The Oral Microbiome – An Intricate Universe

The oral microbiome is a complex and diverse ecosystem that plays a crucial role in maintaining oral and systemic health. Comprising over 700 species of bacteria, fungi, viruses, and protozoa, it is the second largest microbial community in the human body after the gut^1,3.

The oral microbiome is dominated by several major phyla, including Actinobacteria, Bacteroidetes, Firmicutes, Fusobacteria, and Proteobacteria². At the genus level, common inhabitants include Streptococcus, Actinomyces, Veillonella, Fusobacterium, Porphyromonas, and Prevotella, among others^1,2.

This complex micro universe is a network of microorganisms colonizing various niches within the oral cavity, including the teeth, gingival sulcus, tongue, cheeks, and palates. These microbial communities that are adapted to each habitat are initiated and defined by ways of delivery at birth, and later among others by the environment, nutrition, psychological state, oral hygiene practices, medication, are changing  throughout individual lifetime^1,4.

Naturally, the oral cavity provides an ideal environment for microbial growth, with temperature around 37°C and a pH of 6.5-7 maintained by saliva¹. Ample of nutrients from host-derived substances and dietary intake, support the thriving microbial ecosystem.

A Delicate Balance - Functions and Interactions in Health and Sickness

The oral microbiome plays a vital role in maintaining oral homeostasis and protecting against disease development1. In health, the microbial community lives in a symbiosis with us, contributing to various physiological processes. For instance, certain commensals  like S. salivarius and S. mitis help maintain oral health by competing with pathogenic bacteria and stimulating the immune system in the oral cavity and systemically partially by forming immunomodulators like SCFAs (Short Chain Fatty Acids)³.

Microorganisms in the oral cavity often form biofilms, which are complex, multi-species communities adhering to surfaces. These biofilms can be beneficial, forming a protective barrier against pathogenic species. However, they can also contribute to disease when dysbiosis occurs³. Two of the most common oral diseases, dental caries and periodontitis are a result of such dysbiosis. In the case of dental caries, an increase in acid-producing and acid-tolerant species, such as Streptococcus mutans, leads to demineralization of tooth enamel³. Periodontitis, on the other hand, is associated with an overgrowth of anaerobic Gram-negative bacteria, including Porphyromonas gingivalis, Treponema denticola, and Tannerella forsythia².

In more details, if we follow the sequence of caries pathogenesis we see that early colonizers of the tooth are mainly health-associated streptococcal species, such as S. sanguinis and S. gordonii. Poor oral hygiene, a high-sugar diet, and other salivary, immunological, and microbial factors lead to development of pathogenic biofilms (i.e., dysbiosis). S. mutans produces a glucan matrix, which leads to robust biofilm formation and colonization by taxa which could not have bound to the tooth surface unassisted (late colonizers). Production of acid within the biofilm selects for increasingly acid-tolerant cariogenic organisms, such as S. mutans, Lactobacillus spp., Veillonella spp.. Unchecked, the process will destroy the protective enamel coating of the tooth (demineralization) and lead to disease.

Beyond oral health, the oral microbiome has been implicated in various systemic conditions. Research has shown associations between oral dysbiosis and diseases such as diabetes mellitus, cardiovascular disease, and respiratory infections^4,5.

Recent research has uncovered a compelling link between Alzheimer's disease (AD) and the oral microbiome. Studies have demonstrated that oral microbial dysbiosis correlates with increased brain amyloid-β load and AD progression. Periodontal pathogens, including Porphyromonas gingivalis, Treponema denticola, and Fusobacterium nucleatum, have been detected at higher levels in AD patients compared to controls. These microorganisms can promote amyloid-β production and induce neuroinflammation, key factors in AD pathogenesis. Additionally, oral bacteria have been identified in AD brain tissue, suggesting potential direct involvement in neuropathology. The mechanism likely involves systemic dissemination of oral bacteria through the bloodstream, facilitated by periodontal inflammation⁶.

With a little Help of Modern Research

The Human Microbiome Mapping Project with advancements in molecular techniques have revolutionized our understanding of the oral microbiome. Thanks to developments in sequencing-based approaches, multi-omics approaches, including metagenomics, transcriptomics, proteomics, and metabolomics, we now can efficiently explore the diversity, roles, interactions, and metabolic outputs of oral microbes, even if unculturable ^1,4,5.

In conclusion, the oral microbiome is a fascinating and complex ecosystem that plays a critical role in human health. As our understanding of this microbial community continues to grow, so does the potential role of oral microbiome modulation for developing novel strategies for maintaining oral health, overall wellbeing and preventing diseases like diabetes, cardiovascular as well as Alzheimer.