Studying bacteria in a petri dish or test tube has yielded insights into how they function and, in some cases, contribute to disease. But this approach leaves out crucial details about how bacteria act in the real world.
Taking a translational approach, researchers at the University of Pennsylvania School of Dental Medicine and the Georgia Institute of Technology imaged the bacteria that cause tooth decay in three dimensions in their natural environment, the sticky biofilm known as dental plaque formed on toddlers’ teeth that were affected by cavities.
The work, published in the journal Proceedings of the National Academy of Sciences, found that Streptococcus mutans, a major bacterial species responsible for tooth decay, is encased in a protective multilayered community of other bacteria and polymers forming a unique spatial organisation associated with the location of the disease onset.
“We started with these clinical samples—extracted teeth from children with severe tooth decay,” co-author Hyun (Michel) Koo said.
“The question that popped in our minds was, how these bacteria are organised and whether their specific architecture can tell us about the disease they cause?”
To address this question, the researchers used a combination of super-resolution confocal and scanning electron microscopy with computational analysis to dissect the arrangement of S. mutans and other microbes of the intact biofilm on the teeth. These techniques allowed the team to examine the biofilm layer by layer, gaining a three-dimensional picture of the specific architectures.
The researchers discovered that S. mutans in dental plaque most often appeared in a particular fashion: arranged in a mound against the tooth’s surface. But it wasn’t alone. While S. mutans formed the inner core of the rotund architecture, other commensal bacteria, such as S. oralis, formed additional outer layers precisely arranged in a crownlike structure. Supporting and separating these layers was an extracellular scaffold made of sugars produced by S. mutans, effectively encasing and protecting the disease-causing bacteria.
To learn more about how structure impacted the function of the biofilm, the research team attempted to recreate the natural plaque formations on a toothlike surface in the lab using S. mutans, S. oralis, and a sugar solution. They successfully grew rotund-shaped architecture and then measured levels of acid and demineralisation associated with them.
“What we discovered, and what was exciting for us, is that the rotund areas perfectly matched with the demineralised and high acid levels on the enamel surface,” Koo said. “This mirrors what clinicians see when they find dental caries: punctuated areas of decalcification known as ‘white spots’. The domelike structure could explain how cavities get their start.”
In a final set of experiments, the team put the rotund community to the test, applying an antimicrobial treatment and observing how the bacteria fared. When the rotund structures were intact, the S. mutans in the inner core largely avoided dying from the antimicrobial treatment. Only breaking up the scaffolding material holding the outer layers together enabled the antimicrobial to penetrate and effectively kill the cavity-causing bacteria.