Serum IgG antibody levels to periodontal microbiota are associated with incident Alzheimer disease

Mediating and moderating effects of plasma proteomic biomarkers on the association between poor oral health problems and incident dementia: The UK Biobank study

The plasma proteome can mediate poor oral health problems (POHP)'s link to incident dementia. We screened 37,269 UK Biobank participants 50-74 years old (2006-2010) for prevalent POHP, further tested against 1463 plasma proteins and incident dementia over up to 15 years of follow-up. Total effect (TE) of POHP-dementia through plasma proteomic markers was decomposed into pure indirect effect (PIE), interaction referent (INTREF), controlled direct effect (CDE), or mediated interaction (INTMED). POHP increased the risk of all-cause dementia by 17% (P < 0.05). Growth differentiation factor 15 (GDF15) exhibited the strongest mediating effects (PIE > 0, P < 0.001), explaining 28% the total effect of POHP on dementia, as a pure indirect effect. A first principal component encompassing top 4 mediators (GDF15, IL19, MMP12, and ACVRL1), explained 11% of the POHP-dementia effect as a pure indirect effect. Pathway analysis including all mediators (k = 173 plasma proteins) revealed the involvement of the immune system, signal transduction, metabolism, disease, and gene expression, while STRING analysis indicated that top mediators within the first principal component were also represented in the two largest proteomic clusters. The dominant biological GO pathway for the GDF15 cluster was GO:0007169 labeled as "transmembrane receptor protein tyrosine kinase signaling pathway." Dementia is linked to POHP mediated by GDF15 among several proteomic markers.

Association between oral health and cognitive impairment in older adults: Insights from a Six-year prospective cohort study

OBJECTIVE

This study aims to investigate the relationships between four baseline oral conditions (periodontal status, dental caries, tooth wear, and dentition) and repeated global cognition or domain-specific cognition (memory, executive function, attention, and verbal fluency) in non-demented older adults over time.

METHODS

This prospective cohort study (2011-2019) enrolled 516 non-demented community-dwelling older adults (age ≥ 65) to explore the association between oral health and cognitive function. Global and domain-specific cognition were assessed biennially (four repeats) using a battery of neuropsychological tests. The baseline oral health conditions were examined, including periodontal status, dental caries, tooth wear, and dentition. The association of these oral conditions with cognition was evaluated by generalized linear mixed models. Stratified analyses were performed by important covariates.

RESULTS

Over time, dental caries was associated with poor memory in two different logical memory tests (β^= -0.06 and β^= -0.04). Incomplete dentition with less than 28 teeth was associated with poor performance in attention (β^= -0.05) and verbal fluency (β^= -0.03). These associations became more evident in those with an elevated inflammatory marker (IL-6, β^= -0.11 to -0.08). In contrast, tooth wear was associated with better memory in two different logical memory tests (β^= 0.33 and β^= 0.36) and better executive function (β^= 0.06) over time, and this association became more evident in those with the lowest inflammatory marker (IL-6, β^= 0.10).

CONCLUSIONS

Dental caries and incomplete dentition were associated with poor memory, attention, and verbal fluency performance. Conversely, tooth wear was associated with better memory performance and executive function.

CLINICAL SIGNIFICANCE

For early prevention of dementia, an evaluation of multiple dental and periodontal status in older adults helps predict the risk of dementia in the preclinical phase. Maintaining intact tooth structure without caries progression and eventually tooth loss may help prevent the worsening of memory, attention, and verbal fluency over time.

Porphyromonas gingivalis LPS and Actinomyces naeslundii Conditioned Medium Enhance the Release of a Low Molecular Weight, Transcriptionally Active, Fragment of Glycogen Synthase-3 Kinase in IMR-32 Cell Line

Background

Glycogen synthase-3 kinase (GSK3) is one of the major contributors of tau hyperphosphorylation linked to neurofibrillary tangles in Alzheimer's disease (AD).

Objective

To determine a mechanism of GSK-3β activation by two periodontal bacteria consistently confirmed in AD autopsied brains.

Methods

Porphyromonas gingivalis FDC381 and Actinomyces naeslundii ATCC10301 conditioned media were collected. IMR-32 cells were challenged for 48 h with the conditioned media alongside P. gingivalis (ATCC33277) ultrapurified lipopolysaccharide (LPS) designated Pg.LPS under established cell culture conditions either alone or combined. Gene expression and protein analyses for GSK-3β were carried out.

Results

qPCR demonstrated that GSK-3β gene was overexpressed in IMR-32 cells treated with Pg.LPS with a 2.09-fold change (p = 0.0005), while A. naeslundii treated cells demonstrated 1.41-fold change (p = 0.004). Western blotting of the cells challenged with Pg.LPS (p = 0.01) and A. naeslundii conditioned medium (p = 0.001) demonstrated the 37 kDa band for each treatment with variable intensity across the medium control. Immunohistochemistry with the GSK-3β of the IMR-32 cells challenged with Pg.LPS and A. naeslundii alone demonstrated cytoplasmic and nuclear localization.

Conclusions

Exposure to various bacterial factors upregulated the gene expression of GSK-3β. Western blotting for GSK-3β confirmed the presence of the cleaved fragment by Pg.LPS (37 kDa band p = 0.01) and A. naeslundii conditioned medium (37 kDa band p = 0.001). Immunostaining demonstrated both cytoplasmic and nuclear localization of GSK-3β. Therefore, Pg.LPS and an unknown factor from the A. naeslundii conditioned medium mediated GSK-3β activation via its transcriptionally active, cleaved, fragment. These virulence factors in the body appear to be detrimental to brain health.

The emerging role of brain neuroinflammatory responses in Alzheimer's disease

As the most common cause of dementia, Alzheimer's disease (AD) is characterized by neurodegeneration and synaptic loss with an increasing prevalence in the elderly. Increased inflammatory responses triggers brain cells to produce pro-inflammatory cytokines and accelerates the Aβ accumulation, tau protein hyper-phosphorylation leading to neurodegeneration. Therefore, in this paper, we discuss the current understanding of how inflammation affects brain activity to induce AD pathology, the inflammatory biomarkers and possible therapies that combat inflammation for AD.

The neuroinflammatory role of microglia in Alzheimer's disease and their associated therapeutic targets

INTRODUCTION

Alzheimer's disease (AD), the main cause of dementia, is characterized by synaptic loss and neurodegeneration. Amyloid-β (Aβ) accumulation, hyperphosphorylation of tau protein, and neurofibrillary tangles (NFTs) in the brain are considered to be the initiating factors of AD. However, this hypothesis falls short of explaining many aspects of AD pathogenesis. Recently, there has been mounting evidence that neuroinflammation plays a key role in the pathophysiology of AD and causes neurodegeneration by over-activating microglia and releasing inflammatory mediators.

METHODS

PubMed, Web of Science, EMBASE, and MEDLINE were used for searching and summarizing all the recent publications related to inflammation and its association with Alzheimer's disease.

RESULTS

Our review shows how inflammatory dysregulation influences AD pathology as well as the roles of microglia in neuroinflammation, the possible microglia-associated therapeutic targets, top neuroinflammatory biomarkers, and anti-inflammatory drugs that combat inflammation.

CONCLUSION

In conclusion, microglial inflammatory reactions are important factors in AD pathogenesis and need to be discussed in more detail for promising therapeutic strategies.

Periodontitis and brain magnetic resonance imaging markers of Alzheimer's disease and cognitive aging

INTRODUCTION

We examined the association of clinical, microbiological, and host response features of periodontitis with MRI markers of atrophy/cerebrovascular disease in the Washington Heights Inwood Columbia Aging Project (WHICAP) Ancillary Study of Oral Health.

METHODS

We analyzed 468 participants with clinical periodontal data, microbial plaque and serum samples, and brain MRIs. We tested the association of periodontitis features with MRI features, after adjusting for multiple risk factors for Alzheimer's disease/Alzheimer's disease-related dementia (AD/ADRD).

RESULTS

In fully adjusted models, having more teeth was associated with lower odds for infarcts, lower white matter hyperintensity (WMH) volume, higher entorhinal cortex volume, and higher cortical thickness. Higher extent of periodontitis was associated with lower entorhinal cortex volume and lower cortical thickness. Differential associations emerged between colonization by specific bacteria/serum antibacterial IgG responses and MRI outcomes.

DISCUSSION

In an elderly cohort, clinical, microbiological, and serological features of periodontitis were associated with MRI findings related to ADRD risk. Further investigation of causal associations is warranted.

Porphyromonas gingivalis and the pathogenesis of Alzheimer's disease

The cause of Alzheimer's disease (AD), and the pathophysiological mechanisms involved, remain major unanswered questions in medical science. Oral bacteria, especially those species associated with chronic periodontitis and particularly Porphyromonas gingivalis, are being linked causally to AD pathophysiology in a subpopulation of susceptible individuals. P. gingivalis produces large amounts of proteolytic enzymes, haem and iron capture proteins, adhesins and internalins that are secreted and attached to the cell surface and concentrated onto outer membrane vesicles (OMVs). These enzymes and adhesive proteins have been shown to cause host tissue damage and stimulate inflammatory responses. The ecological and pathophysiological roles of P. gingivalis OMVs, their ability to disperse widely throughout the host and deliver functional proteins lead to the proposal that they may be the link between a P. gingivalis focal infection in the subgingivae during periodontitis and neurodegeneration in AD. P. gingivalis OMVs can cross the blood brain barrier and may accelerate AD-specific neuropathology by increasing neuroinflammation, plaque/tangle formation and dysregulation of iron homeostasis, thereby inducing ferroptosis leading to neuronal death and neurodegeneration.

The oral-brain axis: can periodontal pathogens trigger the onset and progression of Alzheimer's disease?

Alzheimer's disease (AD) is the most prevalent form of dementia, characterized by a progressive cognitive decline. Sporadic AD, accounting for more than 95% of cases, may arise due to the influence of environmental factors. It was reported that periodontitis, a common oral ailment, shares several risk factors with AD, including advanced age, smoking, diabetes, and hypertension, among others. Periodontitis is an inflammatory disease triggered by dysbiosis of oral microorganisms, whereas Alzheimer's disease is characterized by neuroinflammation. Many studies have indicated that chronic inflammation can instigate brain AD-related pathologies, including amyloid-β plaques, Tau protein hyperphosphorylation, neuroinflammation, and neurodegeneration. The potential involvement of periodontal pathogens and/or their virulence factors in the onset and progression of AD by the oral-brain axis has garnered significant attention among researchers with ongoing investigations. This review has updated the periodontal pathogens potentially associated with AD, elucidating their impact on the central nervous system, immune response, and related pathological processes in the brain to provide valuable insights for future research on the oral-brain axis.

Potential effects of specific gut microbiota on periodontal disease: a two-sample bidirectional Mendelian randomization study

Introduction

Periodontal disease (PD) presents a substantial global health challenge, encompassing conditions from reversible gingivitis to irreversible periodontitis, often culminating in tooth loss. The gut-oral axis has recently emerged as a focal point, with potential gut microbiota dysbiosis exacerbating PD.

Methods

In this study, we employed a double-sample bidirectional Mendelian randomized (MR) approach to investigate the causal relationship between specific gut microbiota and periodontal disease (PD) and bleeding gum (BG) development, while exploring the interplay between periodontal health and the gut microenvironment. We performed genome-wide association studies (GWAS) with two cohorts, totalling 346,731 (PD and control) and 461,113 (BG and control) participants, along with data from 14,306 participants' intestinal flora GWAS, encompassing 148 traits (31 families and 117 genera). Three MR methods were used to assess causality, with the in-verse-variance-weighted (IVW) measure as the primary outcome. Cochrane's Q test, MR-Egger, and MR-PRESSO global tests were used to detect heterogeneity and pleiotropy. The leave-one-out method was used to test the stability of the MR results. An F-statistic greater than 10 was accepted for instrument exposure association.

Results and conclusion

Specifically, Eubacterium xylanophilum and Lachnoclostridium were associated with reduced gum bleeding risk, whereas Anaerotruncus, Eisenbergiella, and Phascolarctobacterium were linked to reduced PD risk. Conversely, Fusicatenibacter was associated with an elevated risk of PD. No significant heterogeneity or pleiotropy was detected. In conclusion, our MR analysis pinpointed specific gut flora with causal connections to PD, offering potential avenues for oral health interventions.

Oral Health as a Risk Factor for Alzheimer Disease

In patients with Alzheimer's disease pathophysiological changes of the brain that initiate the onset of Alzheimer's disease include accumulation of amyloid-β plaques and phosphorylation of tau-tangles. A rather recently considered risk factor for the onset of Alzheimer's disease is poor oral health. The aim of this systematic review of the literature was to assess the potential association(s) of oral health as a risk factor for the onset of Alzheimer's disease. After a systematic search of Pubmed, Embase and Web of Science. A total of 1962 studies were assessed, of which 17 studies demonstrated possible associations between oral health diseases and Alzheimer's disease. 4 theories could be distinguished that describe the possible links between oral health and the development or onset of Alzheimer's disease; 1) role of pathogens, 2) role of inflammatory mediators, 3) role of APOE alleles and 4) role of Aβ peptide. The main common denominator of all the theories is the neuroinflammation due to poor oral health. Yet, there is insufficient evidence to prove a link due to the diversity of the designs used and the quality of the study design of the included studies. Therefore, further research is needed to find causal links between oral health and neuroinflammation that possibly can lead to the onset of Alzheimer's disease with the future intention to prevent cognitive decline by better dental care.

Oral microbiome and serological analyses on association of Alzheimer's disease and periodontitis

OBJECTIVE

To investigate the association between Alzheimer's disease (AD) and periodontitis in the aspects of periodontal status, serological markers, and oral microbiome.

MATERIALS AND METHODS

Twenty AD and 20 healthy subjects were enrolled in this age- and gender-matched case-control study. Clinical periodontal parameters and serum biomarkers, including amyloid β42 (Aβ42 ), Tau, phosphorylated Tau (pTau), triglyceride, pro-inflammatory cytokines, and anti-Porphyromonas gingivalis lipopolysaccharide (LPS) antibody were examined. The saliva samples were analyzed for oral microbiome composition.

RESULTS

Alzheimer's disease patients with Clinical Dementia Rating (CDR) ≥1 exhibited significantly more clinical attachment loss (CAL) than those with lower CDR. The levels of serum Tau protein, hsCRP and anti-P. gingivalis LPS antibody were markedly elevated in the AD group compared with the control group. Serum pTau protein level was positively correlated with anti-P. gingivalis LPS antibody titer. Moreover, the increased abundances of Capnocytophaga sp ora clone DZ074, Eubacterium infirmum, Prevotella buccae, and Selenomonas artemidis were detected in the AD group. Interestingly, serum levels of Aβ42, pTau, and anti-P. gingivalis LPS antibody were strongly related to the gene upregulation in human pathogen septicemia.

CONCLUSIONS

Our study suggested the association of periodontal infection and oral microbiome with AD. Further large-scale studies with longitudinal follow-up are warranted.

Neuroinflammation in Alzheimer's Disease: A Potential Role of Nose-Picking in Pathogen Entry via the Olfactory System?

Alzheimer's disease (AD) is a complex neurodegenerative disorder characterized by progressive cognitive decline and memory impairment. Many possible factors might contribute to the development of AD, including amyloid peptide and tau deposition, but more recent evidence suggests that neuroinflammation may also play an-at least partial-role in its pathogenesis. In recent years, emerging research has explored the possible involvement of external, invading pathogens in starting or accelerating the neuroinflammatory processes in AD. In this narrative review, we advance the hypothesis that neuroinflammation in AD might be partially caused by viral, bacterial, and fungal pathogens entering the brain through the nose and the olfactory system. The olfactory system represents a plausible route for pathogen entry, given its direct anatomical connection to the brain and its involvement in the early stages of AD. We discuss the potential mechanisms through which pathogens may exploit the olfactory pathway to initiate neuroinflammation, one of them being accidental exposure of the olfactory mucosa to hands contaminated with soil and feces when picking one's nose.

Nisin a probiotic bacteriocin mitigates brain microbiome dysbiosis and Alzheimer's disease-like neuroinflammation triggered by periodontal disease

INTRODUCTION

Periodontitis-related oral microbial dysbiosis is thought to contribute to Alzheimer's disease (AD) neuroinflammation and brain amyloid production. Since probiotics can modulate periodontitis/oral dysbiosis, this study examined the effects of a probiotic/lantibiotic, nisin, in modulating brain pathology triggered by periodontitis.

METHODS

A polymicrobial mouse model of periodontal disease was used to evaluate the effects of this disease on brain microbiome dysbiosis, neuroinflammation, Alzheimer's-related changes, and nisin's therapeutic potential in this context.

RESULTS

16S sequencing and real-time PCR data revealed that Nisin treatment mitigated the changes in the brain microbiome composition, diversity, and community structure, and reduced the levels of periodontal pathogen DNA in the brain induced by periodontal disease. Nisin treatment significantly decreased the mRNA expression of pro-inflammatory cytokines (Interleukin-1β/IL-1 β, Interleukin 6/IL-6, and Tumor Necrosis Factor α/TNF-α) in the brain that were elevated by periodontal infection. In addition, the concentrations of amyloid-β 42 (Aβ42), total Tau, and Tau (pS199) (445.69 ± 120.03, 1420.85 ± 331.40, 137.20 ± 36.01) were significantly higher in the infection group compared to the control group (193.01 ± 31.82, 384.27 ± 363.93, 6.09 ± 10.85), respectively. Nisin treatment markedly reduced the Aβ42 (261.80 ± 52.50), total Tau (865.37 ± 304.93), and phosphorylated Tau (82.53 ± 15.77) deposition in the brain of the infection group.

DISCUSSION

Nisin abrogation of brain microbiome dysbiosis induces beneficial effects on AD-like pathogenic changes and neuroinflammation, and thereby may serve as a potential therapeutic for periodontal-dysbiosis-related AD.

The role of periodontitis-associated bacteria in Alzheimer's disease: A narrative review

Alzheimer's disease causes memory loss and dementia in older adults through a neurodegenerative mechanism. Despite the pathophysiological clarification of this cognitive disorder, novel molecular and cellular pathways should be identified to determine its exact mechanism. Alzheimer's disease (AD) is pathologically characterized by senile plaques comprising beta-amyloid and neurofibrillary tangles (NFTs) formed by hyperphosphorylated tau as a microtubule-associated protein with a key role in the pathogenesis of AD. Periodontitis through inflammatory pathways is a risk factor for deteriorating cognitive impairment in AD patients. Poor oral hygiene coupled with immunocompromised status in older adults causes periodontal diseases and chronic inflammations through an oral bacterial imbalance. Toxic bacterial products, including bacteria themselves, can reach the central nervous system through the bloodstream and evoke inflammatory responses. The present review was conducted to investigate relationships between AD and periodontitis-involved bacteria as a risk factor.

Effect of Periodontal Disease on Alzheimer's Disease: A Systematic Review

The aim of this review was to evaluate the relationship between periodontal disease (PD) and the onset and progression of Alzheimer's disease (AD) and to determine whether patients with PD would be at greater risk of developing AD compared to periodontally healthy subjects. This systematic review was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. An electronic search for cross-sectional, cohort, or case-control studies was conducted on five databases (PubMed, ScienceDirect, EBSCO, Web of Science, and Scopus). No restrictions were applied to the language and year of publication. Exposure was PD, and the outcome of interest was the onset and/or progression of AD. The risk of bias of the included studies was assessed using the Newcastle-Ottawa Scale (NOS) designed for non-randomized studies. Six studies fulfilling the selection criteria were included in this systematic review. Four of the studies were of cohort design and two were of case-control design. All except one showed a significant association between PD and the risk of AD onset and progression. According to the NOS bias risk assessment, three studies were found to be of good quality, and three other cohort studies were of low quality. Data from this systematic review indicate that patients with PD present a significantly higher risk of AD compared to individuals with healthy periodontium. However, results should be interpreted with caution given the methodological limitations found. For future research, powerful and comparable epidemiological studies are needed to evaluate the relationship between PD and AD.

Importance of oral health in mental health disorders: An updated review

Background

Mental disorders are indeed an expanding threat, which requires raised awareness, education, prevention, and treatment initiatives nationally and globally. This review presents an updated review on the relationships between oral health and mental health disorders and the importance of oral health in mental health disorders.

Method

A literature search was done regarding mental disorders and oral health approaches in Google Scholar and PubMed from the year 1995 until 2023. All the English-language papers were evaluated based on the inclusion criteria. Publications included original research papers, review articles and book chapters.

Results

Common mental disorders include depression, anxiety, bipolar disorder, Schizophrenia, dementia, and alcohol and drug use disorders. The interplay of oral health and mental disorders involves dysregulated microbiome, translocated bacteria, and systemic inflammation, among others.

Conclusion

There is a complex relationship between mental disorders and oral diseases. Various oral health problems are associated with mental health problems. The interplay of oral health and mental disorders involves dysregulated microbiome, translocated bacteria, and systemic inflammation, among others. Mental health nurses including physicians and dental professionals should be involved in the oral health care of mental health disorder patients. Therefore, multidisciplinary should be involved in the care of mental health disorders, and they should consider oral health care as an essential part of their care for patients with mental health disorders. Future investigations should strive to elucidate the exact biological relationships, to develop new directions for treatment.

Investigating the relationship between periodontitis and specific memory processes in the search for cognitive markers of Alzheimer's disease risk

The spontaneous retrieval deficit (SRD) hypothesis argues that individuals in the preclinical stages of Alzheimer's disease (AD) are particularly impaired in spontaneous retrieval, which manifests in reduced mind-wandering. Our main purpose was to provide novel evidence to support the SRD hypothesis by investigating, for the first time, the relationship between mind-wandering and periodontitis, the latter being the risk factor for AD. The second objective was to address the lack of deeper understanding of the relationship between oral health and specific cognitive abilities by investigating whether periodontitis would be primarily associated with memory. Sixty community-dwelling dementia-free older adults completed neuropsychological tests that focused on various cognitive abilities and a computerised task, during which mind-wandering was evaluated. Periodontal health was assessed subjectively, and through an oral examination by a qualified dentist that focused on visible periodontitis-related changes in gingival tissues and the number of periodontitis bacteria. In line with our predictions, objective and subjective symptoms of poorer periodontal health were associated with less mind-wandering, providing further support for the SRD hypothesis. Again in line with predictions, poorer periodontal health was associated with worse episodic memory, with no relationship between periodontitis and the measure targeting various cognitive abilities, from which memory was excluded.

Antibodies to oral pathobionts and colon cancer risk in the CLUE I cohort study

Periodontitis has been associated with an increased risk for gastrointestinal cancers. The objective of our study was to investigate the association of antibodies to oral bacteria and the risk of colon cancer in a cohort setting. Using the CLUE I cohort, a prospective cohort initiated in 1974 in Washington County, Maryland, we conducted a nested case-control study to examine the association of levels of IgG antibodies to 11 oral bacterial species (13 total strains) with risk of colon cancer diagnosed a median of 16 years later (range: 1-26 years). Antibody response was measured using checkerboard immunoblotting assays. We included 200 colon cancer cases and 200 controls matched on age, sex, cigarette smoking status, time of blood draw and pipe or cigar smoking status. Controls were selected using incidence density sampling. Conditional logistic regression models were used to assess the association between antibody levels and colon cancer risk. In the overall analysis, we observed significant inverse associations for 6 of the 13 antibodies measured (P-trends <.05) and one positive association for antibody levels to Aggregatibacter actinomycetemcomitans (ATCC 29523; P-trend = .04). While we cannot rule out a role for periodontal disease in colon cancer risk, findings from our study suggest that a strong adaptive immune response may be associated with a lower risk of colon cancer. More studies will need to examine whether the positive associations we observed with antibodies to A. actinomycetemcomitans reflect a true causal association for this bacterium.

The role of microbiome-host interactions in the development of Alzheimer´s disease

Alzheimer`s disease (AD) is the most prevalent cause of dementia. It is often assumed that AD is caused by an aggregation of extracellular beta-amyloid and intracellular tau-protein, supported by a recent study showing reduced brain amyloid levels and reduced cognitive decline under treatment with a beta-amyloid-binding antibody. Confirmation of the importance of amyloid as a therapeutic target notwithstanding, the underlying causes of beta-amyloid aggregation in the human brain, however, remain to be elucidated. Multiple lines of evidence point towards an important role of infectious agents and/or inflammatory conditions in the etiology of AD. Various microorganisms have been detected in the cerebrospinal fluid and brains of AD-patients and have thus been hypothesized to be linked to the development of AD, including Porphyromonas gingivalis (PG) and Spirochaetes. Intriguingly, these microorganisms are also found in the oral cavity under normal physiological conditions, which is often affected by multiple pathologies like caries or tooth loss in AD patients. Oral cavity pathologies are mostly accompanied by a compositional shift in the community of oral microbiota, mainly affecting commensal microorganisms and referred to as 'dysbiosis'. Oral dysbiosis seems to be at least partly mediated by key pathogens such as PG, and it is associated with a pro-inflammatory state that promotes the destruction of connective tissue in the mouth, possibly enabling the translocation of pathogenic microbiota from the oral cavity to the nervous system. It has therefore been hypothesized that dysbiosis of the oral microbiome may contribute to the development of AD. In this review, we discuss the infectious hypothesis of AD in the light of the oral microbiome and microbiome-host interactions, which may contribute to or even cause the development of AD. We discuss technical challenges relating to the detection of microorganisms in relevant body fluids and approaches for avoiding false-positives, and introduce the antibacterial protein lactoferrin as a potential link between the dysbiotic microbiome and the host inflammatory reaction.

Periodontal Health and Disease in the Context of Systemic Diseases

During recent years, considerable progress has been made in understanding the etiopathogenesis of periodontitis in its various forms and their interactions with the host. Furthermore, a number of reports have highlighted the importance of oral health and disease in systemic conditions, especially cardiovascular diseases and diabetes. In this regard, research has attempted to explain the role of periodontitis in promoting alteration in distant sites and organs. Recently, DNA sequencing studies have revealed how oral infections can occur in distant sites such as the colon, reproductive tissues, metabolic diseases, and atheromas. The objective of this review is to describe and update the emerging evidence and knowledge regarding the association between periodontitis and systemic disease and to analyse the evidence that has reported periodontitis as a risk factor for the development of various forms of systemic diseases in order to provide a better understanding of the possible shared etiopathogenetic pathways between periodontitis and the different forms of systemic diseases.

Antibodies against Periodontal Microorganisms and Cognition in Older Adults

INTRODUCTION

Markers of poor oral health are associated with impaired cognition and higher risk of Alzheimer disease (AD) and thus may help predict AD.

OBJECTIVES

The aim of this study was to evaluate the cross-sectional association between empirically derived groups of 19 IgG antibodies against periodontal microorganisms and cognition in middle-aged and older adults.

METHODS

The study population consisted of participants of the third National Health and Nutrition Examination Survey (NHANES III) (1988 to 1994), who were 60 y and older, among whom cognition and IgG antibodies against 19 periodontal microorganisms were measured (N = 5,162).

RESULTS

In multivariable quantile regression analyses, the Orange-Red (Prevotella melaninogenica, Prevotella intermedia, Prevotella nigrescens, Porphyromonas gingivalis) and Yellow-Orange (Staphylococcus intermedius, Streptococcus oralis, Streptococcus mutans, Fusobacterium nucleatum, Peptostreptococcus micros, Capnocytophaga ochracea) cluster scores were negatively associated with cognition. A 1-unit higher cluster score for the Orange-Red cluster was associated on average with a lower cognitive score (β for 30th quantile = -0.2640; 95% confidence interval [CI], -0.3431 to -0.1848). Similarly, a 1-unit higher score for the Yellow-Orange cluster was associated with a lower cognitive score (β for 30th quantile = -0.2445; 95% CI, -0.3517 to -0.1372).

CONCLUSION

Groups of IgG antibodies against periodontal microorganisms were associated with lower cognition among free living adults 60 years and older, who were previously undiagnosed with cognitive impairment. Though poor oral health precedes the development of dementia and AD, oral health information is currently not used, to our knowledge, to predict dementia or AD risk. Combining our findings with current algorithms may improve risk prediction for dementia and AD.

KNOWLEDGE TRANSLATION STATEMENT

IgG antibodies against periodontal microorganisms were associated with lower cognition among adults 60 years and older previously undiagnosed with cognitive impairment. Periodontal disease may predict cognition among older adults.

Genetic correlations between Alzheimer's disease and gut microbiome genera

A growing body of evidence suggests that dysbiosis of the human gut microbiota is associated with neurodegenerative diseases like Alzheimer's disease (AD) via neuroinflammatory processes across the microbiota-gut-brain axis. The gut microbiota affects brain health through the secretion of toxins and short-chain fatty acids, which modulates gut permeability and numerous immune functions. Observational studies indicate that AD patients have reduced microbiome diversity, which could contribute to the pathogenesis of the disease. Uncovering the genetic basis of microbial abundance and its effect on AD could suggest lifestyle changes that may reduce an individual's risk for the disease. Using the largest genome-wide association study of gut microbiota genera from the MiBioGen consortium, we used polygenic risk score (PRS) analyses with the "best-fit" model implemented in PRSice-2 and determined the genetic correlation between 119 genera and AD in a discovery sample (ADc12 case/control: 1278/1293). To confirm the results from the discovery sample, we next repeated the PRS analysis in a replication sample (GenADA case/control: 799/778) and then performed a meta-analysis with the PRS results from both samples. Finally, we conducted a linear regression analysis to assess the correlation between the PRSs for the significant genera and the APOE genotypes. In the discovery sample, 20 gut microbiota genera were initially identified as genetically associated with AD case/control status. Of these 20, three genera (Eubacterium fissicatena as a protective factor, Collinsella, and Veillonella as a risk factor) were independently significant in the replication sample. Meta-analysis with discovery and replication samples confirmed that ten genera had a significant correlation with AD, four of which were significantly associated with the APOE rs429358 risk allele in a direction consistent with their protective/risk designation in AD association. Notably, the proinflammatory genus Collinsella, identified as a risk factor for AD, was positively correlated with the APOE rs429358 risk allele in both samples. Overall, the host genetic factors influencing the abundance of ten genera are significantly associated with AD, suggesting that these genera may serve as biomarkers and targets for AD treatment and intervention. Our results highlight that proinflammatory gut microbiota might promote AD development through interaction with APOE. Larger datasets and functional studies are required to understand their causal relationships.

Subgingival host-microbiome metatranscriptomic changes following scaling and root planing in grade II/III periodontitis

AIM

To assess the effects of scaling and root planing (SRP) on the dynamics of gene expression by the host and the microbiome in subgingival plaque samples.

MATERIALS AND METHODS

Fourteen periodontitis patients were closely monitored in the absence of periodontal treatment for 12 months. During this period, comprehensive periodontal examination and subgingival biofilm sample collection were performed bi-monthly. After 12 months, clinical attachment level (CAL) data were compiled and analysed using linear mixed models (LMM) fitted to longitudinal CAL measurements for each tooth site. LMM classified the sites as stable (S), progressing (P), or fluctuating (F). After the 12-month visit, subjects received SRP, and at 15 months they received comprehensive examination and supportive periodontal therapy. Those procedures were repeated at the 18-month visit, when patients were also sampled. Each patient contributed with one S, one P, and one F site collected at the 12- and 18-month visits. Samples were analysed using Dual RNA-Sequencing to capture host and bacterial transcriptomes simultaneously.

RESULTS

Microbiome and host response behaviour were specific to the site's progression classification (i.e., S, P, or F). Microbial profiles of pre- and post-treatment samples exhibited specific microbiome changes, with progressing sites showing the most significant changes. Among them, Porphyromonas gingivalis was reduced after treatment, while Fusobacterium nucleatum showed an increase in proportion. Transcriptome analysis of the host response showed that interleukin (IL)-17, TNF signalling pathways, and neutrophil extracellular trap formation were the primary immune response activities impacted by periodontal treatment.

CONCLUSIONS

SRP resulted in a significant "rewiring" of host and microbial activities in the progressing sites, while restructuring of the microbiome was minor in stable and fluctuating sites.

Subgingival microbiome at different levels of cognition

Oral health and declining cognition may have a bi-directional association. We characterized the subgingival microbiota composition of subjects from normal cognition to severe cognitive decline in two cohorts. Memory and Periodontitis (MINOPAR) include 202 home-living participants (50-80 years) in Sweden. Finnish Oral Health Studies in Older Adults (FINORAL) include 174 participants (≥65 years) living in long-term care in Finland. We performed oral examination and assessed the cognitive level with Mini Mental State Examination (MMSE). We sequenced the 16S-rRNA gene (V3-V4 regions) to analyse the subgingival bacterial compositions. The microbial diversities only tended to differ between the MMSE categories, and the strongest determinants were increased probing pocket depth (PPD) and presence of caries. However, abundances of 101 taxa were associated with the MMSE score. After adjusting for age, sex, medications, PPD, and caries, only eight taxa retained the significance in the meta-analyses of the two cohorts. Especially Lachnospiraceae [XIV] at the family, genus, and species level increased with decreasing MMSE. Cognitive decline is associated with obvious changes in the composition of the oral microbiota. Impaired cognition is accompanied with poor oral health status and the appearance of major taxa of the gut microbiota in the oral cavity. Good oral health-care practices require special deliberations among older adults.

The impact of study factors in the association of periodontal disease and cognitive disorders: systematic review and meta-analysis

AIM

The aim was to assess study factors that impact the association of cognitive disorders in people with periodontal disease (PD).

METHOD

Medline, EMBASE and Cochrane databases were searched until February 2022 using keywords and MeSH: (periodon* OR tooth loss OR missing teeth) AND (dementia OR Alzheimer's Disease OR cognitive*). Observational studies reporting prevalence or risk of cognitive decline, dementia or Alzheimer's disease (AD) in people with PD compared with healthy controls were included. Meta-analysis quantified the prevalence and risk (relative risk[RR]) of cognitive decline, dementia/AD, respectively. Meta-regression/subgroup analysis explored the impact of study factors including PD severity and classification type, and gender.

RESULTS

Overall, 39 studies were eligible for meta-analysis: 13 cross-sectional and 26 longitudinal studies. PD demonstrated increased risks of cognitive disorders (cognitive decline-RR = 1.33, 95% CI = 1.13-1.55; dementia/AD-RR = 1.22, 95% CI = 1.14-1.31). Risk of cognitive decline increased with PD severity (moderate-[RR] = 1.14, 95% confidence interval [CI] = 1.07-1.22; severe-RR = 1.25, 95% CI = 1.18-1.32). For every 10% population increase in females, the risk of cognitive decline increased by 34% (RR = 1.34, 95% CI = 1.16-1.55). Self-reported PD showed a lower risk of cognitive disorders compared with clinical classification (cognitive decline-RR = 0.77, 95% CI = 0.65-0.91; dementia/AD-RR = 0.86, 95% CI = 0.77-0.96).

CONCLUSION

The prevalence and risk estimates of cognitive disorders in association with PD can be influenced by gender, the disease classification of PD and its severity. Further homologous evidence taking these study factors into consideration is needed to form robust conclusions.

Association Between Oral Bacteria and Alzheimer's Disease: A Systematic Review and Meta-Analysis

BACKGROUND

Pre-clinical evidence implicates oral bacteria in the pathogenesis of Alzheimer's disease (AD), while clinical studies show diverse results.

OBJECTIVE

To comprehensively assess the association between oral bacteria and AD with clinical evidence.

METHODS

Studies investigating the association between oral bacteria and AD were identified through a systematic search of six databases PubMed, Embase, Cochrane Central Library, Scopus, ScienceDirect, and Web of Science. Methodological quality ratings of the included studies were performed. A best evidence synthesis was employed to integrate the results. When applicable, a meta-analysis was conducted using a random-effect model.

RESULTS

Of the 16 studies included, ten investigated periodontal pathobionts and six were microbiome-wide association studies. Samples from the brain, serum, and oral cavity were tested. We found over a ten-fold and six-fold increased risk of AD when there were oral bacteria (OR = 10.68 95% CI: 4.48-25.43; p < 0.00001, I2 = 0%) and Porphyromonas gingivalis (OR = 6.84 95% CI: 2.70-17.31; p < 0.0001, I2 = 0%) respectively in the brain. While AD patients exhibited lower alpha diversity of oral microbiota than healthy controls, the findings of bacterial communities were inconsistent among studies. The best evidence synthesis suggested a moderate level of evidence for an overall association between oral bacteria and AD and for oral bacteria being a risk factor for AD.

CONCLUSION

Current evidence moderately supports the association between oral bacteria and AD, while the association was strong when oral bacteria were detectable in the brain. Further evidence is needed to clarify the interrelationship between both individual species and bacterial communities and the development of AD.

P. gingivalis-LPS Induces Mitochondrial Dysfunction Mediated by Neuroinflammation through Oxidative Stress

Porphyromonas gingivalis (P. gingivalis), a key pathogen in periodontitis, is associated with neuroinflammation. Periodontal disease increases with age; 70.1% of adults 65 years and older have periodontal problems. However, the P. gingivalis- lipopolysaccharide (LPS)induced mitochondrial dysfunction in neurodegenerative diseases remains elusive. In this study, we investigated the possible role of P. gingivalis-LPS in mitochondrial dysfunction during neurodegeneration. We found that P. gingivalis-LPS treatment activated toll-like receptor (TLR) 4 signaling and upregulated the expression of Alzheimer's disease-related dementia and neuroinflammatory markers. Furthermore, the LPS treatment significantly exacerbated the production of reactive oxygen species and reduced the mitochondrial membrane potential. Our study highlighted the pivotal role of P. gingivalis-LPS in the repression of serum response factor (SRF) and its co-factor p49/STRAP that regulate the actin cytoskeleton. The LPS treatment repressed the genes involved in mitochondrial function and biogenesis. P. gingivalis-LPS negatively altered oxidative phosphorylation and glycolysis and reduced total adenosine triphosphate (ATP) production. Additionally, it specifically altered the mitochondrial functions in complexes I, II, and IV of the mitochondrial electron transport chain. Thus, it is conceivable that P. gingivalis-LPS causes mitochondrial dysfunction through oxidative stress and inflammatory events in neurodegenerative diseases.

With Teeth, Broken, or Fixed: The Challenges of Linking Periodontitis, Neuroepidemiology, and Biomarkers of Disease

Periodontitis is a chronic inflammatory, bacterially-driven disease of the tooth-supporting tissues that shares several risk factors and elements of host response with Alzheimer's disease and related dementias (ADRD). Epidemiological studies have identified relatively consistent associations between adverse oral health conditions and ADRD. In this issue of the journal, a large study from the UK Biobank further explores these relationships along with MRI cognitive biomarkers. Despite its strength due to the large sample size, challenges in the study of periodontitis and neuroepidemiology markers include potential sampling bias, appropriate assessment of exposures, and the possibility of reverse causality.

High resolution 16S rRNA gene Next Generation Sequencing study of brain areas associated with Alzheimer's and Parkinson's disease

Introduction

Alzheimer's (AD) and Parkinson's disease (PD) are neurodegenerative conditions characterized by incremental deposition of β-amyloid (Aβ) and α-synuclein in AD and PD brain, respectively, in relatively conserved patterns. Both are associated with neuroinflammation, with a proposed microbial component for disease initiation and/or progression. Notably, Aβ and α-synuclein have been shown to possess antimicrobial properties. There is evidence for bacterial presence within the brain, including the oral pathobiont Porphyromonas gingivalis, with cognitive impairment and brain pathology being linked to periodontal (gum) disease and gut dysbiosis.

Methods

Here, we use high resolution 16S rRNA PCR-based Next Generation Sequencing (16SNGS) to characterize bacterial composition in brain areas associated with the early, intermediate and late-stage of the diseases.

Results and discussion

This study reveals the widespread presence of bacteria in areas of the brain associated with AD and PD pathology, with distinctly different bacterial profiles in blood and brain. Brain area profiles were overall somewhat similar, predominantly oral, with some bacteria subgingival and oronasal in origin, and relatively comparable profiles in AD and PD brain. However, brain areas associated with early disease development, such as the locus coeruleus, were substantially different in bacterial DNA content compared to areas affected later in disease etiology.

Is Periodontitis Associated with Age-Related Cognitive Impairment? The Systematic Review, Confounders Assessment and Meta-Analysis of Clinical Studies

It has been suggested that molecular pathological mechanisms responsible for periodontitis can be linked with biochemical alterations in neurodegenerative disorders. Hypothetically, chronic systemic inflammation as a response to periodontitis plays a role in the etiology of cognitive impairment. This study aimed to determine whether periodontitis (PDS) is a risk factor for age-related cognitive impairment (ACI) based on evidence of clinical studies. A comprehensive, structured systematic review of existing data adhering to the Preferred Reporting Items for Systematic Review and Meta Analyses (PRISMA) guidelines was carried out. Five electronic databases, PubMed, Embase, Scopus, Web of Science, and Cochrane, were searched for key terms published in peer-reviewed journals until January 2021. The Newcastle-Ottawa scale was used to assess the quality of studies and risk of bias. The primary and residual confounders were explored and evaluated. A meta-analysis synthesizing quantitative data was carried out using a random-effects model. Seventeen clinical studies were identified, including 14 cohort, one cross-sectional, and two case-control studies. Study samples ranged from 85 to 262,349 subjects, with follow-up between 2 and 32 years, and age above 45 years, except for two studies. The findings of studies suggesting the PDS-ACI relationship revealed substantial differences in design and methods. A noticeable variation related to the treatment of confounders was observed. Quality assessment unveiled a moderate quality of evidence and risk of bias. The subgroups meta-analysis and pooled sensitivity analysis of results from seven eligible studies demonstrated overall that the presence of PDS is associated with an increased risk of incidence of cognitive impairment (OR = 1.36, 95% CI 1.03-1.79), particularly dementia (OR = 1.39, 95% CI 1.02-1.88) and Alzheimer's disease (OR = 1.03 95% CI 0.98-1.07)). However, a considerable heterogeneity of synthesized data (I2 = 96%) and potential publication bias might affect obtained results. While there is a moderate statistical association between periodontitis and dementia, as well as Alzheimer's disease, the risk of bias in the evidence prevents conclusions being drawn about the role of periodontitis as a risk factor for age-related cognitive impairment.

Interdisciplinary Approaches to Deal with Alzheimer's Disease-From Bench to Bedside: What Feasible Options Do Already Exist Today?

Alzheimer's disease is one of the most common neurodegenerative diseases in the western population. The incidence of this disease increases with age. Rising life expectancy and the resulting increase in the ratio of elderly in the population are likely to exacerbate socioeconomic problems. Alzheimer's disease is a multifactorial disease. In addition to amyloidogenic processing leading to plaques, and tau pathology, but also other molecular causes such as oxidative stress or inflammation play a crucial role. We summarize the molecular mechanisms leading to Alzheimer's disease and which potential interventions are known to interfere with these mechanisms, focusing on nutritional approaches and physical activity but also the beneficial effects of cognition-oriented treatments with a focus on language and communication. Interestingly, recent findings also suggest a causal link between oral conditions, such as periodontitis or edentulism, and Alzheimer's disease, raising the question of whether dental intervention in Alzheimer's patients can be beneficial as well. Unfortunately, all previous single-domain interventions have been shown to have limited benefit to patients. However, the latest studies indicate that combining these efforts into multidomain approaches may have increased preventive or therapeutic potential. Therefore, as another emphasis in this review, we provide an overview of current literature dealing with studies combining the above-mentioned approaches and discuss potential advantages compared to monotherapies. Considering current literature and intervention options, we also propose a multidomain interdisciplinary approach for the treatment of Alzheimer's disease patients that synergistically links the individual approaches. In conclusion, this review highlights the need to combine different approaches in an interdisciplinary manner, to address the future challenges of Alzheimer's disease.

The gut microbiome and Alzheimer's disease: Complex and bidirectional interactions

Structural and functional alterations to the gut microbiome, referred to as gut dysbiosis, have emerged as potential key mediators of neurodegeneration and Alzheimer disease (AD) pathogenesis through the "gut -brain" axis. Emerging data from animal and clinical studies support an important role for gut dysbiosis in mediating neuroinflammation, central and peripheral immune dysregulation, abnormal brain protein aggregation, and impaired intestinal and brain barrier permeability, leading to neuronal loss and cognitive impairment. Gut dysbiosis has also been shown to directly influence various mechanisms involved in neuronal growth and repair, synaptic plasticity, and memory and learning functions. Aging and lifestyle factors including diet, exercise, sleep, and stress influence AD risk through gut dysbiosis. Furthermore, AD is associated with characteristic gut microbial signatures which offer value as potential markers of disease severity and progression. Together, these findings suggest the presence of a complex bidirectional relationship between AD and the gut microbiome and highlight the utility of gut modulation strategies as potential preventative or therapeutic strategies in AD. We here review the current literature regarding the role of the gut-brain axis in AD pathogenesis and its potential role as a future therapeutic target in AD treatment and/or prevention.

Microbiome influences on neuro-immune interactions in neurodegenerative disease

Mounting evidence points to a role for the gut microbiome in a wide range of central nervous system diseases and disorders including depression, multiple sclerosis, Alzheimer's disease, Parkinson's disease, and autism spectrum disorder. Moreover, immune system involvement has also been implicated in these diseases, specifically with inflammation being central to their pathogenesis. In addition to the reported changes in gut microbiome composition and altered immune states in many neurological diseases, how the microbiome and the immune system interact to influence disease onset and progression has recently garnered much attention. This chapter provides a review of the literature related to gut microbiome influences on neuro-immune interactions with a particular focus on neurological diseases. Gut microbiome-derived mediators, including short-chain fatty acids and other metabolites, lipopolysaccharide, and neurotransmitters, and their impact on neuro-immune interactions as well as routes by which these interactions may occur are also discussed.

Outer membrane vesicles of Porphyromonas gingivalis trigger NLRP3 inflammasome and induce neuroinflammation, tau phosphorylation, and memory dysfunction in mice

Background

Porphyromonas gingivalis (Pg), the keystone pathogen in chronic periodontitis, is reported to initiate Alzheimer’s disease pathologies in preclinical studies. However, the specific mechanisms and signaling pathways acting on the brain still need to be further explored. Outer membrane vesicles are derived from Gram-negative bacteria and contain many virulence factors of bacteria. We hypothesized that outer membrane vesicles are an important weapon of Porphyromonas gingivalis to initiate Alzheimer’s disease pathologies.

Methods

The outer membrane vesicles of Porphyromonas gingivalis (Pg OMVs, 4 mg/kg) or saline were delivered to 14-month-old mice by oral gavage every other day for eight weeks. Behavioral alterations were assessed by the open field test, Morris water maze, and Y-maze test. Blood–brain barrier permeability, neuroinflammation, tau phosphorylation, and NLRP3 inflammasome-related protein were analyzed.

Results

Pg OMVs impaired memory and learning ability of mice and decreased tight junction–related gene expression ZO-1, occludin, claudin-5, and occludin protein expression in the hippocampus. Pg OMVs could be detected in the hippocampus and cortex three days after oral gavage. Furthermore, Pg OMVs activated both astrocytes and microglia and elevated IL-1β, tau phosphorylation on the Thr231 site, and NLRP3 inflammasome–related protein expression in the hippocampus. In in vitro studies, Pg OMV (5 µg/ml) stimulation increased the mRNA and immunofluorescence of NLRP3 in BV2 microglia, which were significantly inhibited by the NLRP3 inhibitor MCC950. In contrast, the tau phosphorylation in N2a neurons was enhanced after treatment with conditioned media from Pg OMV-stimulated microglia, which was attenuated after pretreatment with MCC950.

Conclusions

These results indicate that Pg OMVs prompt memory dysfunction, neuroinflammation, and tau phosphorylation and trigger NLRP3 inflammasome in the brain of middle-aged mice. We propose that Pg OMVs play an important role in activating neuroinflammation in the AD-like pathology triggered by Porphyromonas gingivalis, and NLRP3 inflammasome activation is a possible mechanism.

Neuroinflammation in neurodegeneration via microbial infections

Recent epidemiological studies show a noticeable correlation between chronic microbial infections and neurological disorders. However, the underlying mechanisms are still not clear due to the biological complexity of multicellular and multiorgan interactions upon microbial infections. In this review, we show the infection leading to neurodegeneration mediated by multiorgan interconnections and neuroinflammation. Firstly, we highlight three inter-organ communications as possible routes from infection sites to the brain: nose-brain axis, lung-brain axis, and gut-brain axis. Next, we described the biological crosstalk between microglia and astrocytes upon pathogenic infection. Finally, our study indicates how neuroinflammation is a critical player in pathogen-mediated neurodegeneration. Taken together, we envision that antibiotics targeting neuro-pathogens could be a potential therapeutic strategy for neurodegeneration.

Oral Microbiota, Its Equilibrium and Implications in the Pathophysiology of Human Diseases: A Systematic Review

Imbalances of the oral microbiota and dysbiosis have traditionally been linked to the occurrence of teeth and oral diseases. However, recent findings indicate that this microbiota exerts relevant influence in systemic health. Dysbiosis of the oral microbiota is implicated in the apparition and progression of cardiovascular, neurodegenerative and other major human diseases. In fact, the oral microbiota are the second most diverse and largely populated microbiota of the human body and its relationships with systemic health, although widely explored, they still lack of proper integration. The purpose of this systematic review is thus to widely examine the implications of oral microbiota in oral, cardiovascular and neurodegenerative diseases to offer integrative and up-to-date interpretations. To achieve that aim, we identified a total of 121 studies curated in PUBMED from the time interval January 2003–April 2022, which after careful screening resulted in 79 studies included. The reviewed scientific literature provides plausible vias of implication of dysbiotic oral microbiota in systemic human diseases, and encourages further research to continue elucidating the highly relevant and still poorly understood implications of this niche microbiota in systemic health. PROSPERO Registration Number: CRD42022299692. This systematic review follows relevant PRISMA guidelines.

Association of periodontitis and oral microbiomes with Alzheimer’s disease: A narrative systematic review

Background/purpose

Alzheimer’s disease (AD) is a neurodegenerative disorder and the most common form of dementia. The etiology for AD includes age, genetic susceptibility, neuropathology, and infection. Periodontitis is an infectious and inflammatory disease which mainly causes alveolar bone destruction and tooth loss. The evidence of a link between AD and periodontitis remains controversial. Thus far, studies reviewing the association between AD and periodontal disease have been insufficient from the viewpoint of the oral microbiome. The aim of this review was to focus on studies that have explored the relationship between the oral microbiome and AD development by using the next-generation sequencing technique.

Materials and methods

A comprehensive electronic search of MEDLINE via PubMed, EMBASE, Scopus, and Google Scholar was conducted. The keywords included dementia, Alzheimer’s disease, cognitive impairment, periodontitis, periodontal disease, and oral microbiome.

Results

This review included 26 articles based on the eligibility criteria. Epidemiologic researches and post-mortem studies showed that the presence of periodontitis is associated with cognitive decline, suggesting a possible role of periodontal pathogens in the pathogenesis of AD. The reported microbiome was inconsistent with those in gene sequencing studies. Nevertheless, Gram-negative species may be possible candidates.

Conclusion

This review suggests that periodontal infection is associated with AD. The contributing microbiome remains unconfirmed, possibly because of different microbiome sampling sites or methods. Additional large-scale studies with periodontal intervention and longitudinal follow-up are warranted to clarify the relationship between periodontal disease and AD.

Replication of gene polymorphisms associated with periodontitis-related traits in an elderly cohort: the Washington Heights/Inwood Community Aging Project Ancillary Study of Oral Health

AIM

We sought to replicate findings from published genome-wide association studies (GWAS), linking specific candidate gene loci with periodontitis-related clinical/microbial traits.

MATERIALS AND METHODS

In the published GWAS, a total of 2196 single nucleotide polymorphisms associated with periodontitis-related traits at a p ≤ 5 × 10^-6 and mapped to 136 gene loci. The replication cohort included 1124 individuals, 65-98 years old (67% female, 45% Hispanic, 30% Black, 23% White) with available genome-wide genotypes and full-mouth periodontal status. Microbial profiles using checkerboard DNA-DNA hybridization and 16SrRNA sequencing were available from 912 and 739 participants, respectively.

RESULTS

Using gene-specific p-values after linkage disequilibrium pruning, the following gene/phenotype associations replicated successfully: CLEC19A with edentulism and %teeth with pocket depth (PD) ≥4 mm; IL37, HPVC1, TRPS1, ABHD12B, LDLRAD4 (C180rF1), TGM3, and GRK5 with %teeth with PD ≥4 mm; DAB2IP with presence of PD ≥6 mm; KIAA1715(LNPK), ROBO2, RAB28, LINC01017, NELL1, LDLRAD4(C18orF1), and CRYBB2P1 with %teeth with clinical attachment level (CAL) ≥3 mm; RUNX2 and LAMA2 with %teeth with CAL ≥5 mm; and KIAA1715(LNPK) with high colonization by Aggregatibacter actinomycetemcomitans. In addition, CLEC19A, IQSEC1, and EMR1 associated with microbial abundance based on checkerboard data, LBP and NCR2 with abundance based on sequencing data, and NCR2 with microbial diversity based on sequencing data.

CONCLUSIONS

Several gene loci identified in published GWAS as associated with periodontitis-related phenotypes replicated successfully in an elderly cohort.

Periodontal microorganisms and Alzheimer disease - A causative relationship?

In the initiation or exacerbation of Alzheimer disease, the dissemination of oral microorganisms into the brain tissue or the low‐level systemic inflammation have been speculated to play a role. However, the impact of oral microorganisms, such as Porphyromonas gingivalis, on the pathogenesis of Alzheimer disease and the potential causative relationship is still unclear. The present review has critically reviewed the literature by examining the following aspects: (a) the oral microbiome and the immune response in the elderly population, (b) human studies on the association between periodontal and gut microorganisms and Alzheimer disease, (c) animal and in vitro studies on microorganisms and Alzheimer disease, and (d) preventive and therapeutic approaches. Factors contributing to microbial dysbiosis seem to be aging, local inflammation, systemic diseases, wearing of dentures, living in nursing homes and no access to adequate oral hygiene measures. Porphyromonas gingivalis was detectable in post‐mortem brain samples. Microbiome analyses of saliva samples or oral biofilms showed a decreased microbial diversity and a different composition in Alzheimer disease compared to cognitively healthy subjects. Many in‐vitro and animal studies underline the potential of P gingivalis to induce Alzheimer disease‐related alterations. In animal models, recurring applications of P gingivalis or its components increased pro‐inflammatory mediators and β‐amyloid in the brain and deteriorated the animals' cognitive performance. Since periodontitis is the result of a disturbed microbial homoeostasis, an effect of periodontal therapy on the oral microbiome and host response related to cognitive parameters may be suggested and should be elucidated in further clinical trials.

Viruses, periodontitis, and comorbidities

Seminal studies published in the 1990s and 2000s explored connections between periodontal diseases and systemic conditions, revealing potential contributions of periodontal diseases in the initiation or worsening of systemic conditions. The resulting field of periodontal medicine led to the publication of studies indicating that periodontal diseases can influence the risk of systemic conditions, including adverse pregnancy outcomes, cardiovascular and respiratory diseases, as well as Alzheimer disease and cancers. In general, these studies hypothesized that the periodontal bacterial insult and/or the associated proinflammatory cascade could contribute to the pathogenesis of these systemic diseases. While investigations of the biological basis of the connections between periodontal diseases and systemic conditions generally emphasized the bacteriome, it is also biologically plausible, under an analogous hypothesis, that other types of organisms may have a similar role. Human viruses would be logical "suspects" in this role, given their ubiquity in the oral cavity, association with periodontal diseases, and ability to elicit strong inflammatory response, compromise immune responses, and synergize with bacteria in favor of a more pathogenic microbial consortium. In this review, the current knowledge of the role of viruses in connecting periodontal diseases and systemic conditions is examined. We will also delve into the mechanistic basis for such connections and highlight the importance of those relationships in the management and treatment of patients.

Gut- and oral-dysbiosis differentially impact spinal- and bulbar-onset ALS, predicting ALS severity and potentially determining the location of disease onset

BACKGROUND

Prior studies on the role of gut-microbiome in Amyotrophic Lateral Sclerosis (ALS) pathogenesis have yielded conflicting results. We hypothesized that gut- and oral-microbiome may differentially impact two clinically-distinct ALS subtypes (spinal-onset ALS (sALS) vs. bulbar-onset ALS (bALS), driving disagreement in the field.

METHODS

ALS patients diagnosed within 12 months and their spouses as healthy controls (n = 150 couples) were screened. For eligible sALS and bALS patients (n = 36) and healthy controls (n = 20), 16S rRNA next-generation sequencing was done in fecal and saliva samples after DNA extractions to examine gut- and oral-microbiome differences. Microbial translocation to blood was measured by blood lipopolysaccharide-binding protein (LBP) and 16S rDNA levels. ALS severity was assessed by Revised ALS Functional Rating Scale (ALSFRS-R).

RESULTS

sALS patients manifested significant gut-dysbiosis, primarily driven by increased fecal Firmicutes/Bacteroidetes-ratio (F/B-ratio). In contrast, bALS patients displayed significant oral-dysbiosis, primarily driven by decreased oral F/B-ratio. For sALS patients, gut-dysbiosis (a shift in fecal F/B-ratio), but not oral-dysbiosis, was strongly associated with greater microbial translocation to blood (r = 0.8006, P < 0.0001) and more severe symptoms (r = 0.9470, P < 0.0001). In contrast, for bALS patients, oral-dysbiosis (a shift in oral F/B-ratio), but not gut-dysbiosis, was strongly associated with greater microbial translocation to blood (r = 0.9860, P < 0.0001) and greater disease severity (r = 0.9842, P < 0.0001). For both ALS subtypes, greater microbial translocation was associated with more severe symptoms (sALS: r = 0.7924, P < 0.0001; bALS: r = 0.7496, P = 0.0067). Importantly, both sALS and bALS patients displayed comparable oral-motor deficits with associations between oral-dysbiosis and severity of oral-motor deficits in bALS but not sALS. This suggests that oral-dysbiosis is not simply caused by oral/bulbar/respiratory symptoms but represents a pathological driver of bALS.

CONCLUSIONS

We found increasing gut-dysbiosis with worsening symptoms in sALS patients and increasing oral-dysbiosis with worsening symptoms in bALS patients. Our findings support distinct microbial mechanisms underlying two ALS subtypes, which have been previously grouped together as a single disease. Our study suggests correcting gut-dysbiosis as a therapeutic strategy for sALS patients and correcting oral-dysbiosis as a therapeutic strategy for bALS patients.

Association between groups of immunoglobulin G antibodies against periodontal microorganisms and diabetes related mortality

BACKGROUND

Immunoglobulin G (IgG) antibodies against periodontal microorganisms can be markers of periodontal infection because their levels rise following infection and remain elevated several years later.

METHODS

We evaluated the relation between groups of IgG antibodies against 19 periodontal microorganisms and diabetes related mortality over 27 years among participants of the National Health and Nutrition Examination Survey III (1988 to 1992) 40 years and older at the time of examination (N = 8153).

RESULTS

Individuals in the highest versus lowest antibody tertiles were at 86% higher risk of dying due to diabetes related causes in the Red-Green antibody cluster (T. forsythia, T. denticola, A. actinomycetemcomitans, E. corrodens, S. noxia, V. parvula, C. rectus) (HR = 1.86, 95% CI 1.09 to 3.20) and 55% lower in the Orange-Blue antibody cluster (E. nodatum, A. naeslundii) (HR = 0.45, 95% CI 0.32 to 0.63) in multivariable models. In these models, individuals with diabetes at the time of examination had a 16-fold higher risk of dying due to diabetes related causes (HR = 16.4, 95% CI 11.0 to 24.7).

CONCLUSION

As a subset of periodontal microorganisms are associated with adverse systemic outcomes, antibody profiles may help in prediction of diabetes related mortality and identify sub-groups of individuals among whom periodontal treatment may impact diabetes related outcomes. This article is protected by copyright. All rights reserved.

Exercise Modifies the Transcriptional Regulatory Features of Monocytes in Alzheimer's Patients: A Multi-Omics Integration Analysis Based on Single Cell Technology

Monocytes have been reported to be important mediators of the protective effect of exercise against the development of Alzheimer's disease (AD). This study aims explored the mechanism by which monocytes achieve this. Using single cell transcriptome analysis, results showed that CD14 + and CD16 + monocytes interacted with other cells in the circulating blood. TNF, CCR1, APP, and AREG, the key ligand-receptor-related genes, were found to be differentially expressed between exercise-treated and AD patients. The SCENIC analysis was performed to identify individual clusters of the key transcription factors (TFs). Nine clusters (M1-M9) were obtained from the co-expression network. Among the identified TFs, MAFB, HES4, and FOSL1 were found to be differentially expressed in AD. Moreover, the M4 cluster to which MAFB, HES4, and FOSL1 belonged was defined as the signature cluster for AD phenotype. Differential analysis by bulkRNA-seq revealed that the expression of TNF, CCR1, and APP were all upregulated after exercise (p < 0.05). And ATF3, MAFB, HES4, and KLF4 that were identified in M4 clusters may be the TFs that regulate TNF, CCR1, and APP in exercise prescription. After that, APP, CCR1, TNF, ATF3, KLF4, HES4, and MAFB formed a regulatory network in the ERADMT gene set, and all of them were mechanistically linked. The ERADMT gene set has been found to be a potential risk marker for the development of AD and can be used as an indicator of compliance to exercise therapy in AD patients. Using single-cell integration analysis, a network of exercise-regulating TFs in monocytes was constructed for AD disease. The constructed network reveals the mechanism by which exercise regulated monocytes to confer therapeutic benefits against AD and its complications. However, this study, as a bioinformatic research, requires further experimental validation.

Periodontal Disease and Risk of Dementia in Medicare Patients with Hepatitis C Virus

BACKGROUND

Periodontal disease and hepatitis C virus (HCV) represent chronic infectious states that are common in elderly adults. Both conditions have independently been associated with an increased risk for dementia. Chronic infections are thought to lead to neurodegenerative changes in the central nervous system possibly by promoting a proinflammatory state. This is consistent with growing literature on the etiological role of infections in dementia. Few studies have previously evaluated the association of periodontal disease with dementia in HCV patients.

OBJECTIVE

To examine whether periodontal disease increases the risk of developing Alzheimer's disease and related dementias (ADRD) among HCV patients in Medicare claims data.

METHODS

We used Medicare claims data for HCV patients to assess the incidence rate of ADRD with and without exposure to periodontal disease between 2014 and 2017. Cox multivariate regression was used to estimate the association between periodontal disease and development of ADRD, controlling for age, gender, race, ZIP-level income and education, and medical comorbidities.

RESULTS

Of 439,760 HCV patients, the incidence rate of ADRD was higher in patients with periodontal diseases compared to those without (10.84% versus 9.26%, p <  0.001), and those with periodontal disease developed ADRD earlier compared to those without periodontal disease (13.99 versus 21.60 months, p <  0.001). The hazard of developing ADRD was 1.35 times higher in those with periodontal disease (95% CI, 1.30 to 1.40, p <  0.001) after adjusting for all covariates, including age.

CONCLUSION

Periodontal disease increased the risk of developing ADRD among HCV patients in a national Medicare claims dataset.

GSK3β is involved in promoting Alzheimer's disease pathologies following chronic systemic exposure to Porphyromonas gingivalis lipopolysaccharide in amyloid precursor proteinNL-F/NL-F knock-in mice

In line with the strong association between periodontitis and Alzheimer's disease (AD) clinically, preclinical studies have shown that systemic exposure to Porphyromonas gingivalis (Pg) initiates AD pathologies. However, the involvement of periodontitis in promoting AD pathologies is unclear. In the present study, we provided evidence that chronic systemic exposure to lipopolysaccharide derived from Pg (PgLPS, 1 mg/kg, daily, intraperitoneally) prompted neuroinflammation and tau hyperphosphorylation in 10-month-old of amyloid precursor protein (APP) knock-in mice, a model of AD, carrying the Swedish and Beyreuther/Iberian mutation (APP^NL-F/NL-F). The learning and memory function were assessed using the passive avoidance test. The production of APP, Amyloid (A)β_1-42, cytokines, synaptic proteins and the activation of glycogen synthase kinase (GSK)-3β as well as phosphorylation of tau were analyzed by immunohistochemistry, Western blotting or an enzyme-linked immunosorbent assay (ELISA) in the cortex of APP^NL-F/NL-F mice. We found that systemic exposure of PgLPS for three consecutive weeks induced learning and memory deficits with significantly reduced postsynaptic density protein (PSD95). Increased hyperphosphorylation of tau in multiple residues, including Ser202, Thr231 and Ser396, but not the accumulation of Aβ_1-42 was detected in the neurons of APP^NL-F/NL-F mice. Furthermore, PgLPS increased the GSK3β activity by reducing its phosphorylation of the serine residue at position 9 (Ser9) and promoted neuroinflammation by increasing the expression of interleukin-1β (IL-1β) and tumor necrosis factor (TNF-α) while decreasing that of interleukin-10 (IL-10) and transforming growth factor (TGFβ) in the cortex of APP^NL-F/NL-F mice. Moreover, the PgLPS-increased GSK3β activity was detected in both microglia and neurons, while the PgLPS-increased TNF-α expression was mainly detected in the microglia in the cortex of APP^NL-F/NL-F mice. In in vitro studies, PgLPS (1 µg/ml) stimulation increased the mRNA and protein level of TNF-α in MG6 microglia, which were significantly inhibited by the GSK3β-specific inhibitor TWS119. In contrast, the tau hyperphosphorylation and activation of GSK3β in N2a neurons were enhanced after treatment with conditioned medium from PgLPS-stimulated microglia, which was attenuated after pre-treatment with TNF-α inhibitor. Taken together, these findings indicate that GSK3β is involved in prompting microglia (TNF-α)-dependent tau hyperphosphorylation in neurons, resulting in learning and memory deficits in APP^NL-F/NL-F mice without changes in the Aβ expression during chronic systemic exposure to PgLPS. We propose that dampening GSK3β activation may help delay the periodontitis-promoted pathological progression of AD.

Periodontal bacteria in the brain-Implication for Alzheimer's disease: A systematic review

Periodontitis is a chronic non-communicable disease caused by a dysbiotic microbiota. Pathogens can spread to the bloodstream, colonize other tissues or organs, and favor the onset of other pathologies, such as Alzheimer's disease (AD). Pathogens could permanently or transiently colonize the brain and induce an immune response. Thus, we analyzed the evidence combining oral bacteria's detection in the brain, both in animals and humans affected with AD. This systematic review was carried out following the PRISMA guideline. Studies that detected oral bacteria at the brain level were selected. The search was carried out in the Medline, Latindex, SciELO, and Cochrane Library databases. SYRCLE tool and Newcastle-Ottawa Scale were used for the risk of bias assessment. 23 studies were selected according to the eligibility criteria. Infection with oral pathogens in animals was related to developing neuropathological characteristics of AD and bacteria detection in the brain. In patients with AD, oral bacteria were detected in brain tissues, and increased levels of pro-inflammatory cytokines were also detected. There is evidence of a microbiological susceptibility to develop AD when the most dysbiosis-associated oral bacteria are present. The presence of bacteria in the brain is related to AD's pathological characteristics, suggesting an etiological oral-brain axis.

Evidence and Therapeutic Perspectives in the Relationship between the Oral Microbiome and Alzheimer's Disease: A Systematic Review

This review aims to clarify the nature of the link between Alzheimer’s disease and the oral microbiome on an epidemiological and pathophysiological level, as well as to highlight new therapeutic perspectives that contribute to the management of this disease. We performed a systematic review, following the Preferred Reporting Items for Systematic Reviews checklist, from January 2000 to July 2021. The terms “plaque,” “saliva,” and “mouth” were associated with the search term “oral diseases” and used in combination with the Boolean operator “AND”/“OR”. We included experimental or clinical studies and excluded conferences, abstracts, reviews, and editorials. A total of 27 articles were selected. Evidence for the impact of the oral microbiome on the pathophysiological and immunoinflammatory mechanisms of Alzheimer’s disease is accumulating. The impact of the oral microbiome on the development of AD opens the door to complementary therapies such as phototherapy and/or the use of prebiotic compounds and probiotic strains for global or targeted modulation of the oral microbiome in order to have a favourable influence on the evolution of this pathology in the future.

Pathophysiological association between periodontal disease and Alzheimer's disease: Importance of periodontal health in the elderly

BACKGROUND

As the global aging population is rapidly advancing, recognizing the full potential of periodontal disease (PD) in the onset or progression of Alzheimer's disease (AD) is important for reducing geriatric morbidity. This review explores the possible role of PD in the pathogenesis of AD, as the pathological mechanisms underlying AD are the most well-studied among all types of dementia. The investigation was conducted using the electronic academic databases PubMed and ScienceDirect, employing a combination of keywords "periodontal disease," "periodontitis," "Alzheimer's disease," "dementia," and "Porphyromonas gingivalis." After applying the selection and eligibility criteria and removing overlaps, from an initial search finding of 5933 studies, 11 were finally included for qualitative analysis.

HIGHLIGHT

The inflammatory reaction induced by oral pathogenic bacteria related to PD, through complex pathways, may exacerbate inflammation in the central nervous system, thereby contributing to the progression of AD.

CONCLUSION

Maintenance of adequate oral hygiene in patients diagnosed with AD is significant because they suffer from a gradual loss of manual dexterity as the disease advances. Additionally, the evidence presents the potential of systemic inflammation from PD-induced pathogenic bacteria, illustrating the grave cyclical progression of AD.

Dose-Response Meta-Analysis on Tooth Loss With the Risk of Cognitive Impairment and Dementia

OBJECTIVES

To quantify the dose-response associations between tooth loss and risk of cognitive impairment and dementia.

DESIGN

Longitudinal studies that examined the association between tooth loss and cognitive function were systematically searched on 6 databases through March 1, 2020. The study adhered to Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) reporting guidelines. Risk estimates were pooled using random-effects models. The dose-response associations were assessed using generalized least squares spline models.

SETTING AND PARTICIPANTS

Adults from community, institution, outpatient or in-hospital were included in the meta-analysis.

MEASURES

Cognitive impairment and dementia were defined by neuropsychological tests, diagnostic criteria, or medical records. Tooth loss was self-reported or assessed by clinical examinations.

RESULTS

Fourteen studies were entered into the meta-analysis, including 34,074 participants and 4689 cases with diminished cognitive function. Participants with more tooth loss had a 1.48 times higher risk of developing cognitive impairment [95% confidence interval (CI) 1.18-1.87] and 1.28 times higher risk of being diagnosed with dementia (95% CI 1.09-1.49); however, the association was nonsignificant for participants using dentures (relative risk = 1.10, 95% CI 0.90-1.11). Eight studies were included in the dose-response analysis, and data supported the use of linear models. Each additional tooth loss was associated with a 0.014 increased relative risk of cognitive impairment and 0.011 elevated relative risks of dementia. Edentulous participants faced a 1.54 times higher risk of cognitive impairment and a 1.40 times higher risk of being diagnosed with dementia.

CONCLUSIONS AND IMPLICATIONS

Moderate-quality evidence suggested tooth loss was independently associated with cognitive impairment and dementia; risk of diminished cognitive function increased with incremental numbers of teeth lost. Furthermore, timely prosthodontic treatment with dentures may reduce the progression of cognitive decline related to tooth loss.