system
Given the following post, respond as if you're commenting on Reddit:
human
If we know what kind of bacteria causes the majority of cavities, why don't we just make an antibiotic that targets them and distribute it like a vaccine at a doctor's office?
gpt
Antibiotics that target a specific bacterium are tricky. There are a small handful that have been demonstrated, but very few, and only for research purposes. Even then, it's more of a "kills bacteria x very efficiently, while y and z will mostly survive" deal.

The reason is, simply, how targeting can be done. You basically pick a trait or property of that bacterium - for instance, it has a lot of receptors for a specific sugar molecule, or the cell wall is negatively charged, or something. You target that property, by for instance attaching the respective sugar to a drug molecule, and hoping that most of the dose gets taken up by your target. Some will be uptake by other cells though, as there is almost never a completely unique expression or trait.

Also, most of these types of drugs are too expensive, and not specific enough, to be commercially feasible.
Thus the various broad spectrum antibiotics, and the range of somewhat specific antibiotics that each address a niche family or group of bacteria.

Beyond that, antibiotic resistance is a huge problem with overuse. Basically, let's say your antibiotic is super effective, kills 99.9999% of the target bacteria. The remaining 0.0001% is the group that was more resistant, for whatever reason. That is the group that also starts to become dominant, as we kill off the less resistant ones. Eventually, your antibiotic is no longer effective for that bacteria!

This is why we have had to formulate so many modified versions of penicillin, to keep coming up with something new that still works.

Generally speaking, it is best to limit antibiotic use to cases where it is really necessary. While you can certainly argue for it's use against cavities, it would not be a one time (or sporadic) thing like vaccinations but a regular treatment, like mouth wash. That sort of high volume, high frequency usage would give us dental versions of something like MRSA very quickly!