How are the oral and gut microbiomes connected?
The mouth and gut are not isolated ecosystems—they constantly exchange microbes. Strain-level tracking of bacteria across thousands of human samples shows that oral bacteria frequently travel to the gut, and this 'mouth-to-gut' transfer is substantial: about 12% of gut bacterial strains are shared with cohabiting individuals' oral microbiomes, and the rate increases with time spent together [6]. This means your baseline gut ecology is partly shaped by what happens in your mouth, and vice versa.
In disease contexts, this connection becomes even more pronounced. In patients with oral chronic graft-versus-host disease (cGVHD) after bone marrow transplant, researchers found unusually high overlap between oral and gut bacteria, suggesting that oral microbes were ectopically colonizing the gut [4]. Similarly, in type 1 diabetes, a decrease in the oral commensal *Streptococcus salivarius* was linked to its decrease in the gut and to higher levels of gut inflammation markers [8]. These findings show that baseline gut ecology is not static—it can be influenced by oral microbial shifts, and those shifts can be personalized if we know the starting gut state.
Can your baseline gut bacteria predict how your oral microbiome will change?
Yes, and this is the core of personalization. In a clinical trial testing an oral microbiome therapeutic (SER-401) for melanoma patients, researchers stratified participants by their baseline stool levels of *Ruminococcaceae*, a key gut bacterium [1]. They found that patients who responded to the therapy had distinct gut microbial and metabolic profiles before any oral treatment began, and the one patient who achieved a complete response had a unique gut signature at screening [1]. This shows that baseline gut ecology can predict who will benefit from an oral microbiome intervention.
Further evidence comes from dietary intervention studies. In a 6-month trial in pre-diabetic individuals, changes in the gut microbiome explained 12.25% of the variance in serum metabolites, and the oral microbiome showed more genetic-level changes than the gut [5]. This suggests that personalizing diet based on baseline gut microbes could drive specific oral microbiome shifts. Additionally, Mendelian randomization studies have identified causal links between gut bacteria (like *Alistipes* and *Holdemanella*) and periodontitis, an oral disease, confirming that baseline gut ecology can influence oral health outcomes [7].
What does this mean for personalizing oral microbiome treatments?
It means that a one-size-fits-all approach to oral microbiome interventions is unlikely to work—your baseline gut ecology matters. For example, using proton pump inhibitors (PPIs) alters the gut microbiome by increasing oral pathobionts like *Streptococcus* species, which then can affect oral health [2]. Similarly, antibiotics given before surgery disrupt both oral and gut microbiomes, increasing antibiotic resistance genes and reducing diversity for up to 28 days [3]. These disruptions are not random; they depend on your starting gut composition.
The practical takeaway: if you want to personalize an oral microbiome intervention (like a probiotic, diet, or drug), you should first assess the gut microbiome. The strongest evidence here comes from the MCGRAW trial, which used baseline gut *Ruminococcaceae* levels to stratify patients [1], and from the strain-tracking study showing that oral-to-gut transmission is common and influenced by host factors [6]. However, the evidence is still early—most studies are small or observational, and the only randomized trial (MCGRAW) was terminated early due to enrollment challenges and suboptimal engraftment [1]. So while the concept is supported, clinical application is not yet routine.
About These Sources
This answer is built on 8 peer-reviewed studies — published from 2022 to 2025, 3 from 2024 or later, 6 in Q1 journals, collectively cited 581 times — selected as the most relevant from 11 studies that passed quality screening, drawn from 56 papers retrieved from a database of over 500 million.
Sources used in this answer
607 MCGRAW trial: evaluation of the safety and efficacy of an oral microbiome intervention (SER-401) in combination with nivolumab in first line metastatic melanoma patients
In a randomized phase 1b trial (MCGRAW), patients with metastatic melanoma were stratified by baseline gut *Ruminococcaceae* levels; responders to an oral microbiome therapeutic had distinct gut profiles before treatment, but the study was terminated early due to poor engraftment and enrollment challenges.
Abstract MP45: Proton Pump Inhibitors Increase Pathobiont Species in the Gut Microbiome: Findings from the Baltimore Longitudinal Study of Aging
In a longitudinal study of older adults (Baltimore Longitudinal Study of Aging), new users of proton pump inhibitors showed increased gut levels of oral pathobionts like *Streptococcus parasanguinis* (by 0.7%) and *S. salivarius* (by 3.7%), linking gut changes to oral microbes.
Impact of antimicrobial prophylaxis in colorectal cancer surgery on the gut and oral microbiome and resistome: A prospective observational cohort study
In a prospective cohort of 8 colorectal cancer surgery patients, antibiotic prophylaxis significantly reduced oral and gut microbial diversity and increased antibiotic resistance genes, with effects persisting up to 28 days post-surgery.
Oral and Gut Microbiome Alterations in Oral Chronic GVHD Disease: Results from Close Assessment and Testing for Chronic GVHD (CATCH Study)
In the CATCH study of 29 oral chronic GVHD patients and 51 controls, oral cGVHD onset was marked by expansion of *Streptococcus salivarius* and *Veillonella parvula* in the mouth, and high oral-gut microbiome overlap suggested ectopic gut colonization by oral bacteria.
Impact of dietary interventions on pre-diabetic oral and gut microbiome, metabolites and cytokines
In a 6-month dietary intervention in 200 pre-diabetic individuals, changes in the gut microbiome explained 12.25% of serum metabolite variance, and the oral microbiome showed more genetic-level changes than the gut, indicating diet-personalization potential.
The person-to-person transmission landscape of the gut and oral microbiomes
Analyzing over 9,700 human metagenomes, this study found extensive bacterial strain sharing between individuals: 12% median strain-sharing for gut and 32% for oral microbiomes among cohabiting people, with time since cohabitation affecting sharing more than age or genetics.
Exploring the causal relationship between periodontitis and gut microbiome: Unveiling the oral–gut and gut–oral axes through bidirectional Mendelian randomization
Using Mendelian randomization across two large consortia, this study found causal links: gut bacteria *Alistipes* and *Holdemanella* increase periodontitis risk, while periodontitis reduces gut *Ruminococcaceae* UCG014, confirming a bidirectional oral-gut axis.
Alterations of oral microbiota and impact on the gut microbiome in type 1 diabetes mellitus revealed by integrated multi-omic analyses
In 35 individuals from families with type 1 diabetes, multi-omic analysis showed decreased oral *Streptococcus salivarius* linked to its decrease in the gut and to higher gut inflammation, with direct evidence of strain-variant 'mouth-to-gut' transfer.