The human body including the oral cavity is home to 100s of different bacteria, and these bacteria have intra- and inter-species interactions. According to a recent study, bacteria have various effects on living bodies. Just as H. pylori affects gastric cancer, other bacteria affect other cancers. The involvement of bacteria has also been studied in oral cancer. Bacteria are considered to have three ways to involved in cancers. The first is stimulation by chronic inflammation. The sustained release of inflammatory mediators due to chronic inflammation causes cell proliferation, mutagenesis, activation of oncogenes, etc, thereby inducing cancer cell proliferation. The second is inhibition of apoptosis. Inhibition of apoptosis allows the long-term survival of cells. The third is the direct involvement in cancer through the production of carcinogenic substances. The outlines of this review are how Fusobacterium nucleatum, Porphyromonas gingivalis, Candida, and viruses affect oral cancer.

In recent years, the relationship between various cancers and the gut microbiome has been reported. Like the intestine, there are many bacteria in the esophageal mucosa, forming a diverse esophageal flora. The esophagus is affected by various factors, such as the passage of food and drink from the oral cavity and gastric acid reflux from the stomach, and it is thought that these factors also affect the composition of the esophageal flora. This article reviews the esophageal flora in the normal esophagus, and the composition and changes of esophageal flora in esophageal diseases (reflux esophagitis, Barrett’s esophagus) and esophageal cancer.