Membrane Efflux Pumps of Pathogenic Vibrio Species: Role in Antimicrobial Resistance and Virulence

Abstract

Infectious diseases caused by bacterial species of the Vibrio genus have had considerable significance upon human health for centuries. V. cholerae is the causative microbial agent of cholera, a severe ailment characterized by profuse watery diarrhea, a condition associated with epidemics, and seven great historical pandemics. V. parahaemolyticus causes wound infection and watery diarrhea, while V. vulnificus can cause wound infections and septicemia. Species of the Vibrio genus with resistance to multiple antimicrobials have been a significant health concern for several decades. Mechanisms of antimicrobial resistance machinery in Vibrio spp. include biofilm formation, drug inactivation, target protection, antimicrobial permeability reduction, and active antimicrobial efflux. Integral membrane-bound active antimicrobial efflux pump systems include primary and secondary transporters, members of which belong to closely related protein superfamilies. The RND (resistance-nodulation-division) pumps, the MFS (major facilitator superfamily) transporters, and the ABC superfamily of efflux pumps constitute significant drug transporters for investigation. In this review, we explore these antimicrobial transport systems in the context of Vibrio spp. pathogenesis and virulence.

Keywords: antimicrobial resistance; bacteria; cholera; infection; multidrug efflux pump; multidrug resistance.

Figure 1

MATE transporters of bacteria. The transporters are grouped under NorM and DinF subfamilies and are generally composed of 12 TM helices which fold into two distinct N- (TM1-TM6) and C- (TM7-TM12) lobes [76,84,93]. The topology of MATE transporters is characteristically distinct from that of MFS transporters, many of which have similar 12 TM helices [76,94]. In the NorM subfamily (NorM-VC), the C-lobe functions as a cation binding site, and two acidic amino acid residues, Glu255 and Asp371, are located between TM7-12, are critically important for this function [83].

Figure 2

Based on the crystal structures of PfMATE (H⁺-coupled DinF protein from Pyrococcus furiosus [89,98], Kusakizako and colleagues have proposed an “alternating access mechanism” for the DinF-subfamily of MATE transporters [83]. Binding with Na⁺ (or H⁺) ions occurs at the Asp residue in the TM1 (N-lobe), which results in the bent conformation of TM1. In this state, TM1 assumes an outward open state. No substrate-binding takes place due to changes in the conformation at this stage. The following proposed form is a cation-bound occluded state, followed by an inward-open bent form, assuming a straight conformation to allow substrate binding. This step is followed by a substrate-bound occluded state, which finally enters into outward-open conformation.

Figure 3

A rocker-switch mechanism was proposed for MFS transporters. In this proposed antiport system, proton-driven drug efflux occurs through the pump by alternately exposing the drug (D) binding site to either side of the membrane. The drug translocation process involves conformation changes in the two bundles or halves of the MFS pump to form inward open, occluded, and outward-facing open conformations (steps 1–4 in the figure) [118,119].