Beyond systemic inflammation, an emerging and more controversial hypothesis proposes that periodontal pathogens may directly access the central nervous system (CNS) and contribute to Alzheimer’s disease (AD) pathology. Among these pathogens, Porphyromonas gingivalis has received particular attention due to its virulence factors and its established role in chronic periodontitis (How et al., 2016).

Several studies have reported detection of P. gingivalis DNA, antigens, or gingipain enzymes in post-mortem brain tissue of individuals diagnosed with AD (Chacón & Hernández-Hincapié, 2026; El Choueiri et al., 2025). The presence of gingipains in hippocampal regions, which are critically involved in memory processing, has been interpreted as evidence that periodontal pathogens may invade or persist within neural tissue (Shawkatova et al., 2025).

Mechanistically, gingipains have been proposed to influence AD pathology by cleaving host proteins and promoting inflammatory cascades (Li & Collyer, 2011). Some evidence suggests that gingipain activity may exacerbate tau fragmentation and contribute to neuronal dysfunction. Additionally, the development of gingipain inhibitors, such as those evaluated in recent therapeutic investigations (Chacón, 2026), reflects growing interest in targeting microbial factors as a potential intervention strategy for AD.

Despite these findings, several limitations warrant caution. Detection of bacterial DNA in post-mortem brain tissue does not necessarily establish active infection or causation; contamination during sampling or post-mortem translocation cannot be fully excluded. Moreover, many studies reporting microbial presence involve relatively small sample sizes, limiting generalizability and requiring replication across independent cohorts.

The microbial hypothesis offers a compelling explanation for how periodontal pathogens might directly contribute to Alzheimer’s disease; however, important uncertainties remain. While detection of P. gingivalis DNA and gingipain activity in post-mortem brain tissue suggests potential neuroinvasion, questions regarding sample size, reproducibility, and methodological variability limit definitive conclusions. It is also unclear whether the presence of bacterial components represents an initiating factor in neurodegeneration or occurs secondary to already compromised neural tissue. Clinical investigations targeting gingipains have shown promise but have not yet provided conclusive evidence of disease modification. Therefore, although direct microbial involvement remains biologically plausible, further research is required to clarify its significance in human Alzheimer’s disease. 

Alternative Explanations

Although epidemiological and mechanistic studies suggest a potential association between periodontitis and Alzheimer’s disease (AD), several alternative explanations must be considered before inferring causality. Both conditions are multifactorial and share numerous risk factors, which complicates interpretation of observed associations.
Age represents the most significant shared risk factor. The prevalence of both periodontitis and AD increases substantially with advancing age (Eke et al., 2018; National Institute of Health, 2025). Age-related immune decline and increased systemic inflammation may independently increase susceptibility to both conditions.

Lifestyle and metabolic factors also overlap considerably. Smoking, diabetes mellitus, cardiovascular disease, and lower socioeconomic status are well-established risk factors for periodontitis (Gil-Montoya et al., 2017; Gurav, 2014). These same factors are associated with increased risk of cognitive decline and AD. For instance, diabetes and vascular dysfunction can promote both periodontal tissue destruction and cerebral microvascular damage, suggesting that shared systemic vulnerabilities may underlie the observed relationship rather than a direct pathogenic pathway.
Reverse causality is another important consideration. Early cognitive impairment may reduce an individual’s ability to maintain adequate oral hygiene, attend dental appointments, or manage chronic health conditions. As a result, worsening periodontal status may be a consequence rather than a cause of neurodegenerative decline. Kouki et al. (2022) highlights that patients with established dementia often exhibit poorer oral health outcomes, supporting the possibility that cognitive impairment precedes periodontal deterioration in some cases.

Additionally, systemic inflammation itself may act as a common underlying driver. Chronic inflammatory states—regardless of origin—have been implicated in AD progression (Heneka et al., 2025). Periodontitis may therefore represent one of many peripheral inflammatory sources rather than a uniquely causal factor. In this context, periodontal disease could function as a marker of systemic inflammatory burden rather than a direct contributor to neuropathology.
Finally, methodological limitations across studies further complicate interpretation. Diagnostic criteria for both periodontitis and cognitive impairment vary considerably between investigations, and many studies rely on self-reported oral health measures. Small sample sizes, short follow-up periods, and inconsistent control of confounders reduce the strength of causal inference.

Taken together, these alternative explanations underscore the complexity of establishing a direct causal relationship between periodontitis and Alzheimer’s disease. While the association is biologically plausible and supported by emerging evidence, shared risk factors, reverse causality, and systemic inflammatory influences must be carefully accounted for in future research. Well-designed longitudinal studies with rigorous confounder control are essential to clarify whether periodontal disease contributes independently to AD risk or reflects broader systemic vulnerability.