Revelations from Germany
In 2010, researchers at Saarland University in Germany revealed that the long-hypothesized "protective layer" fluoride supposedly forms on teeth is in fact only six nanometers thick. It would take almost 10,000 such layers to span the width of a human hair. Such a minuscule layer is quickly worn away by ordinary chewing, so is unlikely able to shield teeth from decay.
In May 2013, Saarland University's Experimental Physics Department found new evidence for how fluoride knocks out cavity-causing bacteria. Fluoride reduces the adhesion force of bacteria, so they are less able to adhere to teeth, where they produce the acid that causes cavities.
The researchers tested adhesion of Streptococcus mutans, Streptococcus oralis, and Staphylococcus carnosus on hydroxyapatite (main ingredient in tooth enamel) surfaces before and after treatment with fluoride solution. All bacteria species exhibited lower adhesion forces after fluoride treatment of the surfaces. Because fluoride reduces the ability of decay-causing bacteria to stick on teeth, it is easier to wash away these bacteria by saliva, brushing, and other activity. [Loskill et al. 2013]
The researchers said,
"Fluoride appears to weaken bacterial adhesion forces in general."