Then the process begins again and those two cells divide into four. So in order to reproduce, the bacterial cell has to be constantly building new cell walls. The way penicillin and some other antibiotics work is by do stopping the growth of cell walls. These antibiotics bond to protein molecules on the existing wall and prevent the cell from adding any new material as the wall stretches. When the cell wall stretches too far, it becomes weak and eventually bursts, destroying the cell. Since animal cells don’t have cell walls, they are not affected by penicillin. Not all antibiotics work the same way, but to protect a human or animal body, an antibiotic has to attack bacterial cells in some specific way that won’t damage animal cells.