Efflux pumps as antimicrobial resistance mechanisms

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Abstract
Antibiotic resistance continues to hamper antimicrobial chemotherapy of infectious disease, and while biocide resistance outside of the laboratory is as yet unrealized, in vitro and in vivo episodes of reduced biocide susceptibility are not uncommon. Efflux mechanisms, both drug‐specific and multidrug, are important determinants of intrinsic and/or acquired resistance to these antimicrobials in important human pathogens. Multidrug efflux mechanisms are generally chromosome‐encoded, with their expression typically resultant from mutations in regulatory genes, while drug‐specific efflux mechanisms are encoded by mobile genetic elements whose acquisition is sufficient for resistance. While it has been suggested that drug‐specific efflux systems originated from efflux determinants of self‐protection in antibiotic‐producing Actinomycetes, chromosomal multidrug efflux determinants, at least in Gram‐negative bacteria, are appreciated as having an intended housekeeping function unrelated to drug export and resistance. Thus, it will be important to elucidate the intended natural function of these efflux mechanisms in order, for example, to anticipate environmental conditions or circumstances that might promote their expression and, so, compromise antimicrobial chemotherapy. Given the clinical significance of antimicrobial exporters, it is clear that efflux must be considered in formulating strategies for treatment of drug‐resistant infections, both in the development of new agents, for example, less impacted by efflux or in targeting efflux directly with efflux inhibitors.