Amoxicillin for tooth extraction

My sister was prescribed amoxicillin after a minor surgery in her mouth. I'm studying immunology right now and trying to figure out why this antibiotic was chosen.

I'm trying to find out what sort of bacteria is dangerous in the mouth and all I see is there is a ton of different bacteria in the mouth and I can't figure out which one is dangerous.

Anyone got a quick explanation why amoxicillin was used?

Thinking it may because many of these bacteria are anaerobic?