10points! Why is fluorine bad to bacteria

Why is fluorine bad to the bacteria that cause cavities?
I've just heard that "Water fluoridation decreases cavities incidence".

First off, to be chemically exact, there's no fluorine in your drinking water or toothpaste, but fluoride - fluorine is a highly dangerous gas, while fluorides are chemically inert salts containing F- anions. Second, it's not so much that fluorine is bad for bacteria, but that it's good for your teeth. Tooth enamel consists mainly of hydroxyapatite, a mineral with the formula Ca10(PO4)6(OH)2. The mineral can be dissolved by acids (e.g. from soft drinks), this making the underlying tooth structure available as a breeding ground for bacteria, which can bury deeper and form a cavity. Now, if some of the hydroxy anions in hydroxyapatite are replaced by fluorides, we get a mineral that is considerably more resistant to acid.