Antibiotics kill pathogenic, or disease-causing, microbes in the human body by interfering with their cell processes. This interference occurs in one of three ways: by hindering cell wall formation, by disturbing the cell membrane, or by disrupting chemical processes inside the cell. The cellular substances of bacteria are encased within a cell membrane. The cell membrane, in turn, is surrounded by a cell wall. The cell wall is rigid and prevents bacterial cells from splitting open. Some antibiotics, including penicillin, attack invasive microbes by interfering with the formation of these cell walls. Fortunately, these antibiotics do not affecl human body cells, since animal cells do not have cell walls.