Esophageal cancer is one of the most common gastrointestinal malignant diseases worldwide with an estimated 456,000 incident cases annually. Esophageal cancer has a poor prognosis and high mortality rate. The 5-year survival rate is around 15%-25%. The treatment options of esophageal cancer can be divided into curative treatment and palliative treatment, while endoscopy, surgery and chemoradiotherapy were involved. However, 60-70% patients diagnosed with esophageal cancer are not eligible for curative treatment. In these patients, chemoradiotherapy is the standard for unresectable esophageal cancer, but the treatment outcome remains poor. In the literatures, the complete response rate of chemoradiotherapy in advanced esophageal cancer was about 20%, and the 2-year overall survival rate was 40%. Therefore, early detection and prediction of esophageal cancer are needed. Besides, the inconsistency of treatment effect of chemoradiotherapy may indicate some differences of esophageal cancer microenvironment among the patients. Finding out the affecting factors of microenvironment may help the decision making of treatment options and the prediction of disease prognosis. Furthermore, if we can change the affecting factors in microenvironment, we may be able to prevent the esophageal cancer formation or progression.

Esophageal tumor initiation is associated with environmental exposures, chronic inflammation, and immune cells. Several genetic and environmental factors play key roles in the formation and progression of esophageal cancer. Refluxed gastric and bile acids induce chronic inflammation and the development of intestinal metaplasia (Barrett's esophagus), which is the precursor lesion to esophageal adenocarcinoma. Toxic agent like tobacco and alcohol can cause direct esophageal injury and production of reactive oxygen species (ROS). ROS production causes direct DNA damage and tumor-initiating mutations. Besides, some literatures had reported the possible correlation with microbiota and cancer formation. Commensal bacteria (the microbiota) normally live in the gastrointestinal tract with host cell. Disruption of the relationship (dysbiosis) can influence the metabolism, tissue development, and immune response, which may cause damage to epithelial barriers, inflammation, and inducing DNA and pro-oncogenic signaling, leading to carcinogenesis in the gastrointestinal tract. The role of microbiota in the esophagus has not been widely investigated. Increasing of gram-negative bacteria increases the production of lipopolysaccharide (LPS), leading to inflammation and increased gastric reflux. The gut microbiota is associated with nutrition, the immune system, and defense of the host. It produces short chain fatty acids via anaerobic fermentation of dietary fibers in the intestine. Compared with healthy individuals, the abundance of short chain fatty acids -producing bacteria decreased and the abundance of lipopolysaccharide (LPS) -producing bacteria increased in esophageal cancer patients. Butyrate, one of the short chain fatty acids, decreases LPS-induced cytokine expression and NF-κB activation in lamina propria mononuclear cells. Esophageal microbiota theoretically plays a role in esophageal carcinogenesis.

Esophageal cancer is composed of esophageal adenocarcinoma and esophageal squamous cell carcinoma (ESCC). In esophageal adenocarcinoma, a decrease of Firmicutes, and an increase of Proteobacteria, Lactobacillus fermentum, and Tannerella forsythia have been reported. In esophageal squamous cell carcinoma, a reduction of Streptococcus species and an increase of Fusobacterium nucleatum and Porphyromonas gingivalis were observed. In Taiwan, patients with primary oropharyngeal cancer had ten times the risk of second esophageal cancer compared to the general population, and vice versa. Some specific bacteria may be associated with the co-existence of oropharyngeal cancer and esophageal cancer. However, diet is one of the most potent factors in determining microbiome integrity. Owing to the dietary difference between easterners and westerners, the dominant microbiota affecting esophageal cancer may be different. Finding out the esophageal cancer-associated specific bacteria of microbiota in Taiwan is important for further research and application for our patients.

Previously, some microorganisms could not be cultured, which would make the microbiota detection incomplete. Nowadays, 16S ribosomal RNA (16S rRNA) sequences had replaced the culture methods in detection of microbiota. In our study, we aim to compare the microbiota among healthy individuals, patients with esophageal cancer, oropharyngeal cancer, and concurrent esophageal cancer with oropharyngeal cancer in Taiwan. Through the comparison, we may find the potential risky microbiota for cancer formation or progression in Taiwan.