Briefly, antibiotics are chemicals that kill bacteria; there are a few that will kill fungi, protists, or other organisms, but the vast majority of antibiotics prescribed for treatment of human illness will kill bacteria. Mode of action varies; many antibiotics interfere with bacterial cell division, which is a process different enough from how animal cells reproduce (e.g. cell wall synthesis - we don't have cell walls, some bacteria do) to hurt the bacteria without hurting the host (i.e. you). The net result of a course of antibiotics is to kill a very large fraction of the bacteria responsible for an infection, or to prevent them from reproducing long enough for the immune system to get ahead of the bacteria and wipe them out.
Antibiotics are an obvious agent of selection. Any bacterial cell that can avoid or reduce the effects of the antibiotic - for example, by producing an enzyme that destroys the antibiotic before it can interfere with the bacterial cell - will have a clear advantage, and its offspring will carry that resistance. This is why it is VERY IMPORTANT to take the FULL COURSE of a round of therapeutic antibiotics. The symptoms of the infection - pain, puss, whatever - might clear up in a few days, but the antibiotics will be in sufficient number of pills for more than a week. KEEP TAKING THE ANTIBIOTICS. The bacteria are down but not out after those first few days; most of the surviving cells may be resistant. In many cases, resistance means reduced but not eliminated effect of the antibiotics, so those nasty cells are still getting hurt, just not outright killed as their less fortunate compatriots were. Keep hammering them, and the immune system will clean up the mess - few bacteria can withstand the double-blow of compromised reproductive ability and roving bands of killer white blood cells.
Here's a picture of a neutrophil, a type of white blood cell (immune system) that targets bacteria and fungi (photocredit to Bob Blaylock and this Wikipedia page).
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Interestingly, a few years ago I went to talk about antibiotic resistance at McMaster. As should be rather evident based on the short period that we've been using antibiotics, resistance actually evolved long ago, (some bacteria/plants/archaea produce antibiotics naturally, so the selection pressure's been there for a while) and newly arising resistant strains are either a) increasing to large numbers because of selection pressure, or b) acquiring the relevant genes through lateral gene transfer.
What most people don't seem to 'get' is that the antibiotic is simply helping your body take care of a problem that it typically deals with naturally. Some times we get a bigger infection than our immune system can deal with and thus we develop disease (many people 'carry' tuberculosis or leprosy, for example, without manifesting the disease). A small number of of resistant bacteria will be killed by the immune system, but if you switch the balance of infectious bacteria to 50% resistant, then there are too many bacteria for your immune system to do its job, and standard antibiotics can't help you.
Over-the-counter antibiotics should be illegal (e.g., soap).
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