs containing adhesins, toxins, and proteolytic enzymes that can facilitate bacterial invasion of tissues [106,107,108] by destroying tight junctions [109] and modulating the host immune response. The main virulence factors of Treponema denticola are part of the so-called toxin-antitoxin (TA) system [110,111], also known as transposase [112]. The TA system comprises a toxin that inhibits essential cell components and an antitoxin that counteracts its toxic effects. It may therefore influence the biofilm [110,111] and resist environmental influences and the action of drugs [113,114]. Dentilysin is one of the toxic factors of Treponema denticola. It is a protease that operates on the cell surface and is capable of cutting phenylalanyl/alanyl and prolyl/alanyl bonds [115,116,117,118,119]. It can also promote extracellular matrix degradation by activating MMP2 [120] and disrupting the host's intracellular signalling pathway and damaging tight junctions, thus increasing BBB permeability [109]. Treponema denticola can bind to the regulatory protein of the alternative complement activation pathway named factor H (FH) [121]. Some proteins in this family act as cofactors in the cleavage of complement component C3b, so the protein block leads to downregulation of this factor's production [122,123] and C3 convertase impairment. Treponema denticola's complement evasion is further enhanced by FH cleavage due to dentilysin. This leads to local dysregulation of complement activation, severe tissue destruction, and bone resorption [1]. Another mechanism enabling Treponema denticola to evade complement attachment is linked to its neuraminidase enzyme, TDE0471, which can remove sialic acid from serum proteins. This neuraminidase provides Treponema denticola with protection by preventing the deposition of membrane attack complexes on its surface [124].

There is evidence showing that Treponema denticola can destroy the actin cytoskeleton of fibroblasts, epithelial cells, and neutrophils [125,126,127], thereby affecting cellular migration and chemotaxis [128,129]. Cofilin is a small protein essential for regulating the actin cytoskeleton. By binding to F-actin, it depolymerises its filaments to promote renewal and plays a key role in cell motility. However, the binding of cofilin to phosphatidylinositol 4,5 bisphosphate (PIP2) renders cofilin inactive. The major surface protein (MSP) of Treponema denticola interferes with the actin cytoskeleton [130,131] by increasing the production of phosphatase-enhancing PIP₂, which leads to cofilin inactivation [125]. These mechanisms also involve the motility of mitochondria, which travel along the actin cytoskeleton [132]. At the central nervous system (CNS) level, mitochondria perform essential functions in maintaining neuronal vitality and supporting synaptic functions. To perform these activities, they need to migrate towards presynaptic sites. Loss of functional performance leads to progressive depletion of synaptic connections. It is well established that mitochondrial dysfunction in neurons is closely correlated with the onset of damage and neuronal degeneration [133,134,135].

Taste receptors are chemosensory receptors that are expressed in both taste buds and extra-taste tissues. They can identify metabolic products and toxic substances from the oral microbiota and modulate the immune response in the oral cavity, playing a significant role in various physiological and pathological processes [136].

Twenty-five types of receptors called T2R are responsible for recognizing bitter taste [137]. Bitter receptors are sensitive to pH; higher acidity increases the perception threshold of this taste. The expression of bitter receptors has been observed in immune system cells such as monocytes and neutrophils. Various bacterial metabolites can stimulate bitter receptors by activating local calcium flow and inducing the activation of the nitric oxide synthase enzyme, which catalyzes nitric oxide (NO) production. NO is capable of increasing macrophage phagocytic activity and evoking relaxation of the smooth muscles of the airways [138].

The T1R2/T1R3 receptors mediate the perception of sweet tastes [139]. Sweet receptor activation enables intracellular signalling, modulating intestinal hormonal responses and glucose absorption [140,141]. Sweet receptors can also inhibit signals led by bitter receptors, thereby reducing the immune response to microbial metabolites and stimulating T2R receptors.

Umami receptors, which are sensitive to L-glutamate, are linked to taste intensity levels, including sweet, bitter, salty and sour. Umami taste is sensed through the heterodimer T1R1/T1R3 [142]. Our diet is influenced by different taste receptors; individuals with a high sensitivity to sweet tastes tend to reduce their consumption of sweet foods, while a high sensitivity to bitter tastes leads to the avoidance of bitter foods [143]. Therefore, taste receptors influence food choice and regulate general metabolism and internal homeostasis, thereby mediating the transition between health and disease conditions [136]. From a physiological point of view, the sensitivity of taste receptors, which is influenced by polymorphisms in the population, guides individuals' food choices. The introduction of certain foods modifies the oral microbiota, which consequently affects the gut microbiota. Genetic polymorphisms in human taste receptors contribute to variability in taste perception and dietary preferences among individuals. In particular, variants in sweet taste receptors (TAS1R2/TAS1R3) and bitter receptors (e.g., TAS2R38) have been associated with reduced sensitivity to sweet or bitter compounds. This explains why some individuals exhibit a lower preference for sugary or bitter foods, despite similar environmental and nutritional conditions [144,145].

The interaction between oral microbiota and taste receptors is bidirectional, influencing individual dietary preferences. Microbial metabolites, including short-chain fatty acids and other bioactive compounds, can modulate the expression and sensitivity of taste receptors by influencing epithelial signalling pathways [146]. These mechanisms impact the gut microbiota, leading to changes in its composition and function that influence the expression and sensitivity of taste receptors, as well as individual dietary choices (Figure 1). Interestingly, inflammatory mediators such as IL-1β and TNF-α influence the regulation of taste receptors through cytokine-dependent mechanisms, affecting oral epithelial and immune cells [147]. Furthermore, changes in the epithelial microenvironment driven by the microbiota, including shifts in pH and biofilm organisation, impact the accessibility and function of taste receptors [148,149].

As can be seen in Figure 2, the interaction between oral microbiota, taste receptors, oral mucosal immunity and dietary preferences affects the gut microbiota and MALT. This, in turn, affects systemic immunity.

The gut microbiota indirectly influences the immune response related to MALT. It is composed of a vast number of bacteria (approximately 10¹⁵), which are extremely diverse (approximately 70,000 species). Furthermore, microeukaryotes (e.g., fungi, amoebas and flagellates), archaebacteria and viruses interact continuously with each other and can influence the evolutionary trajectory of the host, enabling adaptive processes and the evolution of the psycho-neuro-endocrine-immune (PNEI) system [3].

The composition of the gut microbial population varies according to age, dietary habits, nutritional state, the degree of industrialization of the environment in which a subject lives, and the use of substances that can modify its composition [3].

The gut microbiota performs multiple functions, such as: acting as a barrier against pathogens and pro-inflammatory substances, performing metabolic functions, such as digesting complex polysaccharides and producing short-chain fatty acids (SCFAs), vitamins and amino acids, performing a neuroendocrine function, influencing motility and sensory secretive modes of the gastrointestinal tract, performing a microbial-drug function, affecting the bioavailability, efficacy and toxicity of drugs taken by a subject. It also plays an important role in modulating immune function, which is related to maintaining the mutualistic nature of resident organisms and producing pro-inflammatory and anti-inflammatory molecules [150]. These microorganisms have hormonal receptors and use 'quorum sensing', enabling them to perceive and produce self-inducing molecules that regulate growth, motility and bacterial virulence. They also interact with hormonal signals produced by the host organism.

Microbial dysbiosis resulting in increased gut permeability leads to the destruction of immune homeostasis and a local systemic increase in pro-inflammatory molecules. This process leads to the development of systemic inflammation and LGCI, which is characterized by an increase in many pro-inflammatory cytokines, including IL-18, TNF-α, IFN-γ and IL-17, as well as an increase in M1 macrophages. This process induces multiple degenerative metabolic diseases affecting every organ system in the human body [151]. Additionally, the destruction of IEBs can lead to endotoxemia and bacterial translocation, whereby bacterial components, such as LPS, are transported to the submucosa, thus activating the inflammatory process. The gut tract communicates with the central nervous system (CNS) via the vagus nerve and the blood circulation. This bidirectional neuro-humoral communication system represents a veritable axis, the gut–brain axis (GBA), which is of particular importance as it allows signals from the two systems to be integrated and their mutual functions modulated. GBA contributes to maintaining the homeostatic balance of the gastrointestinal apparatus, its local neuroendocrine-immune structure, and central behavioural functions. GBA perturbations may give rise to changes in the structure of these two systems, with consequences for individuals [152]. Under normal conditions, the gut provides an ideal environment for symbiotic and commensal microorganisms to survive. However, mutations in normal intestinal physiology can destabilise this environment, thereby invalidating the functional structural integrity of the gut. This leads to changes in motility, epithelial cells, mucosal secretion and barrier function, resulting in substantial modifications to the associated mucosal microbial ecosystem [153]. Changes in the physiology of the gastrointestinal tract and the central nervous system (CNS) of the host strongly affect gastrointestinal microbiology, leading to qualitative and quantitative modifications of the microorganisms present. Psychological stress can alter the organization of the gut microbiota, as demonstrated in rodents following food and water deprivation [154], maternal separation of rat pups [155] and maternal contact limitations in Macaca rhesus [156]. This results in increased corticosteroid levels, pro-inflammatory cytokines and increased gut permeability [157].

Therefore, stress-induced activation of the hypothalamic–pituitary–adrenal (HPA) axis disturbs ordinary gut physiology, the homeostasis of existing microbiota and the mucosa-associated immune system, resulting in modification of gut physiology and consequent changes to existing bacteria, as well as increased local systemic inflammation.

The structure of the microbiota strongly affects CNS activity: rats dosed with Bifidobacterium infantis for two weeks showed increased levels of tryptophan in the blood, emphasising the communication between certain microorganisms in the gut and the CNS [158].

The introduction of pathogenic microorganisms into the gut tract promptly causes anxious behaviour in an animal model [159], affecting the vagus nerve and activating the nucleus of the solitary tract and the lateral parabrachial nucleus [160]. When mice are infected with Helicobacter pylori, they exhibit altered food-seeking behaviour and modifications in pro-opiomelanocortin levels [161]. Treating healthy mice with Escherichia coli isolated from colitis-affected mice has caused colitis and memory impairment [162].

Mice that are germ-free show a critical reduction in brain-derived neurotrophic factor (BDNF) in the hippocampus compared to specific pathogen-free (SPF) mice [163], which highlights the crucial role of gut microorganisms in developing and maintaining BDNF-induced trophic and neuroplastic activity in the central nervous system.

These data indicate the crucial role of the gut microbiota in developing the host's behavioural response.

When the NLRP3 inflammasome is activated in response to pathogen- or danger-associated molecular patterns, it leads to the production of pro-inflammatory cytokines such as IL-18 and IL-1β, as well as triggering events such as pyroptosis and apoptosis.

Under physiological conditions, the activation of NLRP3 in epithelial gut cells by resident organisms creates a microenvironment that provides an adequate immune response and controls inflammation to preserve intestinal epithelial barrier (IEB) integrity. However, when dysbiosis occurs, NLRP3 hyperactivation causes a change in local immune responses, increasing the levels of IL-1β, IL-18, IL-17, IL-6 and TNF-α, and inducing IEB damage. Subsequently, as a result of GBA-generated communication, it causes damage to the BBB, increasing kynurenine levels and activating microglia, thereby altering the CNS's homeostatic immune mechanisms and leading to an increase in neuroinflammation and neurodegeneration [164,165].

The oral cavity is home to a variety of microbes, including bacteria, fungi and viruses, which play a key role in maintaining health.

Disruptions to the homeostasis of the oral microbiota can lead to chronic inflammatory diseases of the gums and contribute to the development of systemic diseases such as endocarditis, rheumatoid arthritis, osteoporosis, obesity, diabetes, and psychiatric and neurological disorders. They can also contribute to several degenerative diseases, including CRC and esophageal cancer.

Oral microbiota communities participate in the complex crosstalk generated by the interaction between the microbiota, the gut and the brain (the Microbial–Gut–Brain Axis) [51]. Many species of microorganism are present in specific niches within the host organism; when they leave these niches and settle elsewhere, they can cause illness by altering the existing balance. There is a growing body of evidence supporting the concept of an oral–gut–brain axis as an extension of the traditional gut–brain axis. Both experimental and clinical studies have demonstrated that oral dysbiosis can alter the composition of gut microbes, damage the intestinal barrier's integrity, and trigger systemic inflammation.

Kitamoto et al. (2020) [164] demonstrated that periodontal inflammation can lead to an increase in Enterobacteriaceae in the mouth, particularly Klebsiella and Enterobacter species. These bacteria are able to colonize the gut under inflammatory conditions. However, this process only occurs when gut colonization resistance is disrupted, enabling orally derived pathobionts to displace resident intestinal strains in a competitive manner. Once established, these bacteria interact with the gut microbiota at the strain level and have adapted their metabolism to thrive in an inflamed intestinal environment. These interactions favour the expansion of Th17-inducing microbial consortia, thereby reshaping immune–microbiota interactions, sustaining mucosal inflammation, and causing epithelial barrier dysfunction. Meanwhile, the ectopic colonization of oral pathobionts exacerbates intestinal inflammation both directly, through the activation of innate inflammatory pathways (IL-1β signalling), and indirectly, by serving as cognate antigens for orally primed effector T cells that migrate to the gut. Oral pathogens and their molecular derivatives, such as lipopolysaccharide (LPS) and extracellular vesicles, can translocate to the gastrointestinal tract and enter the systemic circulation. This can lead to blood–brain barrier (BBB) dysfunction, as well as the activation of microglia and inflammatory processes [166,167,168].

The oral microbiota, or its toxic metabolites such as LPS, can reach the brain directly through the vascular system. Alternatively, they can reach the gut and affect cerebral functions indirectly, generating neurovascular alterations, increased BBB permeability, neuroinflammation and neurodegeneration [169].

The oral microbiota can interact with the central nervous system (CNS) not only through the vagal system, but also via the trigeminal nerve, which is responsible for transmitting signals between the oral cavity and the brain. These signals overlap with those from the vagal nerve through the main sensory nucleus [170]. Neuroinflammation is strongly associated with significant cognitive and neurological decline, as well as multiple psychiatric diseases of the CNS, such as depression, anxiety and bipolar disorder, as well as Parkinson's disease, multiple sclerosis and Alzheimer's disease [171]. One of the most relevant oral cavity pathogens is Porphyromonas gingivalis, which can reach the brain via the bloodstream and release a neurotoxic protease called gingipain. This protease is directly involved in generating β-amyloid protein plaques (Aβ) [54]. Experiments on animal models have shown that the oral administration of Porphyromonas gingivalis can lead to gut dysbiosis and cognitive impairment [52]. It has been demonstrated that Porphyromonas gingivalis can promote neuroinflammation and neurodegeneration through the activation of microglia and the stimulation of M1 macrophages, resulting in increased expression of IL-1β, IL-6 and TNF-α. An increase in these molecules at the central level leads to synaptic dysfunction due to Aβ accumulation, which is generated by activation of the NF-κB/cathepsin B pathway; all this is promoted by Porphyromonas gingivalis itself. These processes can lead to dementia and progress to Alzheimer's disease [172]. Evidence has been found that patients with inflammatory bowel disease (IBD) have poorer oral health and a higher risk of periodontitis than those without IBD [172]. Furthermore, individuals with IBD have a higher incidence of anxiety and depression [173].

The oral cavity is home to one of the most important microbial communities in the human body. Consisting of more than 600 bacterial species, this community [173] plays a key role in maintaining oral and overall health. The microenvironment in which this microbial community exists is subject to genetic, environmental and immunological factors that can alter its delicate balance. The resulting dysbiosis creates ideal conditions for periodontal disease to develop. Dysbiosis of the oral cavity is the main cause of periodontal disease, and the chronic inflammatory disorder that develops can lead to the loss of the entire periodontal structure if not treated early.

However, the damage is not limited to the oral cavity; it involves the entire body and becomes a risk factor for chronic extraoral diseases, including cardiovascular disease, respiratory disease, diabetes, non-alcoholic steatohepatitis (NASH), obesity, inflammatory bowel disease, rheumatoid arthritis, allergic disease, Alzheimer's disease and cancer.

The development and maintenance of periodontal disease involves many bacteria, one of the main culprits being Fusobacterium nucleatum, although the triad formed by Porphyromonas gingivalis, Tannerella forsythia and Treponema denticola is also heavily implicated. More recent studies have also highlighted numerous other oral pathogens, including Aggregatibacter actinomycetemcomitans and Filifactor alocis. These microorganisms have one thing in common: they can produce virulence factors, toxins, proteolytic enzymes, LPS, upregulate the NLRP3 inflammasome, increase the expression of MMPs and create the conditions for persistent inflammation.

Oral microbiota dysbiosis is not limited to the mouth; it also affects the intestine, altering the intestinal microbiota. Numerous experimental and clinical studies have demonstrated that oral dysbiosis can affect the composition of the intestinal microbiota, comprising a vast array of bacteria, archaebacteria, microeukaryotes, and viruses that interact continuously in a state of dynamic equilibrium. Alteration to this balance leads to the destruction of IEB, increased intestinal permeability, destruction of immune homeostasis, local and systemic increases in pro-inflammatory cytokines and development of LGCI.

This has repercussions for taste receptors, influencing the host's food choices and creating a feedback loop that is difficult to control.

The inflammatory condition that occurs in the intestine also involves the CNS via the GBA and acts as a trigger for a wide range of neuroinflammatory and neurodegenerative diseases.

All this leads to considering the complex crosstalk generated by the interaction between the oral microbiota, intestinal microbiota, intestine and brain as being of crucial importance for an individual's health.

Interest and research into the oral microbiome and its role in the health of the oral cavity and the individual have increased greatly in recent years, thanks in part to advances in genomic sequencing and metagenomics. This has improved our understanding of the impact of oral microbiota on health, enabling the development of increasingly effective strategies to tackle periodontal disease and prevent and treat systemic diseases such as rheumatoid arthritis, Alzheimer's and cancer.

However, further studies are needed to explore the role of lesser-known and less characterised oral microorganisms. This will help us to understand their specific ecological functions, how they interact with other species, and how they contribute to maintaining human health. It will also be essential to study the interaction between specific strains of symbiotic bacteria, such as Bifidobacteria and Lactobacilli, and pathogenic microorganisms in the oral cavity. Modulating these interactions could help to achieve a stable equilibrium of microorganisms in different mucosal niches, thereby contributing to an individual's state of health.