Microorganism is an important part of human body, although the study of oral microbiome is still in its infancy. However, as the connecting part of the internal and external environment of human body, the microbial community is not only related to oral health, but also affects the health of the whole digestive system, especially the gastrointestinal tract.
Oral microorganism is a collection of microorganisms that are planted in human oral cavity. Its internal environment provides a good atmosphere for the settlement, growth and reproduction of more than 700 kinds of microorganisms . In the normal physiological state, the microorganism in the mouth is beneficial, and it can be used as a biological barrier to inhibit the exogenous microorganism and maintain the ecological balance of the mouth or the whole body.
When the balance between oral microorganism and host is broken, the change of microbiome will change from physiological to pathological. When the ecological relationship between oral microorganism and host is unbalanced, it will cause many kinds of oral infectious diseases, such as caries, periodontitis, halitosis and so on.
Periodontal disease is a typical oral infectious disease caused by the imbalance between the microbial community and the host. It is generally believed that plaque bacteria is the initiating factor of periodontal disease, and subgingival gram-negative anaerobes are the important pathogens of periodontal disease. Bacterial toxins and metabolites can directly destroy periodontal tissue, and the local immune and inflammatory reactions of periodontal tissue can also indirectly cause tissue damage, but the root cause is the imbalance between the host and oral microorganisms .
At present, it is considered that caries is the result of the joint action of a variety of microorganisms. There are 34 different microorganisms in the caries plaque, Streptococcus mutans and s. sorbrinus play a key role in the occurrence of caries, and with the development of caries, the composition and structure of the flora will also change significantly [3,4].
Nearly 90% of people are bothered by halitosis. It is found that the real cause of halitosis is the excessive proliferation of odor producing bacteria in the mouth. The number of anaerobic bacteria and microaerophilic bacteria in patients with halitosis was significantly higher than that in normal people .
Research progress of oral and intestinal microorganisms
Oral cavity and gut are the beginning and end of human digestive tract. A large number of oral microorganisms will enter the digestive tract through saliva. The relationship and pathogenicity between them have become the focus of human microbiome research in recent years.
Periodontal disease is a risk factor for many systemic diseases. The colonization of Porphyromonas gingivalis, the main pathogen, in the oral cavity, can lead to the disorder of intestinal microbial community structure, the increase of Bacteroides, the decrease of pachytenella, the increase of serum endotoxin level, and the occurrence of intestinal inflammation .
Fusobacterium nucleatum is another important bacteria related to periodontal disease. Under normal circumstances, it can hardly be detected in the intestine, but the bacteria can be colonized in the intestine and play an important role in colorectal cancer and inflammatory bowel disease . Recent studies have confirmed that Fusobacterium nucleatum can inhibit T-cell-mediated immune response, promote the occurrence of colorectal cancer, and is closely related to the prognosis of colorectal cancer.
Koren et al.  found that several common bacterial species in atherosclerotic plaque were detected in the same individual oral or intestinal samples, indicating that the bacteria in the plaque were not only from the oral cavity, but also from the intestinal tract. Some bacteria in the oral and intestinal tract are also related to the plasma cholesterol level of patients, suggesting that the oral and intestinal microbial communities are related to the biomarkers of atherosclerosis.
It can be seen that there are countless connections between oral microorganisms, intestinal diseases and intestinal microorganisms. At the same time, they also have certain effects on other organs and diseases of the body. As the two "gathering places" of microorganisms in the body, the oral cavity and intestinal tract are responsible for the important mission of feedback the micro ecological balance in the body.
Application of oral probiotics
At present, the effect of probiotics on human oral health has been gradually recognized, and probiotic therapy as a new method to maintain oral health has been gradually paid attention to. Probiotics play an important role in oral health, especially in reducing the number of Streptococcus mutans in saliva. However, the current research evidence is insufficient to recommend the use of probiotics for caries management .
In conclusion, the structure and function of oral microorganisms are closely related to oral health, and oral microorganisms are closely related to intestinal microorganisms and some systemic diseases. Therefore, we should actively remove the oral pathological state and improve the oral micro ecological environment to maintain the balance of oral microbial community and maintain oral health and physical health.
 Kazor CE, Mitchell PM, Lee AM, et al. Diversity of Bacterial Populations on the Tongue Dorsa of Patients with Halitosis and Healthy Patients [J]. J Clin Microbiol, 2003, 41(2): 558-563
 Xu Chang. The effect of oral microorganisms on health [J]. Health perspective, 2018, (19): 86
 Belda Ferre P, Alcaraz LD, Cabrera Rubio R, et al. The oral metagenome in health and disease [J]. ISMEJ, 2012, 6(1): 46-56
 Xu Xin, he Jinzhi, Zhou Xuedong. The role of oral microbial community in early warning of oral and systemic diseases [J]. West China Journal of Stomatology, 2015,33 (6): 555-560
 Xu Xin, he Jinzhi, Zhou Xuedong. The role of oral microbial community in early warning of oral and systemic diseases [J]. West China Journal of Stomatology, 2015, 33 (6): 555-560
 Nakajima M, Arimatsu K, Kato T, et al. Oral administra-tion of P. Gingivalis Induces Dysbiosis of Gut Microbiota and Impaired Barrier Function Leading to Dissemination of Entero-bacteria to the Liver[J]. PLoS One, 2015, 10(7): e0134234
 Bashir A, Miskeen AY, Bhat A, et al. Fusobacterium Nucleatum: An Emerging Bug in Colorectal Tumorigenesis [J]. EurJ Cancer Prev, 2015, 24(5): 373-385
 Mima K, Nishihara R, Qian ZR. Fusobacterium Nucleatum in Colorectal Carcinoma Tissue and Patient Prognosis [J]. Gut, 2016, 65(12): 1973-1980
 Koren O, Spor A, Felin J, et al. Human oral, Gut and Plaque Microbiota in Patients with Atherosclerosis[J]. Proc Natl Acad Sci USA, 2011, 108(Suppl 1): 4592-4598
 Ma Jingxian, Li Yanhong. Mechanism and Application of Probiotics in Prevention and Treatment of Caries [J]. Medical Review, 2018, 24 (8): 1493-1497