Introduction

The oral microbiota is essential in the human body. It's a complex ecosystem with bacteria, microeukaryotes, archaea, and viruses. There are more than 700 species of oral bacteria alone, making it the second most diverse microbiome. Oral microbes interact not only with each other within the community but also with the host's immune system. A balanced oral microbiome ecosystem is crucial for maintaining oral health.

Composition of the Oral Microbiota

In the oral cavity, bacteria, archaea, eukaryotes, and viruses coexist and interact to regulate the balance of the oral ecosystem.

Fig. 1 Biogeography of the oral microbiome and relative sizes of its members (Baker J. L., et al. 2024).

Oral Bacteria

Oral bacteria are the most typical and widely studied oral microorganisms, as most of the early studies of the oral microbiome were based on 16S rRNA gene amplicon sequencing. It has been shown that the human oral cavity is home to more than 700 species of bacteria from dozens of genera in seven phyla, such as Streptococcus, Staphylococcus, Actinobacillus, Clostridium, and Prevotella. In addition, some other bacteria, like Candidatus Absconditabacteria (SR1) and Candidatus Gracilibacteria (GN02) in the Candidate Phyla Radiation (CPR) bacteria are often detected in the human oral microbiota. Oral bacteria are in different parts of the oral cavity and are involved in oral metabolism and balance. Interestingly, the abundant bacterial species in the oral microbiota are usually similar among individuals.

Oral Eukaryotes

In addition to bacteria, the oral microbiota includes microeukaryotes (fungi, amoebas, and flagellates) and archaea. Individual oral fungal communities are usually dominated by Candida or Malassezia, with Candida Albicans being the most common oral fungus. Studies on bacteria-fungi interactions in the oral cavity have received extensive attention, but fewer studies have been conducted on amoebae, flagellates, and archaea. Although their relatively low abundance in the oral microbiome compared to bacteria, archaea and microeukaryotes may have important ecological regulatory roles in the oral cavity due to their larger size and unique metabolic capabilities.

Oral Virus

Viruses are an important part of oral microorganisms. There are many types of viruses in the oral cavity, like Herpes Simplex Virus (HSV) and Human Papillomavirus (HPV). Most of the oral viruses are phages that infect bacteria, and more than 60,000 oral phages have been identified. It has been suggested that the role of phages in the oral microbiome may not be as important as in other systems, but the diversity and abundance of phages are challenging this view.

Balance of the Oral Microbiota

Oral microbial communities aggregate into complex spatial structures that collectively functionally regulate and adapt by complex physical and chemical signaling systems. Interactions within the oral microbiota and changes in the host and environment determine its equilibrium. In a healthy oral cavity, all types of microorganisms constrain and depend on each other to form a delicate balance.

On the one hand, beneficial microorganisms can inhibit the overgrowth of harmful microorganisms, such as probiotics to prevent the adhesion of cariogenic bacteria, Lactobacillus to produce substances to inhibit the growth of pathogenic bacteria, and different microorganisms to synergize the metabolism.

On the other hand, oral microorganisms interact with the host immune system to maintain the balance of oral microbiota. For example, Bifidobacteria could activate immune cells, promote the secretion of immune factors, and enhance the barrier function of the oral mucosa.

Breakdown of the Oral Microbiota Balance

The oral microbiota balance is often disrupted by poor diet and inadequate oral hygiene, which may lead to various oral diseases. Evidence suggests that oral microbiota is associated with several types of cancers, including oral, pancreatic, colorectal, lung, gastric, and head and neck cancers. Here, we briefly describe the role of oral microbial imbalance in the three most common oral diseases, dental caries, periodontal disease, and oral cancer.

Dental Caries

Dental caries are associated with ecological dysregulation of the plaque microbiota, and S. Mutant is considered a major caries pathogen. S. Mutant can utilize carbohydrates in the oral cavity to produce acidic substances such as lactic acid, which reduces the pH value of the local environment in the oral cavity. When the pH value drops to a certain level, the enamel on the surface of the teeth will be demineralized, and repeated demineralization over a long time will destroy the structure of the teeth and form dental caries.

Periodontal Disease 

Periodontal disease, which primarily consists of gingivitis and periodontitis, is driven by a variety of oral microorganisms, such as Porphyromonas gingivalis and Actinobacillus actinomycetemcomitan. These microorganisms attach to the tooth and gingival surfaces, break through defense barriers to invade periodontal tissues, produce virulence factors that damage cells, and activate the immune system to trigger an inflammatory response. Inflammation is a major factor in the structural characterization of microbiota in the progression of periodontal disease.

Fig. 2 Oral microbiome and extra-oral inflammatory conditions (Hajishengallis G., et al. 2023).

Oral Cancer

Oral cancer is a malignant tumor that occurs in the oral cavity, with Oral squamous cell carcinoma (OSCC) being the most common type. Studies have shown that OSCC samples contain more Fusobacterium and F. nucleatum than normal tissues of the same patient. Moreover, with the development of OSCC, the abundance of Fusobacterium periodonticum increased. Additionally, the abundance of S. mitis, P. pasteri, and H. parainfluenzae negatively correlated with the progression of OSCC. These results indicate that there is a complex regulatory relationship between the oral microbiota and oral cancer.

Conclusion

Oral microbiota balance is closely tied to oral health. Studies have shown it can be a potential biomarker or therapeutic target for oral disease management. Maintaining oral microbiota balance requires good dietary habits, emphasizing oral hygiene, and strengthening the immune system.

The oral microbiota is one of the best-described microbiota due to its accessibility and clinical relevance. Further research is needed to elucidate pathogenic mechanisms and develop microbiota regulation strategies for oral disease prevention and treatment. Only then can oral microbes coexist harmoniously and safeguard oral health.