New research by University of Pennsylvania scientists shows how multiple oral bacteria interact in complex ways to form dental plaque, leading to cavities in children. The study found a little-known bacterium that teams up with a known decay-causing microbe to attack tooth enamel aggressively.
07/21/23 | By Angelo DePalma, Ph.D. | The Defender – CHD
More than half of young children and adolescents in the U.S. have at least one dental cavity, with lower-income children more than twice as likely to be affected as kids from well-to-do households.
Gum disease, an even more serious consequence of oral bacteria running amok, has been detected in up to 73% of children under age 11, with incidence climbing during adolescence.
Cavities may be filled, and gum disease successfully treated — but not addressing the underlying causes of these conditions can lead to serious health problems later in life.
In fact, the Mayo Clinic describes oral health as the “window” to overall health, linking poor oral hygiene to several serious conditions related to inflammation, including cardiovascular disease and endocarditis, plus birthing and maternity complications and pneumonia.
Oral bacteria also have been linked to bone loss (osteoporosis), rheumatoid arthritis and dementia.
Now, new research — focused on preschoolers — has uncovered a more complex picture of how bacteria interact in the mouth to cause tooth decay.
Scientists at the University of Pennsylvania discovered that a relatively obscure oral bacterium, Selenomonas sputigena (S. sputigena), collaborates with the well-known decay-causing Streptococcus mutans (S. mutans) to form a “biofilm,” the technical term for plaque, that attacks tooth enamel more aggressively than either bacterium does on its own.
The findings suggest tooth decay may be caused by teams of bacteria working together, rather than a few individual microbe species.
Understanding these complex bacterial interactions could pave the way for more effective strategies to combat cavities, the researchers wrote.
What the researchers found
The study, published earlier this year in Nature Communications and headed by Hyun Koo, D.D.S., Ph.D., analyzed plaque from 416 preschoolers.
Investigators collected biofilm samples and subjected them to several different genetic analyses to identify the bacteria present, their spatial associations and interactions, and which combinations were most strongly associated with tooth decay.
- mutanshas been known for some time to be a significant bacterial component of dental plaque, a major contributor to tooth decay.
- mutans are oval-shaped cells that grow in chains or pairs. Cavities form when the mouth’s capacity to neutralize the acid produced by S. mutans is overwhelmed.
Among the most prevalent bacteria in humans, S. sputigena are commonly found in the upper respiratory tract. They usually present no problems to their hosts. However, under the right conditions, they become overgrown and cause blood infection, gum inflammation and tooth decay.
That much was known. What Koo discovered was that S. mutans is most destructive when it joins forces with other bacteria, in particular S. sputigena.
Classic case of the whole being greater than the sum of its parts
The University of Pennsylvania investigators uncovered a total of 16 bacterial species that may play a role in tooth decay, and examined their interactions with S. mutans.
They found that S. sputigena, whose role in biofilm formation was previously unknown, becomes trapped inside a honeycomb-shaped structure composed of cellulose-like material excreted by S. mutans.
Humans do not possess the ability to break down cellulose — the major structural component of wood and paper that also is present in many foods — so S. sputigena cannot be cleared or removed from biofilms.
According to the authors, the inability to eliminate or render S. mutans harmless enhances the bacterium’s acid production, which is known to promote dental cavities.
The researchers then turned to an established rodent model of tooth decay to confirm the potential for the newly discovered S. sputigena-S. mutans biofilms to cause cavities.
Fifteen-day-old rats were screened and found to be free of either bacterium, then assigned to one of four groups to receive either S. mutans alone, S. sputigena alone, both bacteria, or no bacteria.
The test animals received daily doses via cotton swabs and were tested to confirm the appropriate infection on days 21, 24 and 30. No cross-infections were noted, and the control group remained free of those bacteria throughout.
All rats ate a standard diet but had only sugar water to drink, in order to mimic a typical sugar-rich, cavity-promoting diet.
After 30 days, the animals were euthanized and examined and evaluated for the development of cavities, according to standard scoring protocols.
Scorers did not know which intervention the test animals received, so this protocol resembles gold standard “double-blind placebo-controlled” studies in humans.
Investigators noted “an unrecognized ability” of S. sputigena to colonize tooth surfaces — but interestingly, this bacterium was “incapable of causing caries [cavities] on its own.”
But when co-infected with S. mutans, S. sputigena “causes extensive tooth enamel lesions and exacerbates disease severity.”
“Pathobiont” is the term applied to bacteria that are incapable of causing illness on their own, but may do so when combined with another agent. In this instance, S. sputigena (non-pathogenic) plus S. mutans (moderately pathogenic) combine to form “a unique spatial structure and heighten biofilm virulence.”
This is a classic case of the whole being greater than the sum of its parts.
Tooth enamel harboring the sputigena-mutans biofilm showed a significantly higher degree of roughness (related to biofilm thickness) compared with plaque induced by either S. mutans alone or biofilms composed of S. mutans and the other bacteria.
And beneath this biofilm investigators found “widespread regions of surface damage.” By contrast, biofilms co-formed from S. mutans and another common oral bacterium, Prevotella salivae, were similar to those formed by S. mutans alone.
- sputigena have tails that provide locomotion. Researchers believe that as these organisms associate with S. mutans they become trapped inside biofilms and eventually lose their ability to move. Since they are “stuck,” the enamel right below them is exposed to the acid these organisms normally give off as a waste product, thus enhancing the bacteria’s potential to damage teeth.
Expanding the list of tooth decay culprits
In addition to discoveries related to the formation of virulent biofilms, the researchers identified several new species “as prime targets for additional studies.” Two of these strongly associated with biofilm formation and dental cavities did not appear to require S. mutans.
Another unexpected suspect, the bacterium Lachnoanaerobaculum saburreum, produces enamel-damaging acids from sugars and is involved in a secondary fermentation process implicated in tooth decay.
The authors noted that studying “multi-species combinations of these candidates may provide additional insights about the biofilm virulence mechanisms.”
Dental science has known for some time that tooth decay involves multiple microbial players. Their interdependence, however, points to even more inclusive strategies for designing future research into dental biofilms.
The authors write that “a limitation of our current multi-omics pipeline is that it is not optimized for studying inter-kingdom interactions (i.e., viruses and fungi, besides bacteria).”
For example, the method they employed for extracting genetic material was not ideal for detecting and analyzing fungi, whose involvement in tooth decay is no longer controversial.
In an earlier meta-analysis review, Koo found that children testing positive for one common fungus, Candida albicans, were between 5 and 7 times more likely to experience early childhood caries.
Viruses represent another possible microbial class to include in future biofilm studies.
A good place to start, Koo suggests, are phages — viruses that reproduce inside bacteria. These could have the effect of promoting the growth of some bacterial species to the detriment of others, thereby promoting or inhibiting tooth decay.
Also, since this is an observational (vs. interventional) study, the association between bacterial populations and cavities may not be causal. The relative levels of bacterial species and how they interact may be another effect, and not a cause, of either tooth decay or its underlying processes.
What does the study mean for kids and dental health?
The more than 700 species of microbes found in the mouth tend to accumulate in different tissues — gums, teeth, tongue — and not all (including those that form biofilms or plaque) are harmful.
However, according to Andrew Mariotti, D.D.S, Ph.D., a professor of periodontology at Ohio State University, the benefits of reducing the burden of harmful oral bacteria outweigh the potential harm caused by killing beneficial bugs through brushing, flossing or using an antibacterial mouthwash.
Steven Lin, D.D.S., a holistic dentist and blogger, also recommends brushing and flossing while advocating lifestyle changes that give healthy oral bacteria a survival advantage over harmful microbes. Those modifications include consuming more dietary fiber and incorporating more prebiotic and probiotic foods into one’s diet.
Probiotics may be taken as pills or in liquid form. Probiotic foods include naturally fermented cabbage, pickles, or yogurt.
Prebiotics are not bacteria but foods that promote the growth of healthy bacteria. Many tasty, healthy foods — like apples, bananas, asparagus and onions — are prebiotic.