Microorganisms living in the oral cavity are connected to our health: not only to that of the mouth itself, but most likely also to that of our respiratory system.
Recently, I had the opportunity to read and study a review from 2022 that talks about this very correlation1. In the world of scientific literature, a review is an article that is not written from a single experimental research paper, but instead does a review of a large number of other experimental papers related to a given topic, to present the state of the art on that topic and to take stock of all the information we have or do not have about it2.
This review analyzes the scientific literature related tothe importance of the oral microecosystem to our respiratory health.
What is the oral ecosystem and what does it include?
In the natural sciences, the concept of“ecosystem” includes two closely related components: a community of living organisms and the physical environment in which they live. Applying this definition to the mouth, then, we can say that theoral ecosystem includes all the anatomical structures of the mouth (teeth, gums, mucous membranes…), saliva, microorganisms living on these surfaces, and their interactions. Thus, a fundamental part of the oral ecosystem, the living part, is precisely the microbiota that inhabits it, that is, the set of all those microorganisms, both eukaryotes and prokaryotes that are present in very large numbers and in very great variety on and in our bodies3.
The oral microbiota includes 700 species of bacteria, 100 species of fungi (including yeasts such as Candida), and viruses as well. These microorganisms enter our mouth through the introduction of air, food or objects we bring to our mouth. Some of them attach to surfaces by forming biofilms, complex aggregations of microbes wrapped in a sticky, protective matrix (an example of biofilms in the oral cavity is plaque, which can mineralize and become tartar, even going so far as to damage tooth enamel and give rise to tooth decay or periodontitis). These biofilms protect microorganisms from changes in their environment, drugs, and the action of our immune system.
Oral microbiota and respiratory system: how do they interact?
The different microbial species that inhabit our oral cavity are in balance with each other. Some are in a mutualistic relationship with each other, that is, they coexist in a symbiosis from which all parties involved benefit; other species are antagonistic, that is, they attempt to harm each other, for example, by releasing harmful substances; still other species simply compete for the same resources. Disturbing this delicate balance can result in alterations in the microbiota or a migration of microorganisms to other districts, for example into the blood or digestive tract4,5.
There are many ways in which the oral cavity and airway can exchange microbes with each other. Breathing is already sufficient for microbes in the air or in the oral cavity to descend to the lungs. In addition, accidental inhalation of saliva, insertion for medical purposes of tracheal cannulas, and coughing contribute instead to the exchange of microflora in the opposite direction.
As evidence of these reciprocal exchanges, researchers have found strong connections between the microorganisms inhabiting our oral cavity and those in our airways, highlighting precisely genetic kinships between the various strains.6
Respiratory diseases caused by poor oral health
Microorganisms from the oral cavity, if they get into the lungs, can have apathogenic action, that is, cause a disease, or they can promote the entry of other pathogenic species. In addition, the mouth can act as a repository for a large number of respiratory pathogenic microorganisms that remain waiting for the right moment to colonize our airways.
Diseases such as pneumonia, chronic obstructive pulmonary disease (also known as COPD), lung cancer , and cystic fibrosis disease have been found to be associated with microflora changes in the oral cavity.
The review goes on to list several diseases that appear to be related to an oral dysbiosis, that is, a condition of imbalance among microbial species in the mouth, including diseases related to pathogen infections, such as the pneumonia or cystic fibrosis disease, but also obstructive diseases, such as chronic obstructive pulmonary disease (in jargon, COPD) andasthma, and even lung cancer. For some of these diseases, there is quite convincing evidence of the relationship with the mouth microbiota, for others the link is yet to be further investigated7,8,9,10.
It is always good to remember, however, that as the English say “correlation is not causation,” that is, the mere fact that two phenomena occur simultaneously does not automatically mean that one is the cause of the other. We need to be cautious and check if there are any mechanisms by which one event can cause the second, that is, in this case check if and how oral dysbiosis can cause respiratory diseases.
It is precisely this issue that the second part of the review discusses, also reviewing other factors that can adversely affect the balance of the oral ecosystem, as well as the lines of defense we have against pathogens and the strategies they put in place to avoid them.
Interested in learning more about these issues? If yes, don’t miss the next article!
1 Dong, J., Li, W., Wang, Q., Chen, J., Zu, Y., Zhou, X., & Guo, Q. (2022). Relationships Between Oral Microecosystem and Respiratory Diseases. Frontiers in molecular biosciences, 8, 718222. https://doi.org/10.3389/fmolb.2021.718222
2 We have already discussed scientific literature in this article
3 We talked about eukaryotes and prokaryotes in this article
4 Parahitiyawa, N. B., Jin, L. J., Leung, W. K., Yam, W. C., & Samaranayake, L. P. (2009). Microbiology of odontogenic bacteremia: beyond endocarditis. Clinical microbiology reviews, 22(1), 46-64. https://doi.org/10.1128/CMR.00028-08
5 Seedorf H., Griffin N. W., Ridaura V. K., Reyes A., Cheng J., Rey F. E., et al. (2014). Bacteria from Diverse Habitats Colonize and Compete in the Mouse Gut. Cell 159, 253-266. 10.1016/j.cell.2014.09.008
6 Bassis, C. M., Erb-Downward, J. R., Dickson, R. P., Freeman, C. M., Schmidt, T. M., Young, V. B., Beck, J. M., Curtis, J. L., & Huffnagle, G. B. (2015). Analysis of the upper respiratory tract microbiota as the source of the lung and gastric microbiota in healthy individuals. mBio, 6(2), e00037. https://doi.org/10.1128/mBio.00037-15
7 Bágyi, K., Klekner, A., Hutóczki, G., and Márton, I. (2006). The Role of the Oral flora in the Pathogenesis of Aspiration Pneumonia. Fogorv Sz 99, 205-212.
8 Bensel, T., Huse, A., Borneff-Lipp, M., Wollschläger, B., Bekes, K., Setz, J., et al. (2010). Periodontal Pockets as Potential Sources of Cystic Fibrosis Lung Infection. J. Cystic Fibrosis 9, S38. doi:10.1016/s1569-1993(10)60147-6
9 Zhou, X., Wang, J., Liu, W., Huang, X., Song, Y., Wang, Z., et al. (2020). Periodontal Status andMicrobiologic Pathogens in Patients with Chronic Obstructive Pulmonary Disease and Periodontitis: A Case-Control Study. Copd 15, 2071-2079. doi:10.2147/ copd.s266612
10 Maddi, A., Sabharwal, A., Violante, T., Manuballa, S., Genco, R., Patnaik, S., et al. (2019). The Microbiome and Lung Cancer. J. Thorac. Dis. 11, 280-291. doi:10.21037/jtd.2018.12.88
