ORNL scientists shine light on 'dark' microbes lurking in our mouths

For the first time, Oak Ridge National Laboratory scientists have cultivated an oral microbe that is present in adults with periodontitis, an advanced gum disease that affects nearly half of all adults in the world.

The microbe, Desulfobulbus oralis, can help scientists better understand how the microbes that form a community within people's mouths interact with each other.

Role in disease

The D. Oralis bacteria is present in the oral cavity, and present more with diseases like periodontitis. But the microbe is not very abundant and is more difficult to get to than other microbes present with the disease because it is anaerobic.

That means it lurks deep within the oral cavity where it is not exposed to oxygen.

D. Oralis exists in the outside environment, too, but ORNL scientists found the version that exists in humans has evolved with its hosts to create cytotoxic molecules. Those molecules can contribute to cell death and loss of teeth, oral bone and oral tissue.

"We have initial evidence that this microbe actually plays a role in periodontal disease, together with other microbes from the oral cavity," ORNL scientist Mircea Podar said.

In isolating D. oralis, Podar and University of Tennessee graduate research assistant Karissa Cross found the microbe can trigger a pro-inflammatory response that activates the immune system.

'Dark microbiota' of the mouth

At present, only about one-third of the mouth's microbes are known.

"There are very distinct microbes that live in the oral cavity," Podar said. "We can culture a significant portion, but there is still around half or maybe a third that we cannot culture."

Those microorganisms are called "dark microbiota."

"We know they are there, but we don't have their picture. We can't culture them in the lab to see how they interact with others," Podar said.

"It's hard to say which are the good microbes and which are the bad ones."

For example, some microbes cause cavities by excreting acids that dissolve teeth. Those are definitely bad, Podar said.

"Some of the really bad ones like that are known, but the process really involves interaction between the different types of microbes and when some of them get out of control."

There are also good microbes that can keep the growth of the bad microbes in check when they are present.

"We now know that a lot of diseases like cavities, gum disease and gingivitis are caused by dysfunction in the normal microbial community composition," Podar said.

Microbes that cause gum diseases like periodontitis are not foreign pathogens like strep or salmonella bacteria that can set up shop in the human body after exposure.

Gum disease microbes actually originate in people's existing oral biomes, which are established shortly after birth.

"It is the way that the microorganisms work together or try to work against each other in their environments that then begins to interact with human hosts," Cross said.

"That can cause this dysbiotic environment that leads to chronic inflammation, which can, in turn, lead to different kinds of diseases in the mouth."

The ORNL scientists' endeavor to study dark microbiota is part of a project funded by the National Institutes of Health to study dark microbiota and how they interact with other microbes and with humans.

"We are continuing to study D. Oralis and isolate other similar microbes to understand how they evolved ... and how they interact with each other and the human host to play a role in oral disease," Podar said."