Mechanisms of antibiotic resistance. Computer artwork showing 4 different ways in which bacteria can be resistant to an antibiotic. From top left: normal interaction between the antibiotic and its target site within the bacterium; alteration of the target site so the antibiotic is ineffective; active efflux of antibiotic so it passes out of the bacterium without having an effect; altered permeability of the bacterial cell wall prevents the antibiotic from entering the bacterium; inactivation of the antibiotic so that it breaks down within the bacterium before it can have an effect.