The Oral-Gut-Brain AXIS: The Influence of Microbes in Alzheimer's Disease

Recent advances in therapeutic probiotics: insights from human trials

SUMMARYRecent advances in therapeutic probiotics have shown promising results across various health conditions, reflecting a growing understanding of the human microbiome's role in health and disease. However, comprehensive reviews integrating the diverse therapeutic effects of probiotics in human subjects have been limited. By analyzing randomized controlled trials (RCTs) and meta-analyses, this review provides a comprehensive overview of key developments in probiotic interventions targeting gut, liver, skin, vaginal, mental, and oral health. Emerging evidence supports the efficacy of specific probiotic strains and combinations in treating a wide range of disorders, from gastrointestinal (GI) and liver diseases to dermatological conditions, bacterial vaginosis, mental disorders, and oral diseases. We discuss the expanding understanding of microbiome-organ connections underlying probiotic mechanisms of action. While many clinical trials demonstrate significant benefits, we acknowledge areas requiring further large-scale studies to establish definitive efficacy and optimal treatment protocols. The review addresses challenges in standardizing probiotic research methodologies and emphasizes the importance of considering individual variations in microbiome composition and host genetics. Additionally, we explore emerging concepts such as the oral-gut-brain axis and future directions, including high-resolution microbiome profiling, host-microbe interaction studies, organoid models, and artificial intelligence applications in probiotic research. Overall, this review offers a comprehensive update on the current state of therapeutic probiotics across multiple domains of human health, providing insights into future directions and the potential for probiotics to revolutionize preventive and therapeutic medicine.

Oral microbiota among treatment-naïve adolescents with depression: A case-control study

BACKGROUND

Adolescent depression has profound impacts on physical, cognitive, and emotional development. While gut microbiota changes have been linked to depression, the relationship between oral microbiota and depression remains elusive. Our study aims to investigate the oral microbiota in treatment-naïve adolescents experiencing depression and examine their potential associations with cognitive function.

METHODS

Our case-control study comprised two groups of adolescents aged 12-17: the depression group, including treatment-naïve individuals diagnosed with DSM-5 major depressive disorder (MDD), and a healthy control group of non-depressed individuals (HC). Participants underwent structured neuropsychiatric assessments, and fasting morning saliva samples were collected for the 16S rRNA sequencing to investigate the oral microbiota.

RESULTS

Significant differences were identified in the α- and β-diversities of the oral microbiota between MDD and HC groups. Specific bacterial taxa, including genera Streptococcus, Neisseria, Hemophilus, Fusobacterium, and g_norank_f_norank_o_Absconditabacteriales_SR1, were significantly associated with MDD. The association extends to cognitive functions, where correlations were observed between certain oral bacteria and cognitive scores, including instant and delayed memory, visual breadth, and speech features for the combined MDD and HC individuals (p < 0.05). Random forest analysis identified ten genera of oral microbes with the highest predictive values for MDD. The area under the curve (AUC) is 0.78 in the receiver operating characteristic (ROC) curve analysis.

CONCLUSION

Our results highlight the oral microbiota's role as a biomarker for adolescent depression and its impact on cognitive functions. These insights underscore the need for further research into the links between oral health, mental health, and cognitive functions.

Ruminal-buccal microbiota transmission and their diagnostic roles in subacute rumen acidosis in dairy goats

BACKGROUND

Subacute rumen acidosis (SARA) is a common metabolic disorder in ruminants that disrupts the rumen microbiome and animal health, but diagnosis is challenging due to subtle symptoms and invasive testing requirements. This study explores the potential of the buccal (oral) microbiome as a diagnostic indicator for SARA, hypothesizing an interaction with the rumen microbiome.

RESULTS

The study involved 47 dairy goats, including 11 on a control diet and 36 on high-concentrate diets with increasing rumen-degradable starch. Animals were grouped based on dietary exposure and ruminal pH: Control, Low-RDS Tolerance/SARA (LRDST/LRDSS), and High-RDS Tolerance/SARA (HRDST/HRDSS). Transcriptomics of rumen epithelium showed heightened inflammatory pathway gene expression in SARA-susceptible goats compared to controls and tolerant groups. Alpha diversity of ruminal bacteria showed lower Shannon diversity in HRDSS goats compared to HRDST whereas buccal bacteria displayed significantly lower Chao1 diversity in LRDSS goats compared to HRDST. Beta diversity analyses revealed distinct patterns between SARA-affected goats and healthy controls in both ruminal and buccal microbiomes. Prevotellaceae_UCG-003 emerged as a candidate biomarker, with reduced abundance in SARA-susceptible goats in both rumen and buccal samples. Machine learning classifiers achieved high accuracy in distinguishing SARA-susceptible goats using this genus (rumen AUC = 0.807; buccal AUC = 0.779). Source tracking analysis illustrated diminished cross-population of bacteria from the buccal to rumen (2.86% to 0.25%) and vice versa (8.59% to 1.17%), signifying compromised microbial interchange in SARA-affected goats. A microbiota transplant experiment verified SARA microbiota's ability to induce pH decline, escalate inflammation-related gene expression (MAPK10, IL17B, FOSB, SPP1), disrupt microbial transfer, and reduce Prevotellaceae_UCG-003 in recipients.

CONCLUSION

Our findings highlight SARA's dual impact on ruminal and buccal microbiota, exacerbating epithelial inflammation gene expression. Shifts in the buccal microbiome, specifically reductions in Prevotellaceae_UCG-003, mirror ruminal changes and can be influenced by inter-compartmental bacterial transmission, thereby offering a non-invasive diagnostic approach for SARA.

Dissecting the immune response of CD4+ T cells in Alzheimer's disease

The formation of amyloid-β (Aβ) plaques is a neuropathological hallmark of Alzheimer's disease (AD), however, these pathological aggregates can also be found in the brains of cognitively unimpaired elderly population. In that context, individual variations in the Aβ-specific immune response could be key factors that determine the level of Aβ-induced neuroinflammation and thus the propensity to develop AD. CD4+ T cells are the cornerstone of the immune response that coordinate the effector functions of both adaptive and innate immunity. However, despite intensive research efforts, the precise role of these cells during AD pathogenesis is still not fully elucidated. Both pathogenic and beneficial effects have been observed in various animal models of AD, as well as in humans with AD. Although this functional duality of CD4+ T cells in AD can be simply attributed to the vast phenotype heterogeneity of this cell lineage, disease stage-specific effect have also been proposed. Therefore, in this review, we summarized the current understanding of the role of CD4+ T cells in the pathophysiology of AD, from the aspect of their antigen specificity, activation, and phenotype characteristics. Such knowledge is of practical importance as it paves the way for immunomodulation as a therapeutic option for AD treatment, given that currently available therapies have not yielded satisfactory results.

Air Pollution and Oral Health: An Overall Insight From Genetic Causality

BACKGROUND

A growing body of epidemiological data consistently links air pollution to various adverse health outcomes. However, the potential connection between air pollution and the risk of oral diseases remains underexplored.

METHODS

This study utilized a two-sample Mendelian randomization approach to assess the causal relationship between air pollution and oral diseases. Six categories of air pollution were considered as exposures: nitrogen oxides (NOx), nitrogen dioxide (NO2), particulate matter (PM2.5, PM2.5-10, PM10), and PM2.5 absorbance. The outcomes included 18 oral health-related diseases drawn from the Finngen R10 dataset, the Gene-Lifestyle Interactions in Dental Endpoints consortium, and the Oncoarray oral cavity and oropharyngeal cancer consortium. Sensitivity analyses were performed to validate the primary inverse-variance weighted estimates using methods such as weighted median, weighted mode, and MR Egger.

RESULTS

The inverse-variance weighted analysis demonstrated a detrimental effect of air pollution on multiple oral health conditions, yielding 5 positive associations including PM2.5 with oral leukoplakia, gingivitis and periodontitis; PM2.5-10 with pulp and periapical diseases, and NO2 with gingivitis and periodontitis, and oral cavity, salivary glands and jaws diseases. Sensitivity tests showed no evidence of heterogeneity or pleiotropy, affirming the robustness of the findings.

CONCLUSION

This study highlights the detrimental impact of air pollution on oral health, emphasizing the need for further research into the underlying mechanisms and interactions. These findings reinforce the importance of implementing environmental interventions to mitigate the associated risks for oral health.

Association of Longitudinal Oral Microbiome Activity and Pediatric Concussion Recovery

Mild traumatic brain injury (mTBI) results in a constellation of symptoms commonly referred to as a concussion. It is unclear why certain individuals experience persistent symptoms. Given the growing evidence linking the microbiome with cognition and inflammation, we examined whether longitudinal microbiome patterns were associated with concussion symptoms. A cohort study of 118 children (aged 7-21 years) was conducted. Symptoms were assessed at three timepoints post-injury (4, 11, and 30 days) using the Post-Concussion Symptom Inventory. Saliva microbial activity was measured at each timepoint using RNA sequencing. A linear mixed model assessed the relationship between microbial activity and symptom burden while controlling for age, sex, and days post-mTBI. The participants' mean age was 16 (±3) years. The symptom burden decreased across all three timepoints (25 ± 22, 13 ± 17, and 5 ± 12). The longitudinal symptom burden was associated with elevated activity of Lactobacillus (F = 5.47; adj. p = 0.020) and Saccharomyces (F = 6.79; adj. p = 0.020) and reduced activity of Micrococcus (F = 7.94, adj. p = 0.015). These results do not establish a causative relationship, or support the use of microbial measures as a concussion test. Further studies are needed to explore the role of the gut-brain axis in mTBI.

Unraveling the mechanisms underlying air pollution-induced dysfunction of the oral-gut-brain axis: implications for human health and well-being

Air pollution exposure has become an international health issue that poses many risks to life and health. The bidirectional regulatory network, known as the oral-gut-brain axis connects the oral cavity, intestine, and central nervous system, as well as its influence on health outcomes from exposure to air pollution is receiving increased attention. This article systematically details the epidemiological evidence linking air pollutants to diseases affecting the oral, respiratory, intestinal, and nervous systems, while also explaining the route of air pollutants via the oral-gut-brain axis. The oral-gut-brain axis anomalies resulting from air pollution and their underlying molecular processes are also covered. The study provides a fresh viewpoint on how exposure to air pollution affects health and investigates cutting-edge preventative and therapeutic techniques.

Iron Homeostasis Dysregulation, Oro-Gastrointestinal Microbial Inflammatory Factors, and Alzheimer's Disease: A Narrative Review

Alzheimer's disease (AD), the most common form of dementia, is a progressive neurodegenerative disorder that profoundly impacts cognitive function and the nervous system. Emerging evidence highlights the pivotal roles of iron homeostasis dysregulation and microbial inflammatory factors in the oral and gut microbiome as potential contributors to the pathogenesis of AD. Iron homeostasis disruption can result in excessive intracellular iron accumulation, promoting the generation of reactive oxygen species (ROS) and oxidative damage. Additionally, inflammatory agents produced by pathogenic bacteria may enter the body via two primary pathways: directly through the gut or indirectly via the oral cavity, entering the bloodstream and reaching the brain. This infiltration disrupts cellular homeostasis, induces neuroinflammation, and exacerbates AD-related pathology. Addressing these mechanisms through personalized treatment strategies that target the underlying causes of AD could play a critical role in preventing its onset and progression.

Shattering the Amyloid Illusion: The Microbial Enigma of Alzheimer's Disease Pathogenesis-From Gut Microbiota and Viruses to Brain Biofilms

For decades, Alzheimer's Disease (AD) research has focused on the amyloid cascade hypothesis, which identifies amyloid-beta (Aβ) as the primary driver of the disease. However, the consistent failure of Aβ-targeted therapies to demonstrate efficacy, coupled with significant safety concerns, underscores the need to rethink our approach to AD treatment. Emerging evidence points to microbial infections as environmental factors in AD pathoetiology. Although a definitive causal link remains unestablished, the collective evidence is compelling. This review explores unconventional perspectives and emerging paradigms regarding microbial involvement in AD pathogenesis, emphasizing the gut-brain axis, brain biofilms, the oral microbiome, and viral infections. Transgenic mouse models show that gut microbiota dysregulation precedes brain Aβ accumulation, emphasizing gut-brain signaling pathways. Viral infections like Herpes Simplex Virus Type 1 (HSV-1) and Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) may lead to AD by modulating host processes like the immune system. Aβ peptide's antimicrobial function as a response to microbial infection might inadvertently promote AD. We discuss potential microbiome-based therapies as promising strategies for managing and potentially preventing AD progression. Fecal microbiota transplantation (FMT) restores gut microbial balance, reduces Aβ accumulation, and improves cognition in preclinical models. Probiotics and prebiotics reduce neuroinflammation and Aβ plaques, while antiviral therapies targeting HSV-1 and vaccines like the shingles vaccine show potential to mitigate AD pathology. Developing effective treatments requires standardized methods to identify and measure microbial infections in AD patients, enabling personalized therapies that address individual microbial contributions to AD pathogenesis. Further research is needed to clarify the interactions between microbes and Aβ, explore bacterial and viral interplay, and understand their broader effects on host processes to translate these insights into clinical interventions.

Mechanistic insight of neurodegeneration due to micro/nano-plastic-induced gut dysbiosis

Despite offering significant conveniences, plastic materials contribute substantially in developing environmental hazards and pollutants. Plastic trash that has not been adequately managed may eventually break down into fragments caused by human or ecological factors. Arguably, the crucial element for determining the biological toxicities of plastics are micro/nano-forms of plastics (MPs/NPs), which infiltrate the mammalian tissue through different media and routes. Infiltration of MPs/NPs across the intestinal barrier leads to microbial architectural dysfunction, which further modulates the population of gastrointestinal microbes. Thereby, it triggers inflammatory mediators (e.g., IL-1α/β, TNF-α, and IFN-γ) by activating specific receptors located in the gut barrier. Mounting evidence indicates that MPs/NPs disrupt host pathophysiological function through modification of junctional proteins and effector cells. Moreover, the alteration of microbial diversity by MPs/NPs causes the breakdown of the blood-brain barrier and translocation of metabolites (e.g., SCFAs, LPS) through the vagus nerve. Potent penetration affects the neuronal networks, neuronal protein accumulation, acceleration of oxidative stress, and alteration of neurofibrillary tangles, and hinders distinctive communicating pathways. Conclusively, alterations of these neurotoxic factors are possibly responsible for the associated neurodegenerative disorders due to the exposure of MPs/NPs. In this review, the hypothesis on MPs/NPs associated with gut microbial dysbiosis has been interlinked to the distinct neurological impairment through the gut-brain axis.

Evidence of microbiota-host dysbiosis between periodontitis and cerebral small vessel disease

OBJECTIVES

To investigate the correlation between periodontitis and cerebral small vessel disease (CSVD) from the clinical and microbiological aspects.

SUBJECTS AND METHODS

Periodontitis patients (CP group, n = 31) and CSVD patients (CSVD group, n = 30) were examined for neurological and periodontal condition. Subgingival plaque was collected and performed using 16S rRNA sequencing. Logistic regression and LASSO regression were used to analyze the periodontal parameters and subgingival microbiota related to CSVD, respectively. Inflammatory factors in gingival crevicular fluid (GCF) were also detected and compared between the two groups.

RESULTS

Clinical attachment level (CAL), teeth number and plaque index demonstrated a significant difference between CP and CSVD group, meanwhile, CAL was independently associated with CSVD. Besides, the microbial richness and composition were distinct between two groups. Five genera related to periodontal pathogens (Treponema, Prevotella, Streptococcus, Fusobacterium, Porphyromonas) were screened out by LASSO regression, suggesting a potential association with CSVD. Finally, the levels of inflammatory factors in GCF were statistically higher in CSVD group than those in CP group.

CONCLUSIONS

Cerebral small vessel disease patients demonstrated worse periodontal condition, meanwhile the interaction between microbiota dysbiosis and host factors (inflammation) leading to a better understanding of the association between periodontitis and CSVD.

Role of oral-gut-brain axis in psychiatric and neurological disorders

The oral cavity and gut are two important microbial habitats in the human body, harboring the most ecologically rich and taxonomically diverse microbial communities in humans, which play a key role in microbiome related diseases. In recent years, the emerging concept of the oral-gut-brain axis has attracted widespread attention in the fields of neuroscience, digestive science, and microbiology. It is not only an anatomical description, but also a comprehensive concept that covers multiple physiological functions and pathological mechanisms. Simply put, the oral-gut-brain axis refers to the complex network that connects the mouth, gut, and brain tightly together through neural connections and hormonal and immune pathways. With the deepening of research on the oral-gut-brain axis theory, more and more evidence shows that it plays an important role in depression, Parkinson's disease, and other neurodegenerative diseases. This article reviews the recent progress in research of the oral-gut-brain axis in psychiatric and neurological diseases.

Every bite counts to achieve oral health: a scoping review on diet and oral health preventive practices

OBJECTIVE

To examine the landscape of preventive strategies and interventions directed to achieve oral health equity, with particular emphasis on the interplay between dental caries prevention, individual behaviors, and population-level strategies across various demographic and geographic regions.

METHODS

This scoping review was guided by Peters et al.'s framework, which incorporates four key concepts aimed at reducing caries: education for individuals and healthcare providers, behavioral modifications, addressing broader social determinants of health, and extending oral health education programs beyond traditional dental settings. A systematic search was conducted across five databases, from 2011 to 2022.

RESULTS

This review identified 107 studies highlighting three main themes: behavioral practices (N = 33), which focused on reducing the prevalence of caries, improving oral hygiene practices, and enhancing overall oral health knowledge; educational interventions (N = 39), which explored strategies to integrate oral health with broader public health initiatives; and dietary interventions (N = 35), which emphasized the critical relationship between diet and oral health.

CONCLUSION

This SR highlights the critical need for comprehensive multilevel approaches that address the complex interplay between nutrition, oral health, and sociodemographic factors, while emphasizing the critical relationship between societal factors and individual health behaviors. Multifaceted interventions that include behavioral change, education, and dietary modifications are crucial for improving oral and overall health outcomes across diverse populations. Comprehensive strategies should prioritize medical-dental integration and data-driven approaches to effectively reduce oral health disparities for vulnerable populations, promoting long-term health equity.

The association between oral diseases and neurodegenerative disorders

BACKGROUND

The association between cognitive neurodegenerative disease and oral diseases has been under great interest recently. Several studies have suggested a connection between periodontitis and Alzheimer's disease (AD) or other neurodegenerative disorders.

OBJECTIVE

This study aimed to review the potential mechanisms between oral diseases and neurodegenerative diseases.

METHODS

The study was executed as a literature review of English-language publications from 2018-2022. The databases used for the search were PubMed, Cochrane, Scopus, and Web of Science. The search phrases used were "neurodegenerative diseases" AND "oral health" and "neurodegenerative diseases" AND "oral diseases."

RESULTS

The linkage between the two disease groups was observed in several distinct publications and several potential mechanisms were found. The link between periodontitis and AD proved to be the most significant. The effect was accentuated in elderly people where individuals possessed also other risk factors for neurodegenerative diseases and had generally worse oral health conditions.

CONCLUSIONS

Oral diseases may be risks for neurodegenerative changes along many different pathways. Good oral health should be acknowledged as a potential preventative or risk-reducing act against neurodegenerative diseases.

The Oral-Gut-Brain Axis: The Influence of Microbes as a Link of Periodontitis With Ischemic Stroke

Periodontitis, a non-communicable chronic inflammation disease resulting from dysbiosis of the oral microbiota, has been demonstrated to have a positive association with the risk of ischemic stroke (IS). The major periodontal pathogens contribute to the progression of stroke-related risk factors such as obesity, diabetes, atherosclerosis, and hypertension. Transcriptional changes in periodontitis pathogens have been detected in oral samples from stroke patients, suggesting a new conceptual framework involving microorganisms. The bidirectional regulation between the gut and the central nervous system (CNS) is mediated by interactions between intestinal microflora and brain cells. The connection between the oral cavity and gut through microbiota indicates that the oral microbial community may play a role in mediating complex communication between the oral cavity and the CNS; however, underlying mechanisms have yet to be fully understood. In this review, we present an overview of key concepts and potential mechanisms of interaction between the oral-gut-brain axis based on previous research, focusing on how the oral microbiome (especially the periodontal pathogens) impacts IS and its risk factors, as well as the mediating role of immune system homeostasis, and providing potential preventive and therapeutic approaches.

Periodontal bacteria influence systemic diseases through the gut microbiota

Many systemic diseases, including Alzheimer disease (AD), diabetes mellitus (DM) and cardiovascular disease, are associated with microbiota dysbiosis. The oral and intestinal microbiota are directly connected anatomically, and communicate with each other through the oral-gut microbiome axis to establish and maintain host microbial homeostasis. In addition to directly, periodontal bacteria may also be indirectly involved in the regulation of systemic health and disease through the disturbed gut. This paper provides evidence for the role of periodontal bacteria in systemic diseases via the oral-gut axis and the far-reaching implications of maintaining periodontal health in reducing the risk of many intestinal and parenteral diseases. This may provide insight into the underlying pathogenesis of many systemic diseases and the search for new preventive and therapeutic strategies.

Brain Injury and Neurodegeneration: Molecular, Functional, and Translational Approach 2.0

The brain is composed of different cells, such as neurons, glia, endothelial cells, etc [...].

Effects of Artemisia asiatica ex on Akkermansia muciniphila dominance for modulation of Alzheimer's disease in mice

The gut microbiome influences neurological disorders through bidirectional communication between the gut and the brain, i.e., the gut-brain axis. Artemisia asiatica ex, an extract of Artemisia asiatica Nakai (Stillen®, DA-9601) has been reported to improve depression by increasing brain-derived neurotropic factor. Therefore, we hypothesized that DA-9601 can be a potential therapeutic candidate for Alzheimer's disease (AD) acting through the gut-brain axis. Four groups of Tg2576 mice were used as the animal model for AD: wild type mice (n = 6), AD mice (n = 6), and DA-9601-administered AD mice given dosages of 30mg/kg/day (DA_30mg; n = 6) or 100mg/kg/day (DA_100mg; n = 6). Microglial activation, blood‒brain barrier integrity, amyloid beta accumulation, cognitive behavior, and changes in the gut microbiome were analyzed. DA-9601 improved the cognitive behavior of mice (DA_30mg **p<0.01; DA_100mg **p<0.01) and reduced amyloid beta accumulation (DA_30mg ***p<0.001; DA_100mg **p<0.01). Increased Iba-1 and upregulation of claudin-5 (DA_30mg *p<0.05) and occludin (DA_30mg **p<0.01; DA_100mg ***p<0.001) indicated altered microglial activation and improved blood‒brain barrier integrity. Akkermansia muciniphila was dramatically increased by DA-9601 administration (DA_30mg 47%; DA_100mg 61%). DA-9601 improved AD pathology with Akkermansia muciniphila dominance in the gut microbiome in a mouse model of AD, inferring that DA-9601 can affect AD through the gut-brain axis.

Mechanisms of oral microflora in Parkinson's disease

Parkinson's disease (PD) is a common neurodegenerative disease with complex pathogenesis and no effective treatment. Recent studies have shown that dysbiosis of the oral microflora is closely related to the development of PD. The abnormally distributed oral microflora of PD patients cause degenerative damage and necrosis of dopamine neurons by releasing their own components and metabolites, intervening in the oral-gut-brain axis, crossing the biofilm, inducing iron dysregulation, activating inter-microflora interactions, and through the mediation of saliva,ultimately influencing the development of the disease. This article reviews the structure of oral microflora in patients with PD, the mechanism of development of PD caused by oral microflora, and the potential value of targeting oral microflora in developing a new strategy for PD prevention, diagnosis and treatment.

Balancing the Oral-Gut-Brain Axis with Diet

Background: The oral microbiota is the second largest microbial community in humans. It contributes considerably to microbial diversity and health effects, much like the gut microbiota. Despite physical and chemical barriers separating the oral cavity from the gastrointestinal tract, bidirectional microbial transmission occurs between the two regions, influencing overall host health. Method: This review explores the intricate interplay of the oral-gut-brain axis, highlighting the pivotal role of the oral microbiota in systemic health and ageing, and how it can be influenced by diet. Results: Recent research suggests a relationship between oral diseases, such as periodontitis, and gastrointestinal problems, highlighting the broader significance of the oral-gut axis in systemic diseases, as well as the oral-gut-brain axis in neurological disorders and mental health. Diet influences microbial diversity in the oral cavity and the gut. While certain diets/dietary components improve both gut and oral health, others, such as fermentable carbohydrates, can promote oral pathogens while boosting gut health. Conclusions: Understanding these dynamics is key for promoting a healthy oral-gut-brain axis through dietary interventions that support microbial diversity and mitigate age-related health risks.

Sensory Dysfunction, Microbial Infections, and Host Responses in Alzheimer's Disease

Sensory functions of organs of the head and neck allow humans to interact with the environment and establish social bonds. With aging, smell, taste, vision, and hearing decline. Evidence suggests that accelerated impairment in sensory abilities can reflect a shift from healthy to pathological aging, including the development of Alzheimer's disease (AD) and other neurological disorders. While the drivers of early sensory alteration in AD are not elucidated, insults such as trauma and infections can affect sensory function. Herein, we review the involvement of the major head and neck sensory systems in AD, with emphasis on microbes exploiting sensory pathways to enter the brain (the "gateway" hypothesis) and the potential feedback loop by which sensory function may be impacted by central nervous system infection. We emphasize detection of sensory changes as first-line surveillance in senior adults to identify and remove potential insults, like microbial infections, that could precipitate brain pathology.

Influence of human gut microbiome on the healthy and the neurodegenerative aging

The gut microbiome plays a crucial role in host health throughout the lifespan by influencing brain function during aging. The microbial diversity of the human gut microbiome decreases during the aging process and, as a consequence, several mechanisms increase, such as oxidative stress, mitochondrial dysfunction, inflammatory response, and microbial gut dysbiosis. Moreover, evidence indicates that aging and neurodegeneration are closely related; consequently, the gut microbiome may serve as a novel marker of lifespan in the elderly. In this narrative study, we investigated how the changes in the composition of the gut microbiome that occur in aging influence to various neuropathological disorders, such as mild cognitive impairment (MCI), dementia, Alzheimer's disease (AD), and Parkinson's disease (PD); and which are the possible mechanisms that govern the relationship between the gut microbiome and cognitive impairment. In addition, several studies suggest that the gut microbiome may be a potential novel target to improve hallmarks of brain aging and to promote healthy cognition; therefore, current and future therapeutic interventions have been also reviewed.

Relationship between human oral microbiome dysbiosis and neuropsychiatric diseases: An updated overview

The role of the gut-brain axis in mental health disorders has been extensively studied. As the oral cavity is the starting point of the digestive tract, the role that the oral microbiota plays in mental health disorders has gained recent attention. Oral microbiota can enter the bloodstream and trigger inflammatory responses or translocate to the brain through the trigeminal nerve or olfactory system. Hence, the concept of the oral microbiota-brain axis has emerged. Several hypotheses have been suggested that the oral microbiota can enter the gastrointestinal tract and affect the gut-brain axis; however, literature describing oral-brain communication remains limited. This review summarizes the characteristics of oral microbiota and its mechanisms associated with mental health disorders. Through a comprehensive examination of the relationship between oral microbiota and various neuropsychiatric diseases, such as anxiety, depression, schizophrenia, autism spectrum disorder, epilepsy, Parkinson's disease, and dementia, this review seeks to identify promising avenues of future research.

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.

The hidden link: How oral and respiratory microbiomes affect multiple sclerosis

BACKGROUND

Extensive research has explored the role of gut microbiota in multiple sclerosis (MS). However, the impact of microbial communities in the oral cavity and respiratory tract on MS is an emerging area of investigation.

PURPOSE

We aimed to review the current literature related to the nasal, oral, and lung microbiota in people with MS (PwMS).

METHODS

We conducted a narrative review of clinical and preclinical original studies on PubMed that explored the relationship between the bacterial or viral composition of the nasal, lung, and oral microbiota and MS. Additionally, to find relevant studies not retrieved initially, we also searched for references in related review papers, as well as the references cited within the included studies.

RESULTS AND CONCLUSIONS

Thirteen studies were meticulously reviewed in three sections; oral microbiota (n = 8), nasal microbiota (n = 3), and lung microbiota (n = 2), highlighting considerable alterations in the oral and respiratory microbiome of PwMS compared to healthy controls (HCs). Genera like Aggregatibacter and Streptococcus were less abundant in the oral microbiota of PwMS compared to HCs, while Staphylococcus, Leptotrichia, Fusobacterium, and Bacteroides showed increased abundance in PwMS. Additionally, the presence of specific bacteria, including Streptococcus sanguinis, within the oral microbiota was suggested to influence Epstein-Barr virus reactivation, a well-established risk factor for MS. Studies related to the nasal microbiome indicated elevated levels of specific Staphylococcus aureus toxins, as well as nasal glial cell infection with human herpes virus (HHV)-6 in PwMS. Emerging research on lung microbiome in animal models demonstrated that manipulating the lung microbiome towards lipopolysaccharide-producing bacteria might suppress MS symptoms. These findings open avenues for potential therapeutic strategies. However, further research is crucial to fully understand the complex interactions between the microbiome and MS. This will help identify the most effective timing, bacterial strains, and modulation techniques.

Bacterial Protein Signatures Identified in Porphyromonas gingivalis Containing-Autophagic Vacuoles Reveal Co-Evolution Between Oral Red/Orange Complex Bacteria and Gut Bacteria

Modern oral bacterial species present as a concoction of commensal and opportunistic pathogens originating from their evolution in humans. Due to the intricate colonization mechanisms shared amongst oral and gut bacteria, these bacteria have likely evolved together to establish and adapt in the human oro-digestive tract, resulting in the transfer of genetic information. Our liquid chromatography-with-tandem-mass-spectrometry (LC-MS-MS) analyses have revealed protein signatures, Elongation Factor Tu, RagB/SusD nutrient uptake outer membrane protein and DnaK, specifically from Porphyromonas gingivalis -containing autophagic vacuoles isolated from the infected human primary gingival epithelial cells. Interestingly, our Mass-Spectrometry analysis reported similar proteins from closely related oral bacteria, Tannerella forsythia and Prevotella intermedia . In our phylogenetic study of these key protein signatures, we have established that pathogenic oral bacteria share extensive relatedness to each other and gut resident bacteria. We show that in the virulence factors identified from gut bacteria, Elongation Factor Tu and DnaK, there are several structural similarities and conservations with proteins from oral pathogenic bacteria. There are also major similarities in the RagB/SusD proteins of oral bacteria to prominent gut bacteria. These findings not only highlight the shared virulence mechanisms amongst oral bacterial pathogens/pathobionts but also gut bacteria and elucidate their co-evolutions in the human host.

Modeling of the brain-lung axis using organoids in traumatic brain injury: an updated review

Clinical outcome after traumatic brain injury (TBI) is closely associated conditions of other organs, especially lungs as well as degree of brain injury. Even if there is no direct lung damage, severe brain injury can enhance sympathetic tones on blood vessels and vascular resistance, resulting in neurogenic pulmonary edema. Conversely, lung damage can worsen brain damage by dysregulating immunity. These findings suggest the importance of brain-lung axis interactions in TBI. However, little research has been conducted on the topic. An advanced disease model using stem cell technology may be an alternative for investigating the brain and lungs simultaneously but separately, as they can be potential candidates for improving the clinical outcomes of TBI.In this review, we describe the importance of brain-lung axis interactions in TBI by focusing on the concepts and reproducibility of brain and lung organoids in vitro. We also summarize recent research using pluripotent stem cell-derived brain organoids and their preclinical applications in various brain disease conditions and explore how they mimic the brain-lung axis. Reviewing the current status and discussing the limitations and potential perspectives in organoid research may offer a better understanding of pathophysiological interactions between the brain and lung after TBI.

Adolescent alcohol use is associated with differences in the diversity and composition of the oral microbiome

BACKGROUND

Adolescence is a sensitive stage of oral microbial development that often coincides with the initiation and escalation of alcohol use. Thus, adolescents may be particularly susceptible to alcohol-induced alterations in the oral microbiome, though minimal research has been done in this area. Understanding the connection between the oral microbiome and alcohol use during adolescence is important to understand fully the biological consequences of alcohol use to mitigate potential adverse outcomes.

METHODS

Saliva samples were collected from adolescents aged 17-19 who used alcohol heavily (n = 21, 52.4% female) and those who did not use alcohol or any other substances (n = 18, 44.4% female). We utilized 16S rRNA sequencing to examine differences in microbial diversity and composition between the groups.

RESULTS

For alpha diversity, evenness was significantly lower in the drinking group than the control group as indicated by Pielou's evenness, Shannon, and Simpson indices. There were no statistically significant findings for beta diversity. Differential abundance analyses revealed higher abundances of Rothia and Corynebacterium in the alcohol-using group using both centered-log-ratio and relative abundance normalization. These genera are known for their high capacity to convert alcohol into acetaldehyde, a toxic metabolite reported to play a role in the neurobiological effects of alcohol. An unclassified Clostridia UCG-014, Streptobacillus, Comamonas, unclassified Lachnospiraceae, and Parvimonas were also identified as significantly different between groups when using only one of the normalization techniques.

CONCLUSIONS

This is the first study designed specifically to compare the oral microbiome of adolescents who use alcohol with that of control participants. Our findings reveal distinct alcohol-related differences in microbial composition and taxon abundance, emphasizing the importance of understanding the impact on the oral microbiome of alcohol use during adolescence. Because the oral microbiome is malleable, this study provides foundational work for future prevention and intervention studies.

Correlation between the gut microbiome and neurodegenerative diseases: a review of metagenomics evidence

A growing body of evidence suggests that the gut microbiota contributes to the development of neurodegenerative diseases via the microbiota-gut-brain axis. As a contributing factor, microbiota dysbiosis always occurs in pathological changes of neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis. High-throughput sequencing technology has helped to reveal that the bidirectional communication between the central nervous system and the enteric nervous system is facilitated by the microbiota's diverse microorganisms, and for both neuroimmune and neuroendocrine systems. Here, we summarize the bioinformatics analysis and wet-biology validation for the gut metagenomics in neurodegenerative diseases, with an emphasis on multi-omics studies and the gut virome. The pathogen-associated signaling biomarkers for identifying brain disorders and potential therapeutic targets are also elucidated. Finally, we discuss the role of diet, prebiotics, probiotics, postbiotics and exercise interventions in remodeling the microbiome and reducing the symptoms of neurodegenerative diseases.

Periodontal disease and risk of Alzheimer's disease: A two-sample Mendelian randomization

BACKGROUND

Evidence from observational studies and clinical trials suggests an association between periodontal disease and Alzheimer's disease (AD). However, the causal relationship between periodontal disease and AD remains to be determined.

METHODS

We obtained periodontal disease data from the FinnGen database and two sets of AD data from the IEU consortium and PGC databases. Subsequently, we conducted a two-sample Mendelian randomization (MR) analysis to investigate the causal relationship between periodontal disease and AD.

RESULTS

The results of the random-effects IVW analysis revealed no evidence of a genetic causal relationship between periodontal disease and AD, regardless of whether the AD data from the IEU consortium or the AD data from the PGC database were utilized. No heterogeneity, multiple effects of levels, or outliers were observed in this study.

CONCLUSIONS

Our findings indicate that there is no causal relationship between periodontal disease and AD at the genetic level.

Examining the relationship between the oral microbiome, alcohol intake and alcohol-comorbid neuropsychological disorders: protocol for a scoping review

INTRODUCTION

Heavy alcohol use and alcohol use disorder (AUD) continues to rise as a public health problem and increases the risk for disease. Elevated rates of anxiety, depression, sleep disruption and stress are associated with alcohol use. Symptoms may progress to diagnosed neurophysiological conditions and increase risk for relapse if abstinence is attempted. Research on mechanisms connecting the gastrointestinal microbiome to neuropsychological disorders through the gut-brain axis is well-established. Less is known how the oral microbiome and oral microbial-associated biomarkers may signal to the brain. Therefore, a synthesis of research studying relationships between alcohol intake, alcohol-associated neurophysiological symptoms and the oral microbiome is needed to understand the state of the current science. In this paper, we outline our protocol to collect, evaluate and synthesise research focused on associations between alcohol intake and AUD-related neuropsychological disorders with the oral microbiome.

METHODS AND ANALYSIS

The search strategy was developed and will be executed in collaboration with a medical research librarian. Studies will be screened by two independent investigators according to the aim of the scoping review, along with the outlined exclusion and inclusion criteria. After screening, data will be extracted and synthesised from the included papers according to predefined demographic, clinical and microbiome methodology metrics.

ETHICS AND DISSEMINATION

A scoping review of primary sources is needed to synthesise the data on relationships between alcohol use, neuropsychological conditions associated with AUD and the oral microbiome. The proposed scoping review is based on the data from publicly available databases and does not require ethical approval. We expect the results of this synthesis will identify gaps in the growing literature and highlight potential mechanisms linking the oral-brain axis to addiction and other associated neuropsychological conditions. The study findings and results will be disseminated through journals and conferences related to psychology, neuroscience, dentistry and the microbiome.

Personalizing Nutrition Strategies: Bridging Research and Public Health

In recent years, although life expectancy has increased significantly, non-communicable diseases (NCDs) continue to pose a significant threat to the health of the global population. Therefore, eating habits have been recognized as key modifiable factors that influence people's health and well-being. For this reason, it is interesting to study dietary patterns, since the human diet is a complex mixture of macronutrients, micronutrients, and bioactive compounds, and can modulate multiple physiological processes, including immune function, the metabolism, and inflammation. To ensure that the data we acquired were current and relevant, we searched primary and secondary sources, including scientific journals, bibliographic indexes, and databases in the last 15 years with the most relevant articles. After this search, we observed that all the recent research on NCDs suggests that diet is a critical factor in shaping an individual's health outcomes. Thus, cardiovascular, metabolic, mental, dental, and visual health depends largely on the intake, habits and patterns, and nutritional behaviors. A diet high in processed and refined foods, added sugars, and saturated fats can increase the risk of developing chronic diseases. On the other hand, a diet rich in whole, nutrient-dense foods, such as vegetables, fruits, nuts, legumes, and a high adherence to Mediterranean diet can improve health's people.

Profiles of subgingival microbiomes and gingival crevicular metabolic signatures in patients with amnestic mild cognitive impairment and Alzheimer's disease

BACKGROUND

The relationship between periodontitis and Alzheimer's disease (AD) has attracted more attention recently, whereas profiles of subgingival microbiomes and gingival crevicular fluid (GCF) metabolic signatures in AD patients have rarely been characterized; thus, little evidence exists to support the oral-brain axis hypothesis. Therefore, our study aimed to characterize both the microbial community of subgingival plaque and the metabolomic profiles of GCF in patients with AD and amnestic mild cognitive impairment (aMCI) for the first time.

METHODS

This was a cross-sectional study. Clinical examinations were performed on all participants. The microbial community of subgingival plaque and the metabolomic profiles of GCF were characterized using the 16S ribosomal RNA (rRNA) gene high-throughput sequencing and liquid chromatography linked to tandem mass spectrometry (LC-MS/MS) analysis, respectively.

RESULTS

Thirty-two patients with AD, 32 patients with aMCI, and 32 cognitively normal people were enrolled. The severity of periodontitis was significantly increased in AD patients compared with aMCI patients and cognitively normal people. The 16S rRNA gene sequencing results showed that the relative abundances of 16 species in subgingival plaque were significantly correlated with cognitive function, and LC-MS/MS analysis identified a total of 165 differentially abundant metabolites in GCF. Moreover, multiomics Data Integration Analysis for Biomarker discovery using Latent cOmponents (DIABLO) analysis revealed that 19 differentially abundant metabolites were significantly correlated with Veillonella parvula, Dialister pneumosintes, Leptotrichia buccalis, Pseudoleptotrichia goodfellowii, and Actinomyces massiliensis, in which galactinol, sn-glycerol 3-phosphoethanolamine, D-mannitol, 1 h-indole-1-pentanoic acid, 3-(1-naphthalenylcarbonyl)- and L-iditol yielded satisfactory accuracy for the predictive diagnosis of AD progression.

CONCLUSIONS

This is the first combined subgingival microbiome and GCF metabolome study in patients with AD and aMCI, which revealed that periodontal microbial dysbiosis and metabolic disorders may be involved in the etiology and progression of AD, and the differential abundance of the microbiota and metabolites may be useful as potential markers for AD in the future.

Characteristics of Oral-Gut Microbiota in Model Rats with CUMS-Induced Depression

PURPOSE

The diversity and composition of the oral and gut microbiota of depressed rats were analyzed to explore the microbiological etiology of major depressive disorder (MDD).

METHODS

The depressed rat model was established by inducing chronic unpredictable mild stress (CUMS). After the establishment of the model, body weight measurements and behavioral tests were conducted. The diversity and composition of oral and gut microbiota were analyzed using 16SrRNA sequencing.

RESULTS

There were significant differences in the alpha and beta diversity of the oral microbiota of rats in the CUMS and control groups. The top three most abundant genera in the oral microbiota were Rothia, Psychrobacter, and Streptococcus. Linear discriminant analysis effect size (LEfSe) analysis showed that the abundance of Rothia decreased and that of Psychrotrophs increased in the CUMS group, and the differences were statistically significant. The top three most abundant genera in the gut microbiota were Lactobacillus, Ruminococcus and Oscillospira. LEfSe analysis showed that the abundance of Ruminococcus decreased in the CUMS group, and the difference was statistically significant. Spearman correlation analysis was performed to analyze the differential microbiota and depression-like behavior, which showed that differential microbiota significantly correlated with body weight, total distance traveled, average speed, and number of rearing. Spearman correlation analysis of oral and gut differential microbiota demonstrated a strong positive correlation between Facklamia in the oral cavity and Enterococcus, Streptococcus in the intestine (r=0.64-0.73, P<0.01); along with a strong negative correlation between Desulfovibrio in the oral cavity and Enterococcus, Turicibacter in the intestine(r=-0.51--0.72, P<0.05).

CONCLUSION

Significant differences were observed in the diversity and composition of oral and gut microbiota between the CUMS depression model and control groups. Modulating the oral and gut microbiota may have positive effects on MDD.

Disease- and stage-specific alterations of the oral and fecal microbiota in Alzheimer's disease

Microbial communities in the intestinal tract are suggested to impact the ethiopathogenesis of Alzheimer's disease (AD). The human microbiome might modulate neuroinflammatory processes and contribute to neurodegeneration in AD. However, the microbial compositions in patients with AD at different stages of the disease are still not fully characterized. We used 16S rRNA analyses to investigate the oral and fecal microbiota in patients with AD and mild cognitive impairment (MCI; n = 84), at-risk individuals (APOE4 carriers; n = 17), and healthy controls (n = 50) and investigated the relationship of microbial communities and disease-specific markers via multivariate- and network-based approaches. We found a slightly decreased diversity in the fecal microbiota of patients with AD (average Chao1 diversity for AD = 212 [SD = 66]; for controls = 215 [SD = 55]) and identified differences in bacterial abundances including Bacteroidetes, Ruminococcus, Sutterella, and Porphyromonadaceae. The diversity in the oral microbiota was increased in patients with AD and at-risk individuals (average Chao1 diversity for AD = 174 [SD = 60], for at-risk group = 195 [SD = 49]). Gram-negative proinflammatory bacteria including Haemophilus, Neisseria, Actinobacillus, and Porphyromonas were dominant oral bacteria in patients with AD and MCI and the abundance correlated with the cerebrospinal fluid biomarker. Taken together, we observed a strong shift in the fecal and the oral communities of patients with AD already prominent in prodromal and, in case of the oral microbiota, in at-risk stages. This indicates stage-dependent alterations in oral and fecal microbiota in AD which may contribute to the pathogenesis via a facilitated intestinal and systemic inflammation leading to neuroinflammation and neurodegeneration.

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.

Active Compounds of Panax ginseng in the Improvement of Alzheimer's Disease and Application of Spatial Metabolomics

Panax ginseng C.A. Meyer (P. ginseng) is one of the more common traditional Chinese medicines (TCMs). It contains numerous chemical components and exhibits a range of pharmacological effects. An enormous burden is placed on people's health and life by Alzheimer's disease (AD), a neurodegenerative condition. Recent research has shown that P. ginseng's chemical constituents, particularly ginsenosides, have a significant beneficial impact on the prevention and management of neurological disorders. To understand the current status of research on P. ginseng to improve AD, this paper discusses the composition of P. ginseng, its mechanism of action, and its clinical application. The pathogenesis of AD includes amyloid beta protein (Aβ) generation and aggregation, tau protein hyperphosphorylation, oxidant stress, neuroinflammation, mitochondrial damage, and neurotransmitter and gut microbiota disorders. This review presents the key molecular mechanisms and signaling pathways of the active ingredients in P. ginseng involved in improving AD from the perspective of AD pathogenesis. A P. ginseng-related signaling pathway network was constructed to provide effective targets for the treatment of AD. In addition, the application of spatial metabolomics techniques in studying P. ginseng and AD is discussed. In summary, this paper discusses research perspectives for the study of P. ginseng in the treatment of AD, including a systematic and in-depth review of the mechanisms of action of the active substances in P. ginseng, and evaluates the feasibility of applying spatial metabolomics in the study of AD pathogenesis and pharmacological treatment.

Salivary microbiome profiles for different clinical phenotypes of pituitary adenomas by single-molecular long-read sequencing

IMPORTANCE

The gut and salivary microbiomes have been widely reported to be significantly associated with a number of neurological disorders. The stability of the microbiome in the oral cavity makes it a potentially ideal sample that can be conveniently obtained for the investigation of microbiome-based pathogenesis in diseases. In the present study, we used a single-molecule long-read sequencing technique to study the distribution of the salivary microbiota in patients with pituitary adenoma (PA) and healthy individuals, as well as among four clinical phenotypes of PA. We found that the diversity of salivary microbes was more abundant in PA patients than in healthy individuals. We also observed some unique genera in different PA phenotypes. The bioinformatics-based functional predictions identified potential links between microbes and different clinical phenotypes of PA. This study improves the existing understanding of the pathogenesis of PA and may provide diagnostic and therapeutic targets for PA.

A reappraisal on amyloid cascade hypothesis: the role of chronic infection in Alzheimer's disease

Alzheimer disease (AD) is a progressive neurological disorder that accounted for the most common cause of dementia in the elderly population. Lately, 'infection hypothesis' has been proposed where the infection of microbes can lead to the pathogenesis of AD. Among different types of microbes, human immunodeficiency virus-1 (HIV-1), herpes simplex virus-1 (HSV-1), Chlamydia pneumonia, Spirochetes and Candida albicans are frequently detected in the brain of AD patients. Amyloid-beta protein has demonstrated to exhibit antimicrobial properties upon encountering these pathogens. It can bind to microglial cells and astrocytes to activate immune response and neuroinflammation. Nevertheless, HIV-1 and HSV-1 can develop into latency whereas Chlamydia pneumonia, Spirochetes and Candida albicans can cause chronic infections. At this stage, the DNA of microbes remains undetectable yet active. This can act as the prolonged pathogenic stimulus that over-triggers the expression of Aβ-related genes, which subsequently lead to overproduction and deposition of Aβ plaque. This review will highlight the pathogenesis of each of the stated microbial infection, their association in AD pathogenesis as well as the effect of chronic infection in AD progression. Potential therapies for AD by modulating the microbiome have also been suggested. This review will aid in understanding the infectious manifestations of AD.

Implications of gut and oral microbiota in neuroinflammatory responses in Alzheimer's disease

A diverse and stable microbiota promotes a healthy state, nevertheless, an imbalance in gut or oral bacterial composition, called dysbiosis, can cause gastrointestinal disorders, systemic inflammatory states and oxidative stress, among others. Recently, gut and oral dysbiosis has been linked to Alzheimer's disease (AD), which is considered the most common form of dementia and a public health priority due to its high prevalence and incidence. The aim of this review is to highlight the implications of gut and oral microbiota in the neuroinflammation characteristic of AD pathology and the subsequent cognitive impairment. It is a systematic review of the current literature obtained by searching the PubMed, Web of Science and Scopus databases. The characteristic intestinal dysbiosis in AD patients leads to increased permeability of the intestinal barrier and activates immune cells in the central nervous system due to translocation of microbiota-derived metabolites and/or bacteria into the circulation leading to increased neuroinflammation and neuronal loss, thus generating the cognitive impairment characteristic of AD. The presence in the central nervous system of Porphyromonas gingivalis can cause an increased neuroinflammation and beta-amyloid peptide accumulation.

The Interaction between the Oral Microbiome and Systemic Diseases: A Narrative Review

Background: The human being is defined as a ‘superorganism’ since it is made up of its own cells and microorganisms that reside inside and outside the human body. Commensal microorganisms, which are even ten times more numerous than the cells present in the body, perform very important functions for the host, as they contribute to the health of the host, resist pathogens, maintain homeostasis, and modulate the immune system. In the mouth, there are different types of microorganisms, such as viruses, mycoplasmas, bacteria, archaea, fungi, and protozoa, often organized in communities. The aim of this umbrella review is to evaluate if there is a connection between the oral microbiome and systematic diseases. Methodology: A literature search was conducted through PubMed/MEDLINE, the COCHRANE library, Scopus, and Web of Science databases without any restrictions. Because of the large number of articles included and the wide range of methods and results among the studies found, it was not possible to report the results in the form of a systematic review or meta-analysis. Therefore, a narrative review was conducted. We obtained 73.931 results, of which 3593 passed the English language filter. After the screening of the titles and abstracts, non-topic entries were excluded, but most articles obtained concerned interactions between the oral microbiome and systemic diseases. Discussion: A description of the normal microbial flora was present in the oral cavity both in physiological conditions and in local pathological conditions and in the most widespread systemic pathologies. Furthermore, the therapeutic precautions that the clinician can follow in order to intervene on the change in the microbiome have been described. Conclusions: This review highlights what are the intercorrelations of the oral microbiota in healthy subjects and in subjects in pathological conditions. According to several recent studies, there is a clear correlation between dysbiosis of the oral microbiota and diseases such as diabetes, cardiovascular diseases, chronic inflammatory diseases, and neurodegenerative diseases.

The Immune System Response to Porphyromonas gingivalis in Neurological Diseases

Previous studies have reported an association between oral microbial dysbiosis and the development and progression of pathologies in the central nervous system. Porphyromonas gingivalis (Pg), the keystone pathogen of the oral cavity, can induce a systemic antibody response measured in patients' sera using enzyme-linked immunosorbent assays. The present case-control study quantified the immune system's response to Pg abundance in the oral cavities of patients affected by different central nervous system pathologies. The study cohort included 87 participants: 23 healthy controls (HC), 17 patients with an acute neurological condition (N-AC), 19 patients with a chronic neurological condition (N-CH), and 28 patients with neurodegenerative disease (N-DEG). The results showed that the Pg abundance in the oral cavity was higher in the N-DEG patients than in the HC (p = 0.0001) and N-AC patients (p = 0.01). In addition, the Pg abundance was higher in the N-CH patients than the HCs (p = 0.005). Only the N-CH patients had more serum anti-Pg antibodies than the HC (p = 0.012). The inadequate response of the immune system of the N-DEG group in producing anti-Pg antibodies was also clearly indicated by an analysis of the ratio between the anti-Pg antibodies quantity and the Pg abundance. Indeed, this ratio was significantly lower between the N-DEG group than all other groups (p = 0.0001, p = 0.002, and p = 0.03 for HC, N-AC, and N-CH, respectively). The immune system's response to Pg abundance in the oral cavity showed a stepwise model: the response diminished progressively from the patients affected with an acute condition to the patients suffering from chronic nervous system disorders and finally to the patients affected by neurodegenerative diseases.

No genetic causal association between dental caries and Alzheimer's disease: a bidirectional two-sample Mendelian randomization analysis

Background

An increasing number of observational studies have suggested an association between dental caries and Alzheimer's disease (AD). The association between dental caries and Alzheimer's disease may be mediated by confounders or reverse causality. In this study, we conducted bidirectional two-sample Mendelian randomization (MR) to estimate the bidirectional causality between dental caries and AD.

Materials and Methods

Genome-wide association study (GWAS) summary statistics of dental caries were extracted from a published meta-analysis which included a total of 487,823 participants. GWAS datasets of AD and AD onset age were obtained from the FinnGen bank. A bidirectional two-sample analysis was performed to explore the causality between dental caries and AD.

Results

For the dental caries-AD causality estimation, there was no significant association between dental caries and AD, neither with the AD GWASs from the FinnGen database (OR: 1.041, p = 0.874) nor with those from the International Genomics of Alzheimer's Project (OR: 1.162, p = 0.409). In addition, the genetic susceptibility to dental caries was not related to the onset age of AD. No causality existed between dental caries and early-onset AD (OR: 0.515, p = 0.302) or late-onset AD (OR: 1.329, p = 0.347). For the AD-dental caries relationship, no causality was detected by the IVW method (OR: 1.000, p = 0.717). Findings from other MR methods were consistent. The pleiotropy test and sensitivity analysis confirmed the validity of these MR results.

Conclusions

In this bidirectional MR study, robust evidence to support a bidirectional causal effect between dental caries and AD from the GWAS results within large-scale European-descent populations was absent. Having dental caries would not alter the onset age of AD.

Impact of Pseudomonas aeruginosa, Bacillus subtilis, Staphylococcus aureus, and Escherichia coli Oral Infusions on Cognitive Memory Decline in Mild Cognitive Impairment

Synaptic plasticity is a result of changes in the neuronal circuits which may result in the formation of protein-dependent (long-term memory (LTM) formation) and protein-independent (short-term memory (STM) formation) memories. This STM formation is based on existing proteins, but LTM formation depends on RNA and protein synthesis within the neuronal cells. This RNA and protein synthesis may depend on stimulus exposure like odour, taste, and other environmental stimuli. The present study is aimed to show the impact of oral bacterial infusions on cognitive memory formation through pre and post-infusive behavioural analysis. The results of the study revealed that oral infusions of Pseudomonas aeruginosa, Bacillus subtilis, Staphylococcus aureus and Escherichia coli result in impaired cognitive learning and memory formation. This impaired cognitive memory formation is shown with the help of two-step (pre and post-infusive) behavioural analysis. Pre-infusive behavioural study shows no decline in cognitive learning and memory formation before oral microbial infusions in a serene habituated environment. After oral microbial infusions, a post-infusive behavioural analysis may reveal a memory decline in the treated group. Comparative two-step behavioural analysis indicates that P. aeruginosa infusions strongly impact cognitive memory decline compared to the other three groups. This cognitive memory decline may happen due to the production of primary/secondary metabolites within the animal gut and their transportation to the CNS  through the blood-brain barrier. The outcome of the present study states that poor oral hygiene plays a significant role in cognitive memory decline concerning mild cognitive impairment (MCI).

Oral and fecal microbiota perturbance in cocaine users: Can rTMS-induced cocaine abstinence support eubiosis restoration?

The effects of cocaine on microbiota have been scarcely explored. Here, we investigated the gut (GM) and oral (OM) microbiota composition of cocaine use disorder (CUD) patients and the effects of repetitive transcranial magnetic stimulation (rTMS). 16S rRNA sequencing was used to characterize GM and OM, whereas PICRUST2 assessed functional changes in microbial communities, and gas-chromatography was used to evaluate fecal short and medium chain fatty acids. CUD patients reported a significant decrease in alpha diversity and modification of the abundances of several taxa in both GM and OM. Furthermore, many predicted metabolic pathways were differentially expressed in CUD patients' stool and saliva samples, as well as reduced levels of butyric acid that appear restored to normal amounts after rTMS treatment. In conclusion, CUD patients showed a profound dysbiotic fecal and oral microbiota composition and function and rTMS-induced cocaine abstinence determined the restoration of eubiotic microbiota.

The interplay between oral microbiota, gut microbiota and systematic diseases

Over the past two decades, the importance of microbiota in health and disease has become evident. The human gut microbiota and oral microbiota are the largest and second-largest microbiome in the human body, respectively, and they are physically connected as the oral cavity is the beginning of the digestive system. Emerging and exciting evidence has shown complex and important connections between gut microbiota and oral microbiota. The interplay of the two microbiomes may contribute to the pathological processes of many diseases, including diabetes, rheumatoid arthritis, nonalcoholic fatty liver disease, inflammatory bowel disease, pancreatic cancer, colorectal cancer, and so on. In this review, we discuss possible routes and factors of oral microbiota to affect gut microbiota, and the contribution of this interplay between oral and gut microbiota to systemic diseases. Although most studies are association studies, recently, there have been increasing mechanistic investigations. This review aims to enhance the interest in the connection between oral and gut microbiota, and shows the tangible impact of this connection on human health.

Gut Microbiota, an Additional Hallmark of Human Aging and Neurodegeneration

Gut microbiota represents a diverse and dynamic population of microorganisms harbouring the gastrointestinal tract, which influences host health and disease. Bacterial colonization of the gastrointestinal tract begins at birth and changes throughout life, with age being one of the conditioning factors for its vitality. Aging is also a primary risk factor for most neurodegenerative diseases. Among them, Alzheimeŕs disease (AD) is probably the one where its association with a state of dysbiosis of the gut microbiota has been most studied. In particular, intestinal microbial-derived metabolites have been associated with β-amyloid formation and brain amyloid deposition, tau phosphorylation, as well as neuroinflammation in AD patients. Moreover, it has been suggested that some oral bacteria increase the risk of developing AD. However, the causal connections among microbiome, amyloid-tau interaction, and neurodegeneration need to be addressed. This paper summarizes the emerging evidence in the literature regarding the link between the oral and gut microbiome and neurodegeneration with a focus on AD. Taxonomic features of bacteria as well as microbial functional alterations associated with AD biomarkers are the main points reviewed. Data from clinical studies as well as the link between microbiome and clinical determinants of AD are particularly emphasized. Further, relationships between gut microbiota and age-dependent epigenetic changes and other neurological disorders are also described. Together, all this evidence suggests that, in some sense, gut microbiota can be seen as an additional hallmark of human aging and neurodegeneration.

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.

Characteristics and Correlations of the Oral and Gut Fungal Microbiome with Hypertension

The mycobiome is an essential constituent of the human microbiome and is associated with various diseases. However, the role of oral and gut fungi in hypertension (HTN) remains largely unexplored. In this study, saliva, subgingival plaques, and feces were collected from 36 participants with HTN and 24 healthy controls for metagenomic sequencing. The obtained sequences were analyzed using the Kraken2 taxonomic annotation pipeline to assess fungal composition and diversity. Correlations between oral and gut fungi and clinic parameters, between fungi within the same sample types, and between different sample types were identified by Spearman's correlation analysis. Overall, the subgingival fungal microbiome had substantially higher alpha diversity than the salivary and fecal fungal microbiomes. The fungal microbiomes of the three sample types displayed distinct beta diversity from each other. Oral fungi but not gut fungi in HTN had beta diversity significantly different from that of controls. Among the fungi shared in the oral cavity and gut, Exophiala was the genus with the most notable changes. Exophiala spinifera was the most abundant salivary species in HTN. Some fungal species directly correlated with blood pressure, including gut Exophiala xenobiotica and Exophiala mesophila. The markedly impaired ecological cocorrelation networks of oral and gut fungi in HTN suggested compromised association among fungal species. Most fungi were shared in the oral cavity and gut, and their correlations suggested the potential interplays between oral and gut fungi. In conclusion, the oral cavity and intestine have unique fungal ecological environments. The fungal enrichment and ecology in HTN, the correlations between oral and gut fungi, and the associations between oral and gut fungi and clinical parameters suggest an important role that the fungal microbiome may play in HTN. IMPORTANCE Our study fills the gap in human studies investigating the oral and gut fungal microbiota in association with blood pressure. It characterizes the diversity and composition of the oral and gut fungal microbiome in human subjects, elucidates the dysbiosis of fungal ecology in a hypertensive population, and establishes oral-gut fungal correlations and fungus-clinical parameter correlations. Targeting fungi in the oral cavity and/or gut may provide novel strategies for the prevention and treatment of hypertension.

Studying ancient human oral microbiomes could yield insights into the evolutionary history of noncommunicable diseases

Noncommunicable diseases (NCDs) have played a critical role in shaping human evolution and societies. Despite the exceptional impact of NCDs economically and socially, little is known about the prevalence or impact of these diseases in the past as most do not leave distinguishing features on the human skeleton and are not directly associated with unique pathogens. The inability to identify NCDs in antiquity precludes researchers from investigating how changes in diet, lifestyle, and environments modulate NCD risks in specific populations and from linking evolutionary processes to modern health patterns and disparities. In this review, we highlight how recent advances in ancient DNA (aDNA) sequencing and analytical methodologies may now make it possible to reconstruct NCD-related oral microbiome traits in past populations, thereby providing the first proxies for ancient NCD risk. First, we review the direct and indirect associations between modern oral microbiomes and NCDs, specifically cardiovascular disease, diabetes mellitus, rheumatoid arthritis, and Alzheimer's disease. We then discuss how oral microbiome features associated with NCDs in modern populations may be used to identify previously unstudied sources of morbidity and mortality differences in ancient groups. Finally, we conclude with an outline of the challenges and limitations of employing this approach, as well as how they might be circumvented. While significant experimental work is needed to verify that ancient oral microbiome markers are indeed associated with quantifiable health and survivorship outcomes, this new approach is a promising path forward for evolutionary health research.