The oral cavity is a primary site for microbial colonization, hosting a complex and diverse microecological environment. Distinct niches, such as the tooth surface, gingival sulcus, saliva, tongue dorsum, and oral mucosa, provide unique habitats for a wide array of microorganisms. Metagenomic studies have revealed that the microbial communities in healthy individuals are predominantly composed of genera such as Streptococcus, Diplococcus, Veillonella, Haemophilus, Neisseria, Porphyromonas, Fusobacterium, Actinomyces, and Prevotella (3). These microorganisms maintain a dynamic equilibrium through nutritional competition and metabolic cooperation, thereby preserving oral microecological stability under physiological conditions.

However, external factors—including poor oral hygiene, diminished host immunity, and radiotherapy or chemotherapy—can disrupt the oral microenvironment, leading to microbial dysbiosis. This imbalance is characterized by the overgrowth of pathogenic bacteria and a decline in beneficial species, increasing the risk of oral diseases. Furthermore, host-specific factors such as age, behavioral habits, salivary composition, and immune status continuously reshape the oral microbiome over time. For instance, the abundance of Bacteroidetes (mainly Prevotella) and Veillonella tends to increase with age, whereas bacteria such as Granulicatella show a decreasing trend (4).

In common oral diseases like periodontitis and dental caries, shifts in microbial composition are central to pathogenesis. Overall dysbiosis, coupled with the overgrowth of pathogens such as Prevotella, Fusobacterium nucleatum (Fn), Porphyromonas gingivalis (Pg), Tannerella forsythia, and Treponema denticola, can trigger host immune responses, leading to gingival inflammation and disease progression. Notably, certain periodontal pathogens, including Pg, have been associated with oral precancerous lesions and oral squamous cell carcinoma (OSCC), potentially through mechanisms involving chronic endogenous infection and modulation of host inflammatory responses (5).

Beyond local oral diseases, oral microecological imbalance is also closely linked to a range of systemic conditions, including Alzheimer’s disease, diabetes, adverse pregnancy outcomes, and multiple malignancies such as oral, gastrointestinal, lung, breast, prostate, and uterine cancers (6). These associations underscore the profound impact of oral microbiota on overall health.

The gut microbiota is a complex microbial community predominantly composed of anaerobic bacteria, with major phyla including Bacteroidetes, Firmicutes, Actinobacteria, and Proteobacteria. Its composition, structure, and functional diversity are shaped by host genetics, dietary patterns, lifestyle, and medication use. As the largest microecosystem in the human body, the gut microbiota profoundly influences intestinal physiology and pathology through mechanisms such as metabolite synthesis and maintenance of mucosal homeostasis (5).

Anatomically, the intestinal barrier—comprising the mucus layer, epithelial cells, and tight junctions—serves as a “gatekeeper,” maintaining internal stability by selectively permitting nutrient absorption while restricting the passage of pathogens and harmful metabolites. Impairment of this barrier, leading to increased permeability, can trigger chronic inflammation, allergic responses, and autoimmune disorders.

Gut dysbiosis has been implicated in numerous diseases, including inflammatory bowel disease, type 2 diabetes, atherosclerosis, obesity, and neuropsychiatric conditions. Recent studies have identified characteristic gut microbial profiles in patients with various cancers, particularly those of the digestive tract, suggesting a potential role for gut microecological imbalance in tumorigenesis (6). Beneficial genera such as Lactobacillus and Bifidobacterium are generally associated with anti-inflammatory effects, whereas opportunistic pathogens like Escherichia coli and Shigella are linked to pro-inflammatory states.

Notably, the gut microbiota exhibits high sensitivity to dietary and environmental influences. Through the fermentation of dietary components, gut microbes produce essential metabolites—including short-chain fatty acids (SCFAs), tryptophan derivatives, and bile acids—that regulate gene expression, immune cell function, and systemic immune balance. For instance, SCFAs derived from dietary fiber not only lower the luminal pH but also promote polyamine synthesis, thereby reinforcing intestinal homeostasis. Probiotics, including bifidobacteria and lactobacilli, contribute to host health by aiding nutrient digestion, inhibiting pathogen colonization, and degrading toxins. Additionally, gut microbes participate in the synthesis of vitamins such as vitamin K2 and B vitamins, with recent evidence suggesting that some strains may even produce vitamin C (6).

Conversely, poor dietary habits—such as high intake of sugar, fat, and protein—may promote the expansion of bile-tolerant microorganisms. These bacteria can produce genotoxic secondary bile acids and pro-inflammatory metabolites (e.g., hydrogen sulfide), creating a favorable microenvironment for colorectal carcinogenesis. Proposed mechanisms include direct DNA damage, activation of pro-tumor signaling pathways, production of oncogenic metabolites, and suppression of anti-tumor immunity. A 2021 study by Deng et al. further revealed an increased abundance of potential pathobionts (e.g., Prevotella and Actinomyces) and a reduction in SCFA-producing beneficial bacteria (e.g., Bacteroides) in cancer patients (7). Such dysbiosis is often accompanied by elevated endotoxin production and impaired barrier function, potentially leading to systemic inflammation that supports tumor development. Importantly, SCFAs like butyrate not only exert anti-inflammatory effects but also enhance intestinal barrier integrity, highlighting the therapeutic potential of modulating gut microbiota in cancer prevention and treatment.

Anatomically connected via the gastrointestinal tract, the oral and gut compartments form a continuous system that facilitates microbial transit. Beyond physical continuity, they interact through saliva and digested food materials. The gastrointestinal barrier exhibits selective permeability, ensuring efficient nutrient absorption while defending against pathogenic invasion (15).

Studies indicate that oral-to-gut microbial translocation occurs primarily through two pathways: the “enteric route” (16, 17), in which oral bacteria traverse the esophagus and stomach via swallowing—with species such as Fn demonstrating gastric acid resistance—and the “hematogenous route” (18), wherein bacteria enter the bloodstream through compromised oral mucosal barriers and disseminate to distant sites including the intestine.

Under physiological conditions, defense mechanisms such as gastric acid, intestinal peristalsis, and digestive enzymes collectively form an efficient “microbial clearance system” that eliminates most orally ingested microbes, maintaining gut microbial homeostasis. However, oral dysbiosis or gastrointestinal barrier impairment can disrupt this fine-tuned regulation, leading to abnormal colonization of oral-derived microbes in the gut. This translocation has been linked to chronic inflammatory conditions such as ileitis, colitis, and colorectal cancer (19). For example, the oral pathobiont Fn can proliferate significantly under poor oral hygiene. Oral administration of Fn in a mouse model of experimental colitis was shown to induce inflammatory responses and tumor formation in the small intestine and colon (20). This microbial crosstalk is bidirectional. Li et al. demonstrated through in vitro studies that gut microbes can translocate retrograde to the oral cavity under conditions of gastrointestinal dysfunction. Clinical observations have identified abnormal colonization of typical oral commensals such as Haemophilus and Veillonella in the intestinal mucosa of patients with inflammatory bowel disease. Notably, this cross-site colonization begins early in life: Bifidobacterium, a dominant gut genus, has been detected in the oral fluid of newborns. In contrast, elderly individuals exhibit a higher prevalence of oral-characteristic bacteria (e.g., Porphyromonas, Fusobacterium, and Pseudobacterium) in the gut compared to healthy adults. Thus, the oral–gut axis represents a critical dimension of human health and is increasingly recognized as a key pathway in disease pathogenesis (3).

The global incidence of HNC continues to rise, posing a significant public health challenge. Beyond established risk factors such as tobacco use, alcohol consumption, and viral infections, growing evidence indicates that specific oral microbial communities play a critical role in HNC pathogenesis (21). Although the overall composition of the oral microbiota in HNC patients shares similarities with that of healthy individuals, significant differences emerge at finer taxonomic resolutions. At the phylum level, Firmicutes often dominate. At the genus level, notable decreases are observed in the abundance of Pseudomonas, Acinetobacter, and Shigella. Conversely, at the species level, Fn and Pg are consistently enriched and have become a major research focus due to their potential involvement in HNC development through diverse mechanisms.

Fn is not only associated with oral conditions like chronic periodontitis but also contributes to HNC progression. Salem et al. proposed that Fn can induce the expression of β-defensin 2 in oral epithelial cells—a molecule implicated in the pathogenesis of oral lichen planus and tongue cancer—thereby stimulating the production of potent pro-inflammatory cytokines such as interleukin 6 (IL-6) and interleukin 8 (IL-8). This process can trigger mast cell degranulation and histamine release, establishing a self-perpetuating cycle of chronic inflammation. This inflammatory microenvironment is characterized by the sustained release of pro-inflammatory cytokines, immune cell recruitment, and progressive angiogenesis (4). The Fn adhesin FadA binds to E-cadherin on epithelial cells, inactivating it and enhancing mucosal permeability. This interaction also leads to the accumulation of free β-catenin, activating the Wnt signaling pathway and driving cellular proliferation. Furthermore, Fn proteins such as Fap-2 can bind to inhibitory immune receptors on T cells and natural killer (NK) cells, blunting their anti-tumor activity (22). Fn infection has also been linked to increased matrix metalloproteinase 9 and matrix metalloproteinase 13 activity, suggesting a potential role in basement membrane degradation and tumor invasion (23). Collectively, Fn infection is associated with high-grade dysplasia, lymph node metastasis, and poor prognosis in HNC (Figure 2).

Pg has been identified as an independent risk factor for oral cancer-related mortality (24).This pathogen activates Toll-like receptor 2 (TLR2) and Toll-like receptor 4 (TLR4), inducing the release of IL-8 and other pro-inflammatory mediators. Intracellularly, Pg suppresses apoptosis through multiple mechanisms, including inhibition of cytochrome c, downregulation of caspase-3 activity, secretion of anti-apoptotic nucleoside diphosphate kinase, and upregulation of specific microRNAs (25).It also upregulates the expression of anti-apoptotic genes such as Bcl-2 and Survivin (26) (308–96). The gingipains produced by Pg activate and mature matrix metalloproteinase 9, facilitating basement membrane degradation and OSCC metastasis (308–98). Moreover, Pg can mediate T cell inhibition by upregulating programmed death-ligand 1 (PD-L1), enabling OSCC cells to evade immune surveillance, and can induce the expression of B7-H1 and B7-dendritic cell receptors, leading to apoptosis of activated T cells (27). Pg has also been shown to induce epithelial-mesenchymal transition primarily through its FimA adhesin, affecting p53, phosphatidylinositol 3-kinase, and cyclin pathways to accelerate cell cycle progression (28). Additionally, Pg reduces the activity of plakophilin, a key protein for epithelial integrity, thereby promoting metastasis (29). As a major producer of the carcinogen acetaldehyde, Pg also contributes to cancer cell autophagy and chemoresistance (308–103). Intriguingly, Pg may induce HIV reactivation through butyrate production and facilitate viral entry into epithelial cells (30, 31), although potential synergistic roles in HNC pathogenesis in HIV-positive patients require further investigation.

Although less frequently detected in HNC, Pseudomonas aeruginosa has attracted attention due to its potential carcinogenicity. This bacterium can induce DNA breaks in epithelial cells (32) and promote tumor cell invasion and metastasis. Its pathogenicity is linked to virulence factors such as flagella and lipopolysaccharides, which directly provoke inflammation, and the ExoU cytotoxin, which activates the NF-κB signaling pathway and stimulates IL-8 secretion, fostering a pro-tumor microenvironment (33). The role of fungi in OSCC progression is also increasingly recognized. Elevated abundance of oral Candida is positively correlated with the severity of oral epithelial dysplasia (34). Candida albicans, in particular, can provide colonization support for pathogenic bacteria like Pg (35), damage keratinocytes by secreting aspartic proteases and inducing endocytosis, and degrade the basement membrane protein laminin in an acidic pH environment. This suggests that the acidification of the OSCC microenvironment may synergize with C. albicans to enhance tissue damage (36).

The dynamic composition of the oral microbiota is also closely linked to HNC treatment responses and postoperative recovery. Chemoradiotherapy often increases the abundance of oral opportunistic pathogens, such as Prevotella and Fusobacterium, elevating the risk of early-onset radiation-induced oral mucositis (RIOM). A lower abundance of Streptococcus is associated with a shorter time to the onset of severe oral mucositis (OM) (37). In a cohort study of patients undergoing head and neck free flap reconstruction, surgical site infection (SSI) within 30 days post-surgery occurred in 67 patients (13.3%). Pathogens derived from the oral flora were identified in 75% of these cases, including Gram-negative bacilli (44%), methicillin-resistant Staphylococcus aureus (MRSA, 20%), and methicillin-sensitive Staphylococcus aureus (MSSA, 16%). From a diagnostic perspective, pioneering work by Mager et al. has advanced the non-invasive detection of HNC. Their saliva-based studies identified characteristic expression patterns of Prevotella melaninogenica and Streptococcus mitis in OSCC patients. A diagnostic model based on these bacterial biomarkers achieved a predictive accuracy of up to 80% (38), providing a crucial foundation for novel, non-invasive diagnostic strategies.

In summary, the oral microbiome plays a significant role in the development, treatment, and diagnosis of HNC. Future research focusing on microbiome-based interventions may open new avenues for precision medicine in HNC.

As the largest and most complex microbial ecosystem in the human body, the gut microbiota plays a crucial role in HNC pathogenesis and progression. Clinical interventions, including the side effects of chemoradiotherapy, dietary changes, and antibiotic use, can significantly alter its composition and function (4). The gut microbiota influences host physiology by participating in dietary metabolism, synthesizing essential nutrients like vitamins, and generating key bioactive metabolites such as SCFAs. SCFAs exert anti-inflammatory effects by inducing interleukin 10 (IL-10) release and suppressing pro-inflammatory cytokines like IL-6 and tumor necrosis factor-s (TNF-r-. They also activate G-protein-coupled receptors (GPCRs) on epithelial and immune cells, modulating broad immune-inflammatory responses and even contributing to the inactivation of head and neck squamous cell carcinoma (HNSCC) cells (39). The gut microbiota may influence HNC through multiple pathways [67: some bacteria can directly cause DNA damage or activate pro-tumor signaling pathways, while others produce cancer-promoting metabolites or suppress anti-tumor immunity. Diet, a major modulator of the gut microbiota, is also linked to HNC risk. SCFAs (e.g., butyrate, acetate, propionate), derived from microbial fermentation of dietary fiber, exhibit well-documented anti-inflammatory and anti-cancer properties. Gut dysbiosis can reduce SCFA production, weakening this protective effect and exacerbating systemic inflammation, which may contribute to OSCC development. Dysbiosis also promotes the production of secondary bile acids, which possess pro-inflammatory and cytotoxic properties. These compounds can enter systemic circulation and potentially affect the oral microenvironment, favoring OSCC initiation (40). Furthermore, gut microbes metabolize tryptophan into indole derivatives that regulate immune responses. Dysbiosis can disrupt this process, leading to enhanced inflammation, immune dysregulation, and promotion of OSCC. Therefore, supplementing beneficial microbial metabolites such as SCFAs—either directly or via interventions that stimulate their endogenous production—represents a promising adjuvant strategy to mitigate inflammation, enhance cytoprotection, and potentially reduce OSCC risk.

HNC patients frequently develop OM and xerostomia following radiotherapy, conditions linked to oral and gut dysbiosis. This dysbiosis is characterized by the overgrowth of cariogenic bacteria (e.g., Streptococcus mutans, Lactobacillus) and opportunistic pathogens (e.g., Staphylococcus, Enterococcus, Candida albicans), alongside a decline in commensals like Neisseria (20, 41). Enterococcus can enhance microbial proteolytic activity against fibronectin, aggravating radiation-induced mucosal damage and inflammation (4). Radiotherapy can also indirectly compromise the intestinal barrier and induce inflammation. For instance, tongue irradiation in mouse models triggers mitochondrial oxidative stress and activates the NLRP3 inflammasome in the small intestine (39). A study of 47 radiotherapy-treated HNC patients found that most developed grade 2–3 OM, with pre-radiotherapy nutritional support preventing grade 4 OM. Microbiota analysis revealed that a higher OM grade was associated with lower abundance of Proteobacteria. As mucositis worsened, the abundance of most differentially abundant bacteria decreased, suggesting that radiotherapy may influence mucositis development by altering the growth patterns of Proteobacteria. Studies in nasopharyngeal carcinoma have reported a higher abundance of Clostridium in patients with mild mucositis and of Actinomyces in those with severe mucositis. Similar microbial signatures have been correlated with quality of life in HNC patients, indicating their potential utility as biomarkers for predicting patient outcomes following radiotherapy (9). Beyond radiotherapy, the gut microbiota also modulates chemotherapy efficacy and toxicity. It can enhance or diminish drug efficacy; for example, it can upregulate pro-apoptotic pathways to improve cisplatin response. In the case of gemcitabine, a common treatment for advanced OSCC, γ-Proteobacteria can confer resistance, while co-administration of ciprofloxacin can reverse this resistance and restore drug efficacy. The gut microbiota also influences immunotherapy outcomes. Beneficial taxa such as Bacteroides and Bifidobacterium can boost immune cell numbers and activity, enhancing the efficacy of immune checkpoint inhibitors targeting CTLA-4 and PD-L1.

In conclusion, the gut microbiota plays a multifaceted role in HNC, profoundly influencing tumor initiation, progression, and treatment response through metabolite production, systemic inflammation, and immune modulation. However, key questions regarding how to precisely modulate the gut microbiota for HNC intervention remain to be answered.

Diet and environmental factors have significant regulatory effects on the composition and function of the oral gut microbiota: a high fiber diet is the core of maintaining gut microbiota diversity and health. The complex carbohydrates it contains can become an energy source for specific bacteria through colonic fermentation, and differences in fiber structure can affect the composition and function of the microbiota; A high-fat diet can disrupt the balance of the microbiota, increase endotoxin production, disrupt the intestinal barrier, and induce inflammation and metabolic disorders. Another study shows that using alcoholic mouthwash twice a day can increase the risk of cancer in smokers by more than 9 times, for drinkers by more than 5 times, and for those who never drink, it is almost 5 times. In addition, environmental factors such as antibiotics, smoking, and pollutants can also disrupt the balance of oral gut microbiota, leading to dysbiosis of the microbiota (3).On this basis, interventions such as diet, lifestyle, prebiotics, and prebiotics can optimize the efficacy of immune checkpoint inhibitors (ICI) by regulating the microbiota. Additionally, Mohammad Reza Shafiei confirmed that probiotic therapy can reduce the severity of OM disease. However, while products containing Bifidobacterium and Lactobacillus can help alleviate the severity of OM, products containing Streptococcus are not effective (10).In terms of dietary intervention, strategies such as ketogenic diet and choline supplementation can enhance anti-tumor immune efficacy by leveraging microbial communities and their metabolites; Fructose, high salt, and Mediterranean diet also have immunomodulatory potential, but diet induced changes in microbiota are reversible, and treatment efficacy is influenced by initial microbiota composition and multi system interactions. Therefore, precise treatment plans need to be developed during critical treatment periods; In terms of lifestyle intervention, fasting and exercise can positively regulate the microbiota to enhance the efficacy of ICI; Probiotics (such as inulin and pectin), active ingredients in traditional Chinese medicine, and specific polyphenolic compounds can target and regulate bacterial communities, synergistically enhancing the efficacy of ICI; Postbiotics (such as SCFAs) can directly regulate host metabolism and immunity, and when combined with anti-PD-1 therapy, can enhance anti-tumor effects. However, the effects of SCFAs on different types of ICI may vary, which may limit the activity of anti-CTLA-4 therapy. The specific mechanism still needs to be further studied (56).

The application of antibiotics can also have an impact on the composition of the gut microbiota. Some antibiotics can enhance the therapeutic effect of ICI by targeting and clearing carcinogenic bacterial strains, regulating the structure of gut microbiota. For example, vancomycin can enhance the tumor suppressive effect of CTLA-4 inhibitors by adjusting the proportion of microbiota; Antibiotics targeting harmful microorganisms can also reprogram the immunogenicity of TME. Currently, relevant clinical trials are actively exploring the potential application of combination therapy with antibiotics and alpha PD-1. From the mechanism of action analysis, antibiotic induced dysbiosis can cause excessive proliferation of pathogenic bacteria, reduce the number of beneficial bacteria such as Bifidobacterium and Lactobacillus, and damage the integrity of the intestinal barrier, downregulate the expression level of mucosal surfactant cell adhesion molecule-1 (MAdCAM-1), and promote the migration of T cell subsets with immunosuppressive function to TME; In addition, this dysregulated state can inhibit the signaling pathway of anti-tumor T cells, impair antigen presentation function and effector T cell activity, ultimately weakening the therapeutic effect of ICI. On the contrary, precise use of antibiotics can target the clearance of tumor promoting bacteria such as Fn, promote the release of microbial neoantigens, and activate CD8+T cell-mediated anti-tumor immune responses, while inhibiting immune suppression related signaling pathways. In addition, smoking and alcohol consumption can significantly reduce the relative abundance of Actinobacteria in the gut of patients with OSCC. However, there is a therapeutic paradox in antibiotic induced gut microbiota imbalance, and its impact on ICI efficacy depends on the type and timing of antibiotic administration. For the use of antibiotics in HNC patients, targeted dosing guidelines need to be developed in the future to achieve a balance between infection control and immunotherapy efficacy (56).

Chronic persistent HPV infection is an important cause of HNC, and its infection characteristics and carcinogenic mechanisms have clear specificity. The overall positive rate of HPV DNA in HNC patients is 15.7%, with HPV16 being the main infectious type, accounting for 88.5%; There is a significant difference in HPV infection rates among different subtypes of HNC, with OSCC having the highest infection rate (66.7%), followed by laryngeal cancer (16.7%), nasal cancer (12.5%). No HPV infection was detected in pharyngeal squamous cell carcinoma (57). From the perspective of pathogenic types, HPV positive HNC caused by HPV16/18 accounts for about 85%, while the remaining 15% is caused by HPV33, 35, 52 and other types, HPV6, 11. A total of 9 strains, including 16, cause 90% of HNC cases (58).

The carcinogenic core of HPV lies in the synergistic effect of non-structural proteins E5, E6, and E7: E5 has anti-apoptotic function and can overactivate the epidermal growth factor receptor signaling pathway to stimulate cell proliferation; E6 binds to the tumor suppressor protein TP53 through the ubiquitin protein ligase E6AP/E3A and mediates its proteasomal degradation; E7 can inhibit retinoblastoma proteins and both can bypass cell cycle checkpoints. In addition, E6 can inhibit the interferon transcription factor IFR3, E7 can inhibit Toll-like receptor 9 in immune cells, jointly weakening the body’s immune response, and E6 and E7 can upregulate telomerase TERT expression to achieve cell immortality. Smoking, immune suppression, and other viral infections are key auxiliary factors that promote the progression of HPV positive HNC. Smoking can downregulate the tumor suppressor gene miRNA-133a-3p, upregulate epidermal growth factor receptor and Hu antigen R, increase the probability of oral HPV infection, damage immune mediators, and promote the integration of HPV DNA with host DNA; According to reports, the risk of HPV infection in immunocompromised populations such as HIV/AIDS patients is three times higher than that of the general population, and they are more prone to multiple co infections (59).

High risk HPV infection is a powerful factor in predicting the specific survival rate of non OSCC patients. Studies have shown that HPV positive OSCC patients are more sensitive to chemotherapy and radiotherapy, with overall survival (OS) and disease-free survival (DFS) better than HPV negative patients. This conclusion has also been confirmed by researchers at Vanessa B. WOOKY (60).

HNC patients exhibit significant dysbiosis of the oral microbiota, which is closely associated with HPV infection status, disease progression, and prognosis. Differentially enriched microbiota are associated with HNC subtypes, T staging, survival outcomes, recurrence risk, HPV infection status, and smoking history. Specifically, the enrichment of symbiotic genera including Corynebacterium and Phocaeicola is negatively correlated with the abundance of pathogenic genera such as Clostridium.; High risk HPV infection is associated with the aggregation of Porphyromonas in OSCC and non OSCC; The enrichment of Clostridium in OSCC tissues is significantly correlated with non-smoking, early T and N staging, and better 3-year disease-specific survival in patients (57). In HPV positive tonsil cancer-related specimens, Pseudomonas aeruginosa is a Gram-negative aerobic bacterium unique to all cancer-related specimens (metastatic/non metastatic tonsil tissue, metastatic lymph node tissue). Among them, the metastatic group specimens only contain Shigella, Staphylococcus aureus, and Neisseria gonorrhoeae, and the number of Gram-negative bacteria in cancer patient samples shows an increasing trend (61). There is a significant difference in the beta diversity of oral microbiota between HPV positive and negative HNC patients and healthy controls, while there is no difference in alpha diversity. Streptococcus is the dominant bacterium in all three groups, and the dominant bacteria in the HNC and HC groups are Streptococcus mucilaginosus and Haemophilus parainfluenzae, respectively; At the level of phylum, family, and genus, the abundance of phyla such as Zygomycota and Bacteroidetes increased in HPV positive samples, while the abundance of phyla such as Proteobacteria decreased. The abundance of genera such as Firmicutes and Treponema increased, while the abundance of genera such as Neisseria and Lactobacillus decreased. Additionally, the abundance of the genus Treponema and its related species increased significantly in adjacent normal oral tissues of HPV-positive individuals. In addition, the relative abundance of 26 periodontal disease related bacteria in the HPV positive HNC group was higher than that in the HPV negative group. Invasive bacteria such as filamentous bacteria can coexist with HPV in oral lesions, suggesting that the two may have a synergistic carcinogenic effect; Spiroplasma contains virulence factors such as chymotrypsin, which may promote epithelial tissue invasion. Clostridium can mediate tumor proliferation by activating chemokines, and its enrichment helps improve the prognosis of OSCC patients (58). The tumor microenvironment and clinical characteristics of HPV positive HNC also exhibit uniqueness, and there are significant differences compared to negative HNC. Dysbiosis of oral microbiota is involved in the pathogenesis of OSCC, and its complex interactions with risk factors such as HPV infection and smoking play an important role in the occurrence and development of HNC. Clarifying these associations can provide important basis for the study of HNC pathogenesis, early diagnosis, and targeted treatment plan formulation.

With the development of tumor immunology, ICI have become an important direction for HNC treatment, and the composition of microorganisms is closely related to the efficacy of ICI therapy, Faecalibacterium,Specific bacterial populations such as Bifidobacterium and Akkermansia muciniphila can enhance the efficacy of ICI by inducing T cell differentiation, regulating the cytokine environment, or indirectly shaping tumor immune responses through their metabolites; Antibiotics can disrupt the diversity and structure of the microbiota, causing dysbiosis and ultimately reducing the effectiveness of ICI. Previous studies have confirmed that various microbial communities and their metabolites (such as polysaccharides A, inosine, deamination tyrosine, SCFAs, etc.) can enhance anti-tumor immune responses by activating dendritic cells, NK cells, and T cells, enhancing antigen presentation, and promoting tumor antigen-specific T cell expansion. Some strains’ metabolites can also improve CAR-T cell efficiency (62).

Griffin’s team’s research indicates that the anti-tumor enhancing effect of peptidoglycan hydrolase SAGA secreted by Enterococcus is achieved through targeted activation of NOD2 receptors; Probiotics such as Bifidobacterium and Staphylococcus aureus regulate through multiple pathways, including promoting dendritic cell activation, inducing interleukin 12 secretion, activating the cyclic GMP-AMP synthase/stimulator of interferon genes signaling pathway, and ultimately enhancing CD8 ⁺ T cell function; In addition, gut probiotics can stimulate immunoglobulin A synthesis and secretion at the mucosal immune level, not only enhancing mucosal immune surveillance efficacy, but also accurately regulating the transport process of symbiotic bacteria; Lactobacillus, Bifidobacterium and other strains can degrade tryptophan into indole derivatives, thereby enhancing the killing effect on tumors and improving the clinical efficacy of ICI; Jia et al. found that the indole-3-propionic acid co-produced by Lactobacillus Johnson and Clostridium sporogenes can enhance the efficacy of anti-PD-1 by regulating T cell stem like characteristics. Metabolites of gut microbiota are also key mediators in regulating immune therapy sensitivity. SCFAs (acetate, propionate, butyrate) can regulate immune function by binding to G protein-coupled receptor or inhibiting histone deacetylase. In addition, butyrate can upregulate/CD28 expression and enhance the efficacy of alpha PD-1 through the histone deacetylase inhibition pathway; Meanwhile, butyrate can also induce Tbx21 transcriptional inhibition of PD-1 and alleviate CD8 ⁺ tumor-infiltrating lymphocyte depletion (56).

The predictive value of microbiota on PD-1/PD-L1 inhibitor response remains controversial: the largest clinical oriented study, CheckMate141, analyzed saliva samples and did not find a significant association between oral microbiota and PD-L1 inhibitor efficacy, but multiple preclinical studies still reveal the potential regulatory role of microbiota. Hu et al. found that the presence of Luteibacter, Flammeovirgo, and Chlamydia trachomatis is significantly correlated with the total number of T cell receptors, clones, and CD8 ⁺ T cell infiltration in HNC patients. The mechanism may be mediated by upregulating the levels of chemokines in TME, promoting T cell recruitment and infiltration, and mediating the response to ICI (63).

In the research system of the interaction between microbiota and head and neck tumors, the mining and validation of microbial toxicity prediction factors have been one of the core directions of clinical translational research in recent years. The Yuchao Li team collected saliva, subgingival plaque, and tongue dorsal mucosa samples from patients with gingival squamous cell carcinoma (GSCC) through a system for microbiota analysis. The results showed that periodontal pathogens were enriched in GSCC lesions and related areas. Meanwhile, the high expression level of atorvastatin in saliva and abnormal biosynthesis of endotoxins may significantly increase the risk of periodontitis in patients. In vitro cell model experiments have confirmed that persistent infection of Gingival pseudomonas can effectively promote the proliferation, migration, and invasion ability of human oral epithelial cells, and induce their malignant transformation (64).

Another study found that the saliva microbiome of OSCC exhibits a phenotype with significantly increased alpha diversity and specific beta diversity characteristics. The specimen is enriched in genera such as Streptococcus, Lactobacillus, Prevotella, Trichomonas and Haemophilus. Among them, Lactobacillus is significantly more abundant in the saliva of oral squamous cell carcinoma patients than in the control group, and is more enriched in advanced T and N stage patients. In contrast, the dominant genera in healthy control samples are Actinobacteria and Corynebacterium , Prevotella and fine haired bacteria are often associated with tumor suppression or good prognosis: Actinobacteria have low virulence and are core bacterial species in healthy populations; Corynebacterium, as a dominant symbiotic bacterium in healthy saliva, is associated with high abundance and low risk of HNC; The abundance of filamentous bacteria decreases with the progression of oral mucositis, and high abundance within the tumor corresponds to higher patient survival rates (65).

The Soyoung Kwak team found a correlation between 13 types of oral bacteria and the risk of HNC through oral microbiota analysis. Among them, four strains of the Proteobacteria phylum (Acinetobacter baumannii, Pseudomonas aeruginosa, Klebsiella pneumoniae, and Pasteurella multocida) are associated with a reduced risk of HNC; The red and orange bacterial pathogen complexes in the oral cavity are moderately positively correlated with the risk of HNC. In addition, no correlation was found between the detected fungal species and the risk of HNC (66).