Quorum sensing signal-response systems in Gram-negative bacteria

Abstract

Bacteria use quorum sensing to orchestrate gene expression programmes that underlie collective behaviours. Quorum sensing relies on the production, release, detection and group-level response to extracellular signalling molecules, which are called autoinducers. Recent work has discovered new autoinducers in Gram-negative bacteria, shown how these molecules are recognized by cognate receptors, revealed new regulatory components that are embedded in canonical signalling circuits and identified novel regulatory network designs. In this Review we examine how, together, these features of quorum sensing signal-response systems combine to control collective behaviours in Gram-negative bacteria and we discuss the implications for host-microbial associations and antibacterial therapy.

Figure 1. Quorum sensing synthases, autoinducers and receptors

This figure shows the structures of various autoinducers together with their corresponding synthases (blue) and receptors (transcription factors are shown as green and pink ovals and transmembrane receptors are shown as orange schematics). a | Homoserine lactone (HSL) autoinducers that are produced by different Gram-negative bacteria. b | 3-hydroxypalmitic-acid-methyl-ester (3-OH PAME) and (R)-methyl-3-hydroxymyristate ((R)-3-OH MAME) are produced and detected by Ralstonia spp. c | Diffusible signal factor (DSF) is used for quorum sensing in Xanthomonas campestris. d | The CAI-1 autoinducer synthase (CqsA) and the CqsS receptor system produces and recognizes various cholera autoinducer 1 (CAI-1) molecules. The Vibrio harveyi and Vibrio cholerae CAI-1 molecules are shown. e | 4,5-dihydroxy-2,3-pentanedione (DPD) is synthesized by all LuxS enzymes and is thus the universal precursor to the widespread family of quorum sensing autoinducers that are collectively designated as autoinducer 2 (AI-2). In the presence of boron, AI-2 forms (2S,4S)-2-methyl-2,3,3,4-tetrahydroxytetrahydrofuran-borate (S-THMF-borate), the active autoinducer in Vibrio spp. In the absence of boron, AI-2 exists as (2R,4S)-2-methyl-2,3,3,4-tetrahydroxytetrahydrofuran (R-THMF), the active autoinducer in enteric bacteria. The LuxPQ and LsrB receptor schematics shown are meant to designate that autoinducer recognition occurs in the periplasm, not the cytoplasm. LuxP and LsrB are homologues of ribose binding proteins. LuxP functions in conjunction with the two-component sensor kinase protein LuxQ and LsrB functions together with a membrane-spanning ATP binding cassette (ABC) transporter complex. f | 2-(2-hydroxyphenyl)-thiazole-4-carbaldehyde (IQS) is produced by Pseudomonas aeruginosa. The IQS receptor is currently unknown. g | The 2-heptyl-3-hydroxy-4-quinolone (PQS) system is one of several quorum sensing systems in P. aeruginosa. h | Photorhabdus asymbiotica uses dialkylresorcinols (DARs) for cell–cell communication. i | PpyS of Photorhabdus luminescens produces several photopyrones, which are sensed by the PluR transcriptional regulator. E. coli, Escherichia coli; RpaI, 4-coumaroyl-homoserine lactone synthase.

Figure 2. Structures of LuxR-type quorum sensing receptors

This figure shows the crystal structures of four LuxR-type receptors. a | TraR from Agrobacterium tumefaciens bound to autoinducer and DNA (Protein Data Bank (PDB) entry 1L3L). b | QscR from Pseudomonas aeruginosa bound to autoinducer (PDB entry 3SZT). c | CviR from Chromobacterium violaceum bound to an inhibitor called chlorolactone (PDB entry 3QP5). The arrows denote the positions of the ligands. The structures of the ligand-binding domains of all three proteins are similar; however, whereas TraR (panel a) adopts an asymmetric dimer, QscR (panel b) and CviR (panel c) form nearly symmetric cross-subunit architectures. The locations and conformations that are adopted by the DNA-binding domains differ substantially, enabling (panels a and b) or preventing (panel c) DNA binding and transcriptional activation of target genes.

Figure 3. Quorum sensing circuits in Pseudomonas aeruginosa

The four autoinducer synthases, LasI, RhlI, PqsABCDH and AmbBCDE, produce the autoinducers, 3-oxo-C12-homoserine lactone (HSL), C4-HSL, 2-heptyl-3-hydroxy-4-quinolone (PQS) and 2-(2-hydroxyphenyl)-thiazole-4-carbaldehyde (IQS), respectively. 3-oxo-C12-HSL, C4-HSL and PQS, are recognized by cytoplasmic transcription factors. The receptor for IQS is currently unknown. The production of the IQS signal is induced under phosphate starvation. The individual circuits are highly interconnected and involve autoinduction (red arrows).

Figure 4. Quorum sensing circuits in Vibrio harveyi

Left panel: Signal transduction at low cell densities. During this stage, autoinducer levels are low and the LuxN, LuxPQ and CqsS receptors act as kinases. LuxO is phosphorylated and the quorum regulatory small RNAs (Qrr sRNAs) Qrr1, Qrr2, Qrr3, Qrr4 and Qrr5 (Qrr1–5) are transcribed. The Qrr sRNAs repress luxR and activate aphA. AphA controls genes that are involved in individual behaviours and activates genes that are required for virulence and the formation of biofilms (in Vibrio cholerae). Right panel: Signal transduction at high cell densities. During this stage, autoinducer levels are high and the LuxN, LuxPQ and CqsS receptors function as phosphatases. LuxO is dephosphorylated, the Qrr1–5 sRNAs are not transcribed; therefore, AphA is not produced, whereas LuxR is produced. LuxR controls genes that are required for group behaviours, including genes that are responsible for bioluminescence (in Vibrio harveyi). AI-2, autoinducer 2; Ea-C8-CAI-1, (Z)-3-aminoundec-2-en-4-one; HAI-1, 3OH-C4-homoserine lactone.

Figure 5. Feedback loops control Vibrio harveyi quorum sensing dynamics

Six different feedback loops are embedded in the Vibrio harveyi quorum sensing circuit. a | LuxO autorepresses its own transcription. b | The quorum regulatory small RNAs (Qrr sRNAs) inhibit luxO translation by mRNA target sequestration. c | LuxR activates qrr transcription. The Qrr sRNAs, in turn, inhibit the production of LuxR by catalytic degradation of the luxR mRNA. d | LuxR represses its own transcription. e | AphA and LuxR reciprocally repress each other’s transcription. f | Base pairing of the Qrr sRNAs with the luxMN mRNA facilitates degradation of the RNA duplex (coupled degradation). The arrows denote activation. Inhibitory arrows denote repression. Grey arrows indicate post-transcriptional regulation. All of these feedback loops except the Qrr-to-luxMN loop also exist in Vibrio cholerae. In V. cholerae, LuxR is known as HapR. HAI-1, 3OH-C4-homoserine lactone; RNAP, RNA polymerase.

Figure 6. AI-2-mediated quorum sensing in the mammalian gut

Gut microorganisms communicate using autoinducer 2 (AI-2). Treatment with antibiotics can alter the composition of the microbiota, which can be ameliorated by modulating levels of AI-2. Eukaryotic cells produce cytokines, such as interleukin-8 (IL-8), in response to AI-2. Hormones (adrenaline and noradrenaline) and AI-2 mimics are produced by the host and can be detected by bacteria. Quorum sensing can alter phenotypic heterogeneity among isogenic members of a bacterial population, which affects virulence-related traits, such as biofilm formation.