AcrB Multidrug Efflux Pump of Escherichia coli : Composite Substrate-Binding Cavity of Exceptional Flexibility Generates Its Extremely Wide Substrate Specificity

Abstract

Gram-negative bacteria have, in general, much higher intrin- sic levels of resistance to various antibiotics, antiseptics, dyes, and detergents than do gram-positive bacteria. This is, in part, due to the effectiveness of the outer membrane as a barrier. The porin channels exclude large compounds (for example, vancomycin) and drastically slow down the influx of most an- tibiotics, which are usually much larger than common nutri- ents. Most antibiotics and chemotherapeutic agents that act on targets in the cytosol must cross the inner, cytoplasmic mem- brane, usually by spontaneous diffusion, and this necessitates their being at least moderately lipophilic. These compounds can in principle diffuse across the lipid bi-layer domain of the outer membrane. Transmembrane diffusion rates across this domain, however, are about two orders of magnitude slower than through the conventional phospholipid bi-layers (29), be- cause the outer leaflet, composed exclusively of lipopolysac- charides (11), acts as an effective barrier. The outer membrane barrier alone, however, only slows down the influx of most of the noxious agents, and the gram- negative bacteria need the additional contribution of multi-