Bacteria and fungi can join forces to wreak havoc on your teeth, according to a somewhat accidental discovery.
When examining the microbes that cause aggressive toddler tooth decay, Zhi Ren, a dental researcher from the University of Pennsylvania, noticed two distinct organisms presenting a united front under the microscope.
Further research in the lab explored how these bacterial-fungal clusters in human saliva might work together in the mouth to cause tooth-rotting disease.
The findings reveal several 'emergent functions' of the clusters, which make the species act almost like an entirely new superorganism – including new abilities to move and spread across our teeth.
The coupled pathogens were caught doing things they could not otherwise accomplish on their own.
The usually sedentary bacteria, Streptococcus mutans, were no longer bound to the whims of saliva. Hitchhiking on the extending arms of the yeast, Candida albicans, the bacteria could now move by 'leaping', continuously growing as it spreads.
"This discovery of a 'bad guy' superorganism is really ground-breaking and unanticipated," says microbiologist Knut Drescher of the University of Basel in Switzerland.
"No one would have predicted this."
It's not that bacteria and fungi have never been found working together before. Multicellular biofilms cause many human infections. It's more that our knowledge of how these communities develop and work remains limited.
In the lab, the authors of the current study found bacterial clusters can attach to the body, branches, and exterior sugars of fungal yeasts.
As a group, this cell network can more easily attach to teeth than the cells on their own. The 'superorganism' also shows stronger tolerance to antimicrobials and brushing.
But the craziest part is how the superorganism moves.
While some bacteria have little arms that they use to swim around, S. mutans is usually immobile. In fact, neither C. albicans nor S. mutans can move as such, but because C. albicans can extend its arms out, it provides the perfect vehicle for a hitchhiker.
When bacteria attach to these probing fungal filaments, they can essentially 'leap' forward to merge with other biofilms.
When testing the superorganisms on tooth-like surfaces, researchers found the bacteria moved at a velocity of more than 40 microns an hour, similar to the speed of moving wound-healing molecules in the human body.
Within hours of the two pathogens binding, the authors caught bacteria jumping to substrates 100 microns away, a distance more than 200 times their usual body length.
To the best of the team's knowledge, nobody else has reported this kind of group-level mobility.
"The dynamic fungal–bacterial interactions lead to biofilm superstructures that cause extensive and more severe damage of the tooth-enamel surface," the authors write.
If bacteria and fungi can somehow be stopped from binding together, researchers think it might help prevent cavities.
But the discovery also matters for reasons outside of dental work.
The new findings could help explain how similar superorganisms spread infectious diseases or cause environmental contamination with such rapidity.
"This collective multicellular migratory mode opens intriguing possibilities," the authors write.
"This could be a stochastic mobility mechanism utilized by the interkingdom colonizers to boost range expansion nearby or possibly a navigation strategy to a desired direction or location."
The study was published in PNAS.