Multidrug efflux pumps have gained notoriety as a major and highly promiscuous class of drug resistance determinants that contribute to the failure of antibiotic therapy and promote the persistence of pathogens in hospitals. Despite their widely-studied roles in drug resistance, for many multidrug efflux pumps drug transport is likely to be a fortuitous side reaction made possible by flexible substrate binding sites that have become beneficial to host organisms living under highly drug selective conditions in hospitals. The core functions of these pumps are likely to be linked to the physiology of the organism and the environments in which they evolved. This is almost certainly true for the AceI transport protein, the prototype for the novel PACE family of efflux pumps. The gene encoding AceI is conserved across all A. baumannii strains to have had their genomes sequenced, indicating an ancient origin and long term pressure for gene maintenance. Paradoxically, its only characterised substrate is chlorhexidine, which, although widely used as an antiseptic today, is purely synthetic and has been produced only since last century. This presentation will describe our progress in deciphering the core physiological functions of the AceI protein, as well as its mode of energisation.