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Tip Sheets

What do we know about the bacteria inhabiting our mouth?

By Mary Otto

Science recently revealed a fascinating new fact about romantic intimacy: a couple exchanges 80 million bacteria during the average French kiss.

The Dutch study “Shaping the Oral Microbiota Through Intimate Kissing,” which was published in the journal Microbiome, suggests that a shared microbiota is able to develop in the mouths of partners. 

Image: Alan via flickr.

The study got quite a bit of attention from the press. And there will be plenty of other tales to tell, as researchers delve deeper into the intricate workings of the microbial communities that inhabit various regions of our bodies, including our mouths.

“The body's flora represents a key area of research interest, as it relates to immunology, nutrition/digestion, among other crucial aspects of human health,” explained organizers of a Dec. 4 webinar, “Introduction to the Microbiome,” hosted by the nonprofit New Jersey-based BioPharma Research Council.

That webinar is now available online. Here are some of the oral health highlights, along with links to relevant research, that will hopefully serve as a useful tip sheet for more stories from the human oral microbiome.

Researchers hope that the genetic sequencing of oral bacteria, made possible by new technologies, will contribute to a deeper understanding of  health and disease. But the study of the oral microbiome actually began centuries ago, noted panelist Floyd E. Dewhirst, a researcher at the prestigious Forsyth Institute in Cambridge, MA.

“Oral microbiology has been studied for the last 340 years, since Anton Van Leeuwenhoek took a toothpick and scraped between his teeth and looked at it under his first crude microscope,” said Dewhirst.

From the mid 19th century until the late 20th century, investigators worked in laboratories to cultivate and name 200 organisms living in the oral cavity. But very recently, advances in DNA sequencing technologies have given rise to the new field of research, called metagenomics, that focuses upon the genetic analysis of material harvested directly from microbial communities without the need to culture the microbes in the lab.

Using molecular methods including gene-based cloning studies, researchers have identified nearly 700 species of bacteria that are endogenous, or native to the human oral microbiome. The Human Oral Microbiome Database which is hosted by the Forsyth Institute and supported by a grant from The National Institute of Dental and Craniofacial Research provides a home for genetic information about all of them.

The work has offered important new information about the unique flora found in the diverse habitats of the mouth: the teeth, pockets of the gums, tongue, cheeks, hard palate, soft palate and tonsils, explained Dewhirst.

“The biogeography of the oral microbiome is quite complex. There are actually 10 or more niches in the oral cavity,” he said. Contiguous areas including the esophagus, middle ear, trachea and lungs provide additional bacterial niches.

“The utility of these genomes is studying the pathogenesis of the organisms,” Dewhirst said. “The oral microbiome is really important for health and disease.”

Oral bacteria have been shown to cause a number of oral diseases including caries (tooth decay) and periodontitis (gum disease).

These common oral infections “affect almost all individuals to some degree,” Dewhirst noted. Strep throat, tonsillitis and middle ear infections, are also widespread.

Middle ear infections are the most common cause for children to visit physicians and the most common reasons for antibiotics to be prescribed,” he said.

Less clear but also compelling are the associations between diabetes and periodontal disease and the possible links between oral infections and systemic problems such as cardiovascular disease and pre-term births. (Oral Health in America, the 2000 landmark report from the Office of the Surgeon General offers an enduring overview.)

Though more study is needed, oral bacteria may contribute to systemic diseases such as bacterial endocarditis, stroke, preterm births and low birth weight. Oral infections have been traced to distant locations in the body, including the brain.

Saliva, which contains a mix of microbial and host markers, increasingly serves as a useful diagnostic fluid, Dewhirst pointed out.

“There are tests for caries, periodontal disease and also tests for viral agents such as HIV, hepatitis, dengue, herpes, Epstein-Barr and many other diseases,” Dewhirst said.

Prebiotics, probiotics and microbial replacement therapy are being examined as potential methods for treating oral and systemic diseases, Dewhirst said.

Streptococcus mutans is the organism most responsible for causing caries,” he noted, observing that researchers at Florida-based Oragenics, Inc have been working for years on a replacement therapy for tooth decay that uses strep mutans that has been genetically modified to be less cariogenic.

On its website, however, the company acknowledged it has struggled with the U.S Food and Drug Administration over regulatory issues related to clinical trials for the treatment known as SmaRT Replacement Therapy and that the project has been at least temporarily shelved.  

Another area of research involving a gluten-degrading organism in the oral microbiome holds promise for people suffering from gluten intolerance.

“It turns out that Rothia aeria produces a protease that breaks down the immunogenic peptides,” said Dewhirst. “It might be useful eventually for treating gluten sensitivity.”

Dewhirst concluded his remarks by acknowledging the new Dutch study on French kissing. While collective bacteria that gathered in the couples’ saliva were eventually washed out, those on the surface of the tongue were able to find “a true niche, allowing long-term colonization” in the partners’ mouths the researchers wrote.

Dewhirst, for one, said he was not surprised by the findings that close partners share oral microbiota.

“It has been known for a long time that children get their oral microbiome primarily from their mother or primary caregiver,” he said. For more on that topic, see Amanda Mascarelli’s 2011 story “Tooth Decay is Contagious from Parents to Children” by that appeared in the Los Angeles Times.

And more background on Dewhirst’s work, see the seminal paper “The Human Oral Microbiome” which was published in 2010 in the Journal of Bacteriology.