Many people do not like to think about the microorganisms in their mouths that can cause cavities. These microbes live on our teeth and consume the same sugars as we do, producing acids that damage our enamel. A recent study published in the Proceedings of the National Academy of Sciences USA reveals that groups of fungi and bacteria can work together to move and spread across the surface of teeth, leading to faster tooth decay than if either organism was present alone.
According to Hyun (Michel) Koo, a microbiologist and dentist at the University of Pennsylvania and co-author of the study, a previous understanding was that cavities were caused by bacteria accumulating gradually on teeth.
However, Koo and his team collected saliva samples from toddlers with advanced tooth decay and found clusters of Streptococcus mutans bacteria and Candida albicans fungi present, which were not found in the saliva of children with healthier teeth. Examining these clusters under a microscope revealed that they seemed to be able to move in complex ways.
Walking Dental Bacteria
The researchers observed that smaller bacterial cells tended to gather around the center of each cluster, creating a sticky substance that held the cluster together. Larger, rod-shaped fungal cells attached to the outside of the cluster acted as moving "limbs" that propelled the structure forward. These front limbs sometimes seemed to walk or jump ahead, with the cluster quickly expanding in one direction while its back limbs remained in place. If two of these clusters were close, they would sometimes extend towards each other in a "handshake" and merge.
The scientists collected saliva samples from children with and without severe tooth decay and analyzed the naturally occurring microbial content using fluorescence in situ hybridization (FISH) and super-resolution confocal imaging. They found that saliva from children with tooth decay had higher clusters of fungal and bacterial cells. These clusters typically consisted of Candida albicans and Streptococcus mutans. The researchers also detected alpha-glucans, a type of extracellular polysaccharide associated with tooth decay, produced predominantly by S. mutans enzymes called glucosyltransferases.
They observed higher levels of alpha-glucans on the cell surfaces of the fungal and bacterial cells within the clusters in the saliva of children with tooth decay and also found higher levels of glucosyltransferase activity in the saliva of these children compared to healthy children. In plaque biofilms from children with tooth decay, both C. albicans and S. mutans were present at high levels, but significantly lower levels were found in healthy samples, suggesting that S. mutans and C. albicans interact dynamically as they move from a fluid environment to a surface of apatite.
"Community Expanding Territory"
According to Zhi Ren, a postdoctoral fellow in Koo's laboratory and co-lead author of the study, the microbes in the mouth "are like a community trying to expand their territory," seeking out new land and sources of sugar. The team found that partnerships between bacteria and fungi grew faster and were more resistant to removal through mechanical force or antimicrobial chemicals than either fungi or bacteria alone.
Judith Behnsen, a microbiologist at the University of Illinois at Chicago who was not involved in the research, notes that this study is notable for its focus on the behavior of living, moving organisms in real-time, rather than examining microbes that have been suspended in place using preservative chemicals. Behnsen says that when she saw the images from the study, she was amazed.
Ren mentioned that future research could help identify the most at risk of developing bacterial-fungal clusters and determine the most effective ways to treat them. Ren notes that tooth decay is a widespread and serious problem globally, and understanding the interactions between bacteria and fungi could help us protect against their spread.
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