Oral bacteria: The microbial world in your mouth

What was among the first objects to be seen under a microscope? Right – oral bacteria! In the 1680s,  Antony van Leeuwenhoek, regarded as the father of microbiology, examined the plaque from his own teeth with his microscope and termed what he could see “little animals”. It was only during the last century that new and better methods of cultivating bacteria allowed us to further explore this microbial universe.

Today, improvements in sequencing tools and bioinformatics allow us to explore the oral microbiome in even greater depth. We have expanded our knowledge about its inhabitants, its organization and possible pathogens responsible for oral diseases such as caries, periodontal diseases or halitosis.

Which microbes live there?

How many microorganisms call the oral cavity their home? Our first information came from culturing samples from different oral sites, i.e. dental plaque, oral mucosa or tongue, on various bacterial growth media and then identifying the microorganisms that grew. Based on these studies, it was estimated that more than 700 different bacterial species may reside in the oral cavity.

The Human Microbiome Project was launched in 2007 to characterize the complexity of the human oral microbiome, amongst others, by sequencing the oral microbial communities from 300 healthy individuals. As of today, the expanded Oral Microbiome Database [1] lists a total of 771 microbial species, a number that is remarkably similar to the estimate obtained by culture methods. Interestingly, “non-oral” bacteria are detected only rarely in the oral cavity and if they are there, they appear to present only transiently.

Where does oral bacteria come from?

Although there is still disagreement among researchers about whether bacterial colonization starts before birth, neonates will definitely come into direct contact with microorganisms during and after birth. When tested within minutes after delivery, the oral microbiome of vaginally and C-section delivered infants were similar to their mother’s vaginal or skin microbiome, respectively. Differences in bacterial profiles can also be due to the method of feeding, i.e. breast-feeding vs. infant formula. Oral lactobacilli, including some with antimicrobial properties, were found exclusively in breast-fed infants. Many other factors, such as the living environment, eating habits, health status, medications, oral hygiene measures or contact with other persons, may then affect the acquisition, composition and evolution of the oral microbiome in children.

In adulthood, the composition of the oral microbiome appears to be relatively stable even if the oral cavity is exposed to numerous external and internal factors. The oral microbiome varies from person to person; however, a “core oral microbiome” shared by all or the majority of humans does exist. In addition, there is a “variable microbiome” that is subject specific and varies over time, depending on the behavioral and genetic factors of its host.

Oral microbiota: who are the bad guys?

Usually we become aware of the presence of our oral microbiome only when diseases such as caries, periodontitis or halitosis will manifest.

You like sweet things to eat? So do certain bacteria! They thrive on carbohydrates, mainly sugars such as sucrose, thereby producing acids. The acid will in turn cause mineral loss in the surface of your teeth. Over time, when too many minerals are lost, the surface of the tooth may break down and a cavity is formed. Until now, tooth decay, also known as dental caries, remains a major health problem globally. Streptococcus mutans and lactobacilli are considered to be the main culprits. However, new research has shown that even in the absence of these bacteria, acids are produced as other oral bacteria can take over the job.

What is Periodontitis?

Periodontitis is an inflammatory disease of the tissues that both surround and support the teeth. It may lead to tooth loss, which affects more than 60 % of a population worldwide. Anaerobic bacteria, i.e. bacteria that can live in an anoxic environment only, will find perfect conditions in the periodontal pocket that is formed between tooth and gum when a person with periodontitis experiences loss of bone and periodontal ligament. Research has focused on Porphyromonas gingivalis, the bacterial species most highly associated with chronic forms of periodontitis. It is considered to be a “key-stone pathogen” as this bacterial species is a very important element in the composition of the periodontal microbiome and there is hope that its specific elimination might lead to improvements in disease severity.

What causes halitosis?

Likewise, in halitosis patients, the medical term for bad breath, oral bacteria can be the culprits. In these patients, the composition of their microbiome resembles that of patients with periodontal diseases and it is not yet clear if indeed a “halitosis-specific” microbiome exists. In any case, your dentists will make sure that no underlying health problems exist. They will then instruct you in oral hygiene measures that will reduce the bacterial load in your mouth.

What can you do to maintain a good oral microbiome?

So, what can YOU do to be on friendly terms with the microorganisms that live in your mouth? Despite years of research, unfortunately, there is still no “magic bullet” that will solve all problems. Therefore, the best thing you can do is to maintain a good personal oral hygiene routine.  This will keep your oral microorganisms at bay, keeping the smile on your face.

[1] http://www.homd.org/; accessed on 01/26/2020.

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