According to new research, a cross-kingdom partnership between bacteria and fungi may result in the two merging to form a “superorganism” with unusual strength and resilience that can effectively colonize teeth.
It may sound like science fiction, but these microbial groups, found in the saliva of toddlers with severe childhood tooth decay, are part and parcel of the here and now.
The assemblies are stickier, more resistant to antimicrobials and more difficult to remove from teeth than bacteria or fungi alone, the researchers report.
What’s more, the assemblies unexpectedly grow “limbs” that cause them to “walk” and “jump” to quickly spread across the tooth surface, although each microbe is itself immobile, reports the team in the Proceedings of the National Academy of Sciences.
“It started with a very simple, almost accidental discovery by examining saliva samples from toddlers who develop aggressive tooth decay,” says corresponding author Hyun (Michel) Koo, a professor in the department of orthodontics at the University. ‘University of Pennsylvania.
“Looking under the microscope, we noticed that bacteria and fungi were forming these assemblies and developing movements that we never thought they would possess: ‘walking-like’ and ‘jumping-like’ mobility. They have a lot of what we call “emerging features” that bring new benefits to this assembly that they couldn’t achieve on their own. It’s almost like a new organism – a superorganism – with new functions.
Assemblies that walk and leap
In the past, Koo’s lab has focused on dental biofilm, or plaque, found in children with severe tooth decay, finding that the two bacteria…Streptococcus mutans—and mushrooms—Candida albicans— contribute to the disease. Cavities, commonly known as cavities, occur when sugars in the diet linger to feed bacteria and fungi in the mouth, resulting in acid-producing dental plaque that destroys enamel.
The new set of findings came when first author Zhi Ren, a postdoctoral fellow in Koo’s group, used microscopy that allows scientists to visualize the behavior of living microbes in real time. The technique “opens up new possibilities for studying the dynamics of complex biological processes,” Ren says.
After seeing the bacterial and fungal clusters present in the saliva samples, Ren, Koo and his colleagues were curious about how the clusters might behave once attached to a tooth surface. They began a series of experiments using live microscopy in real time to observe the process of attachment and eventual growth.
They created a laboratory system to recreate the formation of these assemblies, using the bacteria, fungi and tooth-like material, all incubated in human saliva. The platform allowed researchers to observe clusters and analyze the structure of the resulting assemblies. They found a highly organized structure with bacterial clusters attached in a complex network of fungal yeasts and filamentous projections called hyphae, all entangled in an extracellular polymer, a glue-like material.
Next, the team tested the properties of these cross-kingdom assemblies once they had colonized the tooth surface and found “surprising behaviors and emergent properties,” Ren explains, “including surface adhesion improved, making them very sticky, and increased mechanical and antimicrobial tolerance, making them difficult to remove or kill.
Perhaps the most intriguing feature of the assemblages, according to the researchers, was their mobility. “They displayed ‘jumping’ and ‘walking’ type movements while continuing to grow,” Ren explains.
While some bacteria can propel themselves using appendages like flagella, the microbial species in the current study are both nonmotile. And unlike any known microbial motility, the assemblages used the fungal hyphae to anchor themselves to the surface and then propel the entire superorganism forward, carrying the attached bacteria across the surface, Koo says, ” like bacteria hitchhiking on mushrooms”.
The microbial clusters moved fast and far, the researchers found. On the tooth-like surface, the team measured speeds of over 40 microns per hour, similar to the speed of fibroblasts, a type of cell in the human body involved in wound healing.
During the first hours of growth, scientists observed the assemblies “jumping” more than 100 microns across the surface. “That’s over 200 times their own body length,” Ren explains, “which makes them even better than most vertebrates, relative to body size. For example, tree frogs and grasshoppers can jump forward about 50 times and 20 times their own body length, respectively.
Although the exact mechanisms are unknown, the ability of the assemblages to “move as they grow,” the researchers say, has one clear consequence: it allows them to quickly colonize and spread to new surfaces. When the research team allowed the constructs to attach and grow on real human teeth in a lab model, they discovered more extensive tooth decay due to rapidly spreading biofilm.
“Evil” Superorganism
Because these assemblies are found in saliva, targeting them early could be a therapeutic strategy to prevent tooth decay in children, Koo says. “If you block that bond or disrupt the assembly before it gets to the tooth and causes damage, that could be a preventative strategy.”
And beyond applications to treat this specific disease, the researchers say, the new findings could be applicable to microbial biology in general. For example, aggregate organisms found in other biological fluids or aquatic ecosystems can similarly enhance colonization and surface growth to cause infectious disease or environmental contamination.
“We saw that these two separate organisms come together into a new organic entity that gives each additional benefits and functions that individual cells don’t have on their own,” Koo says.
The findings could even shed light on the evolution of mutualism and multicellularity that enhance the survival and growth of single organisms when they coalesce and work together as a unit in a given environment, the team notes.
“This discovery of a ‘nasty’ superorganism is truly groundbreaking and unexpected,” says Knut Drescher, associate professor at the University of Basel and co-corresponding author of the paper. “No one would have predicted this. Zhi stumbled across it with an open mind.
Other co-authors are from the University of Rochester Medical Center, Indiana University, Philipps-Universität Marburg, University of Basel, and University of Pennsylvania.
The National Institute of Dental and Craniofacial Research and the European Research Council funded the work.
Source: Penn
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