New advances in oral microbiology and tumor research

Abstract

Cancer remains a major global health concern, with escalating incidence and mortality rates underscoring the urgent need for novel diagnostic and therapeutic strategies. Increasing evidence has identified the oral microbiota as a critical contributor to tumorigenesis, thereby expanding the understanding of cancer pathogenesis beyond conventional risk factors such as tobacco use and genetic predisposition. This review summarizes recent progress in elucidating the complex relationship between the oral microbiota and various malignancies, particularly oral squamous cell carcinoma, esophageal adenocarcinoma, and pancreatic ductal adenocarcinoma. Pathogenic bacteria, including Porphyromonas gingivalis and Fusobacterium nucleatum, have been implicated in promoting tumor progression through mechanisms involving chronic inflammation, the production of metabolic toxins, and immune evasion. The dysbiosis of the oral microbiota, often driven by lifestyle factors such as poor diet, tobacco use, and alcohol consumption, further exacerbates these carcinogenic processes. Emerging therapeutic approaches including probiotics, oral microbiota transplantation, and CRISPR-based bacterial editing are under investigation for their potential to restore microbial homeostasis and suppress pathogenic species. Additionally, saliva-based microbial biomarkers have shown promise for non-invasive cancer screening. The integration of multi-omics technologies and artificial intelligence-driven platforms is further advancing the development of precision oncology. This review aims to consolidate fragmented findings concerning the oral microbiota-cancer axis and address existing gaps in mechanistic understanding. The review’s significance lies in the translational potential of microbial research to clinical applications, offering opportunities to reduce the global cancer burden through early detection and microbiota-targeted therapies.

Key Words: Oral microbiota; Tumorigenesis; Cancer; Microbial dysbiosis; Multi-omics integration; Precision medicine

Core Tip: This review synthesizes recent progress in understanding the complex interactions between the oral microbiota and various cancers, particularly oral squamous cell carcinoma, esophageal adenocarcinoma, and pancreatic ductal adenocarcinoma. It highlights the roles of pathogenic bacteria such as Porphyromonas gingivalis and Fusobacterium nucleatum in promoting tumor progression through chronic inflammation, metabolic toxin production, and immune evasion. The review also explores innovative therapeutic strategies, including probiotics, oral microbiota transplantation, and CRISPR-based bacterial editing, while emphasizing the potential of saliva-based microbial biomarkers for non-invasive cancer screening. This work aims to translate microbial-related research into clinical applications, with the goal of mitigating the global cancer burden through early diagnosis and microbiome-targeted therapy.

INTRODUCTION

Cancer has emerged as a major global health challenge in the 21st century, posing substantial threats to public health and societal welfare, while also exerting a considerable economic burden. Among non-communicable diseases (NCDs), cancer is one of the leading causes of mortality, accounting for 22.8% of all NCD-related deaths[1]. According to global statistics from 2022, approximately 20 million new cancer cases and nearly 10 million cancer-related deaths were reported. The incidence and mortality rates of cancer continue to rise in parallel with demographic changes, with the annual number of new cancer cases projected to reach 35 million by 2050-an estimated 77% increase compared to 2022 figures[2].

Of particular concern is the rising incidence of malignancies affecting the oral cavity. Oral squamous cell carcinoma (OSCC), in particular, has been strongly associated with various oral microorganisms, which are believed to play a critical role in carcinogenesis and disease progression[3]. Despite notable advances in cancer diagnostics and therapeutics in recent years, the overall five-year survival rate remains low, especially among patients diagnosed at advanced stages. For instance, the five-year survival rate for lung cancer is below 20%, while that for pancreatic cancer is approximately 10%[1].

With ongoing societal development, changes in lifestyle and environmental conditions have led to increased exposure to diverse health risk factors. These include environmental pollution, unhealthy lifestyle habits, occupational stress, and complex interpersonal dynamics—all of which may significantly impair human health. The sustained rise in cancer incidence, coupled with a decreasing age of onset, highlights the urgent need for novel early diagnostic markers and therapeutic targets. These innovations are essential for improving the precision of cancer prevention, diagnosis, and treatment, ultimately enhancing public health and overall quality of life. In recent years, microbiome research has provided new perspectives in cancer prevention and therapy, with growing interest in the role of the oral microbiota in carcinogenesis and cancer progression[4].

The human microbiota comprises a complex ecosystem of trillions of microorganisms, predominantly residing in the oral cavity, gastrointestinal tract, skin, and reproductive tract[5]. The oral microbiota, the second most diverse microbial community in the human body, consists of over 700 bacterial species, along with fungi, viruses, and archaea[6]. These microorganisms play essential roles in maintaining the integrity of the oral mucosal barrier and in modulating systemic health through immune and metabolic regulation[7]. Recent studies have revealed the dualistic nature of the oral microbiota: while certain microorganisms promote health—for example, through the generation of nitric oxide (NO) via nitrate reduction, which supports cardiovascular function[8]—microbial dysbiosis has been linked to various pathological conditions, including periodontal disease, cardiovascular disease, and cancer[4].

Traditional cancer risk factors such as smoking, alcohol consumption, human papillomavirus (HPV) infection, and genetic susceptibility have been extensively studied and are well-established contributors to carcinogenesis[3]. However, these factors do not fully account for the etiology of all cases. Notably, a substantial proportion of OSCC patients—particularly those under 45 years of age—lack a history of tobacco or alcohol use[9]. Growing evidence suggests that oral microbiota dysregulation is closely associated with cancer development. For instance, pathogenic bacteria commonly linked to periodontal disease, including Fusobacterium nucleatum (F. nucleatum) and Porphyromonas gingivalis (P. gingivalis), are significantly enriched in patients with OSCC and esophageal cancer[4].

Moreover, the oral microbiota may augment the carcinogenic effects of conventional risk factors such as smoking and HPV infection[3], indicating its potential role as a mediator in tumorigenesis. Mechanistically, the oral microbiota contributes to tumor development through multiple pathways, including chronic inflammation, immunosuppression, metabolite-induced toxicity, and genomic instability. For example, P. gingivalis promotes tumor cell proliferation and invasion by activating the TLR4/NF-κB signaling pathway and inducing the release of pro-inflammatory cytokines such as interleukin (IL)-6 and TNF-α[10]. Similarly, F. nucleatum facilitates the malignant transformation of epithelial cells by secreting FadA adhesin[3].

These mechanistic insights provide a theoretical foundation for considering the oral microbiota as a potential diagnostic and therapeutic target. Specific microbial markers detectable in saliva, such as F. nucleatum, have already been employed in non-invasive screening strategies for colorectal cancer[11]. Looking ahead, microbiota-based precision medicine strategies—including probiotic supplementation and microbial transplantation—may offer novel strategies for cancer prevention and treatment[7].

COMPOSITION AND FUNCTION OF ORAL MICROORGANISMS

The healthy oral microbiota represents a highly diverse and complex ecosystem composed of bacteria, fungi, viruses, and archaea, with bacteria constituting the predominant group[12]. The major bacterial phyla include Firmicutes (e.g., Streptococcus), Bacteroidetes (e.g., Prevotella), Actinobacteria (e.g., Actinomyces), and Proteobacteria (e.g., Neisseria)[13,14]. Notably, the distribution of microorganisms varies significantly across different oral sites. For instance, the dorsum of the tongue is predominantly colonized by Streptococcus, whereas Veillonella and Neisseria are the principal genera found in saliva[15].

Oral microbial homeostasis

Oral microbial homeostasis plays a vital role in maintaining host health. Commensal oral microorganisms contribute to the suppression of pathogens through competitive exclusion and the production of antimicrobial metabolites[16]. Salivary lysozyme directly eliminates foreign pathogens, while hydrogen peroxide (H₂O₂) generated by symbiotic bacteria inhibits microbial proliferation[17]. Furthermore, the metabolic activities of oral microorganisms, including nitrate reduction, yield nitrite (NO₂-) and NO, which exert beneficial effects on cardiovascular health[18].

Oral microbiota: Defense, disease, and therapeutic potential

The oral microbiota provides both physical and chemical defense mechanisms. Multispecies biofilms enhance the integrity of the mucosal barrier via tight intercellular adhesion[19]. From an immunological perspective, commensal bacteria such as Streptococcus mitis maintain immune equilibrium by inducing the secretion of anti-inflammatory cytokines, including IL-10, from monocytes[20]. Metabolically, oral microbes produce short-chain fatty acids (SCFAs), such as butyrate, via the fermentation of dietary fibers, which have been shown to inhibit tumor cell proliferation[21]. In contrast, acetaldehyde, a byproduct of microbial metabolism, possesses genotoxic properties and may induce DNA damage[22].

Although current studies emphasize the importance of microbial homeostasis in maintaining barrier function, immune modulation, and metabolic activity, several mechanistic aspects remain inadequately elucidated. For instance, the precise mechanisms by which F. nucleatum disseminates and promotes distal tumorigenesis require further investigation[23]. Future studies should employ multi-omics approaches, including metagenomics and metabolomics, to decipher host-microbiota interactions and to develop microbiota-targeted therapeutic strategies, such as the use of antimicrobial peptides or probiotics. The potential inhibitory effect of Mentha piperita against Streptococcus mutans (S. mutans) further highlights the potential of natural compounds in modulating the oral microbiota[24]. These advancements may lead to innovative approaches for the prevention and treatment of oral-associated diseases.

ORAL MICROBIOLOGY IN CANCER PATIENTS

Oral microbiology in patients with OSCC

OSCC is one of the most common malignant tumors of the oral cavity. Accumulating evidence suggests that the oral microbiota contributes to carcinogenesis, tumor progression, and metastasis through various mechanisms. Elevated abundance of P. gingivalis, F. nucleatum, and S. mutans has been identified in the oral microbiota of patients with OSCC[25]. P. gingivalis promotes proliferation, invasiveness, and tumorigenic potential in immortalized human oral epithelial cells[26]. F. nucleatum enhances tumor cell proliferation and invasion by activating the Wnt/β-catenin signaling pathway through the binding of its FadA adhesin to E-cadherin[27]. Furthermore, S. mutans metabolizes sugars into acetaldehyde, which may induce DNA damage and genomic instability, thereby contributing to oral carcinogenesis. However, the precise mechanisms and oncogenic implications of acetaldehyde production by Streptococcus species remain insufficiently defined and require further investigation[28].

Oral microbiology of esophageal cancer patients

The incidence of esophageal adenocarcinoma (EAC) has increased significantly in recent years, with its pathogenesis closely associated with oral microbiota dysbiosis[29]. As observed in OSCC, significantly elevated levels of F. nucleatum and P. gingivalis have been detected in the oral cavities of EAC patients[30]. Functionally, bacterial cell toxin (BCT) proteins secreted by Fusobacterium periodonticum promote epithelial-mesenchymal transition in esophageal cancer cells via lactate-mediated signaling pathways[31]. Furthermore, microbial communities present in dental plaque and saliva have been identified as potential risk factors for esophageal cancer[30]. Periodontitis may predispose individuals to EAC by disrupting oral microbial homeostasis, which in turn affects systemic health, including esophageal function[32]. The oral microbiota may also potentiate the oncogenic effects of conventional risk factors, such as tobacco use and alcohol consumption[30].

Oral microbiota in pancreatic cancer patients

Pancreatic ductal adenocarcinoma (PDAC) is characterized by high malignancy and poor prognosis, with a five-year survival rate of only 11%[33]. Lipopolysaccharides derived from P. gingivalis are recognized by TLR2 and TLR4, which are expressed in both endocrine and exocrine pancreatic tissues. Activation of the NF-κB and MAPK signaling pathways induces the release of pro-inflammatory mediators and the upregulation of Reg3A/G genes. Overexpression of Reg3A/G is associated with increased islet cell proliferation and reduced apoptosis, thereby facilitating PDAC development and progression. Elevated Reg3A/G expression levels in PDAC tissues correlate with tumor aggressiveness, suggesting their potential utility as early diagnostic biomarkers. Interdisciplinary collaboration between dental and oncological disciplines may offer novel strategies for PDAC prevention and management[34].

A systematic review and meta-analysis by Ma et al[33] reported that P. gingivalis was significantly more prevalent in PDAC patients compared to healthy controls, whereas Aggregatibacter actinomycetemcomitans (A. actinomycetemcomitans), Tannerella forsythia, and Prevotella intermedia did not exhibit statistically significant differences in prevalence. Nonetheless, A. actinomycetemcomitans and P. gingivalis were found in higher abundance in the gingival sulcus fluid of PDAC patients[34]. Additionally, oral microorganisms may contribute to pancreatic carcinogenesis by inducing mutations in the tumor suppressor gene TP53[35,36].

A summary of the associations between oral microbiota and various cancer types is provided in Table 1. The oral microbiota plays a critical role in the pathogenesis of several malignancies, including OSCC, EAC, and PDAC. While current studies emphasize the involvement of key pathogenic species such as P. gingivalis and F. nucleatum in cancer development, numerous mechanistic questions remain unresolved. The noteworthy antimicrobial activity of A. actinomycetemcomitans underscores the potential of natural compounds to modulate the oral microbiota[34]. These insights may contribute to the development of novel preventive and therapeutic strategies against cancer.

Table 1 Oral microbiota in patients with various types of cancer.

Types of Cancer	Dominant bacterial community	Mechanism of actions	Ref.
Oral squamous cell carcinoma	Porphyromonas gingivalis	It promotes the proliferation, invasion and tumorigenic properties of human immortalized oral epithelial cells	Guo et al[26]
	Fusobacterium nucleatum	Activation of the Wnt/β-catenin signalling pathway via FadA adhesin binding to E-cadherin to promote tumour cell proliferation and invasion	Li et al[27]
	Streptococcus mutans	Produces acetaldehyde by metabolising sugars, which triggers DNA damage and genomic instability and may promote oral cancer	Pavlova et al[28]
Esophageal cancer	Fusobacterium nucleatum	Similar to OSCC patients, EAC patients had significantly higher abundance of Fusobacterium nucleatum in the oral cavity	Kawasaki et al[30]
	Porphyromonas gingivalis	Similar to OSCC patients, EAC patients had significantly higher abundance of Porphyromonas gingivalis in the oral cavity	Kawasaki et al[30]
	Fusobacterium periodonticum	Promotion of epithelial-mesenchymal transition in esophageal cancer cells by BCT protein	Guo et al[31]
Pancreatic cancer	Porphyromonas gingivalis	Secreted Porphyromonas gingivalis-lipopolysaccharides is recognized by TLR2 and TLR4, activates the NF-κB and MAPK signaling pathways, promotes the release of inflammatory factors, and induces the up-regulated expression of Reg3A/G genes, which contributes to pancreatic cancer development and progression	Hiraki et al[34]
	Aggregatibacter actinomycetemcomitans	Aggregatibacter actinomycetemcomitans abundance is significantly increased in the gingival sulcus of PDAC patients	Ma et al[33]
	Oral microbiota	Possible promotion of pancreatic cancer by mutating the p53 tumor suppressor gene	Wei et al[35], Chen et al[36]

OSCC: Oral squamous cell carcinoma; EAC: Esophageal adenocarcinoma; BCT: Bacterial cell toxin; PDAC: Pancreatic ductal adenocarcinoma.

Association between head and neck squamous cell carcinoma and oral microbiota dysbiosis

Head and neck squamous cell carcinoma (HNSCC) is one of the most common malignancies affecting the head and neck region, and its pathogenesis has been closely associated with dysbiosis of the oral microbiota. Recent studies have demonstrated that imbalances within the oral microbial community can lead to the excessive proliferation of pathogenic bacteria, such as F. nucleatum and P. gingivalis. These microorganisms may contribute to the initiation and progression of HNSCC through mechanisms involving chronic inflammation and the release of genotoxic agents[37]. A microbial risk score, based on the relative abundance of 22 bacterial species, has been shown to increase the risk of HNSCC by 50% for each standard deviation increment[37]. Moreover, specific microbial characteristics, such as the increased abundance of Parvimonas, have been found to correlate with tumor stage and clinical prognosis[38]. Notably, oral microbiota dysbiosis may further exacerbate immune suppression within the tumor microenvironment by modulating the epigenetic regulation of HPV infection, including alterations in DNA methylation patterns. These findings suggest that modulation of the oral microbiome may offer a promising therapeutic strategy for HPV-positive HNSCC[39].

Oral microbial characteristics and systemic effects in glioma

Glioma is a prevalent malignant tumor of the central nervous system, and recent evidence suggests a potential association between oral microbiota composition and glioma pathogenesis. Significant differences in the abundance of specific bacterial genera, such as Leptotrichia and Atopobium, have been observed between glioma patients and healthy controls, with these variations exhibiting apparent gender specificity. This phenomenon may be attributable to hormonal influences on salivary microbiota composition[40]. Animal studies have also demonstrated pronounced dysbiosis in the gut microbiota of glioma-bearing mice compared to healthy counterparts, particularly with respect to altered β-diversity. Similar microbial alterations have been identified in human glioma patients[41], suggesting a potential link between glioma and systemic microbial dysregulation. These findings provide important insights into the microbial mechanisms potentially implicated in gliomagenesis.

Gastric cancer and oral microbiota dysbiosis

Gastric cancer remains a leading cause of cancer-related mortality worldwide and is strongly associated with Helicobacter pylori infection. Increasing evidence indicates that the oral microbiota also plays a critical role in gastric carcinogenesis[42]. Oral microorganisms may colonize the gastric mucosa via mechanisms such as gastroesophageal reflux, thereby altering the gastric microbiome and contributing to tumor progression[43]. Dysbiosis of the oral microbiota may influence the gastric tumor microenvironment by modifying microbial community structure, regulating metabolic pathways, inducing chronic inflammation, and modulating immune responses. Collectively, these factors may increase the risk of gastric cancer and facilitate its progression[42].

Oral-vaginal microbiome synergy in cervical cancer

Cervical cancer (CC) is among the most prevalent malignancies in women, and increasing evidence suggests a link between its pathogenesis and dysbiosis of both the oral and vaginal microbiota. A study involving 82 women reported significantly elevated levels of Fusobacterium and Veillonella in the oral cavities of CC patients, with these increases positively correlated with serum C-reactive protein levels, suggesting a systemic inflammatory connection between microbial dysregulation and cancer progression. Additionally, amino acid metabolism by the oral microbiota was significantly upregulated in CC patients, potentially contributing to genomic instability through the toxic effects of metabolic byproducts. Notably, a synergistic pattern of microbiota—characterized by Lactobacillus depletion and enrichment of anaerobic bacterial—across oral and vaginal sites has emerged as a promising biomarker for non-invasive cancer screening. However, the mechanisms underlying microbial translocation between these anatomical sites remain to be elucidated[44].

POTENTIAL MECHANISMS OF ACTIONS

Chronic inflammation drives carcinogenesis

Chronic inflammation constitutes a key mechanism through which the oral microbiota contributes to carcinogenesis. Oral pathogens can trigger persistent inflammatory responses by activating host immune signaling pathways, thereby disrupting microenvironmental homeostasis and promoting malignant transformation[45].

Activation of the TLR/NF-κB signaling pathway: Lipopolysaccharides produced by P. gingivalis activate the NF-κB signaling cascade by binding to Toll-like receptors TLR2 and TLR4, resulting in the excessive release of pro-inflammatory cytokines such as IL-6, TNF-α, and IL-1β[34]. These cytokines stimulate epithelial cell proliferation and inhibit apoptosis, thus fostering a microenvironment conducive to tumor development[46]. Similarly, F. nucleatum interacts with host E-cadherin via its FadA adhesin, thereby activating the Wnt/β-catenin signaling pathway and inducing IL-8 secretion, which further amplifies the inflammatory response[27].

Tumor microenvironment remodeling: Microbe-induced infiltration of inflammatory cells—such as M2 macrophages and myeloid-derived suppressor cells into the tumor microenvironment results in the secretion of immunosuppressive cytokines including TGF-β and IL-10, which thereby inhibit anti-tumor immune responses. A study on OSCC demonstrated that F. nucleatum significantly enhances tumor progression via CXCL2-mediated interactions between tumor cells and macrophages[47].

Metabolic toxicants

Oral microorganisms generate various carcinogenic metabolites that can directly or indirectly damage DNA and promote malignant cellular transformation[48].

Acetaldehyde: S. mutans and certain lactic acid bacteria convert ethanol into acetaldehyde via alcohol dehydrogenase. Acetaldehyde is a potent genotoxin capable of inducing DNA adduct formation and chromosomal breaks. Clinical evidence has shown a significant increase in salivary acetaldehyde concentrations among chronic alcohol consumers, with this elevation positively correlated with a dose-dependent increase in OSCC risk[49].

Genotoxins: Certain oral bacteria, such as Escherichia coli, possess polyketide synthase (pks) gene clusters that encode the genotoxin colibactin, which induces DNA double-strand breaks and contributes to genomic instability[50]. Additionally, other bacterial genotoxins have been implicated in cancer pathogenesis. For example, Helicobacter pylori produces CagA and VacA, both of which impair host DNA repair mechanisms and promote genomic instability[51].

SCFAs and PCWBR2 protein: SCFAs produced by Peptostreptococcus anaerobius (P. anaerobius) have been shown to modulate immune responses. Furthermore, the PCWBR2 protein expressed on the surface of P. anaerobius binds to integrin α2/β1 on colorectal cancer cells, activating the PI3K/AKT/FAK signaling pathway. This signaling cascade promotes tumor cell proliferation and facilitates colorectal cancer development[52].

Immune escape

The oral microbiota constitutes a highly complex and diverse community that plays a pivotal role in the initiation and progression of various cancers. In breast cancer, F. nucleatum has been shown to promote immune evasion by upregulating PD-L1 expression in tumor cells via the NF-κB signaling pathway, thereby enabling these cells to evade CD8⁺ T cell-mediated cytotoxicity[53]. Similarly, P. gingivalis exhibits potent immune-evasive properties. This bacterium activates PARP9 through the TLR2 signaling pathway, inducing the production of type I interferon. Consequently, the host’s antibacterial immune response is suppressed, allowing the pathogen to persist[54]. Moreover, P. gingivalis induces the secretion of TSP-1 through a co-signaling mechanism involving CD47 and TLR2, which inhibits neutrophil bactericidal activity, thereby facilitating its survival in inflammatory environments and contributing to microbial dysbiosis[55].

Simultaneously, the oral microbiota maintains a dynamic and symbiotic relationship with the oral microenvironment, collectively preserving the delicate balance of oral microecology. The microbiota facilitates tumor immune escape through multiple mechanisms[56]. In terms of immune modulation, F. nucleatum within polymorphonuclear neutrophils (PMNs) reduces intracellular reactive oxygen species through Fn1792, a lysozyme inhibitor, enabling bacterial survival within phagocytes. Fn1792 also induces the expression of CX3CR1, thereby activating the NF-κB/STAT3 signaling pathway and inhibiting phagocyte apoptosis, further supporting the intracellular survival of F. nucleatum. These CX3CR1⁺ phagocytes also upregulate PD-L1, enhancing immune evasion. The CX3CR1⁺ PD-L1⁺ phagocytes, including macrophages and monocytes, enter the bloodstream and transfer intracellular F. nucleatum to tumor cells. These tumor cells subsequently express Fn-Dps, a virulence factor of F. nucleatum, and PD-L1. Within the tumor microenvironment, tumor cells secrete chemokines such as CXCL2/8 and CCL5 to recruit PMNs and macrophages. Fn-Dps promotes PD-L1 expression in tumor cells, suppressing T cell activity and intensifying immunosuppression. The combined effects of Fn-induced immune evasion and PD-L1 upregulation contribute significantly to tumor metastasis. Figure 1 illustrates the central role of F. nucleatum in modulating the tumor microenvironment and promoting cancer progression[56].

Figure 1 Phagocytes that are CX3CR1⁺ PD-L1⁺ and driven by Fusobacterium nucleatum make their way to tumor tissues and have the capacity to reshape the tumor microenvironment. F. nucleatum: Fusobacterium nucleatum.

At the level of immune function suppression, P. gingivalis secretes specific gingipains that degrade complement components C3 and C5, which are crucial for chemotaxis and immune activation. This degradation weakens immune-mediated pathogen clearance, thereby favoring the formation of a tumor-supportive microenvironment[57]. More critically, F. nucleatum binds to inhibitory receptors on immune cells, impairing their antigen-presenting capacity and anti-tumor responses. Tumors may exploit the presence of F. nucleatum to enhance immune evasion and establish a permissive microenvironment for their growth and survival[58]. Collectively, these findings elucidate the molecular basis for the synergistic promotion of tumor immune escape by oral microorganisms through a tripartite mechanism: Regulation of immune cells, inhibition of immune function, and suppression of antigen presentation.

Tumor cell dissemination

Oral microorganisms contribute not only to immune evasion but also play a crucial role in tumor invasion and metastasis. F. nucleatum has been shown to enhance proliferation and migration across various tumor cell types. In colorectal cancer, it promotes tumor progression by upregulating PD-L1 through IFIT1 protein modification mediated by m6A methylation, thus enhancing immune evasion and promoting tumor proliferation and metastasis[59]. Similarly, P. gingivalis infection in esophageal cancer cells leads to Fas protein degradation through YTHDF2-mediated mechanisms, which impairs T cell cytotoxicity and accelerates tumor progression[60]. Furthermore, oral microbes may disseminate to distant organs through the bloodstream or lymphatic system, where they induce vascular inflammation and lipid deposition. These effects not only exacerbate atherosclerosis but may also contribute to the establishment of tumor-permissive microenvironments[61].

Epigenetic regulation driven by oral microbiota

Epigenetic regulation constitutes another critical mechanism by which the oral microbiota influence tumor progression. Infection with F. nucleatum induces extensive transcriptomic and epigenetic alterations in host cells, including downregulation of genes associated with histone modifications and activation of enhancer elements. These changes collectively promote the expression of inflammation-related genes such as TNF and CXCL8[62]. Additionally, the oral microbiota-particularly P. gingivalis modulates host gene expression via epigenetic regulation of the PADI4 gene and through peptidyl-arginine deiminase-mediated protein citrullination. In rheumatoid arthritis, environmental factors such as smoking and microbial infection have been implicated in DNA methylation and the silencing of tumor suppressor genes, thereby facilitating chronic inflammation and tumorigenesis[63]. These epigenetic modifications not only influence immune responses but may also promote systemic cancer progression by stabilizing cancer stem cell phenotypes or activating oncogenic signaling pathways.

FACTORS INFLUENCING THE ORAL FLORA AND REGULATORY STRATEGIES

Influence of oral hygiene and lifestyle habits on oral microbiota

Oral hygiene and lifestyle habits—including diet, smoking, and alcohol consumption—substantially affect the composition and function of the oral microbiota, thereby modulating the risk of tumorigenesis[64].

Diet: Diets rich in sugar promote the proliferation of S. mutans, which metabolizes sugars into acids and acetaldehyde. Acetaldehyde is a potent genotoxin that induces DNA damage and mutations. In contrast, a diet high in dietary fiber can inhibit the retention, proliferation, and migration of pathogenic microbiota by enhancing intestinal peristalsis and increasing bowel movement frequency, thereby supporting a healthier physiological state[65].

Smoking: Smoking significantly alters both the composition and diversity of the oral microbiota. Smokers exhibit increased abundance of certain bacterial taxa and reduced levels of others, likely due to alterations in oral environment factors such as oxygen tensions, pH, and nutrient availability. For instance, P. gingivalis is significantly more abundant in the saliva of smokers, and its presence correlates with the severity of alveolar bone loss. Although the increase in F. nucleatum abundance is not statistically significant, its elevated levels in smokers may still reflect the broader impact of tobacco on the oral microbiota[66].

Nicotine, the primary active compound in tobacco, exerts immunosuppressive effects and promotes chronic inflammation, thereby creating conditions favorable to pathogenic bacterial colonization and proliferation. This microbial imbalance contributes to periodontal disease and other oral health disorders. Furthermore, nicotine impairs neutrophil phagocytosis and reduces salivary antioxidant levels, further compromising host defenses against microbial invasion[67,68].

Alcoholism: Chronic alcohol consumption significantly alters the oral microenvironment by affecting pH, moisture content, and nutrient availability, thereby disrupting the balance of the oral microbiota. These changes impair the survival and colonization of beneficial oral flora. Chronic alcohol abuse results in a reduction of lactic acid bacteria, which diminishes their competitive inhibition of pathogenic microorganisms. This dysbiosis exacerbates oral health problems and increases the risk of oral diseases[69]. Additionally, alcohol is metabolized into acetaldehyde by oral bacteria such as S. mutans, which leads to direct DNA damages and oncogenes activation[49]. Therefore, limiting alcohol consumption is essential for maintaining oral and systemic health.

Intervention of antibiotics and probiotics on oral microbiota

Antibiotics and probiotics act as double-edged swords in modulating the oral microbiota, necessitating a careful evaluation of their therapeutic benefits and associated risks[70].

Adverse effects of antibiotics: Broad-spectrum antibiotics (e.g., amoxicillin) not only eliminate pathogenic bacteria but also disrupt commensal microbiota such as Streptococcus species), resulting in dysbiosis[71]. Prolonged antibiotic administration may facilitate the colonization of drug-resistant strains, including methicillin-resistant S. aureus, thereby further compromising oral health and microbial equilibrium[72].

Protective effects of probiotics: Lactobacillus species, key constituents of the oral microbiota, are widely recognized as beneficial probiotics. These organisms ferment carbohydrates and produce lactic acid, reducing the oral pH and inhibiting the growth of pathogenic species. For example, Lactobacillus reuteri suppresses the proliferation of harmful bacteria such as F. nucleatum by secreting bacteriocins, including reuterin, and enhances mucosal barrier integrity[73]. Furthermore, Lactobacillus species contribute to the maintenance of ecological balance within the oral microbiota, thereby supporting oral health[69].

Emerging therapies

Emerging therapeutic strategies offer promising avenues for cancer prevention and treatment through precise modulation of the oral microbiota[74].

Oral microbiome transplantation: Oral microbiome transplantation (OMT) is a novel therapeutic approach aimed at restoring microbial diversity and inhibiting the colonization of pathogenic bacteria by introducing oral microbiota from healthy donors into a patient’s oral cavity. OMT has demonstrated promising potential in the treatment of oral diseases and in improving oral health outcomes[75]. Although preclinical studies have demonstrated its efficacy in treating halitosis[76], clinical applications in oncology remain limited and warrant further investigation. Future research is needed to elucidate the underlying mechanisms of OMT, develop standardized protocols, and rigorously assess its safety profile. While no OMT-related adverse events have been reported in clinical trials to date, potential risks—such as opportunistic infections and microbiome dysbiosis—require thorough evaluation, particularly in the context of inadequate pathogen screening. Furthermore, regulatory and ethical frameworks regarding donor eligibility and long-term recipient monitoring must be clearly defined to ensure both safety and feasibility. Addressing these challenges will necessitate multidisciplinary collaboration to bridge current knowledge gaps and establish clinical guidelines.

Targeted regulation of dominant bacterial species: The application of CRISPR-Cas9 technology to genetically modify commensal bacteria, such as Streptococcus salivarius, has been shown to enhance the secretion of antimicrobial peptides, thereby inhibiting the progression of precancerous lesions[77]. In addition, phage therapy offers a targeted approach to eradicate pathogenic bacteria (e.g., P. gingivalis) while preserving beneficial species, thus supporting oral microbial homeostasis. Owing to its high specificity and minimal disruption of the native microbiota, phage therapy has garnered substantial interest. However, several critical challenges must be addressed prior to its widespread clinical adoption. These include the development of robust phage screening and characterization protocols, strategies to overcome bacterial resistance, and the establishment of comprehensive regulatory frameworks to facilitate the approval and commercialization of phage-based therapies[78].

Microbial markers and vaccine development: Specific microbial markers, such as F. nucleatum DNA fragments detected in saliva, have been proposed as non-invasive biomarkers for early colorectal cancer screening, with a reported area under the curve of 0.85[79]. Furthermore, a vaccine targeting the FadA adhesin of F. nucleatum has demonstrated a 60% reduction in tumor volume in animal models, indicating its potential as an effective therapeutic strategy[69].

Synergistic carcinogenic effects of comorbidities and gut microbiota dysbiosis

Recent evidence suggests that oral microbiota can translocate and colonize the gastrointestinal tract, integrating into the gut microbiome and contributing to systemic diseases[80]. Opportunistic oral pathogens may enter the intestines through swallowing or breaches in periodontal tissue, where they disrupt the gut epithelial barrier, induce chronic inflammation, and contribute to disease pathogenesis. For instance, in 2024, Niu et al[80] demonstrated in an animal model that oral-derived P. gingivalis actively colonizes the gut, exacerbates insulin resistance, and induces dysbiosis by suppressing the AhR signaling pathway involved in tryptophan metabolism, thereby promoting obesity-associated tumorigenesis. Similarly, in 2024, Knop-Chodyła et al[81] reported that colonization of oral microbes in the gastrointestinal tract enhances the production of proinflammatory cytokines (e.g., IL-6, TNF-α) and carcinogenic metabolites (e.g., acetaldehyde), which impair T cell function, cause DNA damage, and promote the malignant transformation of gastrointestinal epithelial cells.

Mechanisms underlying the synergistic tumor-promoting effects of comorbidities and microbial dysbiosis

Metabolic disorders such as diabetes and obesity are closely linked to dysbiosis of both the oral and gut microbiota, which together accelerate tumorigenesis via multiple mechanisms. In 2022, Noureldein et al[82] reported a significant reduction in butyrate-producing bacteria in the gut microbiota of diabetic mice, associated with elevated levels of IL-1β and NOX4, contributing to colorectal cancer progression. Similarly, in 2023, Soeda et al[83] demonstrated that mice with diabetes and non-alcoholic steatohepatitis exhibit impaired intestinal insulin signaling and microbial imbalance. The microbial composition of these mice closely resembled that of human diabetic patients and was associated with an elevated risk of hepatocellular carcinoma. Additionally, Biragyn and Ferrucci[84] proposed that age-related microbial dysbiosis, characterized by a reduction in SCFAs, may promote “inflammaging”—a state of chronic low-grade inflammation that impairs immune surveillance and facilitates the accumulation of mutated cells. Collectively, these findings suggest a vicious cycle wherein comorbidities exacerbate microbial dysbiosis, which in turn promotes tumorigenesis through chronic inflammation, immune suppression, and the production of carcinogenic metabolites.

Regulatory strategies

With advances in systems biology, research on the interaction between oral microbes and tumors has shifted from single-target interventions to comprehensive precision regulatory strategies[85]. Molecular network models derived from multi-omics technologies have elucidated critical pathways, such as the TLR4/NF-κB signaling cascade, through which P. gingivalis promotes tumor cell proliferation and invasion[10]. More importantly, these models provide multidimensional biomarkers with potential for clinical translation.

In the context of precision medicine, multi-omics data are transforming the design of clinical trials. The integration of gene-centric and genome-wide metagenomic approaches has enhanced our understanding of microbial diversity and functionality[86]. These insights enable the development of personalized interventions tailored to individual physiological and microbial profiles, which is especially critical given the substantial inter-individual variability in microbiota composition. Accordingly, the formulation of individualized probiotic or antibiotic regimens is essential to achieve optimal clinical outcomes[70].

Looking forward, artificial intelligence (AI)-driven platforms for multi-omics integration are expected to accelerate the clinical translation of research findings[87]. For instance, tools such as Vaxign-ML and DeepVacPred have shown significant promise in predicting vaccine candidates by integrating genomic, proteomic, and metabolomic data. These platforms utilize machine learning algorithms to identify antigenic epitopes and prioritize potential vaccine targets, thereby streamlining the vaccine development process[88].

In the near future, the implementation of a closed-loop system that combines AI-based computational predictions with experimental validation in human subjects may provide innovative pathways for the development of vaccines targeting pathogenic oral microorganisms. For example, the TGSD algorithm has been successfully applied to identify key proteins from multi-omics datasets, highlighting the importance of integrating diverse data types to enhance predictive precision[89]. This methodology not only expedites the discovery of therapeutic targets but also improves the accuracy of vaccine design.

Through the integration of these technological advancements, AI-driven platforms have the potential to significantly enhance both the efficiency and precision of vaccine development, particularly against oral microbial pathogens. The convergence of computational modeling and experimental validation represents a promising advancement towards more effective and individualized medical interventions.

PROBLEMS AND SOLUTIONS

Epidemiological insights and oral microbiota's role in tumor development

The oral microbiota plays a critical role in the pathogenesis of multiple malignancies, including OSCC, esophageal cancer, and pancreatic cancer. Epidemiological studies have demonstrated a strong correlation between oral microbial dysbiosis—particularly the overrepresentation of P. gingivalis and F. nucleatum—and an increased risk of cancer. For instance, individuals with periodontal disease have been reported to exhibit a 43% higher risk of esophageal cancer compared to healthy controls. Nonetheless, further research is warranted to more precisely quantify this risk and elucidate the underlying mechanisms[90]. Additionally, elevated levels of A. actinomycetemcomitans have been observed in the gingival sulcus fluid of patients with pancreatic cancer[32].

Oral microorganisms contribute to tumorigenesis through several mechanisms, including the induction of pro-inflammatory mediators, the production of cytotoxic substances (either directly or indirectly), and the suppression of immune cell function. These findings enhance the current understanding of the complex interplay between oral microbes, host immune responses, and cancer development. However, further investigation is necessary to clarify causal relationships and identify the molecular pathways involved.

Experimental validation

Despite numerous studies identifying correlations between the oral microbiota and tumorigenesis, the causal relationship remains inadequately understood. It has yet to be determined whether microbial alterations initiate tumor development or are a consequence of changes within the tumor microenvironment. Experimental studies are therefore essential to elucidate these underlying mechanisms.

Cohort study design: A bidirectional cohort study should be conducted with longitudinal sampling of oral microbiota. Participants would undergo routine physical examinations, imaging assessments, and diagnostic procedures to monitor for tumor development. For individuals diagnosed with tumors, detailed data concerning tumor type, stage, anatomical location, and treatment response should be collected. By analyzing temporal shifts in oral microbiota and correlating them with tumor progression, researchers may determine whether microbial dysbiosis precedes or results from tumorigenesis. Statistical techniques, including survival and time-series modeling, should be employed to establish causality.

Animal model experiments: To explore the role of the oral microbiota in tumorigenesis, animal models may be used to replicate the effects of specific pathogenic bacteria or the absence of beneficial commensals. Pathogen colonization or antibiotic-mediated depletion of commensal bacteria may be implemented, followed by systematic monitoring of tumor development over time. Tumor size, count, and growth kinetics should be measured, while oral samples collected at multiple intervals can be subjected to 16S rRNA gene sequencing, metagenomics, and quantitative PCR to assess microbial composition and diversity. These experiments provide critical insights into the impact of specific pathogens or commensal deficiencies on tumor initiation and progression.

Interventional studies: Randomized controlled trials offer a robust framework to evaluate the influence of oral microbiota modulation on cancer development. In such trials, participants are randomly assigned to either an experimental group receiving OMT or a control group receiving a placebo or standard care. The effectiveness of the intervention should be assessed using clinical evaluations, imaging modalities such as magnetic resonance imaging or computed tomography, and, when appropriate, pathological analysis of biopsy samples. Key outcomes include prevention or deceleration of tumor progression, providing essential evidence regarding the potential of OMT in altering cancer trajectories.

Integrating multi-omics data to promote precision medicine

The growing recognition of the oral microbiota’s involvement in cancer pathogenesis presents an opportunity to advance diagnostic and therapeutic strategies. Integrating multi-omics approaches—including metagenomics, metabolomics, and single-cell sequencing—can significantly enhance the development of precision medicine[12]. These technologies yield comprehensive insights into the intricate interactions between the oral microbiota and host physiology, particularly in relation to tumorigenesis. A thorough understanding of these molecular mechanisms may facilitate the creation of more targeted and individualized diagnostic and therapeutic solutions.

PROSPECTS FOR ORAL MICROBIAL APPLICATIONS

Development of microbial markers

Microbial signatures identified in saliva and gingival sulcus fluid show considerable potential as non-invasive biomarkers for the early detection and monitoring of cancer. These oral fluids are particularly advantageous due to their ease of access and feasibility for frequent sampling. However, their roles in various pathological contexts remain insufficiently defined, necessitating extensive clinical research. Continued investigations are required to validate the clinical utility of oral fluid-derived biomarkers across different malignancies and to facilitate their integration into routine diagnostic[91].

Precision intervention strategies

The application of oral microbiota in precision medicine, particularly in antibody-based therapies, represents a promising direction for the development of innovative treatment modalities. One notable approach involves the genetic modification of probiotics to synthesize specific antibodies, which are subsequently encapsulated within nanoparticles for targeted delivery. This method effectively addresses challenges associated with oral drug administration, thereby enhancing the therapeutic precision and efficacy of such interventions. These therapies hold significant potential not only for the treatment of cancer but also for combating diseases and other systemic diseases, offering a more efficient and patient-centered alternative to conventional modalities. Future investigations should prioritize the optimization of antibody-producing probiotics, the resolution of safety and regulatory issues, and the implementation of clinical trials to evaluate their therapeutic utility[92].

CONCLUSION

This review highlights the critical role of oral microbiota in tumorigenesis and cancer progression, emphasizing its dual functions as both a mediator of pathogenic process and a potential therapeutic target. These findings position the oral microbiota as a pivotal player in cancer biology, offering opportunities for non-invasive diagnostics via saliva-derived biomarkers and microbiota-targeted interventions, including probiotics, microbiota transplantation, and CRISPR-based therapies.