Your Oral Microbiome Questions Answered

Most conversations about the microbiome start — and stop — in the gut. But your mouth is home to billions of bacteria too, and what lives there has surprising consequences for your digestion, your brain, and your overall health. If you've ever wondered what's actually happening inside your mouth, how it connects to your gut microbiome, and what it means for your long-term wellbeing, you're in exactly the right place.

Jump to a Question

How many bacteria live in the human mouth?

Is the oral microbiome different from the gut microbiome?

How does the oral microbiome affect gut health?

What diseases are linked to an imbalanced oral microbiome?

What is the gut-brain connection, and does the mouth play a role?

Can you improve your oral and gut microbiome naturally?

What is the Human Oral Microbiome Database (HOMD)?

Should people in the UK be paying more attention to their oral microbiome?

How many bacteria live in the human mouth?

The average adult mouth harbours between 50 and 100 billion bacteria — a figure that tends to surprise people who associate microbiome science purely with the gut. According to a peer-reviewed practical guide to the oral microbiome, there are approximately 700 predominant bacterial taxa identified in the oral cavity, with around 200 species detected in any one individual at a given time.

Far from being randomly distributed, these bacteria are highly site-specific. Different species prefer different habitats:

• Tongue: Rothia and Streptococcus salivarius thrive here
• Hard palate: Simonsiella colonises almost exclusively this surface
• Subgingival crevice (below the gumline): Up to 400–500 taxa detected here alone
• Saliva: A mixture sloughed off from all oral surfaces

As of the most recent assessments, roughly 31% of these oral bacterial taxa have never been grown in a laboratory setting — meaning science is still catching up with what lives in your mouth.

Is the oral microbiome different from the gut microbiome?

Yes — the oral microbiome and the gut microbiome are distinct ecosystems with different dominant species, environmental conditions, and functions. The mouth is oxygen-rich and exposed to food, drink, and air, whereas the colon is largely anaerobic (low-oxygen) and subject to very different biochemical pressures.

Researchers at institutions including King's College London and Imperial College London have contributed to our understanding of how these two communities interact — and how disrupting one can affect the other.

How does the oral microbiome affect gut health?

Bacteria from the mouth travel continuously into the digestive system, meaning the oral microbiome directly seeds and influences the gut microbiome. Every time you swallow, you ingest oral bacteria — most are neutralised by stomach acid, but some survive and take up residence further down the digestive tract.

This process becomes clinically significant when oral bacteria associated with disease — a state called oral dysbiosis — enter the gut in abnormal quantities. Studies have detected oral pathogens such as Fusobacterium nucleatum and Porphyromonas gingivalis in gut tissue samples from people with colorectal cancer and inflammatory bowel disease.

The British Gut Project, run in partnership with King's College London, has collected microbiome data from tens of thousands of UK participants and has begun exploring these cross-site microbial relationships. The NHS Eatwell Guide also emphasises fibre intake and dietary diversity — both of which support a balanced microbiome at every level of the digestive tract, starting in the mouth.

Practical implication: improving gut health naturally is not only about what you eat — it's also about maintaining a healthy oral environment through regular dental hygiene and a diverse, plant-rich diet.

What diseases are linked to an imbalanced oral microbiome?

An imbalanced oral microbiome — known as oral dysbiosis — is associated with a wide range of both oral and systemic diseases. The evidence base here is rapidly growing, though researchers are careful to distinguish association from causation.

Oral conditions linked to oral dysbiosis include:

• Dental caries (tooth decay)
• Periodontal (gum) disease
• Halitosis (persistent bad breath)
• Endodontic lesions
• Odontogenic infections

Systemic conditions where the oral microbiome may serve as a biomarker include:

• Type 2 diabetes — altered salivary microbiome profiles have been observed
• Cardiovascular disease — oral pathogens have been detected in arterial plaques
• Pancreatic cancer — specific oral bacterial signatures may predict risk
• Paediatric Crohn's disease — shifts in the oral microbiome mirror gut inflammation
• Preterm birth and low birth weight — periodontal pathogens implicated in some studies

It is important to note that establishing direct causal relationships remains an active area of UK microbiome research, with groups at the University of Cambridge and University of Nottingham among those examining these links rigorously.

What is the gut-brain connection, and does the mouth play a role?

The gut-brain connection — also called the gut-brain axis — is a bidirectional communication network linking the digestive system, the nervous system, and the brain via hormones, immune signals, and the vagus nerve. Research into this axis has exploded over the past decade, with UK Biobank data helping scientists explore how gut microbial diversity correlates with mental health outcomes including anxiety and depression.

While the gut microbiome is the primary player in the gut-brain axis, the oral microbiome contributes in several important ways. Oral bacteria influence systemic inflammation levels, and chronic low-grade inflammation is a well-established pathway through which microbiome imbalance can affect brain function and mood.

Furthermore, nitrate-reducing bacteria in the mouth — found abundantly on the tongue — convert dietary nitrate (from vegetables such as spinach and beetroot) into nitric oxide, a molecule that supports cardiovascular health and has been shown in some studies to influence cerebral blood flow. This positions the oral microbiome as an upstream modulator of the gut-brain connection, not merely a bystander.

The key takeaway: nurturing your oral microbiome UK health professionals increasingly regard as part of a holistic approach to gut-brain wellbeing.

Can you improve your oral and gut microbiome naturally?

Yes — there are well-evidenced, practical steps to improve gut health naturally and support a diverse, balanced oral microbiome simultaneously. Many of the same dietary strategies that nourish the gut microbiome also benefit oral bacteria.

Dietary strategies backed by science:

• Eat more fibre. The UK Eatwell Guide recommends 30g of fibre per day — most adults in the UK consume far less. Fibre feeds beneficial bacteria throughout the digestive tract.
• Include fermented foods. Live yoghurt, kefir, sauerkraut, and kimchi introduce beneficial species at multiple points in the digestive system.
• Diversify plant intake. Research from the British Gut Project suggests eating 30 or more different plant foods per week is associated with greater microbial diversity.
• Avoid excessive mouthwash use. Antibacterial mouthwashes can disrupt beneficial oral bacteria alongside harmful ones — the British Dental Association advises using them at a different time to brushing, not immediately after.
• Reduce ultra-processed foods and added sugars. These are strongly associated with oral dysbiosis and negatively affect gut microbiome diversity.

Lifestyle factors:

• Regular, varied exercise supports microbiome diversity
• Adequate sleep (7–9 hours) reduces systemic inflammation that can disturb microbial balance
• Stress management — important given the gut-brain connection and cortisol's impact on gut permeability

What is the Human Oral Microbiome Database (HOMD)?

The Human Oral Microbiome Database (HOMD) is a freely accessible, curated scientific resource that catalogues approximately 700 predominant bacterial species known to inhabit the human oral cavity. Developed following pioneering work using 16S rRNA gene sequencing — a molecular technique that identifies bacteria by their unique genetic fingerprints — HOMD provides phenotypic, phylogenetic, clinical, and bibliographic information for 701 oral bacterial taxa.

This kind of molecular sequencing represents a revolutionary shift from older culture-based microbiology, where bacteria had to be grown in a lab to be identified. Because roughly a third of oral bacteria cannot currently be cultivated in vitro, culture-independent methods like those catalogued in HOMD are essential to understanding the full picture.

The database also interfaces with data generated by the Human Microbiome Project (HMP), which sequenced microbiomes from 242 healthy volunteers across multiple body sites. This collaborative, open-access approach to microbiome science — supported internationally by funders comparable to the UK's Wellcome Trust and MRC (Medical Research Council) — is precisely how the field has advanced so rapidly.

Should people in the UK be paying more attention to their oral microbiome?

In the UK, oral health and gut health have historically been treated as entirely separate clinical domains — but the science increasingly suggests they should be considered together. NHS dental services focus primarily on structural oral health (cavities, gum disease, tooth loss), whilst NHS gut health pathways focus downstream on the intestinal microbiome. The connection between the two is underserved in public health messaging.

Data from UK Biobank and the British Gut Project are helping to close this gap. UK microbiome research published by teams at King's College London, the University of Reading, and UCL (University College London) is revealing how oral microbial profiles can predict systemic disease risk — insights that could eventually inform NHS screening pathways.

The British Dietetic Association (BDA) and British Nutrition Foundation both advocate for a diverse, plant-rich British diet gut health approach — and that advice, it turns out, benefits your oral microbiome just as much as your gut. For health-conscious adults in the UK, thinking about the microbiome as a whole-body system — from mouth to colon — is a meaningful and evidence-based shift in perspective.

Bottom Line

• Your mouth is a microbiome in its own right, housing up to 100 billion bacteria across approximately 700 species — many yet to be studied in depth.
• The oral and gut microbiomes are connected: bacteria from the mouth continuously seed the gut, meaning oral dysbiosis can disrupt gut health too.
• The gut-brain connection is influenced upstream by oral bacteria, particularly through systemic inflammation and nitric oxide pathways.
• The same dietary habits that improve gut health naturally — high fibre, plant diversity, fermented foods, less sugar — also support a balanced oral microbiome.
• UK microbiome research from institutions like King's College London and the British Gut Project is helping reshape how we understand the mouth-gut-brain relationship.

Always consult your GP or a registered dietitian (via the British Dietetic Association) before making significant changes to your diet or oral health routine, particularly if you have a pre-existing medical condition.

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