1 Background

The buccal cavity is the most diverse reservoir of microbes comprehending symbiotic, commensal, and pathogenic organisms. The microbes in the buccal cavity/oral cavity determine the fitness of oral health. Over time with the progress of techniques, it has been evaluated that about 600 species encompass the oral ecological community. Gerald et al. [1].

The oral cavity is predominantly enriched by Proteobacteria and Firmicutes, which is about 19.26% and 53.83%, respectively. It is espied that there is a shift of oral microbiome in preoral cancer and oral cancer patients compared with normal individuals, and microflora changes metabolic pathway to influence oral health. Zixuan et al. [2].

OSCC—Oral squamous cell carcinoma, is one of the most common types of oral cancer, approximately ninety percent of total oral cancer Tendon et al. [3]. Wang et al. [4] The reoccurrence rate in OSCC ranges from 32.7 to 44.9% [4]. Mathur et al. [5] consumption of alcohol, poor oral hygiene, tobacco smoking, unhealthy diet, and microbial infection are the etiological factor that boosts the risk of OSCC [5]. Some unknown etiological factors are responsible for OSCC development as it has been seen that persons who have never drank or smoked also face OSCC Kruse et al. [6]. It is obtained from the data that genes of OSCC-associated bacterium have a potential role in bacterial chemotaxis, flagellar assembly, and lipopolysaccharide (LPS) synthesis are concentrated in the tumor site [7].

Streptococcus spp. is found to be significantly less in number on the surface of oral squamous cell carcinoma-OSCC lesions. The tumor site of the oral cavity is hiked up with Campylobacter, Fusobacterium, Capnocytophaga, Prevotella, and Peptostreptococcus. Su et al. [8]. Carnobacteriaceae, Actinomycetaceae, Micrococcaceae, and Streptococcaceae are low in number in OSCC patients. Zhang et al. [9].

Periodontitis patients are more prone to develop oral squamous cell carcinoma. Yufei et al. [10]. Considering the carcinogenic property, a study was conducted to determine the relationship between OSCC microflora and periodontitis. The positive and negative regulator microbes of periodontitis were tested against the HSC-3 cell line. Positive regulator microbes of periodontitis express a high amount of IL-6, cyclin-D1, IL-8, and MMP-9 and show more proliferative activity than control and negative regulator microbes of periodontitis. Xiaoyu Hu et al. [11]. This review analyzes the oral microbiota of OSCC patients and the pathway associated with the OSCC advancement to establish the link between the particular positive regulator of OSCC bacteria and molecular mechanisms. These positive regulator microorganisms might be employed as biomarkers to identify and prevent OSCC in its early stages.

2 Pathway used by the positive regulator of oral cancer

2.1 Porphyromonas gingivalis

Porphyromonas gingivalis resides in the subgingival site and is a Gram-negative anaerobic bacteria involved in promoting anti-apoptotic pathway by increasing the expression of anti-apoptotic genes and by blocking the pro-apoptotic genes, as represented in Fig. 1. Porphyromonas gingivalis activates PI3/Akt pathway, which enhances cancer. Mao et al. [12].

Fig. 1
figure 1

Porphyromonas gingivalis carcinogenic signaling pathway

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Sandros et al. [13] Some essential virulent factors like endotoxin, gingipains, and fimbriae of Porphyromonas gingivalis provoke secretion of various chemokines and tumor necrosis factor-alpha (TNF-α). Yilmaz et al. [14] FimA fimbriae found on the surface of Porphyromonas gingivalis, which have a role in rearranging cytoskeleton by interacting with beta 1 integrin on the host cell membrane and cause invasion of the cell. Amano et al. [15] Fimbriae also cause inflammation by inducing the host cell secretion of IL-1, IL-6, IL-8, and TNF-α. Inaba et al. [16] Gingipains convert ProMMP9 to activated MMP9, and MMP9 makes carcinoma cells migrate and show invasiveness property by degrading the basement membrane.

Tang et al. [17] Porphyromonas gingivalis alters the pathway, which is linked with the regulation of cell cycles like cyclins, PI3K, and p53. LPS (lipopolysaccharide) of P. gingivalis is involved in the downregulation of p53. Groeger et al. and Kuboniwa et al. [18, 19] Porphyromonas gingivalis activate NF-κB and MAPK pathways which are associated with cell proliferation.

Metabolites like “Butyric acid” (Chang et al. [20]) and “Acetaldehyde” (Olsen et al. [21]) are synthesized by PG, whose effect can promote cancer by causing mutation in the nucleotides. Kurita-Ochiai et al. [22] Butyric acid also provokes apoptosis of T- cell and B-cells in the absence of P53.

Gabrilovich et al. [23] Myeloid-derived suppressor cells are linked with metastasis and found abundantly at the OSCC site, inhibiting the activation of T cells. PG induces the growth of myeloid-derived suppressor cells at the OSCC site through cytokine and interleukins-mediated pathway.

Nucleoside diphosphate kinase is secreted by P. gingivalis, which blocks the ATP-dependent pathway via purinergic receptor P2X7 [23].

Porphyromonas gingivalis adhere to the host cell membrane through integrin-mediated receptor binding. It activates Akt/ PI3K pathway and promotes the cell proliferation process. It upregulates the anti-apoptosis gene expression and down-regulates pro-apoptotic gene expression.

2.2 Fusobacterium nucleatum

Peyret-Lacombe et al. [24] F. nucleatum is a Gram-negative anaerobic bacteria involved in producing various chemokines like IL-6, IL8, IL1β, IL6, and IL1α interact with oral epithelial cell and promotes carcinogenesis by the 'START' mediated pathway as depleted in Fig. 2. START, in turn, inhibits the expression of apoptotic genes and upregulates the cell survival genes. F. nucleatum also interacts with toll-like receptors present on the oral epithelial cell, and its entry is straightforward for the invasion process. Like PG, F. nucleatum is found copiously at the OSCC site than at the control site [25].

Fig. 2
figure 2

Carcinogenic pathway associated with Fusobacterium nucleatum

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Fardini et al. [26] With the help of FadA, the adhesion molecule, F. nucleatum, binds with the host cell, disrupts the host's membrane integrity by disrupting its tight junction proteins and is also able to invade cells like colonic and placental epithelial cells, keratinocytes, T-cells and macrophages by expressing Fap2 and FadA Han et al. [27]. Abed et al. [28] F. nucleatum attaches to the host cell through an adhesion molecule that is Fap2, which binds with the oligonucleotide that is Gal-GalNAc of the host cell. Coppenhagen-Glazer et al. [29] Fap2 is also associated with oral biofilm formation.

F. nucleatum activates P38, which activates a series of proteins like matrix metalloproteinase-9 and MMP-13 and heat shock protein-27 (HSP-27). MMP-9 and MMP-13 are observed to cause metastasis [30]. F. nucleatum also promotes the JNK and TLR-2 signaling pathways [31].

LPS (lipopolysaccharide) at the cell wall of F. nucleatum upregulates the expression of genes encoding IL-8, which in turn causes inflammation and lesion made in the gingiva cell through cytokine-mediated damaging. [32, 33].

Yost et al. [34] Fusobacterium shows high RNA expression at tumor sites than at healthy sites. Mostly virulent expressing genes transcript more at OSCC site.

BD-1-beta defensin-1and BD-2-beta defensin-2 are antimicrobial peptides generally produced by epithelial cells of salivary glands whose expression is elevated when it detects the presence of LPS and TNF-alpha. [33,34,35].

F. nucleatum and Porphyromonas gingivalis act cooperatively to skip from the host's immune system and found that F. Nucleatum can increase the invasive potential of Porphyromonas gingivalis [36, 37]. Huang et al. [38] F. nucleatum produces ammonia from aspartate and glutamate and makes the environment neutral, which is suitable for P.gingivalis colonization as P. gingivalis is an acid intolerant microbe.

2.3 Candida albicans

Candida albicans is a saprophytic fungus generally found in the oral cavity, vagina, and GI tract (gastrointestinal) [39]. The presence of Candida in the deeper site of OSCC, which is about 74%, clarifies its role in OSCC occurrence [40]. Under certain specific conditions, Candida albicans become pathogenic and can cause acute and chronic infection (candidiasis), especially in immunosuppressive patients [41]. According to Chen et al. [42] , Candida albicans contains zymosan in the cell wall, is glycan in nature and can be easily recognized by pattern recognition receptors. OSCC expresses TLR (mostly TLR3 and TLR4). Activation of TLR leads to activation of adaptor molecules like MyD88, which ultimately triggers the NF-κB pathway that promotes tumor progression. Abdullah et al. [43] Leukoplakic lesions, a pre-cancerous state, are caused by Candida albicans. The ECE1 gene of Candida albicans encodes candidalysin toxin and triggers NF-κB and MAPK pathways. ECE1gene expression was found to be more during macrophage attack by the host cell and escapes from macrophage attack by switching its morphological form in the phagosome [44, 45] (Fig. 3).

Fig. 3
figure 3

Candida albicans carcinogenic signaling pathway

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The ECE1 gene of Candida albicans encodes candidalysin protein, which is very toxic. It damages the host's cell membrane. As a result, cytokine secretion triggers APCs to differentiate and form a lesion, which ultimately turns into cancer. Candida albicans develops hyphae and secretes candidalysin when it is swallowed by macrophages, causing additional harm to the host immune cell. Candida albicans activate the NF-kB and MAPK pathways via TLR and adaptor protein activation.CA: Candida albicans (Table 1).

Table 1 Microbial profile at OSCC site against control site
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3 Positive regulators and their interaction with the host at the OSCC site

It is observed that the deeper tissue is generally sterile, and microbes colonize on the surface layer of mucosal epithelium in healthy individuals. However, microbes can evade the mucosal barrier and seem to colonize themselves on the deeper tissue under certain circumstance. [46, 47]. Aerobic bacteria, obligate anaerobic bacteria, and facultative anaerobic species are abundantly present at the OSCC site, but facultative anaerobic species are more frequently found [48].

The expression of genes regulating bacterial mobility, LPS synthesis, bacterial chemotaxis, and flagella assembly was high and played an essential role at the OSCC site (7). Moghimi et al. [49] found an up-regulation of microbial genes like bspA, fadA, and interpain A, which correlates with CXCL10, MMP9, NCLN, and DIAPH1 genes.

4 Interaction of virulent microbes at the OSCC site

P. gingivalis and F. nucleatum employ different invasion methods. According to studies, P. gingivalis enters host cells using the endocytic route and lipid rafts; likewise, F. nucleatum enters host cells through numerous adhesins and a "zipper" mechanism [50].

FomA of F. nucleatum assists in bacterial co-aggregation and biofilm formation and is advantageous for bacterial invasion into host cells. It binds to the Fc portion of human IgG [51]. F. nucleatum assists P. gingivalis in invading host cells [52].

However, on the other hand, it has been found that P. gingivalis prevented F. nucleatum from invading oral epithelial cells by decreasing the production of the adhesion-related proteins FadA and FomA by secreting proteases [50].

Although P. gingivalis inhibits the invasion process of F. nucleatum, those bacterial mixtures abundantly found at the OSCC site are now a question of research. It may be due to the involvement of a particular strain of P. gingivalis that inhibit the F. nucleatum invasion process. More research is needed focusing on those areas.

A study report observed that C. albicans SN152's hyphal development is inhibited by F. nucleatum ATCC 23,726 in a contact-dependent manner [53]. Whether all strains of C. albicans inhibit F. nucleatum or not, which particular strain of Candida albicans is involved in promoting oral cancer needs to be studied. These are the research gap that needs to be focused on in future research.

5 Conclusion

Although many microbes are found to be more at the OSCC site than the control site, few positive regulator microbes like Porphyromonas gingivalis, Candida albicans, and Fusobacterium nucleatum are associated with host cell membrane damage and tissue invasion, which activates signaling pathways like MAPK and PI3K. Among these microbes, Porphyromonas gingivalis and Fusobacterium nucleatum are found to be more virulent oncogenic bacteria. Moreover, the interrelation between these microbes of different strains needs to be screened. The virulent strains of these microbes may lead to dysplasia of the oral cavity, which might further lead to various inflammatory pathways and later develops into cancer. Positive regulator microbes of OSCC activate pathways associated with cell growth, proliferation, survival, differentiation, and motility, increasing the risk of OSCC. These microbes suppress the pro-apoptotic gene expression and upregulate the anti-apoptotic gene expression; as a result, the survival chance of cells increases. The drug can be developed as a future therapeutic intervention strategy by targeting virulent expressing genes responsible for promoting cancer. The validation of the OSCC-associated microbiota as a biomarker might have significant implications for future oral cancer screening, early detection and treatment.