Efflux pumps are transport proteins involved in the extrusion of toxic substrates (including virtually all classes of clinically relevant antibiotics) from within cells into the external environment. These proteins are found in both Gram-positive and-negative bacteria as well as in eukaryotic organisms. 1 Pumps may be specific for one substrate or may transport a range of structurally dissimilar compounds (including antibiotics of multiple classes); such pumps can be associated with multiple drug resistance (MDR). In the prokaryotic kingdom there are five major families of efflux transporter: 2 MF (major facilitator), MATE (multidrug and toxic efflux), RND (resistance-nodulation-division), SMR (small multidrug resistance) and ABC (ATP binding cassette). All these systems utilize the proton motive force as an energy source, 3 apart from the ABC family, which utilizes ATP hydrolysis to drive the export of substrates. Recent advances in DNA technology and the advent of the genomic era have led to the identification of numerous new members of the above families, and the ubiquitous nature of efflux pumps is remarkable. Transporters that efflux multiple substrates, including antibiotics, have not evolved in response to the stresses of the antibiotic era. All bacterial genomes studied contain several different efflux pumps; this indicates their ancestral origins. It has been estimated that∼ 5–10% of all bacterial genes are involved in transport and a large proportion of these encode efflux pumps. 2, 4

There is some debate as to the ‘normal’physiological role of efflux transporters, as antibiotic susceptible as well as resistant bacteria carry and express these genes. In many cases, efflux pump genes are part of an operon, with a regulatory gene controlling expression. Increased expression is associated with resistance to the substrates, eg resistance to bile salts and some antibiotics in Escherichia coli is mediated by over-expression of acrAB. 5 Although genes encoding efflux pumps can be found on plasmids, the carriage of efflux pump genes on the chromosome gives the bacterium an intrinsic mechanism that allows survival in a hostile environ-