How does an antibody destroy a bacterium?

The antibiotic penicillin block the bacterial processes that build the cell -wall. As a result, the growing bacteria become unable to make cell walls, and die easily. Human cells don't make a cell wall anyway, so penicillin cannot have such an effect on us. Penicillin will have this effect on any bacteria that use such processes for making cell walls. Similarly, many antibiotics work against many species of bacteria rather than simply working against one.