New York, Sep 12 (IANS): Know why your toothpaste is not able to fight bad breath or tooth decay? Blame it on the bacteria itself.

According to new research, bacteria has an in-built mechanism to resist fluoride toxicity.

Although most animal cells are protected from direct exposure to fluoride, this toxic element is a serious threat to single-celled organisms like bacteria and yeast.

As a result, their plasma membranes carry two different types of proteins to help rid the cell of unwanted fluoride.

"The fluoride-specific 'Fluc' ion channels present in the bacterial cell membrane are the key," said Christopher Miller from the Brandeis University in Massachusetts.

Miller and his team looked at "Fluc" channels to find how these passive channels help protect bacteria from fluoride.

The authors found that fluoride accumulates in E coli lacking "Fluc" when the external environment is acidic.

In such acidic environments, fluoride enters the cell in the form of HF (hydrofluoric acid) -- which easily permeates the membrane -- and breaks down in the cell's lower acidity.

"Fluc provides a means of escape for the highly charged fluoride ions," Miller noticed.

They also found that bacteria proliferation was stalled by high fluoride exposure.

Targeting "Fluc" channels with antibiotics could be an effective way to slow bacterial growth, researchers concluded.

The study appeared in the Journal of General Physiology.