Table 1.

Clinical studies investigating the associations between Alzheimer’s disease and periodontitis

Cross-sectional and longitudinal studies based on databases or surveys
Authors	Database and sample size	Alzheimer’s disease diagnosis or parameters	Periodontal disease diagnosis or parameters	Outcomes
Beydounet al. (2021)45	NHANES (1988–1991) 1439 American patients -Incident AD: 277 -Incident all-cause dementia: 549 -Mean follow-up: 10–11 years -Age: ≥ 65 years at baseline	Diagnosis: -ICD-9 code 331.0: Alzheimer’s disease	Parameters: -AL -PD -Serum IgG against 19 periodontal bacteria∗ Site: Two sites on every tooth in 2 quadrants	-The cumulative incidence proportions of AD were significantly higher in the Hp seropositivity group. -P. intermedia, C. Rectus, P. nigrescens, P. melaninogenica, and P. gingivalis interacted synergistically with H. pylori sero-positivity, particularly with respect to AD incidence.
Yu et al. (2008) 28	NHANES (2001–2002) 803 dentate American patients -Age: ≥ 60 years	Parameters recorded: -2-min Digit Symbol Substitution Test	Diagnosis: -AL > 4 mm in at least 10% of sites -PDL > 3 mm in at least 10% of sites Site: -Three sites on each examined tooth in 2 randomly selected quadrants	-Higher cognitive function was associated with lower odds of periodontal disease.
Beydoun et al. (2020) 44	NHANES (1988–1994) linked with National Death Index and Medicare data (2014) 6650 American patients -AD deaths: 52 -Incident AD: 888 -Incident all-cause dementia: 1737 -Up to 26 years of follow-up -Age: ≥ 45 years at baseline	Diagnosis: -ICD-9 code 331.0: Alzheimer’s disease	Parameters recorded: -AL -PD -Serum IgG against 19 periodontal bacteria Site: -Two sites on every tooth in 2 quadrants	AD incident risk -C. rectus (55+ and 65+) -P. gingivalis (55+ and 65+) -Red-Green cluster AD mortality risk -Orange-Red cluster (55+ and 65+) -P. gingivalis IgG (65+) -P. melaninogenica (65+) -S. Oralis (men) -S. intermedius (men) -Factor 2	Inverse AD incident risk+ -S. intermedius (marginally among 55+ women) Inverse AD mortality risk -A. actinomycetemcomitans (65+)
Chen et al. (2017) 27	National Health Insurance Research Database (1996–2013) 27963 Taiwanese patients -Incident CP: 9291 -Control: 18,672 -Mean follow-up: 12 years -Age: ≥ 50 years at baseline	Diagnosis: -ICD-9 code 331.0: Alzheimer’s disease	Diagnosis: -ICD-9 code 523.4: Chronic periodontal disease	-Patients with 10 years of CP exposure exhibited a higher risk of developing AD than unexposed groups
Noble et al. (2009) 46	NHANES (1988–1994) 2355 American patients -Age: ≥ 60 years at baseline	Diagnosis: -Immediate verbal memory/registration: Summary score < 4 -Delayed verbal memory: Summary score < 3 -Serial 3 subtraction test: Summary score < 4	Parameters: -Serum IgG against P. gingivalis	-Mean P. gingivalis IgG was higher among those with impaired performance for each of the 3 cognitive tests. -Individuals in the highest P. gingivalis IgG group (> 119 EU) were more likely to have poor delayed verbal memory and impaired subtraction.
Noble et al. (2014) 47	Washington Heights-Inwood Columbia Aging Project 219 American patients -Incident AD: 110 -Control: 109 -Mean follow-up: 5 years -Age: > 65 years -No AD at baseline	Not mentioned	Parameters: Serum IgG antibody against - P. gingivalis -T. forsythia -A. actinomycetemcomitans Y4 -T. denticola -C. rectus -E. nodatum -A. naeslundii genospecies-2	-High anti-A. naeslundii titer was associated with increased risk of AD. -High anti-E. nodatum IgG was associated with a lower risk of AD.

AD and periodontal clinical parameters in cross-sectional and cohort studies
Authors	Sample size	AD diagnosis or parameters	Periodontal disease diagnosis or parameters	Outcomes
Iwasaki et al. (2015) 63	291 Japanese patients (101 males/190 females) -Average age: 80.9 years -Age: ≥ 75 years Classification: -No periodontal disease -Periodontal disease -Edentulous	Diagnosis: -MMSE: (≤ 23) -HDS-R scores: (≤ 20)	Diagnosis: -Interproximal AL ≥ 5 mm in ≥ 50% of teeth Parameters: -AL: 6 sites of every tooth -Teeth number	Periodontal disease and edentulism were significantly associated with greater odds of low cognitive performance after controlling for potential confounders.
Moriya et al. (2012) 50	152 Japanese patients Inclusion criteria: -Age: 70–74 years -Teeth number: > 6	Parameters -RCPM test -VerPA task -VisPA task -BDT	Diagnosis: -Community Periodontal Index of Treatment Needs	Weak but statistically significant negative correlations were established between the RCPM test, the VerPA task, and the VisPA task and periodontal status, but not the Block Design Test.
Kaye et al. (2013) 48	597 dentate American patients -Followed-up for 32 years -Age: 28–70 years at baseline	Low cognitive statuses: -MMSE: -< 25 points -Age- and education-specific median: < 90% -Spatial Copying Task -< 10 points	Parameters: -Alveolar bone loss progression -Probing pocket depth progression -Tooth loss rate -PD -Caries and restorations	-Each tooth lost per decade since the baseline dental examination increased the risk of low MMSE and Spatial Copying Task scores by 9% to 12%. -Each tooth that had progression of alveolar bone loss or probing pocket depth increased the overall risk of low scores by 2% to 5%. -Development of new caries or restorations was associated with an increased risk of a low Spatial Copying Task score.
Iwasaki et al. (2016) 64	85 Japanese patients -MMSE > 24 at baseline -Age: > 75 years at baseline	Parameters -Difference between MMSE score in 2010 and 2013	Diagnosis: Severe periodontitis: -AL: ≥ 6 mm at ≥ 2 interproximal sites -PD: ≥ 5 mm at ≥ 1 interproximal sites Parameters: -Teeth number	-Severe periodontitis was significantly associated with an increased risk of cognitive decline. -Participants with severe periodontitis had a 1.8-point greater decrease in MMSE score than those without severe periodontitis.
Kamer et al. (2012) 49	152 Danish patients -People born in 1914	Parameters: -DSST -BDT	Diagnosis -Modified Community Periodontal Index score: ≥ 3 for at least 10% of the remaining teeth Parameters: -PD -Teeth number	-Patients with periodontal inflammation obtained lower mean DSST and BDT scores. -Patients with many missing teeth had lower mean DSST and BDT scores. -Patients with periodontal inflammation had significantly lower adjusted mean DSST scores compared to patients without periodontal inflammation. However, for adjusted BDT, the significance held only for patients with few missing teeth.

Clinical studies on AD and inflammatory markers or IgG
Authors	Sample size	Alzheimer’s disease diagnosis or parameters	Periodontal disease diagnosis or parameters	Serology sampling site	Inflammatory markers	IgG	Outcomes
Sochocka et al. (2017) 51	128 Polish patients (45 males/83 females) -Age: 55–90 years	Diagnosis: -DSM-V and NINCDA-ADRDA criteria Parameters: -MMSE	Parameters: -Teeth number -PD -CAL -BoP -Plaque index Record site: -Six sites on all teeth	Sampling: -Peripheral blood leukocytes (PBL)	Inflammatory markers: -IL-1β -IL-6 -IL-10 -TNF-α	Not mentioned	-Worseperiodontal health status as well as cognitive decline were associated with higher Inflammatory state.
Kamer et al. (2009) 52	34 American patients -AD: 18 -Control: 16	Parameters: -MMSE	Not mentioned	Sampling: -Frozen whole blood	Inflammatory markers: -APOE ε4 -TNF-α -IL-1β -IL-6	IgG against - A. actinomycetemcomitans serotype b (ATCC 43718), -T. forsythia (ATCC 43037) -P. gingivalis (ATCC 33277)	-PlasmaTNF-α and antibodies against periodontal bacteria were elevated in AD patients compared with controls and independently associated with AD.
Ide et al. (2016) 53	59 English patients (30 males/29 females) -Mean age: 77.7 years -Followed-up for 6 months -Mild to moderate dementia -Excluded smokers	Parameters: -Alzheimer’s Disease Assessment Scale (ADAS-cog) -MMSE	Diagnosis: -CDC/AAP criteria Parameters: -PD -BOP -PI Record site: -Six sites of each tooth	Sampling: -Venous blood	Inflammatory markers: -CRP -TNF-α -IL-10	IgG against P. gingivalis	-Periodontitis was associated with an increase in cognitive decline in patients with Alzheimer’s disease, independent of baseline cognitive state.
Stein et al. (2012) 54	158 American patients -Incident MCI or AD: 81 -Control: 77 -Mean age: 70.0 years (Control) 72.1 years (MCI) 74.1 years (AD) -Cognitively intact at baseline	Diagnosis: -AD: McKhann et al. criteria -MCI: Petersen et al. criteria	Not mentioned	Sampling: -Venous blood	Not mentioned	IgG against - A. actinomycetemcomitans -P. gingivalis -C. rectus -T. denticola -F. nucleatum -T. forsythia -P. intermedia	-Antibody levels of F. nucleatum and P. intermedia were significantly increased at baseline serum drawing in the AD patients compared to the controls.

Studies on AD and the oral microbiome using PCR
Authors	Sample size	Alzheimer’s disease diagnosis or parameters	Periodontal disease diagnosis or parameters	AD markers	Microbiology	Outcomes
Leblhuber et al. (2020) 55	20 Austrian patients (11 males/9 females) -Mean age: 78.1 ± 2.2 years -Probable AD	Parameters: -MMSE -Clock-Drawing Test -Magnetic resonance imaging	Not mentioned	-Neopterin -Tryptophan -Kynurenine Sampling site: -Serum	Sampling site: -Alveolar fluid Method -RNA-based analysis (PerioPOC) Bacteria: -T. denticola/ T. forsythia/ P. gingivalis/ P. intermedia / A. actinomycetemcomitans	-P. gingivalis was associated with lower MMSE and Clock-Drawing Test scores.
Laugisch et al. (2018) 56	40 German patients -AD: 20 -Other forms of dementia: 20 -Age: 30–70 years -Caucasian origin -Recently diagnosed with dementia	Diagnosis: -MMSE ≥ 19	Diagnosis: -PD of ≥ 4 mm -AL ≥ 3 mm Parameter: -PI PD -AL -BOP Site: -Six sites of each tooth	-Aβ1-42 -Total tau Sampling site: -CSF Inflammatory markers in GCF and serum: -IL-1 -MCP-1/CCL-2	Sampling site: -Subgingival dental biofilm and GCF -Serum -CSF Method: -Real time PCR (High Pure Template Preparation Kit) -Antibodies Bacteria: -A. actinomycetemcomitans/ P. gingivalis/ T. forsythia/ T. denticola/ T. socranskii	-None of the investigated bacteria were detected in the CSF or serum samples. -No significant difference was observed in antibody levels against specific bacteria in the CSF or serum between groups. -In patients with dementia aged up to 70 years, periodontal pathogens did not act as a trigger for developing AD.

Studies on AD and the oral microbiome using NGS or the third-generation technique
Authors	Sample size	Alzheimer’s disease diagnosis or parameters	Periodontal disease diagnosis or parameters	AD markers	Microbiology sampling site and method	Outcomes
Holmer et al. (2021)57	195 Swedish patients -AD: 46 -MCI: 40 -Subjective cognitive decline: 46 -Control: 63 -Age: 50-80 years	Diagnosis: -NIA-AA diagnostic guidelines	Diagnosis: -PD ≥ 6 mm Parameters: -PD -BOP -Radiograph Site: -Six sites of each tooth	Not mentioned	Sampling site: -Subgingival dental biofilm Method: -V3-V4 regions of the 16S rRNA gene -Illumina MiSeq	-Cognitive dysfunction was a significant determinant of subgingival microbial composition and was associated with higher microbial richness and evenness using either alpha or beta diversity measures.
						More abundant in AD group -S. exigua (Gram-positive, anaerobic coccobacillus) -Lachnospiraceae bacterium (Gram-negative, obligately anaerobic) -P. oulorum (Gram-negative, anaerobic)	More abundant in controls -R. aeria (Gram-positive, aerobic) -C. durum (Gram-positive, aerobic) -Actinomyces genus (Gram-positive, facultatively anaerobic)
Wu et al. (2021) 59	35 Taiwanese patients -AD: 17 -Control: 18	Parameters: -Clinical Dementia Rating Scale -SOB -MMSE	Not mentioned	Not mentioned	Sampling site: -Supragingival dental plaque Method: -Full-length 16S rDNA sequencing -PacBio single-molecule real-time sequencing	Overall oral microbial diversity in the AD group tended to be lower than that in the control group.
						More abundant in AD group Order: -Lactobacillales (aerotolerant anaerobes) -Actinomycetales(anaerobic) -Veillonellales (anaerobic) Family: -Lactobacillaceae -Streptococcaceae(aerobic) -Actinomycetaceae -Veillonellaceae Genus: -Lactobacillus -Streptococcaceae -Actinomycetaceae -Veillonella	More abundant in controls Order: -Fusobacteria -Bacteroidetes -Cardiobacteriales Family: -Fusobacteriaceae -Cardiobacteriaceae -Porphyromonadaceae Genus: -Fusobacterium -Cardiobacterium -Porphyromonas
Yang et al. (2021) 58	68 older American patients (27 males/41 females) -MCI: 34 -Control: 34	Diagnosis -ADRC consensus diagnosis Parameters -Uniform Data Set -CSF assays for Aβ42, total-Tau, and phospho-Tau	Not mentioned	Sampling site: -Blood (CRP and LPS) -CSF (91 proteins on the Olink INFLAMMATION panel)	Sampling site: -Oral swab collection from the dorsal tongue, hard palate, buccal mucosa, and keratinized gingiva Method: -V4 regions of the 16S rRNA gene -Illumina MiSeq	-MCI did not appear to shift the taxonomic composition of the oral microbiome. -No difference was identified in alpha or beta diversity between MCI and control groups. -Levels of CRP and LPS in blood were not significantly different between groups.
						More abundant in AD group -Amplicon sequence variants of Pasteurellaceae	More abundant in controls -Amplicon sequence variants of L. mirabilis.
Liu et al. (2019) 60	78 Chinese patients (39 males/39 females) -AD: 39 -Control: 39	Diagnosis: -Mild: MMSE ≥ 20 -Moderate: 10 ≤ MMSE < 20 -Severe: MMSE <10	Not mentioned	-APOE ε4	Sampling site: -Saliva -QIAamp DNA Investigator Kit Method: -V3-V4 regions of the 16S rRNA gene -Illumina Hiseq2500	-Alpha diversity analysis showed that there was lower richness and diversity in AD patients. -No bacteria were found to be associated with the severity of AD.
				-		More abundant in AD group Genus: -Moraxella (Gram-negative bacteria) -Leptotrichia (Gram-negative bacteria) -Sphaerochaeta (Gram-negative bacteria) In AD and APOEε4 (+) patients: -Increase: Abiotrophia and Desulfomicrobium -Decrease: Actinobacillus and Actinomyces	More abundant in controls -Rothia

Post-mortem studies on AD and microbiomes
Authors	Sample size	Microbiology sampling site and method	Outcomes
Emery et al. (2017) 61	26 patients -AD: 14 -Control: 12 Inclusion criteria: -Age: 62–98 years	Sampling site: Temporal cortex Right hemispheres: -Formalin fixed for neuropathological assessment and for immunohistochemical analysis Left hemispheres -Sliced and frozen at −80°C Method -V3 regions of the 16S rRNA gene -Life Technologies Ion Plus Fragment Library Kit (ThermoFisher Scientific)	-Many more bacterial 16S reads were yielded from AD samples, strongly suggesting that contamination was not a major issue for these data. -Contamination: Rhizobiales; Methylobacteriaceae
			More abundant in AD group -Largest component: Actinobacteria Family: -Actinobacteria; Propionibacteriaceae (P. acnes) -Actinobacteria; Corynebacteriaceae	More abundant in controls -Proteobacteria
Siddiqui et al. (2019) 62	20 patients (5 males/15 females) -AD: 10 -Control: 10 -Age: 63–103 years	Sampling site: -Brain tissue adjacent to the lateral ventricle of the parietal lobe -Average storing interval: 16 years Method: -V3-V4 regions of the 16S rRNA gene -Illumina MiSeq	-A wide spectrum of bacteria was detected in samples from both groups, containing both oral and gastrointestinal tract microbiome species.
			More abundant in AD group Phylum -Firmicutes Family: -Pseudomonadaceae: dominating family Order: -Actinomycetales Species -Prevotella -Treponema -Veillonella	More abundant in controls Phylum -Proteobacteria Genus -Fusobacterium
Dominy et al. (2019) 30	58 patients -AD: 29 -Control: 29	Sampling site -Middle temporal gyrus AD markers: -Tau (non-phosphorylated/phosphorylated) -Ubiquitin Methods: -Real-time PCR analysis of P. gingivalis Antibodies to gingipain: -Lysine-gingipain (Kgp) -Arginine-gingipain A (RgpA)	-Both RgpB and Kgp antigens in brain tissue independently demonstrated a significant correlation with AD diagnosis, tau load, and ubiquitin load.
			More abundant in AD group -P. gingivalis strains W83, ATCC33277, and FDC381	More abundant in controls
Poole et al. (2013) 31	20 patients -AD: 10 -Control: 10	Sampling site: -Brain tissue adjacent to the lateral ventricle of the parietal lobe Method: Antibodies against - P. gingivalis (LPS and gingipains) -T. forsythia -T. denticola	-Human brain tissue sections with mouse anti-P. gingivalis revealed strong cellular surface membrane labeling in 4 out of 10 AD cases but not in the non-AD age-matched controls. -No immunolabeling was observed with anti-T. forsythia antibodies nor with anti-T. forsythia. -Four out of 10 AD brains exhibited bands characteristic of LPS.
Riviere et al. (2002) 32	Group 1 34 patients (18 males/16 females) -AD: 16 -Control: 18 Group 2 5 patients (2 males/3 females) -AD: 3 -Control: 2	Group 3 4 patients (2 males/2 females) Group 4 33 living patients -AD: 17 -Control: 16	Sampling site: -Frontal lobe cortexes (Group 1) -Frozen trigeminal ganglia (Group 2) -TG, pons, and hippocampus (Group 3) -Saliva (Group 4) Method: -Treponema species-specific DNA PCR -Treponema species-specific antigens	-Treponema may infect the brain via branches of the trigeminal nerve.

Amyloid β plaques (Aβ), Attachment loss (AL), Apolipoprotein E (APOE), Approximal plaque index (API), Bleeding on probing (BoP), Block Design Test (BDT), Clinical attachment level (CAL), Chronic periodontitis (CP), Cerebrospinal fluid (CSF), Hasegawa Dementia Scale-Revised (HDS-R), Immunoglobulin G (IgG), Interleukin (IL), Mild cognitive impairment (MCI), Mini-Mental State Examination (MMSE), National Health and Nutrition Examination Survey (NHAHES), Next- generation sequencing (NGS), Periodontal inflammation (PI), Polymerase chain reaction (PCR), Raven’s Coloured Progressive Matrices (RCPM), Subjective cognitive decline (SCD), Verbal Paired Associates (VerPA), Visual Paired Associates (VisPA)