Vibrio cholerae RND efflux systems: mediators of stress responses, colonization and pathogenesis

Abstract

Resistance Nodulation Division (RND) efflux systems are ubiquitous transporters in gram-negative bacteria that provide protection against antimicrobial agents and thereby enhance survival in virtually all environments these prokaryotes inhabit. Vibrio cholerae is a dual lifestyle enteric pathogen that spends much of its existence in aquatic environments. An unwitting encounter with a human host can lead to V. cholerae intestinal colonization by strains that encode cholera toxin and toxin co-regulated pilus virulence factors leading to potentially fatal cholera diarrhea and dissemination in the environment. Adaptive response mechanisms to host factors encountered by these pathogens are therefore critical both to engage survival mechanisms such as RND-mediated transporters and to induce timely expression of virulence factors. Sensing of cues encountered in the host may therefore activate more than protective responses such as efflux systems, but also be coordinated to initiate expression of virulence factors. This review summarizes recent advances that contribute towards the understanding of RND efflux physiological functions and how the transport systems interface with the regulation of virulence factor production in V. cholerae.

Keywords: Vibrio cholerae; adaptation; efflux; pathogenesis; virulence.

Figure 1

The Vibrio cholerae ToxR virulence regulon. In response to low cell density and low oxygen tension, the AphA and AphB transcription factors bind to the promoter and activate transcription of the tcpPH operon. TcpP/H and ToxR/S then bind to the toxT promoter to induce its transcription. ToxT then activates the transcription of multiple virulence genes including the genes encoding for production of cholera toxin (CT) and the toxin co-regulated pilus (TCP). ToxR/S, independently of TcpP/H, also modulates transcription of outer membrane porins OmpU and OmpT.

Figure 2

Structure of tripartite RND efflux systems. RND efflux systems consists of an outer membrane pore protein (yellow) orthologous to E. coli TolC, a membrane fusion protein (gray) and an inner membrane RND-family pump protein (blue). The RND systems function as substrate-proton antiporters and collect substrates from the periplasm and cytoplasmic membrane for export into the extremal milieu. Figure derived from PDB ID: 5V5S (Wang et al., 2017).

Figure 3

Genetic arrangement of the V. cholerae RND efflux operons. The six operons encode a membrane fusion protein (gray) upstream of the RND-family pump protein (blue). Five of the operons are encoded on the large chromosome with one on the small chromosome (vexLM). Two TetR-family regulators (red) have been linked to the RND systems, VexR, which regulates the vexRAB operon, and BreR, which regulates the vexCD operon. The outer membrane pore protein is provided by tolC (yellow) is encoded separately in the genome.

Figure 4

Substrates of the RND efflux systems mediate adaptive responses via ToxR. Impaired RND-mediated efflux results in the periplasmic accumulation of endogenous and exogenous RND substrates such as bile acids (green balls), indole (violet diamonds) or malate (blue hexagons) which engage the ToxR periplasmic domain to activate ToxR and downstream adaptive responses.

Figure 5

Reciprocal relationship between the VexAB and VexGH RND systems and the Cpx system. VexAB and VexGH contribute to efflux of the siderophore vibriobactin, and impaired efflux results in vibriobactin accumulation in the periplasm which titrates iron from iron-containing components of the electron transport system (ETS) resulting in protein misfolding and activation of the cpx system. Cpx system activation results in induction of vexAB, vexGH and iron homeostasis genes while repressing virulence gene expression.

Figure 6

RND efflux influences the activation state of the EnvZ-OmpR TCS via efflux of bile acids. Bile acids (green balls) and other membrane intercalating agents induce EnvZ kinase activity and phosphorylation of OmpR which in turn modulates outer membrane porin expression and represses virulence factor production by inhibiting aphB transcription.