system
Given the following post, respond as if you're commenting on Reddit:
human
How do antibiotics destroy foreign bacteria without harming our own body as well?
gpt
It depends on the antibiotic, but in some cases, it can harm the human body.

The easy answer is that antibiotics are selective for specific molecules which exist in bacteria, but not humans. Since humans don't have these molecules on their cells, the antibiotics can't target them, and therefore don't kill the cells. Penicillin, for example, targets and inhibits a protein which is involved in cell wall synthesis in gram positive bacteria (a classification of bacteria which creates cell walls made up of a molecule called peptidoglycan). Since the cell wall of these bacteria can't be finished, basically it becomes too big (it can't divide properly) and therefore the bacteria bursts and dies. Human cells don't make cell walls, therefore penicillin isn't involved in killing human cells.

However, the reality is a bit more complex. Different categories of antibiotics have different targets, and not all of them are as selective as penicillin is. Another class of antibiotics, called aminoglycosides, interacts with the ribosome in a bacterial cell. The ribosome is involved in translating mRNA into amino acid sequences to make proteins, which do a lot of different things in the cell. By binding to the ribosome, aminoglycosides stop the cell's ability to make proteins, which eventually causes it to die.

But both bacteria and have ribosomes. The main reason why aminoglycosides target bacteria selectively, is that bacteria have ribosomes that are slightly different- they have 30S and 50S subunits, but human cells have 40S and 60S subunits. Basically, the bacteria ribosomes are smaller than human ribosomes. This difference makes aminoglycosides more likely to interact with bacteria cells, because the smaller ribosome has the right structure for it to interact better. However, aminoglycosides can and do bind to human ribosomes, and can cause human cell death too. It just binds a lot less frequently, so it causes less human cell death.

I only described how antibiotics directly affect bacteria cells versus human cells, though. Killing bacteria in the body can have effects on human health too (for example, killing bacteria in the intestines can have an effect on digestion, and can cause health problems). And in rare cases, antibiotics can interact and trigger the immune system to attack the body, when it shouldn't, causing harm to the body.