FIG 1.

Schematic of various efflux families and their location in the Gram-negative cell membrane. Efflux pumps that span the inner membrane are of the multidrug and toxic compound extrusion family (MATE), ATP-binding cassette family (ABC), proteobacterial antimicrobial compound efflux family (PACE), major facilitator superfamily (MFS), and small multidrug resistance family (SMR). Efflux pumps that span both the inner and outer membrane belong to the resistance-nodulation-division family (RND) and, as pictured, some ABC transporters are able to do so as well. Substrates are examples to indicate direction of transport.