AcrAB-TolC is the most important multidrug efflux pump system of Gram-negative bacteria, responsible for their resistance to lipophilic and amphiphilic drugs. In this work, a molecular graphics study of the pump components AcrB and TolC, 16 β-lactam antibiotics and 7 other substrates, as well as of AcrB-substrate complexes, was performed in order to give a mechanistic proposal for the efflux process at molecular level. AcrAB-TolC is a proton-dependent electromechanical device which opens to extrude drugs from the bacterial periplasm and perhaps cytoplasm, by means of a series of structural changes within the complex and its components AcrA, AcrB and TolC. These changes are initiated by protonation and disruption of salt bridges and certain hydrogen bonds, and are followed by conformational changes in which a number of intra- and interchain interactions are rearranged. 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