Application of Fusobacterium nucleatum as a biomarker in gastrointestinal malignancies

Abstract

The morbidity and mortality of gastrointestinal (GI) malignancies are among the highest in the world, posing a serious threat to human health. Because of the insidious onset of the cancer, it is difficult for patients to be diagnosed at an early stage, and it rapidly progresses to an advanced stage, resulting in poor treatment and prognosis. Fusobacterium nucleatum (F. nucleatum) is a gram-negative, spore-free anaerobic bacterium that primarily colonizes the oral cavity and is implicated in the development of colorectal, esophageal, gastric, and pancreatic cancers via various intricate mechanisms. Recent development in novel research suggests that F. nucleatum may function as a biomarker in GI malignancies. Detecting the abundance of F. nucleatum in stool, saliva, and serum samples of patients may aid in the diagnosis, risk assessment, and prognosis monitoring of GI malignancies. This editorial systematically describes the biological roles and mechanisms of F. nucleatum in GI malignancies focusing on the application of F. nucleatum as a biomarker in the diagnosis and prognosis of GI malignancies to promote the clinical translation of F. nucleatum and GI tumors-related research.

Key Words: Gastrointestinal malignancies, Fusobacterium nucleatum, Biomarker, Diagnosis, Prognosis

Core Tip: Gastrointestinal (GI) malignancies are characterized by high morbidity and mortality. Early diagnosis of GI malignancies is crucial for disease intervention and treatment. Numerous studies have shown that Fusobacterium nucleatum (F. nucleatum) is closely related to the development of various GI malignant tumors. This paper discusses the mechanism of F. nucleatum in promoting the progression of GI tumors, elaborates its clinical value as a diagnostic and prognostic biomarker, and provides ideas for the development and research of novel biomarkers for GI malignancies.

INTRODUCTION

Gastrointestinal (GI) malignancies, including colorectal cancer (CRC), gastric cancer (GC), esophageal cancer (EC), pancreatic cancer (PC) and so on, are a major cause of cancer-related morbidity and mortality. Globally, GI cancers account for approximately 26% of new cancer cases and 35% of cancer-related deaths[1]. GI malignancies share common risk factors, and their incidence and progression are influenced by various elements, including genetics, environment, diet, and lifestyle. The pathogenesis of GI cancers is intricate and varied[2]. However, due to the absence of specific symptoms during their early stages, it is difficult for patients to be diagnosed with GI cancer in a timely manner, resulting in expedited progression to advanced stages. As a consequence, treatment outcomes, prognosis, and five-year survival rates are poor. The 5-year survival rates for patients with advanced CRC, EC, and PC are below 20%, while the 5-year survival rate for patients with advanced GC is less than 40%[3,4]. Currently, the gold standard for diagnosing malignant GI tumors is a combination of biopsy and pathological examination. However, this method is invasive and time-consuming, making it challenging to implement for population screening purposes[5]. Furthermore, traditional tumor markers in serum like carcinoembryonic antigen (CEA), carbohydrate antigen (CA)-125, and CA19-9, which are commonly utilized for screening because of their convenience, lack the sensitivity and specificity required for consistent diagnosis of GI malignancies resulting in potential cases of missed diagnosis or misdiagnosis[6]. Therefore, it is imperative to explore a novel non-invasive biomarker for early detection, prognostic monitoring, and risk evaluation of GI malignancies.

Fusobacterium nucleatum (F. nucleatum) is a spindle-shaped, gram-negative, anaerobic bacterium devoid of spores with a large middle section and tapered ends[7]. F. nucleatum is a common opportunistic pathogen in the oral cavity that can also be found in the intestinal, urinary, and upper digestive systems. F. nucleatum has been implicated in a variety of inflammatory diseases, such as periodontitis and inflammatory bowel disease[8,9]. Through extensive research conducted on intestinal flora in recent years, it has been determined that F. nucleatum exhibits a close correlation with GI malignant tumors. F. nucleatum affected the occurrence and progression of GI malignancies by colonizing the digestive tract, contributing to the proliferation, metastasis, and chemoresistance of cancer cells. Thus, F. nucleatum may serve as a biomarker for the diagnosis and prognosis monitoring of GI malignancies[10-12].

In fact, certain patterns in oral and gut microbiota have been shown to be predictive of the development of GI malignancies[13,14]. Increasing evidence suggests that F. nucleatum is more common in the saliva and stool samples of patients with GI malignancies compared to healthy controls[15]. Moreover, quantitative polymerase chain reaction (qRCR) and bacterial 16S rDNA sequence analysis revealed increased levels of F. nucleatum in GI malignancies tissues compared to adjacent normal tissues[16,17]. Notably, a high abundance of F. nucleatum was inversely associated with overall survival and exhibited high specificity and potential value as a diagnostic and prognostic marker for GI malignancies[18]. This editorial set out to summarize the latest developments regarding the potential involvement of F. nucleatum in carcinogenesis (Figure 1 and Table 1) and the significance of F. nucleatum as a diagnostic and prognostic biomarker for GI malignancies.

Figure 1 Underlying mechanism of Fusobacterium nucleatum pathogenesis in gastrointestinal malignancies. Fusobacterium nucleatum (F. nucleatum) adheres to and invades tumor cells through adhesins FadA on its surface, releasing virulence factors that promote the progression of gastrointestinal malignancies. Binding of F. nucleatum to receptors on the surface of tumor cells promotes the malignant transformation of tumor cells through a variety of mechanisms, including the induction of DNA damage, modulation of the immune microenvironment of tumors, and regulation of non-coding RNA expression. GC cells: Gastric cancer cells; GI cancer cells: Gastrointestinal cancer cells; Fn-GCEx: Exosomes secreted by F. nucleatum-infected GC cells; TLR4: Toll-like receptor 4; Gal-GalNAc: D-galactos-β (1-3)-N-acetyl-D-galactosamine; ESCC cells: Esophageal squamous carcinoma cells; MDSCs: Myeloid suppressor cells; PBMCs: Peripheral blood mononuclear cells; PMNs: Polymorphonuclear cells; NK cell: Natural killer cell.

Table 1 Potential mechanisms of Fusobacterium nucleatum in gastrointestinal malignancies.

Mechanisms	Type of cancer	Potential pathway	Ref.
Release of virulence factors	CRC	Myloid-like FadA	[19]
		FadA/VE-cadherin	[20]
		FadA/E-adherin/β-catenin signaling	[21]
		Fap2/Gal-GalNAC	[22]
		Fn-DPS/ATP3/PD-L1	[23]
		Succinic acid/cGAS-interferon-β pathway	[24]
	EC	Fn-DPS/CCL2/CCL7/EMT	[11]
		Putrescine	[70]
Induction of DNA damage	CRC	E-cadherin and chk2	[26]
		γ-H2AX	[27]
		NEIL2	[28]
	EC	NOD1/RIPK2/NF-κB	[76]
		Senescence-associated secretory phenotype	[25]
Dysregulation of non-coding RNAs	CRC	TLR4/MYD88/NFκB/miRNA21/MAPK	[30]
		miR34a/miR22/miR28	[29]
		miR-31/eukaryotic initiation factor 4f-binding protein 1/2	[32]
		miR-4717/METTL3	[77]
		miR-4474/miR-4717/CREBBP	[78]
		miR-122-5p/FUT8/TGF-1/Smads axis	[36]
		lncRNA EVADR/YBX1	[34]
		lncRNA ENO1-IT1	[33]
	GC	HOTTIP/miR-885-3p	[35]
Modulation of tumor immune microenvironment	CRC	CD11b+ myeloid cells/arginase-1 and iNOS/CD4+T cell	[79]
		miR-1322/polarization of M2 macrophages	[31]
		CEACAM1/T cell and NK cell depletion	[38]
		Fap2/TIGIT/T cell and NK cell depletion	[39]
	EC	NLRP3/MDSCs	[40]
		KIR2DL1/CD8+ T cells	[80]
	PC	CXCL1/CXCR2/MDSCs/CD8+ T cells	[73]
		GM-CSF, CXCL1, IL-8 and MIP-3α	[81]
		AI-2/TNFSF9/CD8+ T cells	[82]
Cooperation with other intestinal microbiota	CRC	Bacteroides fragilis/PKS+E. coli	[42]
		Enterobacteriaceae/Stenotrophomonas	[43]
		Gemella morbillorum/fiber intake	[44]
		RadD/Clostridioides difficile	[83]

EC: Esophageal cancer; CRC: Colorectal cancer; PC: Pancreatic cancer; lncRNAs: Long noncoding RNAs; miRNAs: MicroRNAs; TLR: Toll-like receptor; ENO1-IT1: Enolase1-intronic transcript 1; YBX1: Y-box binding protein 1; MDSCs: Myeloid suppressor cells; NLRP3: NOD-like receptor protein 3; NK cell: Natural killer cell.

MECHANISMS OF CARCINOGENESIS OF F. NUCLEATUM

Release virulence factors

F. nucleatum adheres to and colonizes tumor cells by releasing adhesins, virulence factors, and metabolites, inducing the oncogenic response. Under conditions of stress and disease, it was revealed that F. nucleatum secreted adhesion FadA with amyloid properties on its surface acting as a scaffold for biofilm formation to protect F. nucleatum and promote survival in acidic environments enhancing its virulence[19]. Research has indicated that FadA bound to the vascular cell receptor VE-cadherin resulting in weakened cell-cell connections and increased endothelial cell permeability, aiding F. nucleatum to invade endothelial cells and ultimately spread throughout the body[20]. In addition to binding VE-cadherin, FadA further was shown to localize to the E-cadherin receptor, activating the β-catenin signaling pathway, mediating F. nucleatum adhesion and invasion on epithelial cells, and promoting CRC proliferation by upregulating the expression of specific oncogenes and inflammatory genes[21]. Furthermore, Abed et al[22] discovered that microbial protein Fap2 mediated the binding of F. nucleatum to overexpressed D-galactos-β (1-3)-N-acetyl-D-galactosamine (Gal-GalNAc) in CRC. This resulted in the enrichment of F. nucleatum in CRC, thus strengthening its adhesion and invasion into the tumor, and promoting immune-mediated inflammation. Furthermore, a novel F. nucleatum virulence factor, DNA hunger/stationary phase protective proteins (Fn-Dps), promoted CRC metastasis by inducing epithelial-mesenchymal transition (EMT) through upregulation of chemokine CCL2/CCL7 expression[23]. Notably, succinic acid, a metabolite of F. nucleatum, was revealed to inhibit the cyclic GMP-AMP synthase-interferon-β pathway, resulting in reduced levels of chemokines CCL5 and CXCL10 in the tumor microenvironment and attenuation of the aggregation of CD8+ T cells thereby suppressing the anti-tumor response[24].

Induce DNA damage

F. nucleatum infection has been shown to promote the malignant transformation of GI tumor cells by inducing DNA damage and regulating the expression of DNA damage response genes, a crucial step in the process of carcinogenesis. It has been shown that F. nucleatum invaded chemotherapy-induced senescent esophageal squamous carcinoma (ESCC) cells and induced DNA damage and activation of DNA damage response pathways resulting in increased secretion of proinflammatory factors, exacerbating ESCC progression and chemoresistance[25]. In the C57BL/6 J-adenomatous polyposis coli Min/J mouse model, Guo et al[26] demonstrated that F. nucleatum inoculation upregulated E-cadherin and chk2 expression through FadA, resulting in DNA damage and cell proliferation of CRC cells. Moreover, it was revealed that F. nucleatum infection of CRC cells increased the level of the DNA damage marker γ-H2AX, causing DNA double-strand breaks (DSBs)[27]. In addition, the presence of F. nucleatum downregulated the expression of the DNA repair protein NEIL2, which in turn promoted DSBs and the secretion of inflammatory factors, contributing to the development of CRC[28].

Dysregulation of non-coding RNAs

Recently, an increasing number of studies have identified dysregulation of non-coding RNAs (ncRNAs), especially microRNAs (miRNAs) and long noncoding RNAs (lncRNAs), as being key to the molecular mechanisms by which F. nucleatum induces the development of GI malignancies. F. nucleatum induced pro-inflammatory cytokine secretion by recognizing Toll-like receptor (TLR) 4 and TLR2 receptors and upregulation of miR-34a and miR-135b expression[29]. Similarly, F. nucleatum infection of intestinal epithelial cells increased the proliferation and invasive ability of tumor cells through activation of the MAPK signaling pathway by inducing the expression of miRNA21 and downregulating the expression of RASA1 through the TLR4/ MYD88/NF-κB pathway[30]. Furthermore, downregulation of miRNA-1322 expression through activation of the NF-kB signaling pathway following F. nucleatum infection increased CCL20 expression to provide an immune microenvironment suitable for tumor development and metastasis[31]. Moreover, activation of the NF-kB signaling pathway induced the expression of miRNA-31, which directly acted upon eukaryotic initiation factor 4f-binding protein 1/2 to promote CRC cell proliferation by inhibiting its expression. On the other hand, miRNA-31 enhanced the inhibitory effect of F. nucleatum on autophagic flux by targeting SYNTAXIN-12 and increased the intracellular survival of F. nucleatum resulting in continuous infection of CRC cells and enhanced tumorigenicity[32].

Intriguingly, lncRNAs have emerged as important mediators of interactions between tumor cells and F. nucleatum. F. nucleatum was shown to activate lncRNA enolase1-intronic transcript 1 (ENO1-IT1) transcription by promoting the binding of transcription factor SP1 to the promoter region of lncRNA ENO1-IT1 resulting in increased glucose metabolism in CRC cells and tumor cell carcinogenesis[33]. Additionally, F. nucleatum infection caused increased expression of lncRNA-EVADR, which was revealed to interact with Y-box binding protein 1 to translocate to polyribosomes, leading to increased translation of EMT-related factors and ultimately promotion of CRC metastasis[34].

Exosomes are important mediators of intercellular communication. As such, exosome content and secretion were shown to be influenced by F. nucleatum, playing a key role in tumor communication networks. Moreover, it has been reported that exosomes secreted by F. nucleatum-infected GC cells (Fn-GCEx) could enhance the proliferation, invasion, and metastasis of uninfected GC cells. LncRNA-HOTTIP, an important cancer-promoting signal in Fn-GCEx, was observed to upregulate the expression of EphB2 by sponging miRNA-885-3p resulting in activation of the PI3K/ AKT signaling pathway and promotion of the progression of GC[35]. Similarly, in F. nucleatum-infected CRC cells we observed that the RNA-binding protein hnRNPA2B1 mediated the secretion of oncogene miR-122-5P by F. nucleatum into exosomes, leading to the downregulation of its expression in CRC cells and in turn an upregulation of FUT8 expression promoting the migratory and proliferative capacities of tumor cells. Moreover, it has been further revealed that F. nucleatum activated the TGF-β1/Smads signaling pathway by regulating the miRNA-122-5p/FUT8 axis to promote EMT, resulting in tumorigenesis and development[36].

Modulation of tumor immune microenvironment

F. nucleatum has been shown to regulate the tumor immune microenvironment and suppress anti-tumor immune responses in cancer tissues through two pathways. Firstly, F. nucleatum was revealed to regulate the immune microenvironment by inhibiting the function of anti-tumor immune cells. Jewett et al[37] reported that F. nucleatum activated the NF-kB/interleukin-converting enzyme pathway through bacterial apoptosis-inducing molecules on its surface, mediated apoptosis of immune cells, such as peripheral blood mononuclear cells and polymorphonuclear cells, and suppressed their immune function. These phenomena created an immune microenvironment suitable for tumor development and metastasis. Similarly, F. nucleatum was shown to suppress the cytotoxicity of immune cells and regulate anti-tumor immunity through the interaction of Fap2 with the inhibitory receptors on natural killer cells, TIGIT and CEACAM1, which protected F. nucleatum from immune cell attack thereby promoting tumor cell metastasis[38,39].

Secondly, F. nucleatum selectively recruits immunosuppressive myeloid suppressor cells (MDSCs) and M2 macrophages to promote the malignant transformation of tumor cells. Liang and colleagues discovered that elevated expression of NOD-like receptor protein 3 in the tissues of F. nucleatum-infected ESCC patients induced the enrichment of MDSCs in the tumor microenvironment, resulting in weakened anti-tumor immunity and ESCC resistance to cisplatin[40]. In addition, F. nucleatum infection was observed to inhibit tumor cell miR-1322 expression through activation of the NF-κB signaling pathway. In turn, this caused upregulation of CCL20, promoted macrophage infiltration, induced M2 macrophage polarization, and participated in CRC cell metastasis[31].

Cooperation with other intestinal microbiota

A multitude of studies have reported that F. nucleatum interacts with other gut microbes implicated in the occurrence and development of GI malignancies. To investigate bacterial symbiosis and its potential interactions, Tran et al[41] constructed a correlation network using the gut microbiomes of CRC patients and identified that multiple subspecies of Fusobacterium cluster in the tumor microbiome, and furthermore, that genes encoding key virulence factors (Fap2 and RadD) may undergo frequent horizontal gene transfer or recombination. Gong et al[42] postulated that in the early stages of CRC, Bacteroides fragilis-mediated inflammation induced intestinal epithelial degradation and intestinal mucosal injury, leading to an imbalance in intestinal ecology and providing the basic conditions for PKS⁺E. coli colonization and induction of oncogenic mutations. Cancerous intestinal epithelial cells can further recruit F. nucleatum to colonize the lesion site, and F. nucleatum in turn was shown to promote cancer cell proliferation through Fap2-mediated immune evasion. Wu et al[43] identified that in azoxymethane/dextran sulfate sodium salt mice F. nucleatum infection altered the structure of the intestinal mucosal microbial community, this was primarily manifested as an enrichment of potentially pathogenic flora and the depletion of probiotic bacteria, particularly Enterobacteriaceae and Stenotrophomonas. The structural dysregulation of the intestinal microbial community may affect the transcriptional activity of tumor-associated metabolic pathways and cause malignant transformation of tumor cells. In addition, F. nucleatum and Gemella morbillorum were shown to play important roles in modulating the relationship between dietary fiber intake and CRC pathogenesis, but the precise mechanisms require further exploration[44]. Importantly, Figure 1 and Table 1 further summarized the potential mechanisms by which F. nucleatum may be involved in the progression of GI malignancies.

APPLICATION OF F. NUCLEATUM DETECTION IN THE DIAGNOSIS AND PROGNOSTIC MONITORING OF GI MALIGNANCIES

At present, the detection of F. nucleatum is primarily reliant upon the following sample types: Tissue samples, stool samples, saliva samples and serum samples, as we illustrate in Table 2.

Table 2 The detection efficiency of Fusobacterium nucleatum in different gastrointestinal malignancies samples.

Cancer	Samples	Risk assessment and detection rate of F. nucleatum	Method	Ref.
CRC	Tissue	72% (proximal) and 35% (distal)	qPCR	[84]
CRC	Tissue	38.8% (54/139)	ddPCR	[85]
CRC	Tissue	56.30% (18/32)	qPCR	[86]
CRC	Tissue	57.1% (8/14)	AP-PCR	[62]
	Saliva	100% (14/14)
CRC	Stool	Worse overall survival	16S rRNA sequencing	[10]
	Tissue
CRC	Tissue	50%	PCR	[87]
	Stool	53.5%
GC	Tissue	26% (19/80)	qPCR	[88]
GC	Tissue	44% (40/91)	qPCR	[89]
GC	Tissue	28.75% (23/80)	qPCR	[66]
		Worse overall survival
EC	Tissue	69.4% (68/98)	qPCR	[71]
PC	Tissue	15.5% (13/84)	qRT-PCR	[73]
PC	Serum	52.4%	ELISA and qPCR	[75]

qPCR: Quantitative polymerase chain reaction; ddPCR: Droplet digtial polymerase chain reaction; AP-PCR: Arbitarily primed polymerase chain reaction; qRT-PCR: Quantitative reverse-transcription polymerase chain reaction; ELISA: Enzyme-linked immunosorbent assay; EC: Esophageal cancer; CRC: Colorectal cancer; PC: Pancreatic cancer.

F. nucleatum and CRC

Application of F. nucleatum detection in tissues for the diagnosis and prognosis of CRC: Presently, tissue samples are the commonest sample type used in the study of F. nucleatum and CRC. Utilizing tissue, it is possible to directly analyze the relationship between F. nucleatum abundance and pathological features of the tumor as well as epigenetic alterations. Metagenomic analyses using whole-genome sequencing, transcriptome sequencing, and DNA sequencing of bacterial 16S ribosomal RNA genes revealed enrichment of Fusobacterium species in CRC compared to in adjacent normal tissues[45-47]. Moreover, it was demonstrated that the abundance of F. nucleatum in tumor tissues was correlated with the site, metastasis, and invasion depth of CRC[48,49]. Mima et al[50] measured the relative amount of F. nucleatum DNA in tumor tissues from 1069 CRC cases by qPCR and observed that in terms of epigenetics, F. nucleatum abundance was closely related to CpG island methylation phenotype, microsatellite instability and BRAF mutations in CRC.

Several studies have revealed that CRC patients with a high tissue abundance of F. nucleatum have a significantly worse prognosis than those with a low abundance of F. nucleatum. A previous clinical study suggested that high levels of F. nucleatum may be a prognostic biomarker for CRC and positively associated F. nucleatum with higher CRC-specific mortality[50]. A recent Meta-analysis revealed that high levels of F. nucleatum in tumor tissue were strongly associated with lower overall survival, disease-free survival, and cancer-specific survival in CRC patients[51]. Several studies conducted in different geographical Settings (Europe, China, and Japan) have confirmed these results[52,53]. Notably, high levels of F. nucleatum were further significantly associated with low survival in metastatic CRC[54].

In summary, the detection of F. nucleatum in tissues may be an important tool for the early detection of CRC as well as for its prognostic assessment.

Application of F. nucleatum detection in stool for the diagnosis of CRC: Recent and accumulating evidence has been reported that the ecological dysbiosis of intestinal flora is closely associated with CRC. Since bacteria account for approximately 60% of fecal dry weight, non-invasive fecal flora analysis has become widely applied in the study of intestinal flora and the diagnosis of CRC. Tunsjø et al[55] revealed markedly increased levels of F. nucleatum in fecal samples from patients with CRC in comparison to healthy controls and a polyp group, indirectly reflecting the abundance of F. nucleatum in tumor tissues. Moreover, Liang et al[56] tested the faecal samples of 203 CRC patients and 236 healthy controls by duplex qPCR and observed that F. nucleatum levels were significantly higher in CRC than in the control group, with an area under the receiver operator characteristic curve (AUC) of 0.868, 77.7% sensitivity, and 79.5% specificity. Moreover, a meta-analysis of data collected from multiple studies also revealed that the combined AUC of fecal F. nucleatum for the diagnosis of CRC was 0.8695, and the sensitivity and specificity were 81.95% and 77.95%, respectively, indicating good diagnostic value for CRC[57].

It is noteworthy that combining F. nucleatum detection with other assays has exhibited improved diagnostic performance for CRC. Currently, fecal immunochemical testing (FIT) is widely used for noninvasive screening of CRC, but it is limited by its low sensitivity for detecting advanced tumors[58]. Wong et al[59] revealed that combining FIT with F. nucleatum measurement significantly improved the detection rate of CRC with sensitivity and specificity of 92.3% and 93.0%, respectively, and of advanced adenomas with a detection rate of 38.6% and specificity of 89.0%, respectively. This combination approach was capable of detecting advanced lesions missed by FIT alone. Meanwhile, Guo et al[60] discovered that the microbial ratio of F. nucleatum to Bifidobacterium bifidum had a sensitivity of 92.3% and a specificity of 91.1% for detecting CRC, with an AUC of 0.846. Moreover, the diagnostic AUC reached 0.943 when using a combination of F. nucleatum/Bifidobacterium bifidum and F. nucleatum/Faecalibacterium prausnitzii. Overall, these results suggest that fecal quantification of F. nucleatum can contribute to the diagnosis of CRC and be used as a novel non-invasive diagnostic biomarker for CRC.

Application of F. nucleatum detection in saliva for the diagnosis and prognosis of CRC: The microbial community in the oral cavity can be anatomically linked to the microbial community in the colon through saliva. As such, fluctuations in the oral microbiome can indirectly reflect dysregulation of the intestinal microbial community. F. nucleatum is widely distributed in the oral cavity, and whole genome sequencing by Abed et al[61] identified a high level of homology between F. nucleatum in tissues and saliva from the same CRC patient, consistent with the F. nucleatum random primer multiplex PCR results of Komiya et al[62], which suggests that the F. nucleatum found in CRC originated from the oral cavity. Guven et al[63] previously demonstrated that the abundance of F. nucleatum in the saliva of CRC patients was significantly higher than that of the control group, but lacked diagnostic value. Our research group systematically analyzed F. nucleatum in the saliva of CRC patients, patients with proliferative intestinal polyps, adenoma patients and healthy controls using a multiple qPCR method, and observed that the level of F. nucleatum in the saliva of CRC patients was significantly increased. Furthermore, analysis of the receiver operator characteristic curve (ROC) revealed that F. nucleatum DNA was significantly superior to CEA, CA19-9, and their combination in the diagnosis of CRC, with an AUC of 0.841, and a sensitivity and specificity of 71.5% and 82.1%, respectively. Moreover, we identified that F. nucleatum DNA levels were negatively associated with overall survival and disease-free survival in CRC patients, serving as an independent prognostic factor[18]. Overall, salivary F. nucleatum DNA may be a noninvasive diagnostic and prognostic biomarker for CRC patients.

Application of antibody detection of F. nucleatum in serum for the diagnosis of CRC: It is often considered that some cancer-associated microorganisms could act as antigens to induce host-specific antibody production for infection diagnosis and tumor screening. As such, it has been shown that F. nucleatum induces a strong humoral response and the production of a high level of specific antibodies in CRC patients, which enables the possibility for CRC diagnosis based on serological detection of F. nucleatum antibodies[64,65]. Wang et al[64] discovered significantly higher serum levels of anti-F. nucleatum-IgA and anti-F. nucleatum-IgG in CRC patients infected with F. nucleatum compared to healthy controls and patients with benign colon disease. Remarkably, the titer of IgA was significantly higher than that of IgG, suggesting that IgA may have a more sensitive and specific diagnostic value for CRC. Additionally, they detected serum anti-F. nucleatum antibodies by ELISA and evaluated their diagnostic performance. The results revealed that anti-F. nucleatum-IgA had a sensitivity of 36.43% and a specificity of 92.71% for the diagnosis of CRC, and the diagnostic efficacy of anti-F. nucleatum-IgA was further improved by combining it with CEA (sensitivity of 53.10%, specificity of 96.41%, AUC = 0.848). The combined detection of anti-F. nucleatum-IgA with CEA and CA19-9 further improved the diagnostic efficacy for early CRC. Therefore, anti-F. nucleatum-IgA is of great value in mass screening for early CRC.

F. nucleatum and GC

Application of F. nucleatum detection in tissues for the diagnosis and prognosis of GC: Several studies have reported that the abundance of F. nucleatum in the tumor tissues of GC patients was significantly correlated with prognosis. Boehm et al[66] detected the abundance of F. nucleatum in the tumor tissues and paracancer tissues of GC patients; the positivity rate for F. nucleatum in tumor samples was 28.75%, while in normal mucosal tissue samples it was 23.08%. Further survival analysis indicated a significantly shorter overall survival time in the positive group (524.5 d) compared to the negative group (1287 d). Overall survival was significantly longer in patients with F. nucleatum negative GC (244.5 vs 1229.5 d, P = 0.009) compared with patients with F. nucleatum positive diffuse Laurentian G. Although, no significant difference in survival was observed between intestinal and mixed-type GC. An additional study revealed that compared with healthy individuals, patients with GC had a higher abundance of F. nucleatum in gastric tissue. Furthermore, a higher F. nucleatum abundance in tumor tissues was associated with significantly poorer overall survival, however there was no significant correlation between F. nucleatum abundance in paracarcinoma tissues and overall survival[67]. Moreover, Liu et al[68] identified that the disease course of patients with F. nucleatum colonization in tumor tissues was more likely to be complicated by visceral vein thrombosis and pulmonary embolism, a finding that resulted in speculation that F. nucleatum colonization may induce pro-inflammatory and pro-thrombotic states, thus affecting the prognosis of this patient population.

Application of F. nucleatum detection in saliva for the diagnosis of GC: Considering the application of F. nucleatum in the diagnosis of GC, Chen et al[69] demonstrated that salivary F. nucleatum levels were significantly elevated in GC compared to atrophic gastritis, non-atrophic gastritis, gastric polyp, and healthy controls. ROC analysis indicated that salivary F. nucleatum abundance had diagnostic efficacy, with an AUC of 0.813, a sensitivity of 73.33%, and a specificity of 82.14% under the optimal cut-off value. The diagnostic efficacy was higher than that of traditional serum tumor markers like CEA, CA19-9, CA72-4, ferritin and sialic acid.

F. nucleatum and EC

Application of F. nucleatum detection in tissues for the diagnosis and prognosis of EC: The abundance of F. nucleatum in ESCC tumor tissues has been revealed to be significantly higher than in adjacent normal tissues[40,70-72]. Research by Li et al[71] reported that in 98 tumor tissue samples, 69.4% (68/98) were positive for F. nucleatum, and the relative abundance of F. nucleatum in tumor tissues was significantly higher than that in paracancer tissues. Moreover, F. nucleatum was significantly enriched in patients with advanced ESCC compared to in those with early ESCC. Furthermore, F. nucleatum infection was significantly associated with reduced survival of ESCC patients, and the 5-year survival rate and median and inter-quartile survival time in the positive F. nucleatum infection group were significantly lower than in the negative group[72].

Studies have reported that F. nucleatum infected patients were more likely to develop chemotherapy resistance, thus further affecting the survival and prognosis of ESCC patients. As such, the sensitivity of ESCC to cisplatin progressively decreased with the extension of F. nucleatum infection time[40]. Similarly, patients with high intratumoral levels of F. nucleatum exhibited greater resistance to neoadjuvant chemotherapy treatment[72]. It was therefore suggested that F. nucleatum may be a potential target for antibiotic intervention to improve treatment response rate in ESCC patients.

F. nucleatum and PC

Application of F. nucleatum detection in tissues for the diagnosis and prognosis of PC: Hayashi et al[73] attempted to identify F. nucleatum by detecting lipopolysaccharides using immunohistochemistry and observed that lipopolysaccharides could be detected in tumor tissues of patients with PC, whereas it was not detected in normal pancreatic tissues. Moreover, it has been suggested that F. nucleatum may be associated with poor prognosis in patients with PC. In a univariate Cox regression analysis, mortality was reported to be significantly higher and survival significantly shorter in Fusobacterium positive PC cases compared with Fusobacterium negative cases[74]. Hayashi et al[73] observed that tumors colonized by F. nucleatum were significantly larger than tumors absent of F. nucleatum colonization and subsequently were more likely to have retroperitoneal infiltration and multiple lymph node metastases. Furthermore, patients with F. nucleatum detected in their tumors had shorter disease-free and overall survival, suggesting that F. nucleatum may promote the growth and metastasis of pancreatic tumors, leading to poor prognosis.

Application of antibody detection of F. nucleatum in serum for the diagnosis of PC: Notably, Alkharaan et al[75] attempted to detect F. nucleatum using serum and saliva antibodies in PC patients. The results suggested that patients with intraductal papillary mucinous neoplasms (IPMN) with highly atypical hyperplasia or progression to invasive cancer had adequate antibody reactivity to oral microorganisms, and that salivary antibody reactivity to F. nucleatum and Fap2 was highly correlated. It was therefore suggested that the immunological detection of pancreatic-associated oral microorganisms may reflect the severity of IPMN, with the potential to contribute to the discovery of novel biomarkers.

CONCLUSION

In this editorial, we discuss the mechanisms of action of F. nucleatum in the development of four GI malignancies and the significance of F. nucleatum as a biomarker in the diagnosis, prognosis, and risk assessment of GI malignancies. Overall, the current literature strongly supports that F. nucleatum is involved in the development of GI malignancies primarily by releasing virulence factors, inducing DNA damage, modulating ncRNA expression, and remodeling the tumor immune microenvironment. An in-depth study of the oncogenic mechanisms of F. nucleatum may provide new strategies for the targeted treatment and prevention of GI malignancies. Importantly, the detection of F. nucleatum in feces, saliva, or serum can provide valuable new avenues for more efficacious early diagnosis and treatment of GI malignancies, greatly improving the prognosis of patients. Overall, F. nucleatum showed good predictive ability in areas such as diagnosis and prognosis determination, and is a promising diagnostic marker for GI malignancies. However, its detection efficiency is unstable, which severely limits its clinical application, and further studies are needed to improve the detection performance of this technique. In addition, whether F. nucleatum is independently involved in the development of GI malignancies needs further investigation. Therefore, further investigation is required to determine whether F. nucleatum alone can cause cancer and there is a necessity for the development of accessible methods to reliably detect F. nucleatum.