Other antibiotics disturb the function rather than the formation of cell walls of various microbes. The membrane of a bacterial cell controls the flow of nutrients in and out of the cell. By disturbing yhe membrane, these antibiotics allow necessary nutrients to leave the cell, or they allow poisonous substances to enter and kill the cell. These antibiotics attack only membranes containing elements unique to microbial cells. Consequently, the membranes of humen cells remain undisturbed.