You can now see that the life of a cell is dependent on a rich soup of enzymes that float in the cell's cytoplasm. Many different poisons work by disrupting the balance of the soup in one way or another.
For example, diphtheria toxin works by gumming up the action of a cell's ribosomes, making it impossible for the ribosome to walk along the mRNA strand. The toxin in a death-cap mushroom, on the other hand, gums up the action of RNA polymerase and halts the transcription of DNA. In both cases, the production of new enzymes shuts down and the cells affected by the toxin can no longer grow or reproduce.
An antibiotic is a poison that works to destroy bacterial cells while leaving human cells unharmed. All antibiotics take advantage of the fact that there are many differences between the enzymes inside a human cell and the enzymes inside a bacterium. If a toxin is found, for example, that affects an E. coli ribosome but leaves human ribosomes unharmed, then it may be an effective antibiotic. Streptomycin is an example of an antibiotic that works in this way.
Penicillin was one of the first antibiotics. It gums up a bacterium's ability to build cell walls. Since bacterial cell walls and human cell walls are very different, penicillin has a big effect on certain species of bacteria but no effect on human cells. The sulfa drugs work by disabling an enzyme that manages the creation of nucleotides in bacteria but not in humans. Without nucleotides, the bacteria cannot reproduce.
You can see that the search for new antibiotics occurs down at the enzyme level, hunting for differences between the enzymes in human and bacterial cells that can be exploited to kill bacteria without affecting human cells.
The unfortunate problem with any antibiotic is that it becomes ineffective over time. Bacteria reproduce so quickly that the probability for mutations is high. In your body, there may be millions of bacteria that the antibiotic kills. But if just one of them has a mutation that makes it immune to the antibiotic, that one cell can reproduce quickly and then spread to other people. Most bacterial diseases have become immune to some or all of the antibiotics used against them through this process.