Bacterial Multidrug Efflux Pumps: Much More Than Antibiotic Resistance Determinants

Abstract

Bacterial multidrug efflux pumps are antibiotic resistance determinants present in all microorganisms. With few exceptions, they are chromosomally encoded and present a conserved organization both at the genetic and at the protein levels. In addition, most, if not all, strains of a given bacterial species present the same chromosomally-encoded efflux pumps. Altogether this indicates that multidrug efflux pumps are ancient elements encoded in bacterial genomes long before the recent use of antibiotics for human and animal therapy. In this regard, it is worth mentioning that efflux pumps can extrude a wide range of substrates that include, besides antibiotics, heavy metals, organic pollutants, plant-produced compounds, quorum sensing signals or bacterial metabolites, among others. In the current review, we present information on the different functions that multidrug efflux pumps may have for the bacterial behaviour in different habitats as well as on their regulation by specific signals. Since, in addition to their function in non-clinical ecosystems, multidrug efflux pumps contribute to intrinsic, acquired, and phenotypic resistance of bacterial pathogens, the review also presents information on the search for inhibitors of multidrug efflux pumps, which are currently under development, in the aim of increasing the susceptibility of bacterial pathogens to antibiotics.

Keywords: MDR; antibiotic resistance; bacteria/plant interactions; multidrug efflux pumps; quorum sensing; solvent tolerance.

Figure 1

Schematic representation of the main types of bacterial efflux systems. Schematic illustration of the five major families of efflux transporters: the resistance- nodulation-division (RND) family, the small multidrug resistance (SMR) family, the major facilitator superfamily (MFS), the multidrug and toxic compound extrusion (MATE) family and the adenosine triphosphate (ATP)-binding cassette (ABC) superfamily. IM: Inner membrane. OM: Outer membrane. OMP: Outer membrane protein.

Figure 2

Role of multidrug efflux pumps in antibiotic resistance. Expression of efflux pumps is frequently down-regulated by transcriptional repressors encoded upstream of the pump operon. Consequently, MDR efflux pumps can contribute to the phenotype of antibiotic resistance at different levels, depending on their expression level. Intrinsic resistance: Some MDR efflux pumps, such as P. aeruginosa MexAB-OprN [62] or E. coli AcrAB-TolC [63], present a basal level of expression, enough for contributing to the intrinsic antimicrobial resistance of these microorganisms (blue in the Figure). Acquired resistance: De-repression of the expression of the efflux pumps can be achieved by mutations at the regulatory proteins, rendering stable acquired resistance (yellow in the Figure). Phenotypic resistance: The expression of efflux pumps can by triggered in the presence of specific inducers, rendering transient phenotypic resistance (pink in the Figure).

Figure 3

Schematic representation of the regulation of the expression of the E. coli acrAB multidrug efflux pump. AcrAB-TolC is a tripartite complex formed by AcrA, a membrane fusion protein, AcrB, a cytoplasmic membrane protein, and TolC, an outer membrane protein. acrA and acrB are part of the same operon, which is negatively regulated by the dimeric protein AcrR. The global regulators MarA, SoxS, and Rob can activate acrAB expression in response to different environmental signals. MarA is encoded by the second gene of the marRAB operon, the expression of which is repressed by MarR through its binding to its operator marO. The presence of salicylate inactivates MarR, leading to the expression of marRAB. MarA increases its own transcription and activates the expression of acrAB. SoxR and SoxS constitute an oxidative response system [136]. In the absence of signals, the homodimer SoxR represses soxS expression, but under oxidative stress conditions, SoxR is oxidized and becomes an activator of soxS transcription [137]. SoxS binds to the acrAB promoter region and induces its expression. Rob is constitutively expressed but remains in an inactive form, unless an effector such as decanoate or bile salts is present. Those effectors bind and produce conformational changes that activate Rob, inducing acrAB expression.