Resistance Is Not Futile: The Role of Quorum Sensing Plasticity in Pseudomonas aeruginosa Infections and Its Link to Intrinsic Mechanisms of Antibiotic Resistance

Abstract

Bacteria use a cell-cell communication process called quorum sensing (QS) to orchestrate collective behaviors. QS relies on the group-wide detection of extracellular signal molecules called autoinducers (AI). Quorum sensing is required for virulence and biofilm formation in the human pathogen Pseudomonas aeruginosa. In P. aeruginosa, LasR and RhlR are homologous LuxR-type soluble transcription factor receptors that bind their cognate AIs and activate the expression of genes encoding functions required for virulence and biofilm formation. While some bacterial signal transduction pathways follow a linear circuit, as phosphoryl groups are passed from one carrier protein to another ultimately resulting in up- or down-regulation of target genes, the QS system in P. aeruginosa is a dense network of receptors and regulators with interconnecting regulatory systems and outputs. Once activated, it is not understood how LasR and RhlR establish their signaling hierarchy, nor is it clear how these pathway connections are regulated, resulting in chronic infection. Here, we reviewed the mechanisms of QS progression as it relates to bacterial pathogenesis and antimicrobial resistance and tolerance.

Keywords: antibiotic resistance; quorum sensing; virulence.

Figure 1

What regulates the regulators? Six steps from global gene regulation to infection and antibiotic tolerance. (a) A selection of well-characterized global transcriptional regulators that directly regulate the expression of the las and rhl QS systems. (b) The three main branches of QS regulators all regulate their own signal production through an autofeedback mechanism that results in the upregulation of their respective AI signals, as shown in (c). There is an extensive interplay between the three QS pathways. PqsE is colored light purple, distinct from the rest of the PQS pathway, because of its role outside of the biosynthesis of PQS related to regulating RhlR function. (c) The three main AI signals that bind to their cognate receptors in (b). HHQ, the precursor to PQS synthesis is shown because it can also act as an activator of PqsR. (d) Downstream regulators may or may not be QS regulated, but they exert their influence on QS by repressing the activation of QS through transcriptional regulation or binding to the receptors to disrupt their activation. (e) The totality of the regulation shown in a-d manifests itself in the timely expression of QS-dependent public goods. For simplicity, we highlight only the genes discussed in this review. (f) A summary of the effects each of the QS-dependent public goods has during host infection. Red bars indicate transcriptional repression. Green arrows indicate transcriptional activation. Blue arrows indicate AI production and binding. Orange bars and arrows indicate protein-protein interactions that result in activation and repression, respectively. Dotted green lines indicate indirect transcriptional activation.

Figure 2

The interplay between QS and intrinsic mechanisms of antibiotic resistance. Select regulators of efflux pump and porin function as they relate to QS regulation. RhlR binds to C₄HSL (blue arrow) and activates the transcription of the synthase responsible for C₄HSL (green arrow). RhlR and C₄HSL directly regulate the expression of mexGHI-opmD (green arrow) and indirectly regulates mexAB-oprM expression via an unknown mechanism. MexGHI-OpmD plays a role in secreting important virulence factors (blue arrow). MexAB-OprM expels fluoroquinolones, β-lactams, aztreonam, and AI signaling from the cell. MexT indirectly represses (dotted red bar) the production of C₄HSL, while simultaneously upregulating the expression of mexEF-oprN. MexEF-OprN expels C₄HSL, fluoroquinolones, chloramphenicol, trimethoprim, and HHQ from the cell (blue arrows). MexT also represses oprD expression, which contains a carbapenem binding pocket (blue arrow).

Figure 3

The future of anti-infective therapeutics to treat P. aeruginosa infections. (a) Schematic representation of how disrupting the PqsE-RhlR interaction could affect RhlR-dependent transcription. An inhibitor that binds in the catalytic pocket could disrupt the PqsE-RhlR interface by allostery (middle) or via a direct perturbation of the buried surface area between PqsE and RhlR (right). (b) Flow chart detailing how combinatorial treatment with phage therapy, anti-QS therapy, and antibiotic therapy could enhance the efficacy of each individual therapy, creating a new viable option for the treatment of antibiotic-resistant bacteria.