The role of microbiota in esophageal squamous cell carcinoma: A review of the literature

Abstract

Esophageal squamous cell carcinoma (ESCC) exhibits high incidence with poor prognosis. Alcohol drinking, cigarette smoking, and betel nut chewing are well‐known risk factors. Dysbiosis, an imbalance of the microbiota residing in a local environment, is known to be associated with human diseases, especially cancer. This article reviews the current evidence of esophageal microbiota in ESCC carcinogenesis, including initiation, progression, and drug resistance. Articles involving the esophageal microbiota, diagnosis, treatment, and the progression of esophageal cancer were acquired using a comprehensive literature search in PubMed in recent 10 years. Based on 16S rRNA sequencing of human samples, cell, and animal studies, current evidence suggests dysbiosis of the esophagus promotes ESCC progression and chemotherapy resistance, leading to a poor prognosis. Smoking and drinking are associated with esophageal dysbiosis. Specific bacteria have been reported to promote carcinogenesis, involving either progression or drug resistance in ESCC, for example Porphyromonas gingivalis and Fusobacterium nucleatum. These bacteria promote ESCC cell proliferation and migration via the TLR4/NF‐κB and IL‐6/STAT3 pathways. F. nucleatum induces cisplatin resistance via the enrichment of immunosuppressive myeloid‐derived suppressor cells (MDSCs). Correcting the dysbiosis and reducing the abundance of specific esophageal pathogens may help in suppressing cancer progression. In conclusion, esophageal dysbiosis is associated with ESCC progression and chemoresistance. Screening the oral and esophageal microbiota is a potential diagnostic tool for predicting ESCC development or drug‐resistance. Repairing esophageal dysbiosis is a novel treatment for ESCC. Clinical trials with probiotics in addition to current chemotherapy are warranted to study the therapeutic effects.

INTRODUCTION

Esophageal cancer is one of the most common cancers worldwide with a poor prognosis. ¹ The annual incidence of newly diagnosed esophageal cancer is approximately 450 000 cases per year. ² The 5‐year survival rate of esophageal cancer is poor at 15%–25%. ³ Esophageal squamous cell carcinoma (ESCC) accounts for 88% of cases of which 12% are esophageal adenocarcinoma. ² ESCC can initiate in any esophageal region, while esophageal adenocarcinoma typically develops from Barrett's esophagus at the distal esophagus. ⁴ Esophageal adenocarcinoma has a higher prevalence in the West, while ESCC is predominant in the East. ¹

ESCC tumorigenesis typically results from the direct contact of a carcinogen with the esophageal mucosa. Risk factors for ESCC include chemical injury (swallowing strong acid or base), tobacco smoking, alcohol drinking, and betel nut chewing. ¹ Carcinogens irritate and inflame the esophageal mucosa interfering with the natural protective barriers and defense mechanisms. Chronic exposure with carcinogens to the esophageal mucosa increases the risk of ESCC. ⁵

ESCC develops in multiple steps from normal squamous epithelium, to low‐grade dysplasia, then high‐grade dysplasia, and finally to invasive squamous cell carcinoma. ⁵ Exposure of normal esophageal mucosa to carcinogens can induce chronic inflammation which in turn promotes ESCC development. ⁵ The esophageal microbiota is an assemblage of commensal living microorganisms in the esophagus, which present with the longest contact time to the esophageal mucosa. Normally, commensal esophageal microbiota live in the esophageal mucosa in harmony. While some bacteria have a protective effect, other bacteria can be harmful to the esophageal mucosa. An increase in harmful bacteria can tip the balance towards a disease state. The relationship between commensal microbiota and the esophageal mucosal immunity can be interfered with in multiple ways, for example, food, chemical exposure, alcohol, infection, or medications. ⁶ When the microbiota in the environment become imbalanced, also known as dysbiosis, it leads to immune activation and chronic inflammation of esophageal mucosa. ⁷

The literature shows chronic inflammation of esophageal mucosa increases cytokines, chemokines, and reactive oxygen species (ROS) in the microenvironment, which are important for the initiation and progression of cancer. ⁶ From this point of view, a change in esophageal microbiota plays a role in the development of ESCC. Specific pathogens in the esophagus are reported to be associated with the initiation, progression, and prognosis of ESCC. ⁶ This review will provide the current evidences from human, animal and cell studies that certain bacterial species are involved in ESCC development and progression.

Tumor biomarkers can provide information for diagnosis and help predict treatment response and prognosis of ESCC. ⁸ , ⁹ Currently, serum squamous cell carcinoma antigen (SCC‐Ag) and cytokeratin 19 fragments (CYFRA 21‐1) are widely used conventional ESCC tumor markers in evaluating treatment response and predicting cancer recurrence. ⁹ Circulating cell‐free‐DNA and mRNA‐based biomarkers are potential markers to detect early ESCC. ¹⁰ Due to the poor prognosis of late‐stage ESCC, detection of early ESCC can improve a patient's outcome. The microbiota is a potential biomarker for detecting early cancer and predicting cancer patient prognosis. ¹¹ , ¹² The specific microbiota composition or the presence of specific bacteria that can predict early ESCC is developing.

Despite recent advances in immunotherapy in addition to traditional chemotherapy for late‐stage ESCC, the median overall survival of advanced ESCC remains poor at 1 year. ¹³ In order to improve the ESCC outcome, medical professionals are trying to enhance treatment response and reduce treatment resistance. Since the microbiota plays an important role in the development and progression of cancer, changing the esophageal microbiota from a pathological to a healthy type may benefit ESCC treatment.

MICROBIOTA ANALYSIS

The microbiota is a community of microorganisms in a tissue, including bacteria, viruses, fungi, and other microorganisms acquired from the environment. The number of bacterial microorganisms in the human body is 10 times more than the number of human cells. ¹⁴ Microorganisms in the gastrointestinal tract can interact with nearby organs and release functional metabolites (e.g., bile acids, short‐chain fatty acids, and branched‐chain amino acids), helping digestion and the construction of adaptive immunity via the regulation of T cells, B cells, dendritic cells, and macrophages. ¹⁵ , ¹⁶ , ¹⁷ As an example, short‐chain fatty acids can inhibit histone deacetylases (HDACs) and thereby modify the number and function of regulatory T cells in vivo. ¹⁸ An animal study also demonstrated inhibition of HDAC can increase regulatory T cell numbers. ¹⁹ With multiple physiological functions, the gastrointestinal microbiota acts similar to a human organ, modulating the immune system. ²⁰ Figure 1 exhibits the microbiota composition and distribution for different regions of the human digestive tract. When the microbiota composition changes or the diversity reduces, dysbiosis can occur resulting in aberrant activation of the immune system. ²⁰

FIGURE 1.

Microbiota regional specific distribution in the human digestive tract. Each section of the digestive tract contains specific ratios of bacterial species. These ratios can change and result in a disease state or these ratios can be perturbed by a disease.

Traditionally, the analysis of bacterial types of the gastrointestinal tract relies on culture‐based methods. However, fragile bacteria and anaerobic bacteria are difficult to grow, thus previous knowledge of the gastrointestinal microbiota is incomplete. Analyzing the bacterial 16S ribosomal RNA (rRNA) gene is a novel approach to discovering the bacteria species in a microbiota community. ²¹ The rRNA gene in bacteria consists of nine variable regions (V1–V9), which are specific for individual bacterial species. With 16S rRNA sequencing, bacteria species in a specific microbiota community can be identified and quantized, including dead microorganisms.

INTERACTION BETWEEN THE MICROBIOTA AND HUMAN DISEASE

The gastrointestinal microbiota is implanted at birth and during breastfeeding, ²² and becomes relatively stable in late childhood, and through adolescence to adulthood. ²³ A healthy gastrointestinal microbiota can produce short‐chain fatty acids, protect gut barrier, synthesize specific lipopolysaccharides, essential amino acids and vitamins. ²⁴

However, environmental exposure to food or stress and a decrease in immune function reduce microbiota diversity at an advanced age. ²² The decline of microbiota diversity is reported to be associated with multiple human metabolic disorders, for example an increase in adiposity, insulin resistance, and chronic inflammation. ²⁵ In this state, the prevalence of obesity, type 2 diabetes mellitus, and metabolic liver disease increases. ²⁴

The association between microbiota and cancer is another important issue. The relationship between Helicobacter pylori and gastric cancer development is well‐known. ²⁶ When H. pylori invade the gastric wall, virulence factors are released leading to increased reactive oxygen species, activation of angiogenesis, and epithelial‐mesenchymal transition. ²⁶ Through toxin release and subsequent inflammation, H. pylori are linked to the development of chronically active gastritis and atrophic gastritis. ²⁷ Atrophic gastritis reduces gastric acidity and microbiota diversity, and increases the colonization of pathogenic bacteria. ²⁸ , ²⁹ Further interaction between pathogenic bacteria and gastric mucosa leads to chronic gastric inflammation and gastric cancer development. In recent years, increasing evidence has linked specific bacteria to other types of cancers, for example Streptococcus bovis and colon cancer, Porphyromonas gingivalis and oral cancer, and so on. ³⁰

DYSBIOSIS IN ESOPHAGEAL SQUAMOUS CELL CARCINOMA

Similar to other regions in the gastrointestinal tract, the esophageal microenvironment is also composed of epithelial cells, immune cells, and microbiota. ³¹ Normally, the number of bacteria in the esophagus is approximately 10⁴ bacteria per gram of esophageal content, which is less than the ileum and colon (10^12–13 bacteria per gram) of the gastrointestinal tract. ³² Streptococcus viridans was the most commonly found microorganism in esophageal cultures in previous studies. ³³ However, when 16S rRNA gene sequencing was introduced, more bacteria were found in the normal esophagus. Via the 16S rRNA sequencing, the most common normal esophageal microbial flora was Streptococcus, Prevotella, and Veillonella, adding new information to earlier reports. ³⁴ , ³⁵

Normally, the commensal microbiota lives in the esophagus with the host cell harmoniously and helps inhibit the growth of pathogenic bacteria. Disruption of this relationship, termed dysbiosis, causes the concomitant decrease of Streptococcus with an increase in gram‐negative bacteria (GNB). ³³ The lipopolysaccharide (LPS) of GNB pathogens interacts with the TLR4 receptor and activates NF‐κB. ³¹ The increased NF‐κB activation causes epithelial barrier interruption and inflammation, inducing DNA damage and pro‐oncogenic signals, for example mutations in p53 and or Myc, leading to carcinogenesis in the esophagus. ³¹ , ³⁶ , ³⁷ , ³⁸

Several case–control studies (Table 1) have revealed a different population of esophageal microbiota in ESCC patients compared to healthy individuals by quantitative polymerase chain reaction (qPCR) ³⁹ or 16S rRNA sequencing. ⁴⁰ , ⁴¹ , ⁴² , ⁴³ , ⁴⁴ , ⁴⁵ , ⁴⁶ Reduced diversity of esophageal microbiota has previously been observed in a patient with ESCC. ⁴³ The conclusion is a reduction of normal flora with an increase in pathogens contributes to chronic inflammation and a subsequent tumor‐promoting microenvironment. Recent studies found a reduction of Streptococcus species and an increase of Porphyromonas gingivalis and Fusobacterium nucleatum in ESCC. ³³ , ⁴⁷ Emerging evidence also showed these specific pathogens were associated with the progression and a poorer prognosis of ESCC. ⁴⁸ , ⁴⁹ , ⁵⁰

TABLE 1.

Case‐control studies investigating the compositional change of esophageal microbiota in esophageal squamous cell carcinoma (ESCC) patients.

Author (year)	Country	Cases (N)	Controls (N)	Sample type	Method	Increased abundance	Decreased abundance
Shao et al. 44	China	66 ESCC	67 non‐ESCC site	Tumor biopsy	16S rRNA	Fusobacterium	Streptococcus
Li et al. 42	China	17 ESCC	16 healthy controls	Tumor biopsy	16S rRNA	Lactobacillus, Prevotella, Fusobacterium	Actinobacteria
Yang et al. 43	China	18 ESCC	11 healthy controls	Tumor biopsy	16S rRNA	N/A	Fusobacteria, Bacteroidetes, Spirochaetes
Kawasaki et al. 39	Japan	61 ESCC	62 healthy controls	Oral swab	qPCR	Tannerella forsythia, Streptococcus anginosus, Aggregatibacter actinomycetemcomitans	N/A
Jiang et al. 41	China	32 ESCC	21 healthy controls 15 esophagitis	Tumor biopsy	16S rRNA	Streptococcus, Actinobacillus, Peptostreptococcus, Fusobacterium, Prevotella	Bacteroidetes, Faecalibacterium, Bacteroides, and Blautia
Li et al. 45	China	32 ESCC	35 healthy controls	Saliva	16S rRNA	Streptococcus, Prevotella	Neisseria
Chen et al. 46	China	90 ESCC	50 healthy controls	Saliva	16S rRNA	Leptotrichia, Fusobacterium nucleatum, Porphyromonas gingivalis, Streptococcus salivarius, Rothia, Lactobacillus, and Peptostreptococcus	Haemophilus, Alloprevotella, Prevotella_7, Pasteurellaceae and Pasteurellales
Gao et al. 47	China	100 ESCC	100 non‐ESCC site	Tumor biopsy	qPCR	Porphyromonas gingivalis	N/A

Note: The bold words denote bacterial species related to ESCC development.

Porphyromonas gingivalis and ESCC

P. gingivalis is a Gram‐negative bacterium, a known pathogen causing periodontal diseases. ⁵¹ P. gingivalis can promote immortalized oral epithelial cell proliferation, migration, and invasion by activating ERK1/2‐Ets1, and proteinase‐activated receptor 2 (PAR2)/NFκB pathways. ⁵² Based on the close anatomy between the oral cavity and esophagus, P. gingivalis in the esophagus also causes chronic inflammation of the normal esophageal mucosa and may promote tumor progression and chemotherapy resistance. ⁴⁸ , ⁵³ , ⁵⁴

In a cell‐line study, Meng et al. demonstrated that P. gingivalis promoted proliferation and motility of ESCC cells by activating the NF‐κB signaling pathway. ⁵⁵ In an animal study, Chen et al. showed P. gingivalis infection was associated with advanced stages and a poor prognosis in a carcinogen‐induced mice esophageal cancer model through the IL‐6/STAT3 pathway. ⁴⁸ This bacterium also promoted chemotherapy resistance in a xenograft mice model. ⁵⁴ In ESCC patients, there was an increased abundance of salivary P. gingivalis compared to healthy controls, ⁴⁶ and an increased abundance of tumor site P. gingivalis compared to adjacent nontumor site. ⁴⁷ Furthermore, the presence of P. gingivalis infection was associated with ESCC severity and poor prognosis. ⁴⁷ , ⁵⁴ These data suggest P. gingivalis infection is associated with ESCC progression, chemotherapy resistance and poor prognosis via the NF‐κB and IL‐6/STAT3 pathway.

Fusobacterium nucleatum and ESCC

Fusobacterium nucleatum, another Gram‐negative bacterium, is a common bacterial infection in the oral cavity. Similar to P. gingivalis, F. nucleatum is a pathogen responsible for inflammatory periodontal diseases. ⁵⁶ In addition, F. nucleatum was reported to be associated with colonic inflammation and progression of colorectal cancer via TLR4, IL‐6, TNF‐α, and NF‐κB pathways. ⁵⁷ , ⁵⁸ In a cell‐line study, Nomoto et al. demonstrated that F. nucleatum promotes ESCC cell growth and migration by activating the NOD1/RIPK2/NF‐κB pathway. ⁵⁹ Liang et al. revealed F. nucleatum induces immunosuppressive myeloid‐derived suppressor cell (MDSC) enrichment via activation of the NOD‐like receptor protein 3 (NLRP3) inflammasome. ⁶⁰ , ⁶¹ , ⁶² F. nucleatum can also induce chemoresistance in ESCC cells by modulating autophagy. ⁶³ In female NOD SCID gamma (NSG) mice inoculated with ESCC tissue, F. nucleatum infection enriched MDSCs and promoted cisplatin‐resistance. ⁶⁰ In ESCC patients, there was an increased abundance of salivary F. nucleatum compared to healthy controls. ⁴⁶ Li et al. found an abundance of F. nucleatum at the tumor site, and this was related to a more advanced pathological T stage and clinical stage. ⁵⁰ Higher intratumoral F. nucleatum levels were related to lower chemotherapy response ⁶⁰ , ⁶³ , ⁶⁴ and shortened survival in ESCC patients. ⁶⁰

Based on the above literature, the abundance of P. gingivalis or F. nucleatum in the oral cavity and esophagus may promote ESCC invasion, metastasis, and chemotherapy resistance, and decreases the overall survival in ESCC patients.

DIET EFFECT ON THE ESOPHAGEAL MICROBIOTA

Diet has a great impact on the development and shaping of commensal microbiota during human growth. ²² , ²³ In adults, the type of ingested food also affects the composition of intestinal microbiota. For example, there is a great difference in diet habits between the West and East. The Western diet contains excessive amounts of processed foods, red meats, dairy, and sugary beverages, while the Eastern diet contains more fried foods, pork, fiber, fruits, and vegetables. Western diet consumption is associated with systemic low‐grade inflammation and increased incidence of metabolic diseases. ⁶⁵ Dietary changes to the Western style can cause gut inflammation and disrupt the commensal microbial community. ⁶⁶ , ⁶⁷ In an animal study, Kaakoush et al. found that mice fed with a regular or high‐fat diet developed different esophageal microbiota. ⁶⁸ Increased esophageal Fusobacterium, Rothia, and Granulicatella were observed in mice fed with a high‐fat diet. ⁶⁸ In a human study, Nobel et al. demonstrated that increasing fiber intake could change esophageal microbiota. ⁶⁹ An increased fiber intake was associated with increased Firmicutes, while a low fiber intake was associated with increased Gram‐negative bacteria, such as Prevotella, Neisseria, and Eikenella. ⁶⁶ While a healthy diet brings a healthy esophageal microbiota, an unhealthy dietary habit could lead to unhealthy esophageal microbiota. Since alcohol and betel nuts are risk factors for ESCC, these ingested substances could also affect the composition of esophageal microbiota. ⁷⁰ , ⁷¹ Alcohol is metabolized in the liver from alcohol into acetaldehyde and causes toxic damage to gut microbiota, while betel nuts release harmful chemicals to bacteria directly. ⁷⁰ , ⁷¹

Oral hygiene and esophageal disease

The oral cavity is juxtaposed to the esophageal tract, where both mucosae are composed of squamous cells. However, these two regions contain quite different microorganisms. While the number of esophageal bacteria is approximately 10 ⁴ /g, the oral cavity contains about 10⁹ bacteria/g. ³² , ⁷² The junction between teeth and gingiva provides a suitable microenvironment for bacteria to thrive. Studies have shown the connection between poor oral hygiene and oral diseases, including tooth decay, periodontal disease, and oral cancer. ⁷³ Therefore, good oral hygiene is important for oral health. Saliva is sterile during secretion. When it is secreted into the oral cavity, the microbiota in the oral cavity is brought into the saliva. Most saliva is swallowed into the stomach bringing oral bacteria to the esophagus. Studies showed poor oral hygiene was associated with a higher risk of esophageal disease, including reflux esophagitis, Barrett's esophagus, and esophageal cancer. ⁷⁴ Chen et al. found reduced tooth brushing per day and increasing numbers of lost teeth enhanced the risk of ESCC. ⁷⁵ Thus, good oral hygiene is important for both oral and esophageal health.

Interaction between tobacco/alcohol and the esophageal microbiota

Tobacco smoking and alcohol drinking are important risk factors for ESCC initiation and development. ² , ⁷⁶ Tobacco contains more than 60 carcinogens, while the alcohol metabolite acetaldehyde promotes carcinogenesis. ⁷⁷ Interestingly, not all people who drink or smoke develop ESCC, suggesting other factors play a role in ESCC development. The esophageal microbiota could be one of the factors. In addition, previous studies found combined use of alcohol and tobacco increases the ESCC risk more than single‐use. ⁷⁷ It is possible that the carcinogenic esophageal microbiota further raises ESCC risk in addition to drinking and smoking.

Since environmental and food exposure can influence the composition of microbiota, ²² , ³⁴ it is believed that drinking and smoking can also alter the oral and esophageal microbiota. Li et al. demonstrated a different composition of esophageal and saliva microbiota between drinking/smoking and nondrinking/nonsmoking people. ⁷⁸ The saliva of people who smoke and drink has a higher prevalence of Neisseria, Prevotella, Porphyromonas, and Fusobacterium. ⁷⁸ Porphyromonas and Fusobacterium have been shown to be associated with ESCC progression. ⁴⁶ , ⁴⁷ , ⁴⁸ , ⁵⁰ , ⁵¹ , ⁵² , ⁵³ , ⁵⁴ , ⁵⁵ Rao et al. also found a significantly different microbiota composition in both saliva and esophageal biopsy between drinking and nondrinking ESCC patients. ⁷⁹ Thus, the altered esophageal microbiota in drinkers or/and smokers may be involved in ESCC development or progression. On the other hand, whether some protective bacteria can reduce the harm of alcohol and smoking is worthy of further investigation. Studies investigating the interaction between bacteria and alcohol/smoking in ESCC risk are warranted.

MICROBIOTA AS AN ESCC BIOMARKER

Tumor biomarkers can help predict the risk, treatment response, and prognosis of ESCC. ⁸ , ⁹ Recent studies implied an increase in abundance of oral and esophageal Porphyromonas gingivalis and Fusobacterium nucleatum are associated with higher ESCC risk. ³³ , ⁴⁷ Based on these results, examining the abundance of these specific pathogens from the oral cavity could be a potential convenient biomarker for ESCC risk stratification. Peters et al. found the abundance of the periodontal Porphyromonas gingivalis trended with a higher ESCC risk. ⁸⁰ Chen et al. demonstrated a higher ratio of Porphyromonas gingivalis/Prevotella and Porphyromonas gingivalis/all bacteria in the oral cavity associated with the prediction of early ESCC. ⁴⁶ Furthermore, higher intratumoral Fusobacterium nucleatum levels were found to be related to poor chemotherapy response of ESCC to cisplatin, docetaxel, and 5‐fluorouracil. ⁶³ , ⁶⁴

However, most microbiota studies were limited to studying the association of the pathogens with the presence of ESCC. The data regarding the association between microbiota and treatment response or prognosis are insufficient. In addition, no prospective study using the oral microbiota as a predictive marker of ESCC development has been conducted yet. The predictive value of oral microbiota in clinical use is limited at this current stage and requires further investigation.

MICROBIOTA IN THE TREATMENT OF ESCC

Both Porphyromonas gingivalis and Fusobacterium nucleatum are associated with increased ESCC cancer stage and chemoresistance. Based on current evidence, adding an antibacterial treatment to standard chemotherapy is worth investigating. In colorectal cancer, the interaction of probiotics with toll‐like receptors led to the inhibition of NF‐κB signaling. ⁸¹ In this way, probiotic administration can reduce colorectal tumor incidence, modulate the immune system by promoting the production of anti‐inflammatory factors and antioxidant enzymes, and improve quality of life. ⁸¹ , ⁸² However, using antibiotics or probiotics to decrease the amount of esophageal pathogens to prevent ESCC development/progression, increase chemotherapy sensitivity, or improve patient outcomes remains uncertain at this time. Animal studies and clinical trials are warranted for evidence. Unfortunately, the bacterial species in prophylactic probiotics varies in each of the studies. Discovering which bacteria exhibit a tumor‐suppressing effect is vital in probiotic therapy.

FUTURE PERSPECTIVE AND CONCLUSION

Dysbiosis in the esophagus promotes ESCC progression and treatment resistance and subsequently leads to poor prognosis. P. gingivalis and F. nucleatum are pathogenic for esophageal carcinogenesis through the TLR4/NF‐κB and IL‐6/STAT3 pathways (Figure 2). Several studies have pointed out that esophageal P. gingivalis and F. nucleatum can promote ESCC progression from the cell, animal to human studies. As shown in Table 1, there are many other bacteria with increased amounts in the microenvironment of ESCC. Although most studies discovered increased oral or esophageal Fusobacterium in ESCC patients, ⁴¹ , ⁴² , ⁴⁴ one study found decreased Fusobacterium in ESCC. ⁴³ The inconsistent results can be attributed to the inadequate depth of sequencing by 16S rRNA analysis to only genus level rather than species level. Therefore, to clarify the role of specific bacterial species, such as F. nucleatum and P. gingivalis, the sequencing depth should be expanded to the species level in future studies. Basic and human studies investigating the association of those specific pathogens with ESCC development, progression, and chemoresistance are needed. In contrast, there are some bacteria with decreased amounts in ESCC. Whether or not those bacteria have a protective effect from ESCC progression is worth investigating. The interaction between two or more types of bacteria and the effect on ESCC is still an unknown area. What is more, current studies have only focused on the bacterial effect on ESCC. However, virus and fungus also composed of the esophageal microbiota, and their effect on esophageal diseases have not yet been investigated. Diets and carcinogenic food also play roles in both changing the esophageal microbiota and promoting ESCC. Discovering the whole esophageal microbiota map can develop novel diagnostic and therapeutic methods.

FIGURE 2.

Esophageal dysbiosis promotes esophageal squamous cell carcinoma (ESCC) progression. (a) Esophageal dysbiosis increases gram‐negative bacteria (GNB) such as Porphyromonas gingivalis and Fusobacterium nucleatum. Their lipopolysaccharides (LPS) activate reactive oxygen species (ROS) and NF‐κB through interaction with TLR‐4. The activated NF‐κB triggers TNF‐α and IL‐6 gene expression and cytokine release; while ROS increases inflammasome, resulting in a protumor local environment. (b) Locally increased Porphyromonas gingivalis can accumulate IL‐6 to activate JAK2 and subsequent STAT3 to induce cell proliferation and migration. (c) Locally increased Fusobacterium nucleatum can infect esophageal cell and increase intracellular muramyl dipeptide (MDP) to activate NOD‐RIPK2. The activated NOD‐RIPK2 subsequently activates NF‐κB to induce DNA damage and gene mutation. Such microenvironment may promote tumor proliferation, metastasis, and chemoresistance, leading to a poor prognosis.

Screening the oral microbiota is a potential diagnostic tool for ESCC; however, accurate cutoff values and prospective studies are needed. Moreover, there is a lack of standard operating procedures for sampling the microbiota from oral cavity and esophagus. Different positions of the oral cavity and esophagus can yield different proportions of bacteria, even in the same person. The sample size, endoscope sterilization, and patient preparation before an endoscopy are factors that can affect the microbiota results. A standard operating procedure for esophageal microbiota tests should be formulated.

Studies have shown that administration of probiotics can reduce colorectal tumor incidence and improve quality of life. Repairing the dysbiosis and reducing the abundance of specific esophageal pathogens may help in suppressing ESCC progression and reducing chemotherapy resistance, but limited human studies have been conducted. Clinical trials testing the effect of probiotics or antibiotics in improving ESCC outcomes are urgently required.

AUTHOR CONTRIBUTIONS

Hsueh‐Chien Chiang: study concept and design; acquisition of data; analysis and interpretation of data; drafting of the manuscript. Michael Hughes: methodology, review, editing, and revision of the manuscript. Wei‐Lun Chang: study concept and design; technical and material support; study supervision, revision of the manuscript.

CONFLICT OF INTEREST STATEMENT

To the best of our knowledge, the named authors have no conflict of interest, financial or otherwise.

Chiang H‐C, Hughes M, Chang W‐L. The role of microbiota in esophageal squamous cell carcinoma: A review of the literature. Thorac Cancer. 2023;14(28):2821–2829. 10.1111/1759-7714.15096

DATA AVAILABILITY STATEMENT

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.