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 Penn Dental Medicine and the Georgia Institute of Technology imaged the bacteria that cause tooth decay in humans in three dimensions. The work, published in Proceedings of the National Academy of Sciences, found that Streptococcus mutans, a bacterial species associated with tooth decay, is encased in a protective multilayered community of other bacteria and polymers forming a unique spatial organization associated with the location of the disease onset.
"We started with these clinical samples, extracted teeth from children with severe tooth decay," says Dr. Michel Koo of Penn Dental Medicine, a co-senior author on the work. "The question that popped in our minds was, how these bacteria are organized and whether their specific architecture can tell us about the disease they cause?"
To address this question, the researchers, including lead author Dongyeop Kim of Penn Dental Medicine, used a combination of 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.
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. Other commensal bacteria, such as S. oralis, formed additional outer layers precisely arranged in a crown-like 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.
"What was exciting for us is that the rotund areas perfectly matched with the demineralized and high acid levels on the enamel surface," says Koo. "This mirrors what clinicians see when they find dental caries: punctuated areas of decalcification known as 'white spots.' The crown-like structure could explain how cavities get their start."
The study's findings may help researchers more effectively target the pathogenic core of dental biofilms but also have implications for other fields.
"It demonstrates that the spatial structure of the microbiome may mediate function and the disease outcome, which could be applicable to other medical fields dealing with polymicrobial infections," says Koo.
