Fundamentals

Oral Microbiome

Evidence: High Fundamentals Reading time: approx. 5 min. Reviewed: 11.08.2026 · KEERN Editorial Team

The oral microbiome is a complex, site-specific ecosystem comprising hundreds of bacterial taxa alongside fungi, archaea, viruses, and other microorganisms. Distinct communities occupy different oral niches, including the tongue, tooth surfaces, gingival sulcus, and saliva. A health-associated microbiome is functionally balanced and adapted to its ecological niche; microbial diversity alone is not a reliable marker of oral health. Such disease-associated ecological shifts are commonly described as dysbiosis and are closely involved in the development of caries and periodontal disease, through different mechanisms.

Related terms:BiofilmPlaqueGingivitisPeriodontitis

Definition

The oral microbiome comprises the microorganisms that colonize the oral cavity, predominantly bacteria, alongside fungi, viruses, and archaea. Several hundred bacterial species alone have been described. This is not a single, uniform community: the tongue, tooth surfaces, gingival pockets, and saliva each host distinct microbial communities adapted to their specific site.

The oral microbiota refers to the community of microorganisms itself that colonize the oral cavity. The oral microbiome is the broader concept: it encompasses these microorganisms together with their genes, functions, interactions, and surrounding ecological context. In scientific and everyday usage, however, the two terms are sometimes used interchangeably, including in the remainder of this article.

A substantial part of these communities forms the biofilm on teeth and mucosal surfaces, a structured community adhering to surfaces that differs markedly from free-floating microorganisms.

In short

A health-associated oral microbiome is functionally balanced and adapted to its particular ecological niche. Microbial diversity can contribute to stability, but on its own is not a reliable marker of oral health: in caries, the balance often shifts toward a smaller, acid-tolerant group of species, while periodontal disease tends to involve a more complex community that is not necessarily lower in diversity. What matters is not sterility, but a stable, functionally balanced microbial ecosystem.

A common misconception

More antibacterial action is often equated with better oral health, on the assumption that fewer bacteria is always better. This falls short. Many members of the health-associated oral microbiota are commensal and contribute to ecological stability, including through so-called colonization resistance: they compete for nutrients and binding sites, which can make it harder for potentially disease-promoting communities to establish themselves. Antibacterial agents can be clinically useful in clearly defined situations, but maximizing germ reduction is not a general goal of daily oral care. Such agents can affect health-associated microorganisms as well as disease-associated ones; the long-term clinical significance of these changes in individual cases is not yet fully established.

When the balance tips

When the composition and activity of the microbial community change such that disease-promoting properties predominate, this is called dysbiosis. It can be favored by frequent sugar intake throughout the day, insufficient cleaning, or altered saliva flow. Such disease-associated ecological shifts are closely involved in both caries and gingivitis and periodontal disease, through different microbial mechanisms. In gingivitis and periodontitis, the body's inflammatory response also comes into play: biofilm accumulation triggers an immune response, and this inflamed environment in turn alters the microbial community, an interplay that runs in both directions rather than a one-way cause.

What this means for oral care

  • Regular, thorough biofilm control rather than attempting complete sterility
  • Limiting the frequency of sugary food and drink throughout the day, since frequent acid production can shift the balance toward acid-tolerant species
  • Adequate saliva flow as a natural regulator of the microbial balance
  • Using strongly antiseptic products based on indication and according to professional or product-specific guidance, not continuously without a specific reason

Key points at a glance

  • The oral microbiome comprises several hundred bacterial species along with other microorganisms, forming distinct communities depending on the site in the mouth
  • Not sheer diversity, but the functional balance of the community is decisive for oral health
  • ‘More antibacterial equals better’ is a common but overly simple assumption
  • Dysbiosis is closely involved in both caries and gingivitis and periodontitis, with the latter also involving an interplay with the body's own inflammatory response
  • Regular biofilm control, limited sugar frequency, and adequate saliva flow contribute to stable oral conditions

Frequently Asked Questions

Are all bacteria in the mouth harmful?

No. Many members of the health-associated oral microbiota are commensal and contribute to ecological stability, including through so-called colonization resistance, competing for nutrients and binding sites in ways that can make it harder for potentially disease-promoting communities to establish themselves.

Does that make antibacterial mouthwashes problematic?

Not inherently; they can be clinically useful for certain indications. However, they can alter not just specific target organisms but potentially the composition of the entire community. The long-term clinical significance of this depends on the active ingredient, application, and starting situation, and is not yet fully established.

Does more diversity automatically mean better oral health?

No. In caries, for example, the balance often shifts toward a smaller, acid-tolerant group of species, meaning less diversity, not more. What matters is the functional balance of the community, not species count alone.

What is the difference between the microbiome and the biofilm?

The microbiome refers to the totality of all microorganisms in the mouth. The biofilm is the structured form, adhering to surfaces, in which a large part of these microorganisms is organized.

Can the oral microbiome be deliberately ‘built up’ or ‘repaired’?

Current evidence supports maintaining conditions associated with oral ecological stability, including effective biofilm control, limiting frequent fermentable-carbohydrate exposure, and supporting normal salivary function, more strongly than claims that a specific consumer product can ‘rebuild’ an individual's oral microbiome. Probiotics and related approaches are an area of active research: a 2025 systematic review found signals of benefit from specific probiotic strains for caries, but the effect remains dependent on strain, delivery method, and the endpoint studied, and no standardized clinical model for ‘rebuilding’ the microbiome yet exists.

Is the microbiome the same in everyone?

There is a shared core set of species, but the exact composition varies individually, depending among other things on diet, saliva, oral hygiene, lifestyle, medical conditions, and other individual host factors.

KEERN Perspective

The oral microbiome shows why oral care does not aim for maximum germ reduction. What matters are stable ecological conditions, regular biofilm control, and indication-appropriate use of antimicrobial agents.

Questions about recurring oral conditions or the appropriate use of antiseptic products should be assessed by a dentist.

A healthy oral ecosystem is not defined by the absence of microbes, but by the relationships between microbes, their environment, and the host.

Professional perspective

Clinical relevance

  • Communicating a functional-ecological, rather than purely eliminatory, understanding of oral health to patients
  • Framing dysbiosis as a shared but mechanistically distinct factor in caries and periodontal disease
  • Critically assessing strongly antibacterial products in terms of benefit-risk ratio with long-term use, without categorically rejecting indicated applications
  • Contextualizing commercial ‘microbiome’ claims against the backdrop of limited intervention evidence

Ecological model: from eubiosis to dysbiosis

The ecological plaque hypothesis (Marsh, 1994) describes oral disease not as the result of individual specific pathogens, but as the outcome of ecological shifts within an existing, largely symbiotic microbial community. Frequent acid exposure from fermentable carbohydrates selects for acidogenic, acid-tolerant species and favors caries, typically with a reduction in species diversity in favor of fewer dominant taxa. In periodontal disease, by contrast, the subgingival community tends to shift toward a more complex, proteolytic, obligately anaerobic, gram-negative composition, often not with reduced diversity but with altered diversity. Current reviews (Lamont, Koo, Hajishengallis, 2018) therefore emphasize that the collective function of the microbial community, not species count alone, is the decisive driver of homeostasis or dysbiosis.

In gingivitis and periodontitis, this shift is additionally closely linked to the host response: biofilm accumulation triggers an inflammatory response, and the altered local environment (including an increased supply of gingival crevicular fluid and inflammatory mediators) in turn favors the growth of proteolytic, dysbiotic taxa, a self-reinforcing interplay (feed-forward loop) between microbiota and host, as described by Lamont, Koo, and Hajishengallis (2018) and the EFP/ORCA consensus report (Sanz et al., 2017). In caries, by contrast, the ecological shift is driven mainly by frequent intake of fermentable carbohydrates and repeated pH drops.

Diversity and composition

Dewhirst et al. (2010) established the Human Oral Microbiome Database (HOMD), a curated, site-specific taxonomy of over 600 prokaryotic taxa of the oral cavity, based on 16S rRNA sequencing. Kilian et al. (2016), in a more recent assessment, put the number at over 700 bacterial taxa or species; the discrepancy reflects ongoing taxonomic additions rather than a contradiction, and a substantial proportion of taxa also remain as yet uncultured or incompletely classified. Even this figure is not a final state: the taxonomy of the oral microbiome is continually being added to and revised, so the exact species count is better understood as a snapshot than a fixed number.

Baker et al. (2024, online 2023), in a current Nature Reviews Microbiology review, synthesize the present state of knowledge and confirm a markedly structured biogeography of oral communities: composition follows consistent, site-specific patterns across niches, with bacteria, microeukaryotes, archaea, and viruses as members. This more recent work complements the foundational taxonomic work of Dewhirst and Kilian with a more ecologically structural perspective, without calling their findings into question.

The oral cavity comprises several distinct ecological niches, including teeth, the gingival sulcus, the tongue, buccal mucosa, hard and soft palate, and tonsils, each with characteristic microbial communities. Despite considerable interindividual variation, a shared core set of species can be identified.

What current evidence supports

  • The site-specific organization of the oral microbiome and its taxonomic characterization, consistent across multiple independent bodies of work (Dewhirst 2010, Kilian 2016, Baker 2024)
  • The general significance of ecological shifts for the development of caries and periodontal disease, though through different mechanisms
  • The feed-forward model between biofilm accumulation, host inflammation, and microbial shift in gingivitis and periodontitis

Why this matters

This distinction matters because microbiome measurements are increasingly used in commercial tests and product claims. Detecting a difference in microbial composition does not by itself establish disease, causality, or clinical benefit from changing that composition.

What the data currently doesn't show

  • No established, standardized definition of an ‘optimal’ individual microbiome profile
  • Limited controlled evidence on targeted, product-based interventions to durably alter the microbial composition
  • No definitive clarification of the extent to which observed associations between specific taxa and disease states are causal versus a consequence of the disease
  • No conclusively established long-term clinical significance of microbial compositional changes from regular use of antiseptic agents
  • No validated, universal microbiome profile that can currently define oral health or disease for an individual consumer, independent of clinical findings
  • No basis for assuming that a product-induced change in microbiome composition necessarily represents a clinically beneficial change

Evidence summary

Marsh (1994): Microbial ecology of dental plaque and its significance in health and disease. Advances in Dental Research, 8(2), 263-271. Foundational work; establishes the ecological plaque hypothesis.

Dewhirst, Chen, Izard, Paster, Tanner, Yu, Lakshmanan, Wade (2010): The Human Oral Microbiome. Journal of Bacteriology, 192(19), 5002-5017. Establishes the Human Oral Microbiome Database (HOMD); site-specific taxonomic characterization.

Kilian, Chapple, Hannig, Marsh, Meuric, Pedersen, Tonetti, Wade, Zaura (2016): The oral microbiome, an update for oral healthcare professionals. British Dental Journal, 221(10), 657-666. Updated overview for dental practice; basis for the commonly cited figure of over 700 species.

Lamont, Koo, Hajishengallis (2018): The oral microbiota: dynamic communities and host interactions. Nature Reviews Microbiology, 16(12), 745-759. Key source for the functional, not purely species-count-based, understanding of dysbiosis and the feed-forward model in caries and periodontitis.

Baker, Mark Welch, Kauffman, McLean, He (2024, online 2023): The oral microbiome: diversity, biogeography and human health. Nature Reviews Microbiology, 22(2), 89-104. Current review article; confirms a markedly structured biogeography of oral communities across bacteria, microeukaryotes, archaea, and viruses, and frames the taxonomy as continually evolving.

Sanz, Beighton, Curtis, Cury, Dige, Dommisch, Ellwood, Giacaman, Herrera, Herzberg, Könönen, Marsh, Meyle, Mira, Molina, Mombelli, Quirynen, Reynolds, Shapira, Zaura (2017): Role of microbial biofilms in the maintenance of oral health and in the development of dental caries and periodontal diseases. Journal of Clinical Periodontology, 44(Suppl 18), S5-S11. Consensus report of the joint EFP/ORCA working group on ecological interactions in the dental biofilm.

Inchingolo, Inchingolo, Palumbo, Guglielmo, Riccaldo, Morolla, Inchingolo, Palermo, Dipalma (2025): The role of probiotics in preventing dental caries: a systematic review of clinical evidence. Frontiers in Oral Health, 6, 1720036. Current systematic review; finds signals of strain-specific benefit, alongside continued heterogeneity in strain, dosage, and delivery method.

Sources

📚 Marsh PD (1994): Microbial ecology of dental plaque and its significance in health and disease. Advances in Dental Research, 8(2), 263-271.
📚 Dewhirst FE, Chen T, Izard J, Paster BJ, Tanner ACR, Yu WH, Lakshmanan A, Wade WG (2010): The Human Oral Microbiome. Journal of Bacteriology, 192(19), 5002-5017.
📚 Kilian M, Chapple ILC, Hannig M, Marsh PD, Meuric V, Pedersen AML, Tonetti MS, Wade WG, Zaura E (2016): The oral microbiome, an update for oral healthcare professionals. British Dental Journal, 221(10), 657-666.
📚 Lamont RJ, Koo H, Hajishengallis G (2018): The oral microbiota: dynamic communities and host interactions. Nature Reviews Microbiology, 16(12), 745-759.
📚 Baker JL, Mark Welch JL, Kauffman KM, McLean JS, He X (2024): The oral microbiome: diversity, biogeography and human health. Nature Reviews Microbiology, 22(2), 89-104 (online 2023).
📚 Sanz M, Beighton D, Curtis MA, et al. (2017): Role of microbial biofilms in the maintenance of oral health and in the development of dental caries and periodontal diseases. Journal of Clinical Periodontology, 44(Suppl 18), S5-S11.
📚 Inchingolo AD, Inchingolo AM, Palumbo I, Guglielmo M, Riccaldo L, Morolla R, Inchingolo F, Palermo A, Dipalma G (2025): The role of probiotics in preventing dental caries: a systematic review of clinical evidence. Frontiers in Oral Health, 6, 1720036.

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The KEERN Lexicon provides evidence-based educational information about oral health. It is intended to support, not replace, individual advice from a dentist, physician, or pharmacist. Diagnosis and treatment decisions should always be based on a personal clinical assessment.