www.nature.com/scientificreports

OPEN

Received: 10 July 2019
Accepted: 6 September 2019
Published: xx xx xxxx

Infrequent Denture Cleaning
Increased the Risk of Pneumonia
among Community-dwelling Older
Adults: A Population-based Crosssectional Study
Taro Kusama1, Jun Aida

1

, Tatsuo Yamamoto2, Katsunori Kondo3,4 & Ken Osaka

1

Pneumonia is a leading cause of death among older adults. The effectiveness of oral care in preventing
pneumonia in nursing homes and hospitals has been reported. However, in community-dwellingolder adults, the role of denture cleaning in preventing pneumonia remains unknown. We aimed
to investigate the association between infrequent denture cleaning and the risk of pneumonia
in community-dwelling older adults. This cross-sectional study was based on the self-reported
questionnaire targeting towards community-dwelling older adults aged ≥65 years. Responses of71,227 removable full/partial denture users were included. The incidence of pneumonia within thelast one-year and the frequency of denture cleaning (daily/non-daily) were treated as dependentand independent variables, respectively. The odds ratio (OR) and 95% confidence interval (CI) were
calculated by the inverse probability weighting (IPW) method based on the logistic regression
model. The mean age of the participants was 75.2 ± 6.5 years; 48.3% were male. Overall, 4.6% of theparticipants did not clean their dentures daily; 2.3% and 3.0% who did and did not clean their dentures
daily, respectively, experienced pneumonia. After IPW, infrequent denture cleaning was significantly
associated with pneumonia incidence (OR = 1.30, 95% CI = 1.01–1.68)). This study suggests that
denture cleaning could prevent pneumonia among community-dwelling older adults.
Pneumonia is a leading cause of hospitalization and death1,2. It is widely prevalent among the older population
because of the decline in immune system and respiratory function with advancing age3. Aspiration is one of themechanisms that explains the onset of pneumonia among older adults4,5. In fact, oral bacteria have been identified in the lungs of the patients who developed pneumonia; therefore, a relationship between aspiration of oral
bacteria and pneumonia is strongly suggested6. In addition, because a substantial proportion of older adults areaffected by dysphagia7,8, the risk of pneumonia through aspiration may increase4. To reduce the risk of aspiration
pneumonia, oral care has been implemented in nursing homes and has successfully decreased the incidence ofpneumonia among the nursing home residents9–11.
Although the role of oral care in reducing the risk of pneumonia has been recognized, the importance ofdenture cleaning has been relatively neglected. The presence of fewer or no teeth is prevalent among the older
adults12; therefore, the use of removable dentures is a common treatment option. On the surface of the denture, abiofilm composed of microorganisms called “denture plaque” rapidly develops upon insertion after cleansing13.
There is a possibility that the denture plaque may reach the lungs by aspiration, causing aspiration pneumonia.
Although generally included as a part of oral care11, previous studies have not focused on denture cleaning alone.
In addition, most of the previous studies on the relationship between oral hygiene and pneumonia were carried
1

Department of International and Community Oral Health, Tohoku University Graduate School of Dentistry, Aoba-ku,
Sendai, Miyagi, Japan. 2Department of Disaster Medicine and Dental Sociology, Graduate School of Dentistry,
Kanagawa Dental University, Yokosuka, Kanagawa, Japan. 3Department of Social Preventive Medical Sciences, Center
for Preventive Medical Sciences, Chiba University, Chuo Ward, Chiba-shi, Chiba, Japan. 4Department of Gerontological
Evaluation, Center for Gerontology and Social Science, National Center for Geriatics and Gerontology, Obu city, Aichi,
Japan. Correspondence and requests for materials should be addressed to J.A. (email: j-aida@umin.ac.jp)
Scientific Reports |(2019) 9:13734 | https://doi.org/10.1038/s41598-019-50129-91www.nature.com/scientificreports/

www.nature.com/scientificreports

out in nursing homes and hospitals14,15. However, the risk of aspiration pneumonia is considered to be highin community-dwelling older adults. To the best of our knowledge, no study has investigated the association
between denture cleaning and pneumonia among community-dwelling older adults. From a public health viewpoint, as the majority of older adults are community-dwellers and not institutionalized, the prevention of pneumonia among community-dwelling older adults is important. In this study, we investigate whether infrequent
denture cleaning is associated with the risk of developing pneumonia among community-dwelling older adults.

Methods

Settings and participants. This cross-sectional study is based on a self-reported questionnaire. The data
were obtained from the survey of the 2016 Japan Gerontological Evaluation Study (JAGES). JAGES targeted thecommunity-dwelling older adults aged ≥65 years, who were not certified to be eligible for long term public care.
Information on social, behavioral, and health factors were collected. JAGES in 2016 was conducted in 39 municipalities in Japan. The questionnaire was sent by post and was retrieved by mail.
Dependent variable. We used the self-reported incidence of pneumonia within the last one-year as adependent variable. We asked the question “Did you experience the following diseases within the last one year?”Those who answered “pneumonia” were considered to be the individuals who suffered from pneumonia within
the last one year.
Independent variable.

We used the frequency of denture cleaning as an independent variable. To those
who used removable dentures, we asked the question “Do you clean your dentures daily?”; the choices provided
were “Yes” or “No.” We defined people chose “Yes” as those who cleaned their denture daily and “No” as those whocleaned their dentures infrequently (non-daily).

Covariates. We selected possible cofounders as covariates based on previous studies and clinical knowledge1,16,17; this included age, sex, smoking status, educational status, equivalent income, number of teeth, activities
of daily living (ADL), comorbidity related to stroke or dementia, and experience of pneumococcal vaccination
within last five-year.
Statistical analysis. We estimated the propensity score for the independent variable. The stabilized averagetreatment effect (ATE) on the risk of pneumonia was calculated using the inverse probability weighting (IPW)
method. To predict the propensity score for infrequent denture cleaning, we used the logistic regression model; all
the covariates were included as possible confounders and the stabilized ATE weight was calculated. The stabilized
ATE weight was used to avoid instability of the estimated effect size due to extreme weighting18. We compared thestandardized difference between the categories of independent variable before and after stabilized ATE weighting19,20. The standardized difference was used to check the balance of the covariates between the treated and control groups. If standardized difference of all covariates was <0.1 after weighting, it was regarded as well balanced.
We developed the logistic regression model; the odds ratios (ORs) and 95% confidence intervals (95% CIs) were
calculated using IPW with stabilized ATE weights (stabilized ATE-IPW). For missing responses, we presumed
that the missing pattern of the original data set was missing at random. Multiple imputation by chained equation(MICE) was used to generate 20 imputed datasets. We calculated the stabilized ATE weighted OR for each data
set and combined all estimators by Rubin’s rule21. In the sensitivity analysis, the participants were stratified into
two age groups (<75 or ≥75 years) for IPW. Then, the interaction effect of age and frequency of denture cleaning
was confirmed by using the relative excess risk due to interaction (RERI) as additive scales and the ratio of OR
as multiplicative scale between them22. We used Stata/MP version 15 (Stata Corp., College Station, TX, USA) for
statistical analysis.

Ethical issue.

In this study, the process of obtaining informed consent was as follows: the questionnaire wassent by mail along with the explanation of the study; the participants read the written explanation about the aimof study and replied. Hence, we considered that informed consent was provided by those who replied and sent
back the questionnaire. The JAGES protocol in 2016 was approved by the ethics committee of National Center for
Geriatrics and Gerontology (No. 992) and the ethics committee of Chiba University (No. 2493). We followed the
STROBE Statement to report our observational study.

Results

From a target population of 279,661, 180,021 individuals participated in the survey (response rate = 70.2%). Of
these, 88,994 (49.4%) participants who used removable dentures (including both removable full/partial dentures)
were included in this analysis. However, 17,767 participants with missing information regarding the dependent
variable were excluded. Finally, data of 71,227 participants were included in the analysis. Table 1 shows the characteristics of the participants. The mean age was 75.2 years (SD = 6.5); 48.3% were male. Overall, 2.3% (n = 1,666)
and 97.7% (n = 69,561) of the participants, respectively, did and did not experience pneumonia within the last
one year.
Table 2 shows the proportion of participants who experienced pneumonia based on the frequency of denturecleaning and stratified by age group. Pneumonia was more prevalent among the participants who did not clean
their dentures daily, especially those aged ≥75 years. Among these participants aged ≥75 years, 2.9% and 4.3% of
those who did and did not clean their dentures daily, respectively, experienced pneumonia.
To reduce the possibility of selection bias, we estimated the propensity score for denture cleaning after MICE.
After multiple imputation, the missing values of 22,020 participants were imputed. The propensity scores werepredicted using the logistic regression model separately for the entire data (all participants) and stratified data
(participants aged <75 years or ≥75 years) for each imputed data sets. After using stabilized ATE weight, the
Scientific Reports |(2019) 9:13734 | https://doi.org/10.1038/s41598-019-50129-92

www.nature.com/scientificreports

www.nature.com/scientificreports/

Characteristics

All participants(n = 71,227)

Experienced pneumonia
within last one-year(n = 1,666)

Not experienced pneumonia
within last one-year(n = 69,561)

n

n

n

%

%

%

Frequency of denture cleaning
Daily

67,208

94.4

1,547

92.9

65,661

94.4

Non-daily

3,293

4.6

100

6.0

3,193

4.6

Missing

726

1.0

19

1.1

707

1.0

Age
65–69 years

16,770

23.5

248

14.9

16,52223.8

70–74 years

18,579

26.1

365

21.9

18,21426.2

75–79 years

17,347

24.4

425

25.5

16,922

24.3

80–84 years

11,858

16.6

369

22.2

11,48916.5≥85 years

6,673

9.4

259

15.6

6,414

9.2

Sex
Male

34,393

48.3

984

59.1

33,40948.0

Female36,825

51.7

682

40.9

36,14352.0

Missing

9

0.0

0

0.0

9

0.0

Education≤9 years

25,133

35.3

706

42.4

24,42735.1

10–12 years

28,513

40.0

596

35.8

27,91740.1≥13 years

16,611

23.3

331

19.9

16,28023.4

Missing

970

1.4

33

2.0

937

1.4

Equivalent income (100 JPY ≈ 1 USD)
<1,000,000 JPY

7,568

10.6

230

13.8

7,338

10.6

1,000,0000–1,999,999 JPY

21,017

29.5

455

27.3

20,56229.6

2,000,0000–2,999,999 JPY

13,401

18.8

274

16.5

13,12718.9

3,000,0000–3,999,999 JPY

8,055

11.3

124

7.4

7,931

11.4

≥4,000,000 JPY

5,701

8.0

117

7.0

5,584

8.0

Missing

15,485

21.8

466

28.0

15,019

21.6

Smoking status
Never

39,027

54.8

702

42.2

38,325

55.1

Quite

22,368

31.4

772

46.3

21,596

31.1

Current

8,726

12.3

145

8.7

8,581

12.3

Missing

1,106

1.5

47

2.8

1,059

1.5

Dementia
Yes

484

0.7

16

1.0

468

0.7

No

68,468

96.1

1,620

97.2

66,84896.1

Missing

2,275

3.2

30

1.8

2,245

3.2

Stroke
Yes

2,197

3.1

76

4.6

2,121

3.1

No

66,755

93.7

1,560

93.6

65,195

93.7

Missing

2,275

3.2

30

1.8

2,245

3.2

Activities of daily living
No need for personal assistance

63,052

88.5

1,300

78.0

61,75288.8

Require some personal assistance

4,787

6.7

255

15.3

4,532

6.5

Missing

3,388

4.8

111

6.7

3,277

4.7

Number of teeth
0

10,620

14.9

337

20.2

10,283

14.8

1–4

7,577

10.6

217

13.0

7,360

10.6

5–9

11,707

16.4

299

18.0

11,408

16.4

10–19

20,687

29.1

437

26.2

20,25029.1≥20

19,096

26.8

320

19.2

18,77627.0

Missing

1,540

2.2

56

3.4

1,484

2.1

Experience of pneumococcal vaccination within last five-year
Yes

30,174

42.4

1,016

61.0

29,15841.9

No39,349

55.2

565

33.9

38,784

55.8

Missing

1,704

2.4

85

5.1

1,619

2.3

Table 1. Characteristics of the participants (n = 71,227).

Scientific Reports |(2019) 9:13734 | https://doi.org/10.1038/s41598-019-50129-93

www.nature.com/scientificreports

www.nature.com/scientificreports/
All participants (n = 70,501)

65–74 years (n = 35,062)

Frequency of denture cleaning

Frequency of denture cleaning

Frequency of denture cleaning

Daily

Non-daily

Daily
972 (2.9)n (%) Daily

Non-daily≥75 years (n = 35,439)
Non-daily

Incidence of pneumonia within the last one year
Yes

1,547 (2.3)

100 (3.0)575 (1.7)

34 (1.9)

No65,661 (97.7)

3,193 (97.0)

32,733 (98.3)

1,720 (98.1) 32,928 (97.1)

66 (4.3)
1,473 (95.7)

Table 2. The incidence of pneumonia within the last one year based on the frequency of denture cleaning
stratified by age groups.

All participants (n = 71,227)

65–74 years (n = 35,349)≥75 y (n = 35,878)

Stabilized ATE weighted

Stabilized ATE weighted

Stabilized ATE weighted

Frequency ofdenture cleaning

OR (95% CI)

OR (95% CI)

OR (95% CI)

Daily

Ref.

Ref.

Ref.

Non-daily

1.30 (1.01–1.68)

0.98 (0.64–1.50)

1.58 (1.15–2.17)

Table 3. The association between the incidence of pneumonia within the last one year and the frequency ofdenture cleaning. Note: ATE = average treatment effect, OR = odds ratio, 95%CI = 95% confidence interval,
Ref. = reference.

standardized differences of all covariates were <0.1 (Supplementary Table 1). Therefore, by using the estimatedpropensity score, we confirmed that the all the covariates are well balanced between those who did and did
not cleaned their dentures daily. Table 3 shows the results of the logistic regression analysis using the stabilized
ATE-IPW method; infrequent denture cleaning was significantly associated with the incidence of pneumonia
among all participants (OR = 1.30, 95% CI = 1.01–1.68). In addition, the sensitivity analysis based on stratification by age groups showed that infrequent denture cleaning was significantly associated with the occurrence ofpneumonia among those aged ≥75 years (OR = 1.58, 95% CI = 1.15–2.17). In contrast, a significant association
between infrequent denture cleaning and the incidence of pneumonia was not observed among those aged <75
years (OR = 0.98, 95% CI = 0.64–1.50). However, the additive and multiplicative scale of interaction effect was
not significant (Supplementary Table 2).

Discussion

The present study revealed that infrequent denture cleaning was associated with the incidence of pneumonia
within the last one year among community-dwelling older adults. This result suggests the importance of denturecleaning in reducing the risk of pneumonia among community-dwelling older adults. From the public healthviewpoint, this is an important finding because the number of community-dwelling older adults is increasing inthis aging world.
As mentioned in the introduction, previous studies suggested that oral hygiene including denture cleaning
was associated with the incidence of pneumonia among nursing homes residents11; the present study showed asimilar association among the community-dwelling older adults. A study conducted in nursing home reported a
reduction of death due to pneumonia among older residents by oral care including denture cleaning23. We addedthat frequent denture care could reduce the incidence of pneumonia in community-dwelling older adults.
Denture plaque is composed from many species of bacteria and fungus; some of them are regarded as pathogen of pneumonia24,25. Infrequent denture cleaning causes accumulation of denture plaque26, and therefore, thepossibility of the pathogens reaching the lung by aspiration might increase27. Consequently, it may be presumed
that the pathogens from denture plaque accumulated due to infrequent cleaning were aspirated and may haveincreased the risk of pneumonia. In the present analysis, a strong association was observed among those aged
≥75 years, although a statistical significance was not clearly observed. With advancing age, the immune systemdeclines3 and aspiration is more likely to occur in older adults rather than those who are younger28. The mortality
rate of pneumonia is increasing among the older adults2. Therefore, the results of the present study are reasonable: those aged ≥75 years were more likely to develop pneumonia and the harmful effect of infrequent denturecleaning was stronger than that observed in younger participants. These results are supported by the biologicalexplanations mentioned above. Further study considering the effect modification of dysphagia on the association
between poor oral hygiene and pneumonia incidence would strengthen our explanation of the results of thepresent study.
The strength of this study was the inclusion of over 70,000 participants; this sample size was large enoughto detect the association between infrequent denture cleaning and pneumonia. The incidence of pneumonia
among community-dwelling older adults is lower than that in nursing homes where frail older adults live29.
Therefore, it is difficult to have sufficient statistical power to detect the association in smaller epidemiologicalstudies. This study, however, has several limitations. As this was a cross-sectional study, we could not evaluate
the causal relationship between denture cleaning and pneumonia. However, it is less likely that the occurrenceof pneumonia would lead to infrequent denture cleaning. In addition, the self-reported incidence of pneumonia
causes reporting bias. However, the incidence of pneumonia in this study is similar to that previously reported30.
Therefore, the reporting bias caused by the self-reporting of pneumonia was considered to be relatively small.
Scientific Reports |(2019) 9:13734 | https://doi.org/10.1038/s41598-019-50129-94www.nature.com/scientificreports/

www.nature.com/scientificreports

The self-reported independent variable, denture cleaning, also created bias. A wide variety of denture cleaning
methods and techniques may be used by the participants. Our questionnaire could not obtain information on
the details regarding the denture cleaning methods. However, this reporting bias could widen the 95% confidence interval of our estimates. Despite this situation, there was a significant association of denture cleaning
with pneumonia; therefore, we consider the present results to be robust. Furthermore, those who died because ofpneumonia were not included in this study. This selection bias is considered to cause an underestimation of theassociation between denture cleaning and pneumonia. In the present results, the benefit of denture cleaning was
remarkable among only older adults aged ≥75 years. The individuals who died from pneumonia are considered
to be frail and very old27,29; therefore, the impact of denture cleaning on these individuals is larger than those who
experienced pneumonia but are alive. The previous study revealed an association between denture wearing during
sleep and pneumonia incidence among community-dwelling older adults31. The results of this previous study
were similar to those from our study. There was a possibility of multicollinearity between denture wearing during
sleep and infrequent denture cleaning. In our survey, a question about denture wearing during sleep was asked
to only one-eighth of all participants (n = 8,316), so we did not include this variable in the present analysis to
avoid decreasing the sample size. When analyzing this variable alone, we confirmed that the proportions of thosewearing dentures during sleep were similar among those participants who did/did not clean their dentures daily(17.3% among those who cleaned their dentures daily and 18.5% among those who did not clean their dentures
daily wore dentures during sleep; chi-square test, p = 0.544). Therefore, infrequent denture cleaning is associatedwith pneumonia incidence and is independent of denture wearing during sleep.

Conclusion

The present study revealed that infrequent denture cleaning was associated with the incidence of pneumonia
within the last one year among community-dwelling older adults. Daily cleaning of dentures may reduce the riskof pneumonia among community-dwelling older adults. In the chair side, dental professionals need to instructtheir patients to keep their dentures clean to prevent pneumonia. Even for community-dwelling older adults,
dental professionals should pay more attention to oral hygiene for pneumonia prevention.

Data Availability

All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials. The JAGES data used in this study will be made available upon request. The authors require the applicant tosubmit an analysis proposal to be reviewed by an internal JAGES committee to avoid duplication. Confidentiality
concerns prevent us from depositing our data in a public repository. Proposals submitted by outside investigators
will be discussed during the monthly investigators’ meeting to ensure that there is no overlap with ongoing analyses. If approval to access the data is granted, the JAGES researchers will request the outside investigator to help
financially support our data manager’s time to prepare the data for outside use.

References

1. Jackson, M. L. et al. The Burden of Community-Acquired Pneumonia in Seniors: Results of a Population‐Based Study. Clin. Infect.
Dis. 39, 1642–1650 (2004).
2. Ewig, S. et al. New perspectives on community-acquired pneumonia in 388 406 patients. Results from a nationwide mandatory
performance measurement programme in healthcare quality. Thorax 64, 1062–1069 (2009).
3. Lowery, E. M., Brubaker, A. L., Kuhlmann, E. & Kovacs, E. J. The aging lung. Clin. Interv. Aging 8, 1489–1496 (2013).
4. Mandell, L. A. & Niederman, M. S. Aspiration Pneumonia. N. Engl. J. Med. 380, 651–663 (2019).
5. Kikuchi, R. et al. High incidence of silent aspiration in elderly patients with community-acquired pneumonia. Am. J. Respir. Crit.
Care Med. 150, 251–3 (1994).
6. Imsand, M., Janssens, J. P., Auckenthaler, R., Mojon, P. & Budtz-Jorgensen, E. Bronchopneumonia and oral health in hospitalized
older patients. A pilot study. Gerodontology 19, 66–72 (2002).
7. Kawashima, K., Motohashi, Y. & Fujishima, I. Prevalence of dysphagia among community-dwelling elderly individuals as estimatedusing a questionnaire for dysphagia screening. Dysphagia 19, 266–271 (2004).
8. Serra-Prat, M. et al. Prevalence of oropharyngeal dysphagia and impaired safety and efficacy of swallow in independently living
older persons. J. Am. Geriatr. Soc. 59, 186–187 (2011).
9. Sjogren, P., Wardh, I., Zimmerman, M., Almstahl, A. & Wikstrom, M. Oral Care and Mortality in Older Adults with Pneumonia in
Hospitals or Nursing Homes: Systematic Review and Meta-Analysis. J. Am. Geriatr. Soc. 64, 2109–2115 (2016).
10. Kaneoka, A. et al. Prevention of Healthcare-Associated Pneumonia with Oral Care in Individuals Without Mechanical Ventilation:
A Systematic Review and Meta-Analysis of Randomized Controlled Trials. Infect. Control Hosp. Epidemiol. 36, 899–906 (2015).
11. Liu, C. et al. Oral care measures for preventing nursing home-acquired pneumonia. Cochrane database Syst. Rev. 9, CD012416(2018).
12. Kassebaum, N. J. et al. Global Burden of Severe Tooth Loss: A Systematic Review and Meta-analysis. JDR clínical Res. Suppl. 93,
20s–28s (2014).
13. Nikawa, H., Hamada, T. & Yamamoto, T. Denture plaque–past and recent concerns. J. Dent. 26, 299–304 (1998).
14. El-Solh, A. A. Association between pneumonia and oral care in nursing home residents. Lung 189, 173–180 (2011).
15. Salamone, K., Yacoub, E., Mahoney, A.-M. & Edward, K.-L. Oral care of hospitalised older patients in the acute medical setting.
Nurs. Res. Pract. 2013, 827670 (2013).
16. Loeb, M. B. Community-acquired pneumonia in older people: the need for a broader perspective. J. Am. Geriatr. Soc. 51, 539–543(2003).
17. Evren, B. A., Uludamar, A., Iseri, U. & Ozkan, Y. K. The association between socioeconomic status, oral hygiene practice, denturestomatitis and oral status in elderly people living different residential homes. Arch. Gerontol. Geriatr. 53, 252–257 (2011).
18. Xu, S. et al. Use of stabilized inverse propensity scores as weights to directly estimate relative risk and its confidence intervals. Value
Health 13, 273–277 (2010).
19. Ali, M. S. et al. Reporting of covariate selection and balance assessment in propensity score analysis is suboptimal: a systematic
review. J. Clin. Epidemiol. 68, 112–121 (2015).
20. Austin, P. C. An Introduction to Propensity Score Methods for Reducing the Effects of Confounding in Observational Studies.
Multivariate Behav. Res. 46, 399–424 (2011).

Scientific Reports |(2019) 9:13734 | https://doi.org/10.1038/s41598-019-50129-9

5www.nature.com/scientificreports/

www.nature.com/scientificreports

21. Leyrat, C. et al. Propensity score analysis with partially observed covariates: How should multiple imputation be used? Stat. Methods
Med. Res. 28, 3–19 (2019).
22. Knol, M. J. & VanderWeele, T. J. Recommendations for presenting analyses of effect modification and interaction. Int. J. Epidemiol.
41, 514–520 (2012).
23. Bassim, C. W., Gibson, G., Ward, T., Paphides, B. M. & Denucci, D. J. Modification of the risk of mortality from pneumonia with oral
hygiene care. J. Am. Geriatr. Soc. 56, 1601–1607 (2008).
24. Przybylowska, D., Mierzwinska-Nastalska, E., Swoboda-Kopec, E., Rubinsztajn, R. & Chazan, R. Potential respiratory pathogens
colonisation of the denture plaque of patients with chronic obstructive pulmonary disease. Gerodontology 33, 322–327 (2016).
25. Sumi, Y., Miura, H., Sunakawa, M., Michiwaki, Y. & Sakagami, N. Colonization of denture plaque by respiratory pathogens in
dependent elderly. Gerodontology 19, 25–29 (2002).
26. Nishi, Y. et al. Examination of denture-cleaning methods based on the quantity of microorganisms adhering to a denture.
Gerodontology 29, 259–266 (2012).
27. Janssens, J. P. & Krause, K. H. Pneumonia in the very old. Lancet Infect. Dis. 4, 112–124 (2004).
28. Sura, L., Madhavan, A., Carnaby, G. & Crary, M. A. Dysphagia in the elderly: management and nutritional considerations. Clin.
Interv. Aging 7, 287–298 (2012).
29. Sue Eisenstadt, E. Dysphagia and aspiration pneumonia in older adults. J. Am. Acad. Nurse Pract. 22, 17–22 (2010).
30. Morimoto, K. et al. The burden and etiology of community-onset pneumonia in the aging Japanese population: a multicenterprospective study. PLoS One 10, e0122247 (2015).
31. Iinuma, T. et al. Denture wearing during sleep doubles the risk of pneumonia in the very elderly. J Dent Res 94, 28S–36S (2015).

Acknowledgements

This study used data from JAGES (the Japan Gerontological Evaluation Study), and this work was supported
by MEXT(Ministry of Education, Culture, Sports, Science and Technology-Japan)-Supported Program for
the Strategic Research Foundation at Private Universities (2009–2013), JSPS(Japan Society for the Promotionof Science) KAKENHI Grant Numbers (18390200, 22330172, 22390400, 23243070, 23590786, 23790710,
24390469, 24530698, 24683018, 25253052, 25870573, 25870881, 26285138, 26882010,15H01972, 18KK0057,
19H03860), Health Labor Sciences Research Grants (H22-Choju-Shitei-008, H24-Junkanki [Seishu]-Ippan-007,
H24-Chikyukibo-Ippan-009, H24-Choju-Wakate-009, H25-Kenki-Wakate-015, H25-Choju-Ippan-003, H26Irryo-Shitei-003 [Fukkou], H26-Choju-Ippan-006, H27-Ninchisyou-Ippan-001, H28-choju-Ippan-002, H28Ninchisyou-Ippan-002, H30-Kenki-Ippan-006, H30-Junkankitou-Ippan-004), Japan Agency for Medical
Research and development (AMED) (JP17dk0110017, JP18dk0110027, JP18ls0110002, JP18le0110009), theResearch Funding for Longevity Sciences from National Center for Geriatrics and Gerontology (24-17, 24-23,
29-42). The views and opinions expressed in this article are those of the authors and do not necessarily reflect theofficial policy or position of the respective funding organizations.

Author Contributions

T.K.: conception and design. J.A.: conception and design, acquisition of data. T.Y., K.K., K.O.: acquisition ofdata. All authors: analysis and interpretation of data, drafting the article, critical revision and approval of final
manuscript.

Additional Information

Supplementary information accompanies this paper at https://doi.org/10.1038/s41598-019-50129-9.
Competing Interests: The authors declare no competing interests.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps andinstitutional affiliations.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International
License, which permits use, sharing, adaptation, distribution and reproduction in any medium or
format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in thisarticle are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to thematerial. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the
copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.
© The Author(s) 2019

Scientific Reports |(2019) 9:13734 | https://doi.org/10.1038/s41598-019-50129-9

6