The oral microbiome and inflammation in mild cognitive impairment

https://doi.org/10.1016/j.exger.2021.111273Get rights and content

Highlights

  • Pasteurellacae and L. mirabilis were differentially abundant between participants with MCI and those without.

  • Five CSF inflammatory markers differentiated participants with mild cognitive impairment from those without.

  • Two microbiome features were associated with several CSF inflammatory markers.

  • Modest evidence suggests both an oral microbiome and neuroinflammatory difference between MCI and normal cognition cohorts.

Abstract

Inflammation and immune mechanisms are believed to play important roles in Alzheimer's disease pathogenesis. Research supports the link between poor oral health and Alzheimer's disease. Periodontal disease and dental caries represent the two most common infections of the oral cavity. This study focused on a precursor to Alzheimer's disease, mild cognitive impairment (MCI). Using 16S rRNA sequencing, we characterized and compared the oral microbiome of 68 older adults who met the criteria for MCI or were cognitively normal, then explored relationships between the oral microbiome, diagnostic markers of MCI, and blood markers of systemic inflammation. Two taxa, Pasteurellacae and Lautropia mirabilis were identified to be differentially abundant in this cohort. Although systemic inflammatory markers did not differentiate the two groups, differences in five cerebrospinal fluid inflammatory mediators were identified and had significant associations with MCI. Because inflammatory markers may reflect CNS changes, pursuing this line of research could provide opportunities for new diagnostic tools and illuminate mechanisms for prevention and mitigation of Alzheimer's disease.

Introduction

Alzheimer's disease is the most frequent cause of dementia in Western societies with recent United States (U.S.) estimates of 5.5 million people affected (Mayeux and Stern, 2012). As the population ages, a rapid rise in the prevalence of Alzheimer's disease is expected with a possible 16 million people affected by 2050 (Lim et al., 2015). Persons with this disease have a life expectancy of 5–9 years after diagnosis (Lim et al., 2015), and the progressive cognitive decline experienced during that time makes it among one of the most disabling diseases incurred by aging populations (Ferri et al., 2006). Annual costs for dementia care are assessed to be between $41,000 to $56,000 per person, with a total cost of up to $215 billion in the U.S. in 2010 (Hurd et al., 2013). To date, there is no effective, mechanism-based treatment strategy for the condition (Lim et al., 2015).

Inflammation and immune mechanisms are believed to play important roles in Alzheimer's disease pathogenesis (Krstic and Knuesel, 2013). Systemic inflammation may dysregulate the normal mechanism for clearing out misfolded or damaged neuronal proteins in aging brains. This would result in a neurotoxic inflammatory environment marked by the build-up of amyloid beta plaques and neurofibrillary tangles leading to neurodegeneration and the cognitive decline that marks Alzheimer's Disease (Krstic and Knuesel, 2013; Lim et al., 2015). What then could trigger this systemic inflammation and possible subsequent chain of events? Given that persistent low level bacterial infection may play a role in inducing chronic inflammation among the elderly (Licastro et al., 2014), a possible source for this inflammation is oral cavity infection.

As an individual ages, immunosenescence increases susceptibility to chronic and common infections (Targonski et al., 2007) like oral infection. Risk for oral infection is related to declining general health, medication use, and impaired access to oral healthcare (Singhrao et al., 2014). Periodontal disease and dental caries represent the two most common infections of the oral cavity (Löe, 2000). Periodontal disease affects 64% of adults in the U.S. aged 65 years and older (Eke et al., 2012) and includes two conditions: gingivitis and periodontitis. Gingivitis represents an inflammatory response to the microorganisms that inhabit the biofilm at the interface of the tooth and the gingiva (Kinane, 2001). For some, progressive gingivitis leads to periodontitis, defined as a polymicrobial inflammatory disease involving the irreversible destruction of supportive soft tissue and bone, ultimately leading to tooth loss (Darby et al., 2000). Dental caries occur on the exposed surfaces of the teeth, developing through a complex interaction over time between acid-producing bacteria and fermentable carbohydrates (Selwitz et al., 2007).

Several studies supporting the link between oral health and Alzheimer's disease have reported that poor oral health, chronic periodontal disease, and tooth loss are associated with worsening cognitive impairment (Gatz et al., 2006; Noble et al., 2009; Stein et al., 2007; Stewart et al., 2008). A recent literature review cites epidemiologic links between poor oral health and dementia including post-mortem evidence of higher quantities of oral organisms in the brains of people with Alzheimer's disease compared to controls and higher levels of systemic inflammatory markers in Alzheimer's disease patients compared to controls (Shoemark and Allen, 2015). These culture based and immunologic investigations have identified a few oral bacterial suspects, including Porphyromonas gingivalis (Poole et al., 2013), Fusobacterium nucleatum, and Prevotella intermedia (Stein et al., 2012). It is becoming increasingly clear, however, that oral infectious etiology is based on a complex set of bacterial species (Holt and Ebersole, 2005) and that human disease and inflammation related disorders are linked to bacterial community imbalance or dysbiosis of the oral cavity (Schippa and Conte, 2014).

Despite the suggested link between compromised oral health and Alzheimer's disease and the complexity and diversity of the oral microbiome (Wade, 2013), few studies have profiled the oral microbiome of individuals with Alzheimer's disease, and no previous study has considered the relationship between the oral microbiome and inflammation as a contributor to early cognitive decline related to Alzheimer's Disease. This pilot study focuses on a precursor to Alzheimer's disease, mild cognitive impairment (MCI). MCI refers to a transitional period between normal aging and Alzheimer's disease in which individuals demonstrate subtle signs of cognitive decline, not yet meeting the criteria for Alzheimer's dementia, but still greater in extent than what would be expected for their age (Petersen et al., 2001). Individuals with MCI progress to clinically probable Alzheimer's disease at an accelerated rate compared with healthy age-matched individuals (Petersen et al., 2001), and those with MCI that is linked to underlying Alzheimer's pathology have a near 100% likelihood of progressing to mild dementia within five years (Hansson et al., 2006). Currently there is no cure for MCI, however, given the potential for progression, the condition is a suitable target for prevention and/or mitigation of Alzheimer's disease.

Using 16S rRNA sequencing, this pilot study characterized, for the first time, the oral microbiome of individuals with MCI providing fundamental knowledge for future explorations of the role of the oral microbiome in the inflammatory mechanisms underlying early stages of Alzheimer's disease. The hypothesis of the project was that the oral microbiome would be associated with Alzheimer's disease and inflammation, with our aims being to characterize and compare the composition and diversity of the oral microbiome among individuals with MCI and their aged-matched controls, and to explore associations between the composition of the oral microbiome, central nervous system (CNS), systemic inflammation, and presence of MCI.

Section snippets

Design

Institutional review board approval was obtained. A total of 92 older adults were enrolled for this pilot study, however to control for significant differences in race between our case and control groups as well as DNA library size (see 2.3.7), data from 68 participants were included for analysis.

Setting and sample

Participants were recruited from a large, ongoing longitudinal observational study of 500 participants in the Emory Goizueta Alzheimer's Disease Research Center (ADRC) Clinical Core and ADRC-associated

Results

The mean age of all the participants was 72.0 ± 5.8 years. Other sociodemographic, oral health behavioral, and oral symptom characteristics can be found in Table 1. Of the 68 participants included, 34 met the criteria for MCI due to biomarker-proven Alzheimer's disease. All had either single- or multi-domain amnestic MCI. There were no significant differences between groups on age, sex, or other demographic, oral hygiene behavioral, or symptomatic characteristics listed in Table 1.

Discussion

Using 16S rRNA sequencing, this pilot study characterized the oral microbiome of individuals with normal cognition and MCI and hypothesized that the oral microbiome would be associated with Alzheimer's disease and markers of systemic and neuroinflammation.

The top five phyla represented in the oral microbiome of both groups were Firmicutes, Proteobacteria, Bacteroidetes, Fusobacteria, and Actinobacteria. This is consistent with previous studies that used 16S rRNA sequencing which reported that

CRediT authorship contribution statement

Irene Yang: Conceptualization; methodology; data curation; formal analysis; writing – original draft; visualization; supervision; funding acquisition.

Robert Adam Arthur: Formal analysis; data curation; writing – original draft; visualization.

Liping Zhao: Formal analysis; writing – original draft.

Jasmine Clark: Writing – original draft; writing – review and editing.

Yijuan Hu: Formal analysis; writing – review and editing.

Elizabeth Corwin: Conceptualization; writing – review and editing; funding

Funding sources

NIH – NIA, Emory Alzheimer's Disease Research Center, P50 AG025688, Robert W. Woodruff Health Science Center Fund Synergy Award (2016).

References (79)

  • M. Shi et al.

    The subgingival microbiome of periodontal pockets with different probing depths in chronic and aggressive periodontitis: a pilot study

    Front. Cell. Infect. Microbiol.

    (2018)
  • DS Shouval et al.

    Interleukin 10 receptor signaling: Master regulator of intestinal mucosal homeostasis in mice and humans

    Adv. Immunol.

    (2014)
  • P.S. Stein et al.

    Tooth loss, dementia and neuropathology in the Nun study

    J. Am. Dent. Assoc.

    (2007)
  • P.V. Targonski et al.

    Immunosenescence: role and measurement in influenza vaccine response among the elderly

    Vaccine

    (2007)
  • W.G. Wade

    The oral microbiome in health and disease

    Pharmacol. Res.

    (2013)
  • S.F. Ziegler et al.

    The biology of thymic stromal lymphopoietin (TSLP)

    Adv. Pharmacol.

    (2013)
  • L. Abusleme et al.

    The subgingival microbiome in health and periodontitis and its relationship with community biomass and inflammation

    The ISME journal

    (2013)
  • H.S. AlMoharib et al.

    Oral fluid based biomarkers in periodontal disease: part 1. Saliva

    Journal of international oral health: JIOH

    (2014)
  • S. Andrews

    FastQC: a quality control tool for high throughput sequence data

  • E. Assarsson et al.

    Homogenous 96-plex PEA immunoassay exhibiting high sensitivity, specificity, and excellent scalability

    PLoS One

    (2014)
  • S.M. Ben Dekhil et al.

    Isolation of Lautropia mirabilis from sputa of a cystic fibrosis patient

    J Clin Microbiol. Apr

    (1997)
  • L. Besser et al.

    Version 3 of the national Alzheimer’s coordinating center’s uniform data set

    Alzheimer Dis. Assoc. Disord.

    (2018)
  • S. Bizzarro et al.

    Subgingival microbiome in smokers and non-smokers in periodontitis: an exploratory study using traditional targeted techniques and a next-generation sequencing

    J. Clin. Periodontol.

    (2013)
  • E. Bolyen et al.

    QIIME 2: reproducible, interactive, scalable, and extensible microbiome data science

    PeerJ

    (2018)
  • E. Bolyen et al.

    Reproducible, interactive, scalable and extensible microbiome data science using QIIME 2

    Nat. Biotechnol.

    (2019)
  • S.T. Chambers et al.

    HACEK infective endocarditis: characteristics and outcomes from a large, multi-national cohort

    PLoS One

    (2013)
  • T. Chen et al.

    The human oral microbiome database: a web accessible resource for investigating oral microbe taxonomic and genomic information

    Database

    (2010)
  • A Cianferoni et al.

    The importance of TSLP in allergic disease and its role as a potential therapeutic target

    Expert. Rev. Clin. Immunol.

    (2014)
  • A.P. Colombo et al.

    Comparisons of subgingival microbial profiles of refractory periodontitis, severe periodontitis, and periodontal health using the human oral microbe identification microarray

    J. Periodontol.

    (2009)
  • L D’Anna et al.

    Serum interleukin-10 levels correlate with cerebrospinal fluid amyloid beta deposition in Alzheimer disease patients

    Neurodegener. Dis.

    (2017)
  • I. Darby et al.

    Microbial comparison of smoker and non-smoker adult and early-onset periodontitis patients by polymerase chain reaction

    J. Clin. Periodontol.

    (2000)
  • SM de la Monte et al.

    Altered serum and cerebrospinal fluid inflammatory cascades in mild cognitive impairment and Alzheimer’s disease

    J. Neuroinflammation Neurodegener. Dis.

    (2017)
  • T.Z. DeSantis et al.

    Greengenes, a chimera-checked 16S rRNA gene database and workbench compatible with ARB

    Appl. Environ. Microbiol.

    (2006)
  • F.E. Dewhirst et al.

    The human oral microbiome

    J. Bacteriol.

    (2010)
  • P. Eke et al.

    Prevalence of periodontitis in adults in the United States: 2009 and 2010

    J. Dent. Res.

    (2012)
  • D.H. Fine et al.

    Aggregatibacter actinomycetemcomitans and its relationship to initiation of localized aggressive periodontitis: longitudinal cohort study of initially healthy adolescents

    J. Clin. Microbiol.

    (2007)
  • D. Gate et al.

    Clonally expanded CD8 T cells patrol the cerebrospinal fluid in Alzheimer’s disease

    Nature

    (2020)
  • P. Gerner-Smidt et al.

    Lautropia mirabilis gen. nov., sp. nov., a gram-negative motile coccus with unusual morphology isolated from the human mouth

    Microbiology

    (1994)
  • A.L. Griffen et al.

    Distinct and complex bacterial profiles in human periodontitis and health revealed by 16S pyrosequencing

    The ISME journal

    (2012)
  • Cited by (31)

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

      2022, Journal of Dental Sciences
      Citation Excerpt :

      Wu et al. found that Firmicutes, Lactobacillales, Actinomycetales, and Veillonellales are more common in the MCI group, whereas Fusobacteria, Bacteroidetes, and Cardiobacteriales are more common in the control group.59 Yang et al. found that Pasteurellaceae is more common in the MCI group, whereas Lautropia mirabilis is more common in the control group.58 Liu et al. reported that Moraxella, Leptotrichia, and Sphaerochaeta are more common in the AD group, whereas Rothia is more common in the control group.60

    View all citing articles on Scopus
    View full text