The molecular tricks used by bacteria to fight the effect of fluoride in toothpaste and mouth wash have been uncovered by scientists.
In the new study, researchers at Yale University report that sections of RNA messages called riboswitches, which control the expression of genes, detect the build-up of fluoride and activate the defences of bacteria, including those that contribute to tooth decay.
"These riboswitches are detectors made specifically to see fluoride," Ronald Breaker, senior author of the study, said.
Fluoride in over-the-counter and prescription toothpastes is widely credited with the large reduction in dental cavities seen since these products were made available beginning in the 1950s.
This effect is largely caused by fluoride bonding to the enamel of our teeth, which hardens them against the acids produced by bacteria in our mouths.
However, it has been known for many decades that fluoride at high concentrations also is toxic to bacteria, causing some researchers to propose that this antibacterial activity also may help prevent cavities.
The riboswitches work to counteract fluoride's effect on bacteria.
"If fluoride builds up to toxic levels in the cell, a fluoride riboswitch grabs the fluoride and then turns on genes that can overcome its effects," Breaker said.
Breaker also said that since both fluoride and some RNA sensor molecules are negatively charged, they should not be able to bind.
"We were stunned when we uncovered fluoride-sensing riboswitches.
"Scientists would argue that RNA is the worst molecule to use as a sensor for fluoride, and yet we have found more than 2000 of these strange RNAs in many organisms," he said.
By tracking fluoride riboswitches in numerous species, the research team concluded that these RNAs are ancient, meaning that many organisms have had to overcome toxic levels of fluoride throughout their history.
Organisms from at least two branches of the tree of life are using fluoride riboswitches, and the proteins used to combat fluoride toxicity are present in many species from all three branches.
"Cells have had to contend with fluoride toxicity for billions of years, and so they have evolved precise sensors and defense mechanisms to do battle with this ion," Breaker added.
The study has been published in the online issue of the journal Science Express.