Surface Sensing and Adaptation in Bacteria.

Bacteria thrive both in liquids and attached to surfaces. The concentration of bacteria on surfaces is generally much higher than in the surrounding environment, offering bacteria ample opportunity for mutualistic, symbiotic, and pathogenic interactions. To efficiently populate surfaces, they have evolved mechanisms to sense mechanical or chemical cues upon contact with solid substrata. This is of particular importance for pathogens that interact with host tissue surfaces. In this review we discuss how bacteria are able to sense surfaces and how they use this information to adapt their physiology and behavior to this new environment. We first survey mechanosensing and chemosensing mechanisms and outline how specific macromolecular structures can inform bacteria about surfaces. We then discuss how mechanical cues are converted to biochemical signals to activate specific cellular processes in a defined chronological order and describe the role of two key second messengers, c-di-GMP and cAMP, in this process.

Second messenger-mediated adjustment of bacterial swimming velocity.

Bacteria swim by means of rotating flagella that are powered by ion influx through membrane-spanning motor complexes. Escherichia coli and related species harness a chemosensory and signal transduction machinery that governs the direction of flagellar rotation and allows them to navigate in chemical gradients. Here, we show that Escherichia coli can also fine-tune its swimming speed with the help of a molecular brake (YcgR) that, upon binding of the nucleotide second messenger cyclic di-GMP, interacts with the motor protein MotA to curb flagellar motor output. Swimming velocity is controlled by the synergistic action of at least five signaling proteins that adjust the cellular concentration of cyclic di-GMP. Activation of this network and the resulting deceleration coincide with nutrient depletion and might represent an adaptation to starvation. These experiments demonstrate that bacteria can modulate flagellar motor output and thus swimming velocity in response to environmental cues.

Combining CRISPRi and metabolomics for functional annotation of compound libraries.

Molecular profiling of small molecules offers invaluable insights into the function of compounds and allows for hypothesis generation about small-molecule direct targets and secondary effects. However, current profiling methods are limited in either the number of measurable parameters or throughput. Here we developed a multiplexed, unbiased framework that, by linking genetic to drug-induced changes in nearly a thousand metabolites, allows for high-throughput functional annotation of compound libraries in Escherichia coli. First, we generated a reference map of metabolic changes from CRISPR interference (CRISPRi) with 352 genes in all major essential biological processes. Next, on the basis of the comparison of genetic changes with 1,342 drug-induced metabolic changes, we made de novo predictions of compound functionality and revealed antibacterials with unconventional modes of action (MoAs). We show that our framework, combining dynamic gene silencing with metabolomics, can be adapted as a general strategy for comprehensive high-throughput analysis of compound functionality from bacteria to human cell lines.

A Synthetic Cumate-Inducible Promoter for Graded and Homogenous Gene Expression in Pseudomonas aeruginosa.

Inducible gene expression systems are powerful genetic tools to study bacterial physiology, probing essential and toxic gene functions, gene dosage effects, and overexpression phenotypes. For the opportunistic human pathogen Pseudomonas aeruginosa, dedicated inducible gene expression systems are scarce. In the current study, we developed a minimal synthetic 4-isopropylbenzoic acid (cumate)-inducible promoter, called PQJ, that is tunable over several orders of magnitude. This was achieved by combining semirandomized housekeeping promoter libraries and control elements from the Pseudomonas putida strain F1 cym/cmt system with powerful fluorescence-activated cell sorting (FACS) to select functionally optimized variants. Using flow cytometry and live-cell fluorescence microscopy, we demonstrate that PQJ responds rapidly and homogenously to the inducer cumate in a graded manner at the single-cell level. PQJ and cumate are orthogonal to the frequently used isopropyl ß-d-thiogalactopyranoside (IPTG)-regulated lacIq-Ptac expression system. The modular design of the cumate-inducible expression cassette together with the FACS-based enrichment strategy presented here facilitates portability, thus serving as a blueprint for the development of tailored gene expression systems for a wide range of bacteria. IMPORTANCE Reverse genetics is a powerful approach to study bacterial physiology and behavior by relying on well-developed genetic tools, such as inducible promoters. For the human pathogen Pseudomonas aeruginosa, well-characterized inducible promoters are scarce. In the current work, we used a synthetic biology-based approach to develop a cumate-inducible promoter for P. aeruginosa, termed PQJ, that shows excellent induction properties at the single-cell level. This genetic tool provides the means for qualitative and quantitative gene function studies describing P. aeruginosa's physiology and virulence in vitro and in vivo. Because this synthetic approach to constructing species-specific inducible promoters is portable, it can serve as a blueprint for similar tailored gene expression systems in bacteria largely lacking such tools, including, for example, representatives of the human microbiota.

Allosteric regulation of histidine kinases by their cognate response regulator determines cell fate.

The two-component phosphorylation network is of critical importance for bacterial growth and physiology. Here, we address plasticity and interconnection of distinct signal transduction pathways within this network. In Caulobacter crescentus antagonistic activities of the PleC phosphatase and DivJ kinase localized at opposite cell poles control the phosphorylation state and subcellular localization of the cell fate determinator protein DivK. We show that DivK functions as an allosteric regulator that switches PleC from a phosphatase into an autokinase state and thereby mediates a cyclic di-GMP-dependent morphogenetic program. Through allosteric activation of the DivJ autokinase, DivK also stimulates its own phosphorylation and polar localization. These data suggest that DivK is the central effector of an integrated circuit that operates via spatially organized feedback loops to control asymmetry and cell fate determination in C. crescentus. Thus, single domain response regulators can facilitate crosstalk, feedback control, and long-range communication among members of the two-component network.

Intercepting second-messenger signaling by rationally designed peptides sequestering c-di-GMP.

The bacterial second messenger cyclic diguanylate (c-di-GMP) regulates a wide range of cellular functions from biofilm formation to growth and survival. Targeting a second-messenger network is challenging because the system involves a multitude of components with often overlapping functions. Here, we present a strategy to intercept c-di-GMP signaling pathways by directly targeting the second messenger. For this, we developed a c-di-GMP-sequestering peptide (CSP) that was derived from a CheY-like c-di-GMP effector protein. CSP binds c-di-GMP with submicromolar affinity. The elucidation of the CSP⋅c-di-GMP complex structure by NMR identified a linear c-di-GMP-binding motif, in which a self-intercalated c-di-GMP dimer is tightly bound by a network of H bonds and π-stacking interactions involving arginine and aromatic residues. Structure-based mutagenesis yielded a variant with considerably higher, low-nanomolar affinity, which subsequently was shortened to 19 residues with almost uncompromised affinity. We demonstrate that endogenously expressed CSP intercepts c-di-GMP signaling and effectively inhibits biofilm formation in Pseudomonas aeruginosa, the most widely used model for serious biofilm-associated medical implications.

Hybrid histidine kinase activation by cyclic di-GMP-mediated domain liberation.

Cytosolic hybrid histidine kinases (HHKs) constitute major signaling nodes that control various biological processes, but their input signals and how these are processed are largely unknown. In Caulobacter crescentus, the HHK ShkA is essential for accurate timing of the G1-S cell cycle transition and is regulated by the corresponding increase in the level of the second messenger c-di-GMP. Here, we use a combination of X-ray crystallography, NMR spectroscopy, functional analyses, and kinetic modeling to reveal the regulatory mechanism of ShkA. In the absence of c-di-GMP, ShkA predominantly adopts a compact domain arrangement that is catalytically inactive. C-di-GMP binds to the dedicated pseudoreceiver domain Rec1, thereby liberating the canonical Rec2 domain from its central position where it obstructs the large-scale motions required for catalysis. Thus, c-di-GMP cannot only stabilize domain interactions, but also engage in domain dissociation to allosterically invoke a downstream effect. Enzyme kinetics data are consistent with conformational selection of the ensemble of active domain constellations by the ligand and show that autophosphorylation is a reversible process.

Photoaffinity Capture Compounds to Profile the Magic Spot Nucleotide Interactomes.

Magic Spot Nucleotides (MSN) regulate the stringent response, a highly conserved bacterial stress adaptation mechanism, enabling survival under adverse external challenges. In times of antibiotic crisis, a detailed understanding of stringent response is essential, as potentially new targets for pharmacological intervention could be identified. In this study, we delineate the MSN interactome in Escherichia coli and Salmonella typhimurium applying a family of trifunctional photoaffinity capture compounds. We introduce MSN probes covering a diverse phosphorylation pattern, such as pppGpp, ppGpp, and pGpp. Our chemical proteomics approach provides datasets of putative MSN receptors both from cytosolic and membrane fractions that unveil new MSN targets. We find that the activity of the non-Nudix hydrolase ApaH is potently inhibited by pppGpp, which itself is converted to pGpp by ApaH. The capture compounds described herein will be useful to identify MSN interactomes across bacterial species.

In situ structure of the Caulobacter crescentus flagellar motor and visualization of binding of a CheY-homolog.

Bacterial flagellar motility is controlled by the binding of CheY proteins to the cytoplasmic switch complex of the flagellar motor, resulting in changes in swimming speed or direction. Despite its importance for motor function, structural information about the interaction between effector proteins and the motor are scarce. To address this gap in knowledge, we used electron cryotomography and subtomogram averaging to visualize such interactions inside Caulobacter crescentus cells. In C. crescentus, several CheY homologs regulate motor function for different aspects of the bacterial lifestyle. We used subtomogram averaging to image binding of the CheY family protein CleD to the cytoplasmic Cring switch complex, the control center of the flagellar motor. This unambiguously confirmed the orientation of the motor switch protein FliM and the binding of a member of the CheY protein family to the outside rim of the C ring. We also uncovered previously unknown structural elaborations of the alphaproteobacterial flagellar motor, including two novel periplasmic ring structures, and the stator ring harboring eleven stator units, adding to our growing catalog of bacterial flagellar diversity.

De- and repolarization mechanism of flagellar morphogenesis during a bacterial cell cycle.

Eukaryotic morphogenesis is seeded with the establishment and subsequent amplification of polarity cues at key times during the cell cycle, often using (cyclic) nucleotide signals. We discovered that flagellum de- and repolarization in the model prokaryote Caulobacter crescentus is precisely orchestrated through at least three spatiotemporal mechanisms integrated at TipF. We show that TipF is a cell cycle-regulated receptor for the second messenger--bis-(3'-5')-cyclic dimeric guanosine monophosphate (c-di-GMP)--that perceives and transduces this signal through the degenerate c-di-GMP phosphodiesterase (EAL) domain to nucleate polar flagellum biogenesis. Once c-di-GMP levels rise at the G1 → S transition, TipF is activated, stabilized, and polarized, enabling the recruitment of downstream effectors, including flagellar switch proteins and the PflI positioning factor, at a preselected pole harboring the TipN landmark. These c-di-GMP-dependent events are coordinated with the onset of tipF transcription in early S phase and together enable the correct establishment and robust amplification of TipF-dependent polarization early in the cell cycle. Importantly, these mechanisms also govern the timely removal of TipF at cell division coincident with the drop in c-di-GMP levels, thereby resetting the flagellar polarization state in the next cell cycle after a preprogrammed period during which motility must be suspended.

Regulation of Bacterial Cell Cycle Progression by Redundant Phosphatases.

In the model organism Caulobacter crescentus, a network of two-component systems involving the response regulators CtrA, DivK, and PleD coordinates cell cycle progression with differentiation. Active phosphorylated CtrA prevents chromosome replication in G1 cells while simultaneously regulating expression of genes required for morphogenesis and development. At the G1-S transition, phosphorylated DivK (DivK∼P) and PleD (PleD∼P) accumulate to indirectly inactivate CtrA, which triggers DNA replication initiation and concomitant cellular differentiation. The phosphatase PleC plays a pivotal role in this developmental program by keeping DivK and PleD phosphorylation levels low during G1, thereby preventing premature CtrA inactivation. Here, we describe CckN as a second phosphatase akin to PleC that dephosphorylates DivK∼P and PleD∼P in G1 cells. However, in contrast to PleC, no kinase activity was detected with CckN. The effects of CckN inactivation are largely masked by PleC but become evident when PleC and DivJ, the major kinase for DivK and PleD, are absent. Accordingly, mild overexpression of cckN restores most phenotypic defects of a pleC null mutant. We also show that CckN and PleC are proteolytically degraded in a ClpXP-dependent way before the onset of the S phase. Surprisingly, known ClpX adaptors are dispensable for PleC and CckN proteolysis, raising the possibility that as yet unidentified proteolytic adaptors are required for the degradation of both phosphatases. Since cckN expression is induced in stationary phase, depending on the stress alarmone (p)ppGpp, we propose that CckN acts as an auxiliary factor responding to environmental stimuli to modulate CtrA activity under suboptimal conditions.IMPORTANCE Two-component signal transduction systems are widely used by bacteria to adequately respond to environmental changes by adjusting cellular parameters, including the cell cycle. In Caulobacter crescentus, PleC acts as a phosphatase that indirectly protects the response regulator CtrA from premature inactivation during the G1 phase of the cell cycle. Here, we provide genetic and biochemical evidence that PleC is seconded by another phosphatase, CckN. The activity of PleC and CckN phosphatases is restricted to the G1 phase since both proteins are degraded by ClpXP protease before the G1-S transition. Degradation is independent of any known proteolytic adaptors and relies, in the case of CckN, on an unsuspected N-terminal degron. Our work illustrates a typical example of redundant functions between two-component proteins.

Regulatory cohesion of cell cycle and cell differentiation through interlinked phosphorylation and second messenger networks.

In Caulobacter crescentus, phosphorylation of key regulators is coordinated with the second messenger cyclic di-GMP to drive cell-cycle progression and differentiation. The diguanylate cyclase PleD directs pole morphogenesis, while the c-di-GMP effector PopA initiates degradation of the replication inhibitor CtrA by the AAA+ protease ClpXP to license S phase entry. Here, we establish a direct link between PleD and PopA reliant on the phosphodiesterase PdeA and the diguanylate cyclase DgcB. PdeA antagonizes DgcB activity until the G1-S transition, when PdeA is degraded by the ClpXP protease. The unopposed DgcB activity, together with PleD activation, upshifts c-di-GMP to drive PopA-dependent CtrA degradation and S phase entry. PdeA degradation requires CpdR, a response regulator that delivers PdeA to the ClpXP protease in a phosphorylation-dependent manner. Thus, CpdR serves as a crucial link between phosphorylation pathways and c-di-GMP metabolism to mediate protein degradation events that irreversibly and coordinately drive bacterial cell-cycle progression and development.

The Diguanylate Cyclase HsbD Intersects with the HptB Regulatory Cascade to Control Pseudomonas aeruginosa Biofilm and Motility.

The molecular basis of second messenger signaling relies on an array of proteins that synthesize, degrade or bind the molecule to produce coherent functional outputs. Cyclic di-GMP (c-di-GMP) has emerged as a eubacterial nucleotide second messenger regulating a plethora of key behaviors, like the transition from planktonic cells to biofilm communities. The striking multiplicity of c-di-GMP control modules and regulated cellular functions raised the question of signaling specificity. Are c-di-GMP signaling routes exclusively dependent on a central hub or can they be locally administrated? In this study, we show an example of how c-di-GMP signaling gains output specificity in Pseudomonas aeruginosa. We observed the occurrence in P. aeruginosa of a c-di-GMP synthase gene, hsbD, in the proximity of the hptB and flagellar genes cluster. We show that the HptB pathway controls biofilm formation and motility by involving both HsbD and the anti-anti-sigma factor HsbA. The rewiring of c-di-GMP signaling into the HptB cascade relies on the original interaction between HsbD and HsbA and on the control of HsbD dynamic localization at the cell poles.

Correction: The Diguanylate Cyclase HsbD Intersects with the HptB Regulatory Cascade to Control Pseudomonas aeruginosa Biofilm and Motility.

[This corrects the article DOI: 10.1371/journal.pgen.1006354.].

Quorum-Quenching Human Designer Cells for Closed-Loop Control of Pseudomonas aeruginosa Biofilms.

Current antibiotics gradually lose their efficacy against chronic Pseudomonas aeruginosa infections due to development of increased resistance mediated by biofilm formation, as well as the large arsenal of microbial virulence factors that are coordinated by the cell density-dependent phenomenon of quorum sensing. Here, we address this issue by using synthetic biology principles to rationally engineer quorum-quencher cells with closed-loop control to autonomously dampen virulence and interfere with biofilm integrity. Pathogen-derived signals dynamically activate a synthetic mammalian autoinducer sensor driving downstream expression of next-generation anti-infectives. Engineered cells were able to sensitively score autoinducer levels from P. aeruginosa clinical isolates and mount a 2-fold defense consisting of an autoinducer-inactivating enzyme to silence bacterial quorum sensing and a bipartite antibiofilm effector to dissolve the biofilm matrix. The self-guided cellular device fully cleared autoinducers, potentiated bacterial antibiotic susceptibility, substantially reduced biofilms, and alleviated cytotoxicity to lung epithelial cells. We believe this strategy of dividing otherwise coordinated pathogens and breaking up their shielded stronghold represents a blueprint for cellular anti-infectives in the postantibiotic era.

Functionalized Proline-Rich Peptides Bind the Bacterial Second Messenger c-di-GMP.

c-di-GMP is an attractive target in the fight against bacterial infections since it is a near ubiquitous second messenger that regulates important cellular processes of pathogens, including biofilm formation and virulence. Screening of a combinatorial peptide library enabled the identification of the proline-rich tetrapeptide Gup-Gup-Nap-Arg, which binds c-di-GMP selectively over other nucleotides in water. Computational and CD spectroscopic studies provided a possible binding mode of the complex and enabled the design of a pentapeptide with even higher binding strength towards c-di-GMP. Biological studies showed that the tetrapeptide inhibits biofilm growth by the opportunistic pathogen P. aeruginosa.

Allosteric activation of exopolysaccharide synthesis through cyclic di-GMP-stimulated protein-protein interaction.

In many bacterial pathogens, the second messenger c-di-GMP stimulates the production of an exopolysaccharide (EPS) matrix to shield bacteria from assaults of the immune system. How c-di-GMP induces EPS biogenesis is largely unknown. Here, we show that c-di-GMP allosterically activates the synthesis of poly-ß-1,6-N-acetylglucosamine (poly-GlcNAc), a major extracellular matrix component of Escherichia coli biofilms. C-di-GMP binds directly to both PgaC and PgaD, the two inner membrane components of the poly-GlcNAc synthesis machinery to stimulate their glycosyltransferase activity. We demonstrate that the PgaCD machinery is a novel type c-di-GMP receptor, where ligand binding to two proteins stabilizes their interaction and promotes enzyme activity. This is the first example of a c-di-GMP-mediated process that relies on protein-protein interaction. At low c-di-GMP concentrations, PgaD fails to interact with PgaC and is rapidly degraded. Thus, when cells experience a c-di-GMP trough, PgaD turnover facilitates the irreversible inactivation of the Pga machinery, thereby temporarily uncoupling it from c-di-GMP signalling. These data uncover a mechanism of c-di-GMP-mediated EPS control and provide a frame for c-di-GMP signalling specificity in pathogenic bacteria.

High intracellular c-di-GMP levels antagonize quorum sensing and virulence gene expression in Burkholderia cenocepacia H111.

The opportunistic human pathogen Burkholderia cenocepacia H111 uses two chemically distinct signal molecules for controlling gene expression in a cell density-dependent manner: N-acyl-homoserine lactones (AHLs) and cis-2-dodecenoic acid (BDSF). Binding of BDSF to its cognate receptor RpfR lowers the intracellular c-di-GMP level, which in turn leads to differential expression of target genes. In this study we analysed the transcriptional profile of B. cenocepacia H111 upon artificially altering the cellular c-di-GMP level. One hundred and eleven genes were shown to be differentially expressed, 96 of which were downregulated at a high c-di-GMP concentration. Our analysis revealed that the BDSF, AHL and c-di-GMP regulons overlap for the regulation of 24 genes and that a high c-di-GMP level suppresses expression of AHL-regulated genes. Phenotypic analyses confirmed changes in the expression of virulence factors, the production of AHL signal molecules and the biosynthesis of different biofilm matrix components upon altered c-di-GMP levels. We also demonstrate that the intracellular c-di-GMP level determines the virulence of B. cenocepacia to Caenorhabditis elegans and Galleria mellonella.