system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do antibiotics kill unwanted bacteria but not the body's own cells?

I know about enzymes and such in a basic sense, but the fact that an antibiotic actually *works* still doesn't quite make sense to me. Wouldn't we need a specifically engineered antibiotic for each strain of bacteria? Why does the antibiotic only harm bacteria?
gpt
This principle is called "selective toxicity". 

It utilises the fact that bacterial cells are different from human/animal cells. Specifically, they have different biochemistry, different anatomy, etc.

For example, a very popular group of antibiotics called beta-lactams (an example is penicillin, but there are lots of synthetic variants) work by blocking the biochemical pathway that builds the bacterial cell wall. Because the bacteria cannot maintain or extend their cell walls, eventually the wall degrades and the bacteria die.

Other antibiotics affect different chemical processes. For example, the antibiotic tetracycline, blocks the process that bacteria use to translate their DNA code into protein manufacture. The ribosomes, the molecular machines that build proteins, are different between bacteria and mammals, so the mammal ribosomes are less affected (also bacteria actively pump tetracycline from outside to insude because the tetracycline tricks the bacterial molecular pumps which absorb nutrients, so the bacterial ribosomes get a much bigger dose than mammal cells, which don't have pumps which accept tetracycline). The other interesting thing about tetracycline is that it doesn't kill bacteria. It just stops them, or slows down,  reproducing because it slows down protein production. This is still useful, because it gives the immune system time to catch up.