antibiotic have a great range of action depending on the familly. Some antibiotic form pores on the cell wall and disrupt important gradient. therefore molecules and important leaks out and the bacteria dies. Some antibiotics will enter the cell via membrane receptor and interfere with the cell mechanism such as protein synthesis, DNA replication and RNA synthesis by binding to specific proteins. Most of these mechanism are require for the cell to live or to replicate.
Also not all antibiotic kill bacteria, (bacteriacide), some are bacteriastatics (prevent the grow of bacteria rather than killing).
To add more information, antibiotic resistance either arise from mutation in membrane receptor gene, enzyme that degrade antibitotic and pump that expell the antibitic out of the bacteria cell.
So far so good.
Now suppose we have found a new kind of bacteria.
How will we develop an antibiotic for it ?
Will it by trial or there are some ways or knowing that a certain antibiotic will work on a certain bacteria (depending maybe on some charachteristics in the bacteria).
Keep in mind that if a new bacteria is discover very little information will be available. Most information will probably come from related bacteria but it does not mean that function observed in group are present in the new bacteria.
One way of knowing what kind of antibiotic to used would be to classified the bacteria (gram +, gram -, etc) because some antibitotic have narrow spectrum ( ex: penicillin is bacteriacidal for gram + only) whereas other have broad spectrum (ex: chloramphenicol and tetracyclines bacterialstatics against gram +, gram -, rickettsia and chlamydia). The classification would help to eliminate the antibitotic that do not work against this particular group.
The second step would be to test antibiotic sensivity of the new bacteria species. The most common antibiotics would be test first. These are perform by using small disc with a known concentration of a certain antibitotics on agar plates. It is also important to note that antibiotics can have nasty side effects on human and animals if used above a certain concentration.
This method is usually used by medical microbiology labs.
It is important to add that antibiotics are refferred to as antibacterial agents of microbial origin such as fungi and bacteria. Nowadays most antibiotic are synthetic and derived from original antibiotics.
The next step would be to study the bacteria and its function. It is also important to note that people developing new antibiotics used will use other tools such a bacterial genome and proteome. Bacterial genome and proteome give insigth of what the bacteria is producing and potential antibiotic and vaccine target (membrane receptor, ribosome, DNA polymerase, etc ) can be find. Synthetic antibiotic could be developed to attack these target. Antibiotic resistance can also be identify using the genome and proteome of bacteria.