Answer:
Producing oxygen
Explanation:
Bacteria actually add to nutrients in soil because they decompose the organic matter such as dead bodies and such. I do not know if bacteria heal sick animals or if they prevent tooth decay rather I am sure they cause both of those.
Therefore, most plausible answer is producing oxygen because there are bacterias like cyanobacteria, or blue-green algae who go through photosynthesis and produce oxygen which is beneficial for all living beings.
