More than 90% of oral cancers, which rank 16th among all the common malignant tumors, are oral squamous cell carcinomas (OSCCs) originating from the squamous tissues [1]. Advances in medical imaging and therapy have improved the 5-year overall survival from 59% during 1990–2000 to 70% during 2001–2010 [2]; however, there were 377,713 cases of oral and lip cancer and 177,757 deaths in 2020 [3]. The etiology of OSCC is attributed to genetics; microbes; and unhealthy habits, including alcoholism [4], smoking [5], and chewing betel [6]. Periodontal diseases and tooth loss are also risk factors for OSCC ^[7][8][7,8], indicating that several oral bacteria (Streptococcus, Peptostreptococcus, and Prevotella) may be related to the development of OSCC [9]. Microorganisms that colonize the human body were found to be associated with 20% of cancers and are known to modulate tumor occurrence and development since Helicobacter pylori (H. Pylori) was found to contribute to gastric cancer in the 1990s [10]. The number of bacteria is almost equal to that of human cells in the body, and these microorganisms have at least 100 times more metagenomes than humans, which can be utilized to modulate the biological behavior of cancer cells [11] by promoting cell proliferation, resisting cell death, inducing angiogenesis, reprogramming energy metabolism, and evading immune destruction [12]. Increasing evidence suggests that microorganisms play a significant role in oral cancer development. Therefore, the mechanisms of microbial carcinogenesis should be explored, and a cure must be developed for oral cancer.

Several advanced approaches can detect oral cancer, such as lab-on-chip, microfluidics, nanodiagnostics, liquid biopsy, omics technology, and synthetic biology ^[13][78]. To date, numerous studies have applied different methods to describe the differences in oral microbiota between normal tissue and OSCC sites ^{[14][15][16][17]}[28,79,80,81], including the surface of the tumor tissue, within the tumor, and saliva. Allan Radaic etc. made an exhaustive summary of potential oral microbiome-based biomarkers for OSCC ^[18][82]. It is possible to distinguish cancerous lesions from normal tissues and perform tumor staging by noninvasively detecting microbes ^[19][83].

Intratumoral bacteria potentially originate from normal adjacent tissues ^[20][84] and play the role of immunomodulation in the tumor microenvironment ^[21][85]. Hooper et al. were the first to study microorganisms in OSCC and suggested that they are mainly aciduric and saccharolytic secondary colonizers, such as Micrococcus luteus, Prevotella melaninogenica, Exiguobacterium oxidotolerans, and Staphylococcus aureus, because of their acidic and hypoxic environments ^[22][86]. The abundances of the phylum Fusobacteria, genus Fusobacterium, and phylum Bacteroidetes were found to be elevated in the saliva of patients with OSCC and were believed to be diagnostically specific ^[23][15]. Aside from viruses and bacteria, a variety of molecular and genetic markers can be detected for treatment and monitoring ^[24][87]. A recent study reported that oral-cancer-related microorganisms in the mucosa, other than in gingival plaque or saliva samples, have the most diverse species differences and functional changes and are the most suitable sites for observing microbial dysregulation ^[25][88]. Another study that investigated unstimulated salivary microbial profiles found significant differences in Bacillus, Enterococcus, Parvimonas, Peptostreptococcus, and Slackia between epithelial precursor lesions and cancer groups ^[26][89]. As early as 2005, Madhura et al. found that with Capnocytophaga gingivalis, Prevotella melaninogenica, and Streptococcus mitis as diagnostic markers, the sensitivity and specificity for the three species are 80% and 82%, respectively ^[27][90]. Zhou et al. adopted random forests and cross-validations to build a diagnostic model based on oral microbiota and found that Actinobacteria, Fusobacterium, Moraxella, Bacillus, and Veillonella species were strongly correlated with OSCC ^[28][91]. However, Parvimonas micra and Streptococcus mitis have been implicated in the reduced risk of OSCC development ^{[29][30][31][32]}[27,92,93,94]. The presence of Corynebacterium and Kingella is also associated with a low incidence of head and neck squamous cell carcinoma, possibly because they are involved in the degradation of cancer-inducing metabolites ^[29][33][27,95]. Similarly, Shen et al. found that periodontitis-negative-associated bacteria (Neisseria sicca and Corynebacterium matruchotii) play an anti-cancer role in OSCC by upregulating DDR to repair DNA damage, inducing pyroptosis, and decreasing CD4+ T cells ^[34][96]. P. gingivalis IgG and IL-6 are also used as potential serum biomarkers for OSCC diagnosis ^[35][97]. As the expression patterns of CXCL10, DIAPH1, NCLN, and MMP9 genes are significantly correlated with interpain A, fadA, and bspA in OSCC cases, gene expression is an alternative target to detect OSCC ^[36][98].

With the progression of OSCC, the abundance of these bacteria increases, indicating that the microbiome can serve as a marker for staging and predicting prognosis. Yang et al. reported that F. periodonticum, S. mitis, and P. pasteri are bacterial marker panels that can be used to distinguish patients with stage 4 OSCC from healthy individuals ^[37][24]. Tumor MMP-9 expression is associated with poor outcomes in OPSCC, especially in HPV-negative disease, whereas Rgp immunoexpression in inflammatory cells is associated with better disease-specific survival, which can be utilized to predict prognosis ^[38][99].

In addition to diagnosing the disease, microorganisms can also be used to distinguish healthy from diseased mucosa. Su et al. demonstrated that Fusobacterium spp. is a successful marker species for identifying noncancerous tissues. However, compared with Fusobacterium spp., Streptococcus spp., especially Streptococcus pneumoniae, are more accurate when classifying lesion sites ^[16][80].

Multidisciplinary therapeutic strategies are generally used for OPMDs to prevent OSCC progression and prolong survival. However, there is no consensus on the treatment of OPMDs owing to the variety and complex mechanisms of OPMDs. Tacrolimus, which is used as the first-line drug after transplantation, was found to be effective in treating OLP ^[39][40][100,101] by downregulating immunity ^[41][102] and downregulating cell-cycle-related proteins ^[42][103]. Tacrolimus treatment also significantly altered the proportion of Allobaculum, Bacteroides, and Lactobacillus in the colonic mucosa and the circulation ^[43][104], which indicated that it may improve microbial dysregulation of mucosal surfaces. Some studies reported that tacrolimus promotes tumorigenesis and leads to adverse events ^[44][45][105,106], while others have reached a different conclusion ^[46][47][48][107,108,109]. Topical tacrolimus can be an effective second-line therapy for patients who do not respond to corticosteroids. However, further studies on its adverse effects are required.

A combination of surgery, chemotherapy, and radiotherapy has been administered to patients with OSCC. Recently, more attention has been paid to the oral microbiome, as the structural, metabolic, and virulence characteristics of microbes are potential targets. The use of pre- or probiotics and salivary substitutes was also suggested based on differences in the salivary microbiota between patients with OSCC and healthy controls ^[49][50][110,111]. The concept of oncolytic bacterial immunotherapy has long been popular, as commonly used radiotherapy and chemotherapy have side effects owing to damage to healthy tissues, while microorganism therapy shows the merits of accurate target specificity, tissue penetration, and less treatment expense. Many engineered bacterial strains were generated to overcome potential safety problems and improve tumor targeting ^[51][112]. Salmonella typhimurium ^[52][113], Escherichia coli ^[53][114], and Bifidobacterium ^[54][115] showed outstanding anti-cancer activities in both preclinical and clinical trials. Bifidobacterium, Streptococcus, Caulobacter, and Clostridium spp. are commonly found in the oral cavity and are promising candidates for OSCC tumor-targeting therapies ^[55][116]. Oncolytic or “cancer-killing” viruses have been highly used as immunotherapeutic drugs for the treatment of cancer as well ^[56][117] and are combined with radiotherapy, chemotherapy drugs, or other strategies. Nonetheless, concerns remain regarding the use of microorganisms in tumor therapies. Owing to the weakened immune system, bacteria-mediated tumor therapy is ineffective in patients who have undergone chemotherapy ^[57][118] and can cause serious infections. Moreover, bacterial monotherapy does not completely cure cancer, and bacterium-mediated synergistic cancer therapy was proposed to have promising potential ^[58][59][119,120]. Further studies are warranted to understand the interactions between oncolytic immunotherapies and other therapies.

Increasing evidence indicates that the presence of a microbiome can affect treatment outcomes ^[60][121]. Recent research found an increase in Lactobacillaceae and Bifidobacteriaceae families and a decrease in Porphyromonadaceae and Prevotellaceae after OSCC treatment. Furthermore, they observed a change in DMBT1 expression accompanied by the microbiome change, suggesting DMBT1 to be a possible treatment indicator ^[61][122]. And a certain genus, namely, Leptotrichia, was shown to improve patient prognosis ^[62][123]. Another prediction model with five microbial signatures, namely, Leptotrichia trevisanii, Capnocytophaga sputigena, Capnocytophaga, Cardiobacterium, and Olsenella, displayed high accuracy ^[63][124]. Cancer-related intratumoral bacteria and gut microbiota influence the effectiveness of chemotherapy. Lehouritis et al. found that bacteria can both decrease or increase the effectiveness of chemotherapeutic drugs via enzymatic biotransformation and chemical modification ^[64][125]. In a mouse model of colon cancer, intratumor Gammaproteobacteria converted gemcitabine into its inactive form by expressing the bacterial enzyme cytidine deaminase ^[65][126]. Another study found that 5-FU is metabolized by preTA-encoding bacteria ^[66][127]. Microbes modulate drug toxicity and side effects. In mice raised in a germ-free environment and administered antibiotic prophylaxis, oxyaliplatin-induced mechanical hyperalgesia was reduced, indicating that the gut microbiota enhanced chemotherapy-induced mechanical hyperalgesia ^[67][128]. Simultaneously, because microorganisms have a regulatory effect on immune-inflammatory responses, they also affect immunotherapy. Fusobacteria species are associated with the high expression of IL-12 and TGF-β, ultimately promoting the differentiation of T cells ^[68][129]. As for radiotherapy, antibiotic-mediated fungal reduction enhances the response to radiation, whereas antibiotic-mediated bacterial reduction presents the opposite results ^[69][130]. Significant microbiome changes can also affect radiation-induced osteoradionecrosis ^[70][131]. Current research on the microbial effects of cancer therapy mainly focuses on digestive tract cancers, requiring further exploration for oral cancer. Although the mechanism by which these microbes affect the efficacy of anti-cancer treatments remains unclear, a growing number of studies have shown that microbes are inextricably linked to anti-cancer treatments.

In addition to regulating cancer treatment, the microbiome can also be a target for regulating immune-related adverse events and even the prognosis of cancer treatment, as cancer treatments can lead to microflora dysregulation. Almost 75% of patients with head and neck cancer who undergo chemotherapy or radiotherapy treatment experience oral mucositis (OM) ^[71][132]. After receiving a 5-fluorouracil (5-FU) i.v. for 6 d, an increase in facultative and strictly anaerobic bacteria in the oral cavity and facultative anaerobes in the colon is observed ^[72][133]. Hong et al. also demonstrated that OM severity is associated with 5-FU ^[73][134]. Modification of oral microbiome is a promising preventive treatment for OM ^[74][75][135,136]. Palifermin, which is the only pharmacological agent approved by the FDA to treat OM, is a recombinant human keratinocyte growth factor (KGF) that targets the KGF receptor to enhance the differentiation and maturation of epithelial cells ^[71][76][132,137]. A recent meta-analysis showed that palifermin reduces the incidence of severe mucositis by up to 30% in patients treated with chemotherapy and radiotherapy ^[77][138]. Therefore, palifermin can be used as a prophylactic to prevent severe OM in patients with oral cancer. Recent research found that palifermin affects the oral microbial community composition, though more studies are warranted to figure out the correlations between palifermin and community composition changes ^[78][139]. However, in the 2021 European clinical practice guidelines, palifermin is not recommended for pediatric patients receiving cancer treatment ^[79][140] because of its short-term adverse effects, potential long-term negative effects on cancer outcomes, high costs, and restricted availability. Palifermin can be used as a prophylactic to prevent severe OM in patients with oral cancer; more reliable evidence is needed on the safety and efficacy of palifermin. Interestingly, Lactococcus strains were shown to be effective in the control of 5-FU-dysbiosis ^[80][141], indicating that probiotic supplementation may be a prophylaxis to reduce the adverse effects of cancer therapies and improve the quality of patients’ lives. Table 1 summarizes the promising clinical applications of oral microbes in OSCC.