What is the strongest evidence that oral microbiome changes cause disease?
The most compelling evidence comes from Mendelian randomization (MR) studies, which use genetic variants as natural experiments to test causality. A 2025 MR study analyzed genetic data from East Asian populations and found that 8 bacterial species on the tongue dorsum and 10 in saliva had causal relationships with chronic obstructive pulmonary disease (COPD) [6]. This means that genetically predicted changes in these bacteria directly influence COPD risk, not just correlate with it. The study identified 9 genera involved, including Prevotella, Streptococcus, and Haemophilus, supporting a causal oral-lung axis [6].
Another MR study from 2022, stratified by sex, found abundant causal relationships between oral microbiome composition and serum metabolites, including sex-specific hormonal interactions [11]. For example, specific oral microbes causally influenced aldosterone levels in females and androstenedione in males, suggesting the oral microbiome can directly affect systemic hormone regulation [11]. These genetic approaches provide the strongest evidence for causality because they minimize confounding and reverse causation.
How can oral microbes actually cause disease in other parts of the body?
Oral microbes can directly cause disease through several well-documented mechanisms. First, they can travel from the mouth to the gut via saliva swallowing or through the bloodstream, especially when the oral barrier is compromised by gum disease [1][2]. Once in the gut, oral-derived pathogens like Porphyromonas gingivalis can disrupt the intestinal microbiota, damage mucosal barriers, and trigger inflammatory responses that contribute to inflammatory bowel disease and other conditions [1].
Second, oral dysbiosis can promote systemic inflammation and oxidative stress. A 2026 review on chronic kidney disease (CKD) explains that periodontitis-associated oral bacteria can affect kidney function through systemic inflammation, endothelial dysfunction, and abnormal immune responses [5]. Similarly, oral pathogens have been linked to Alzheimer's disease through inflammatory pathways and direct bacterial invasion [3][7]. The oral microbiome can also influence cancer risk, cardiovascular disease, and respiratory diseases through these shared inflammatory and immune mechanisms [8][10].
When are oral microbiome changes just a marker rather than a cause?
In many cases, oral microbiome changes are likely markers of underlying disease or risk factors rather than direct causes. For example, a 2024 study of over 7,000 salivary samples from children with autism spectrum disorder (ASD) found that the oral microbiome could discriminate ASD from neurotypical siblings (AUC = 0.66), but the authors explicitly state that causal relationships could not be established [9]. The observed associations may reflect differences in diet, oral hygiene, or other lifestyle factors rather than a causal role.
Similarly, studies on preterm birth [4] and Alzheimer's disease [3][7] show altered oral microbiomes, but causality remains unproven. The preterm birth study identified Veillonella, Prevotella, and Capnocytophaga as associated with preterm birth, but the authors call for further research to clarify their role [4]. In Alzheimer's disease, while specific bacteria like Porphyromonas gingivalis and Veillonella parvula are associated, the evidence is still correlational [3][7]. The key distinction is that when genetic or interventional studies demonstrate a causal pathway (as with COPD), the evidence for causation is stronger; when only observational associations exist, the changes may be markers of disease or shared risk factors.
About These Sources
This answer is built on 11 peer-reviewed studies — published from 2021 to 2026, 6 from 2024 or later, 8 in Q1 journals, collectively cited 500 times — selected as the most relevant from 15 studies that passed quality screening, drawn from 74 papers retrieved from a database of over 500 million.
Sources used in this answer
Regulatory effects of oral microbe on intestinal microbiota and the illness
This review summarizes how oral microbes can enter the gut via swallowing or bloodstream, disrupt intestinal microbiota and mucosal barriers, and trigger inflammation, supporting an oral-gut microbiome axis in disease.
The oral–gut microbiome axis in health and disease
This review describes how oral microorganisms can colonize the gastrointestinal tract, particularly during gut dysbiosis, and may cause or exacerbate diseases in other organs, though mechanisms are not fully elucidated.
Oral Microbiome Stamp in Alzheimer’s Disease
In a case-control study of 64 Alzheimer's disease patients and 71 healthy seniors, the oral microbiome of AD patients showed higher diversity, increased Firmicutes, decreased Bacteroidetes, and reduced periodontitis-associated bacteria.
Oral microbiome and preterm birth
In 152 pregnant women (61 preterm, 91 term), oral microbiome genera Veillonella, Prevotella, and Capnocytophaga were enriched in preterm birth, independent of other complications, suggesting a potential causal role.
Relationship between oral microbiota and chronic kidney disease: facts and perspectives
This review synthesizes evidence that oral dysbiosis may contribute to chronic kidney disease through systemic inflammation, endothelial dysfunction, and oxidative stress, but causality remains unproven due to limited interventional studies.
Genetic evidence from Mendelian randomization strengthens the causality between oral microbiome and chronic obstructive pulmonary disease
Using Mendelian randomization on genetic data from East Asian populations, this study found causal relationships between 18 oral bacterial species (9 genera) and COPD, supporting a causal oral-lung axis.
Profiling the oral microbiomes in patients with Alzheimer's disease
In 26 Alzheimer's disease patients and 26 controls, Streptococcus oralis and Porphyromonas gingivalis were predominant in AD, and Veillonella parvula was associated with AD in both saliva and gingival fluid.
Oral microbiome in human health and diseases
This review systematically covers how oral microbiome variations impact overall health and contribute to diseases including cancer, cardiovascular disease, and respiratory disease, and how diseases exacerbate dysbiosis.
Large-scale metagenomic analysis of oral microbiomes reveals markers for autism spectrum disorders
In over 7,000 salivary samples from 2,025 families, the oral microbiome discriminated autism spectrum disorder from neurotypical siblings (AUC 0.66), with 108 differentiating species correlated with cognitive impairment, but causality not established.
Oral Microbiota: A Major Player in the Diagnosis of Systemic Diseases
This review summarizes how oral microbiota dysbiosis triggers or aggravates chronic systemic diseases, making it a target for diagnosis and treatment, and emphasizes collaboration between oral and general health specialists.
Sex differences in the oral microbiome, host traits, and their causal relationships
In 4,478 individuals, sex-stratified Mendelian randomization identified causal relationships between oral microbiome and serum metabolites, including sex-specific hormonal interactions (e.g., aldosterone in females, androstenedione in males).