Identifying several new bacteria that cause dental caries

New study genetically sequences the oral microbial communities of Japanese university students to reveal more microbes that could cause dental cavities

OKAYAMA UNIVERSITY

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IMAGE: THE RED AREAS OF THIS CLADOGRAM INDICATE A GREATER ABUNDANCE OF THE PREVOTELLACEAE AND VEILLONELLACEAE BACTERIAL FAMILIES AND ALLOPREVOTELLA AND DIALISTER BACTERIAL GENERA IN THE GROUP OF PEOPLE WHO HAD... view more 

CREDIT: THE INTERNATIONAL JOURNAL OF ENVIRONMENTAL RESEARCH AND PUBLIC HEALTH

The human body is home to trillions of microbes. Through its natural functioning, much of the time, this ecosystem regulates our health. But like the environment of the world at large, this bodily ecosystem is delicate, and any change in the composition of the microbial community, also called the "microbiome," can cause an overall imbalance in their collective functioning, resulting in disease.

Now, advances in research in this field have yielded a technique called next-generation DNA sequencing, which allows for very accurate identification of the members of this microbial community, thereby offering insights into microbial community composition. For several diseases, knowing which microbes densely populate the organ/tissue in question or become absent from it during disease can help develop effective treatments. Such is the case for dental caries, a type of tooth decay in which acid-producing bacteria eat away at the out layer of teeth and cause cavities.

A type of bacteria called the mutans streptococci are the most commonly implicated microbes in dental caries. Their increase causes dental decay. But, could other microbes be responsible as well?

Scientists globally have looked into this question. However, focus on the younger demographic has been low. Meanwhile, in Japan, the number of young adults developing dental caries is increasing.

Spurred by this increase and this insufficient literature, a team of researchers from Japan, led by Dr. Uchida-Fukuhara from Okayama University, called for Japanese university student volunteers for oral examinations at the Health Service Center in Okayama University.

The students answered a survey about their dental health at the beginning of the study and during a follow-up after three years. This told the researchers which students had significantly increased dental caries after this time and who didn't. The researchers grouped the students accordingly during the follow-up (let's say, Groups A and B respectively). They then collected saliva samples of randomly selected students from these groups, which they analyzed via next-generation DNA sequencing to obtain microbial profiles.

It turned out that very similar oral microbial diversities existed in both groups. But in Group A, the abundances of the bacterial families Prevotellaceae and Veillonellaceae, and genera Alloprevotella and Dialister, were greater than those in Group B. These two families are known to comprise species that produce acid as well. This finding, therefore, suggests new prevention possibilities for dental caries that does not focus on keeping mutans streptococci populations in check.

Interestingly, both groups had low levels of mutans streptococci. Should the focus of research on what causes dental caries change?

The striking results of the study, published in the International Journal of Environmental Research and Public Health, underscore the necessity of updating current knowledge on the oral microbial community and its role in the development of dental caries. But Dr. Uchida highlights limitations in the study's applicability and advises taking these findings with a pinch of salt. "Among other things, all our participants were from Okayama University, so our results may not be generalizable to the wider population," she says.

Yet, Dr. Uchida is hopeful, "For many years our group has been conducting population studies to reduce oral diseases. We believe that the results of this new study will help us develop novel strategies to prevent dental caries and our students will achieve greater life satisfaction because of better teeth and oral health."

Perhaps, in the future, students' teeth will be clean as a hound's tooth.

Stopping tooth decay before it starts -- without killing bacteria

AMERICAN CHEMICAL SOCIETY

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WASHINGTON, Aug. 17, 2020 -- Oral bacteria are ready to spring into action the moment a dental hygienist finishes scraping plaque off a patient's teeth. Eating sugar or other carbohydrates causes the bacteria to quickly rebuild this tough and sticky biofilm and to produce acids that corrode tooth enamel, leading to cavities. Scientists now report a treatment that could someday stop plaque and cavities from forming in the first place, using a new type of cerium nanoparticle formulation that would be applied to teeth at the dentist's office.

The researchers will present their progress toward this goal today at the American Chemical Society (ACS) Fall 2020 Virtual Meeting & Expo. ACS is holding the meeting through Thursday. It features more than 6,000 presentations on a wide range of science topics.

The mouth contains more than 700 species of bacteria, says Russell Pesavento, D.D.S., Ph.D., the project's principal investigator. They include beneficial bacteria that help digest food or keep other microbes in check. They also include harmful streptococcal species, including Streptococcus mutans. Soon after a cleaning, these bacteria stick to teeth and begin multiplying. With sugar as an energy source and building block, the microbes gradually form a tough film that can't easily be removed by brushing. As the bacteria continue metabolizing sugar, they make acid byproducts that dissolve tooth enamel, paving the way for cavities.

Dentists and consumers can fight back with products including stannous fluoride to inhibit plaque, and silver nitrate or silver diamine fluoride to stop existing tooth decay. Researchers have also studied nanoparticles made of zinc oxide, copper oxide or silver to treat dental infections. Although bactericidal agents such as these have their place in dentistry, repeated applications could lead to both stained teeth and bacterial resistance, according to Pesavento, who is at the University of Illinois at Chicago. "Also, these agents are not selective, so they kill many types of bacteria in your mouth, even good ones," he explains.

So, Pesavento wanted to find an alternative that wouldn't indiscriminately kill bacteria in the mouth and that would help prevent tooth decay, rather than treat cavities after the fact. He and his research group turned to cerium oxide nanoparticles. Other teams had examined the effects of various types of cerium oxide nanoparticles on microbes, though only a few had looked at their effects on clinically relevant bacteria under initial biofilm formation conditions. Those that did so prepared their nanoparticles via oxidation-reduction reactions or pH-driven precipitation reactions, or bought nanoparticles from commercial sources. Those prior formulations either had no effect or even promoted biofilm growth in lab tests, he says.

But Pesavento persevered because the properties and behavior of nanoparticles depend, at least partially, on how they're prepared. His team produced their nanoparticles by dissolving ceric ammonium nitrate or sulfate salts in water. Other researchers had previously made the particles this way but hadn't tested their effects on biofilms. When the researchers seeded polystyrene plates with S. mutans in growth media and fed the bacteria sugar in the presence of the cerium oxide nanoparticle solution, they found that the formulation reduced biofilm growth by 40% compared to plates without the nanoparticles, though they weren't able to dislodge existing biofilms. Under similar conditions, silver nitrate -- a known anti-cavity agent used by dentists -- showed no effect on biofilm growth.

"The advantage of our treatment is that it looks to be less harmful to oral bacteria, in many cases not killing them," Pesavento says. Instead, the nanoparticles merely prevented microbes from sticking to polystyrene surfaces and forming adherent biofilms. In addition, the nanoparticles' toxicity and metabolic effects in human oral cells in petri dishes were less than those of silver nitrate.

Pesavento, who was awarded a patent in July, would like to combine the nanoparticles with enamel-strengthening fluoride in a formulation that dentists could paint on a patient's teeth. But, he notes, much work must be done before that concept can be realized. For now, the team is experimenting with coatings to stabilize the nanoparticles at a neutral or slightly basic pH -- closer to the pH of saliva and healthier for teeth than the present acidic solution. His team has also begun working with bacteria linked to the development of gingivitis and has found one particular coated nanoparticle that outcompeted stannous fluoride in limiting the formation of adherent biofilms under similar conditions. Pesavento and his team will continue to test the treatment in the presence of other bacterial strains typically present in the mouth, as well as test its effects on human cells of the lower digestive tract to gain a better sense of overall safety for patients.

Using physics to improve root canal efficiency

Temperature is a critical parameter in determining cleansing efficiency during a root canal

AMERICAN INSTITUTE OF PHYSICS

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IMAGE: LEFT: STREAMLINES OF THE IRRIGATION FLUID; RIGHT: VELOCITY VECTORS AT THE RECIRCULATION ZONE NEXT TO THE NEEDLE TIP.view more 

CREDIT: HANHUI JIN

WASHINGTON, August 11, 2020 -- Scientists used computational fluid dynamics to determine the effect of temperature on root canal cleaning efficiency. Higher temperatures can, to a point, improve cleansing, but this benefit falls off if the temperature gets too high.

In Physics of Fluids, by AIP Publishing, scientists from China and the U.S. report calculations with a model of the conical-shaped root canal inside a tooth. This cavity is usually filled with pulp. When the pulp becomes inflamed or infected, an endodontist removes the infected pulp, and then cleans, shapes and fills the canal. The apex is then sealed.

A crucial step in this common dental procedure is irrigation, or rinsing, of the root canal cavity with an antibacterial solution, such as sodium hypochlorite. Efficient cleaning and successful destruction of any bacteria or other microbes in the cavity depend on the penetration and cleaning ability of the irrigation fluid.

The computational investigation used a structured mesh as a model of the conical root canal cavity. More than 1 million cells in the mesh completely and accurately described both the root canal and the side-vented needle through which the hypochlorite solution is injected. Fluid dynamics equations were used to model flow of the hypochlorite solution.

The scientists varied the fluid velocity, temperature and input power to determine the most efficient cleansing technique. As expected, higher fluid velocities lead to better cleansing. Perhaps counterintuitively, cleansing efficiency is higher on the wall behind the needle vent.

"The effective area on the root canal wall, in which the shear stress exceeds the critical value to clean the wall, is usually larger behind the needle outlet than in front of it," said author Hanhui Jin.

The maximum shear stress also usually occurs on the wall behind the needle outlet, Jin explained.

The investigators also looked at the effect of temperature on cleansing. They considered four different temperatures: 22 Celsius, which is room temperature; 37 C, which is body temperature; and two higher temperatures of 45 C and 60 C. Temperatures above 60 C are painful for the patient and tend to cause root canal damage.

Increasing the temperature to 45 C, while holding the fluid velocity fixed, improved the depth of cleansing and the cleansing span across the canal's width, but further increases in temperature beyond 45 C actually decreased the cleansing efficiency.

The investigators considered the effect of power consumption by the irrigation device. If the power consumption is held at a fixed value, the effect of temperature on cleansing efficiency is much more pronounced.

"The fluid circulation within the canal is clearly enlarged when the temperature is increased," said Jin. Therefore, careful control of both power consumption and temperature leads to increased cleaning efficiency.

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The article, "Effect of inflow temperature on root canal irrigation: A computational fluid dynamics study," is authored by Mingzhou Yu, Zhengqiu Huang, Na Zhou, Zihan Xu, Shuli Deng and Hanhui Jin. The article will appear in Physics of Fluids on Aug. 11, 2020 (DOI: 10.1063/5.0014737). After that date, it can be accessed at https://aip.scitation.org/doi/10.1063/5.0014737.

Mouthwashes could reduce the risk of coronavirus transmission

Cell culture experiments show that commercially available preparations have an effect on Sars-Cov-2 viruses

RUHR-UNIVERSITY BOCHUM

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Sars-Cov-2 viruses can be inactivated using certain commercially available mouthwashes. This was demonstrated in cell culture experiments by virologists from Ruhr-Universität Bochum together with colleagues from Jena, Ulm, Duisburg-Essen, Nuremberg and Bremen. High viral loads can be detected in the oral cavity and throat of some Covid-19 patients. The use of mouthwashes that are effective against Sars-Cov-2 could thus help to reduce the viral load and possibly the risk of coronavirus transmission over the short term. This could be useful, for example, prior to dental treatments. However, mouth rinses are not suitable for treating Covid-19 infections or protecting yourself against catching the virus.

The results of the study are described by the team headed by Toni Meister, Professor Stephanie Pfänder and Professor Eike Steinmann from the Bochum-based Molecular and Medical Virology research group in the Journal of Infectious Diseases, published online on 29 July 2020. A review of laboratory results in clinical trials is pending.

Eight mouthwashes in a cell culture test

The researchers tested eight mouthwashes with different ingredients that are available in pharmacies or drugstores in Germany. They mixed each mouthwash with virus particles and an interfering substance, which was intended to recreate the effect of saliva in the mouth. The mixture was then shaken for 30 seconds to simulate the effect of gargling. They then used Vero E6 cells, which are particularly receptive to Sars-Cov-2, to determine the virus titer. In order to assess the efficacy of the mouthwashes, the researchers also treated the virus suspensions with cell culture medium instead of the mouthwash before adding them to the cell culture.

All of the tested preparations reduced the initial virus titer. Three mouthwashes reduced it to such an extent that no virus could be detected after an exposure time of 30 seconds. Whether this effect is confirmed in clinical practice and how long it lasts must be investigated in further studies.

The authors point out that mouthwashes are not suitable for treating Covid-19. "Gargling with a mouthwash cannot inhibit the production of viruses in the cells," explains Toni Meister, "but could reduce the viral load in the short term where the greatest potential for infection comes from, namely in the oral cavity and throat - and this could be useful in certain situations, such as at the dentist or during the medical care of Covid-19 patients."

Clinical studies in progress

The Bochum group is examining the possibilities of a clinical study on the efficacy of mouthwashes on Sars-Cov-2 viruses, during which the scientists want to test whether the effect can also be detected in patients and how long it lasts. Similar studies are already underway in San Francisco; the Bochum team is in contact with the American researchers.