Dental cavity microbes create cluster and lead to illness, scientists suggest

This coverage, which you will read shortly, will once again remind you how important oral health is.

A study led by researchers from the University of Pennsylvania School of Dental Medicine shows that the fungus Candida albicans and the bacteria Streptococcus mutans are both involved in cavity formation.

Published in PNAS on October 3, the study also reveals that bacteria cluster together and can sprout limbs to crawl and even leap across teeth. This means that microbes can spread to other parts of your body, and you can get sick.

Found in the saliva of toddlers with severe childhood tooth decay, these assemblages can effectively colonize teeth.

“This started with a very simple, almost accidental discovery while looking at saliva samples from toddlers who develop aggressive tooth decay,” says Hyun (Michel) Koo, a professor at Penn Dental Medicine and a co-corresponding author on the paper.

"Looking under the microscope, we noticed the bacteria and fungi forming these assemblages and developing motions we never thought they would possess: a ‘walking-like’ and ‘leaping-like’ mobility."

Many experiments were carried out

Ren, Koo, and colleagues were intrigued by the bacterial-fungal clusters found in the saliva samples and wondered how the groupings would behave once attached to the surface of a tooth. Thus they began a series of experiments using real-time live microscopy to study the attachment and eventual growth process.

They developed a laboratory system to mimic the formation of these assemblages by incubating bacteria, fungi, and a tooth-like material in human saliva. The platform allowed the researchers to observe the groupings as they formed and analyze the structure of the resulting assemblages. 

They discovered a highly organized structure that was enmeshed in an extracellular polymer, a glue-like material, with bacterial clusters attached in a complex network of fungal yeast and filament-like projections called hyphae.

The researchers discovered that the microbial communities moved quickly and far. The team measured velocities of more than 40 microns per hour on the tooth-like surface, which is comparable to the speed of fibroblasts, a type of cell in the human body involved in wound healing.

The scientists observed the assemblages "leaping" more than 100 microns across the surface within the first hours of growth. "That is more than 200 times their own body length," Ren says, "making them superior to most vertebrates in terms of body size."

Abstract:

Fungi and bacteria often engage in complex interactions, such as the formation of multicellular biofilms within the human body. Knowledge about how interkingdom biofilms initiate and coalesce into higher-level communities and which functions the different species carry out during biofilm formation remain limited. We found native-state assemblages of Candida albicans (fungi) and Streptococcus mutans (bacteria) with highly structured arrangement in saliva from diseased patients with childhood tooth decay. Further analyses revealed that bacterial clusters are attached within a network of fungal yeasts, hyphae, and exopolysaccharides, which bind to surfaces as a preassembled cell group. The interkingdom assemblages exhibit emergent functions, including enhanced surface colonization and growth rate, stronger tolerance to antimicrobials, and improved shear resistance, compared to either species alone. Notably, we discovered that the interkingdom assemblages display a unique form of migratory spatial mobility that enables fast spreading of biofilms across surfaces and causes enhanced, more extensive tooth decay. Using mutants, selective inactivation of species, and selective matrix removal, we demonstrate that the enhanced stress resistance and surface mobility arise from the exopolymeric matrix and require the presence of both species in the assemblage. The mobility is directed by fungal filamentation as hyphae extend and contact the surface, lifting the assemblage with a “forward-leaping motion.” Bacterial cell clusters can “hitchhike” on this mobile unit while continuously growing, to spread across the surface three-dimensionally and merge with other assemblages, promoting community expansion. Together, our results reveal an interkingdom assemblage in human saliva that behaves like a supraorganism, with disease-causing emergent functionalities that cannot be achieved without coassembly.