Turns out, not much. It is keeping the dentifrice clean through the mechanical action of the toothbrush. And brushing more than once a day does the job more than the fluoride in the toothpaste we all watched fighting germs in television commercials.
Fluoride is the agent used in both toothpastes and mouthwashes to fight germs that cause cavity. Fluoride works by bonding to our tooth enamel. Once bonded, it hardens the enamel and strengthens it to help fight acids that come in through food (soda, juice, fruits etc) harmful as well as those produced by bacteria in our oral cavity.
It was observed in a study conducted by Yale researchers that oral germs have what is known as 'riboswitches' that detect the presence of fluoride in the teeth and turn on/activate the defenses of the bacteria/germ gene against the fluoride.
A 'riboswitch' is a part of an RNA molecule that can directly bind a small target molecule in response to the high concentrations of its target molecule (in this case, a fluoride molecule) and whose binding of the target affects the gene's activity.
It has been common knowledge among scientists that bacteria succumb to higher concentrations of fluoride. However, the amount available in toothpaste may not measure up to that level due to fluoride's adverse side effects in humans in long-term usage at high concentrations.
As per the Henry Ford II Professor and chair of the Department of Molecular, Cellular and Developmental Biology and senior author of the study, Ronald Breaker, “These riboswitches are detectors made specifically to see fluoride. 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.”
The scientists were very surprised to discover the fluoride-sensing 'riboswitches'. However, examination of other types of organisms pointed at the presence of such 'riboswitches' against fluoride in most of them.
This implies that many organisms have had to overcome toxic levels of fluoride throughout their history and have retained and evolved this mechanism to detect/sense and fight fluoride to be able to survive.
The research team also observed protein channels in the germs that help flush out fluoride from the cells. However, this could be countered by blocking these channels with another molecule would cause fluoride to accumulate in bacteria, making it more effective as a cavity fighter.
It must be kept in mind nonetheless, that fluoride used at current levels in oral hygiene preparations has significant benefits in maintaining good dental health. Its introduction in toothpastes in 1950s has since brought down the cases of dental cavities drastically.