Chemotaxis of Escherichia coli to norepinephrine (NE) requires conversion of NE to 3,4-dihydroxymandelic acid

Microbiota-gut-brain axis in health and neurological disease: Interactions between gut microbiota and the nervous system

Along with mounting evidence that gut microbiota and their metabolites migrate endogenously to distal organs, the 'gut-lung axis,' 'gut-brain axis,' 'gut-liver axis' and 'gut-renal axis' have been established. Multiple animal recent studies have demonstrated gut microbiota may also be a key susceptibility factor for neurological disorders such as Alzheimer's disease, Parkinson's disease and autism. The gastrointestinal tract is innervated by the extrinsic sympathetic and vagal nerves and the intrinsic enteric nervous system, and the gut microbiota interacts with the nervous system to maintain homeostatic balance in the host gut. A total of 1507 publications on the interactions between the gut microbiota, the gut-brain axis and neurological disorders are retrieved from the Web of Science to investigate the interactions between the gut microbiota and the nervous system and the underlying mechanisms involved in normal and disease states. We provide a comprehensive overview of the effects of the gut microbiota and its metabolites on nervous system function and neurotransmitter secretion, as well as alterations in the gut microbiota in neurological disorders, to provide a basis for the possibility of targeting the gut microbiota as a therapeutic agent for neurological disorders.

Deciphering Bacterial Chemorepulsion: The Complex Response of Microbes to Environmental Stimuli

Bacterial motility relying on flagella is characterized by several modes, including swimming, swarming, twitching, and gliding. This motility allows bacteria to adapt remarkably well to hostile environments. More than 50% of bacteria naturally contain flagella, which are crucial for bacterial chemotaxis motility. Chemotaxis can be either positive, where bacteria move towards a chemical source, or negative, known as chemorepulsion, where bacteria move away from the source. Although much is known about the mechanisms driving chemotaxis towards attractants, the molecular mechanisms underlying chemorepulsion remain elusive. Chemotaxis plays an important role in the colonization of the rhizosphere by rhizobacteria. Recently, researchers have systematically studied the identification and recognition mechanisms of chemoattractants. However, the mechanisms underlying chemorepellents remain unclear. Systematically sorting and analyzing research on chemorepellents could significantly enhance our understanding of how these compounds help probiotics evade harmful environments or drive away pathogens.

Bacterial vampirism mediated through taxis to serum

Bacteria of the family Enterobacteriaceae are associated with gastrointestinal (GI) bleeding and bacteremia and are a leading cause of death, from sepsis, for individuals with inflammatory bowel diseases. The bacterial behaviors and mechanisms underlying why these bacteria are prone to bloodstream entry remain poorly understood. Herein, we report that clinical isolates of non-typhoidal Salmonella enterica serovars, Escherichia coli, and Citrobacter koseri are rapidly attracted toward sources of human serum. To simulate GI bleeding, we utilized an injection-based microfluidics device and found that femtoliter volumes of human serum are sufficient to induce bacterial attraction to the serum source. This response is orchestrated through chemotaxis and the chemoattractant L-serine, an amino acid abundant in serum that is recognized through direct binding by the chemoreceptor Tsr. We report the first crystal structures of Salmonella Typhimurium Tsr in complex with L-serine and identify a conserved amino acid recognition motif for L-serine shared among Tsr orthologues. We find Tsr to be widely conserved among Enterobacteriaceae and numerous World Health Organization priority pathogens associated with bloodstream infections. Lastly, we find that Enterobacteriaceae use human serum as a source of nutrients for growth and that chemotaxis and the chemoreceptor Tsr provide a competitive advantage for migration into enterohemorrhagic lesions. We define this bacterial behavior of taxis toward serum, colonization of hemorrhagic lesions, and the consumption of serum nutrients as 'bacterial vampirism', which may relate to the proclivity of Enterobacteriaceae for bloodstream infections.

Bacterial vampirism mediated through taxis to serum

Bacteria of the family Enterobacteriaceae are associated with gastrointestinal (GI) bleeding and bacteremia and are a leading cause of death, from sepsis, for individuals with inflammatory bowel diseases. The bacterial behaviors and mechanisms underlying why these bacteria are prone to bloodstream entry remains poorly understood. Herein, we report that clinical isolates of non-typhoidal Salmonella enterica serovars, Escherichia coli, and Citrobacter koseri are rapidly attracted toward sources of human serum. To simulate GI bleeding, we utilized a custom injection-based microfluidics device and found that femtoliter volumes of human serum are sufficient to induce the bacterial population to swim toward and aggregate at the serum source. This response is orchestrated through chemotaxis, and a major chemical cue driving chemoattraction is L-serine, an amino acid abundant in serum that is recognized through direct binding by the chemoreceptor Tsr. We report the first crystal structures of Salmonella Typhimurium Tsr in complex with L-serine and identify a conserved amino acid recognition motif for L-serine shared among Tsr orthologues. By mapping the phylogenetic distribution of this chemoreceptor we found Tsr to be widely conserved among Enterobacteriaceae and numerous World Health Organization priority pathogens associated with bloodstream infections. Lastly, we find that Enterobacteriaceae use human serum as a source of nutrients for growth and that chemotaxis and the chemoreceptor Tsr provides a competitive advantage for migration into enterohaemorrhagic lesions. We term this bacterial behavior of taxis toward serum, colonization of hemorrhagic lesions, and the consumption of serum nutrients, as 'bacterial vampirism' which may relate to the proclivity of Enterobacteriaceae for bloodstream infections.

Sensing the environment by bacterial plant pathogens: What do their numerous chemoreceptors recognize?

Bacteria have evolved multiple sensing strategies to efficiently adapt to their natural hosts and environments. In the context of plant pathology, chemotaxis allows phytopathogenic bacteria to direct their movement towards hosts through the detection of a landscape of plant-derived molecules, facilitating the initiation of the infective process. The importance of chemotaxis for the lifestyle of phytopathogens is also reflected in the fact that they have, on average, twice as many chemoreceptors as bacteria that do not interact with plants. Paradoxically, the knowledge about the function of plant pathogen chemoreceptors is scarce. Notably, many of these receptors seem to be specific to plant-interacting bacteria, suggesting that they may recognize plant-specific compounds. Here, we highlight the need to advance our knowledge of phytopathogen chemoreceptor function, which may serve as a base for the development of anti-infective therapies for the control of phytopathogens.

Three unrelated chemoreceptors provide Pectobacterium atrosepticum with a broad-spectrum amino acid sensing capability

Amino acids are important nutrients and also serve as signals for diverse signal transduction pathways. Bacteria use chemoreceptors to recognize amino acid attractants and to navigate their gradients. In Escherichia coli two likely paralogous chemoreceptors Tsr and Tar detect 9 amino acids, whereas in Pseudomonas aeruginosa the paralogous chemoreceptors PctA, PctB and PctC detect 18 amino acids. Here, we show that the phytobacterium Pectobacterium atrosepticum uses the three non-homologous chemoreceptors PacA, PacB and PacC to detect 19 proteinogenic and several non-proteinogenic amino acids. PacB recognizes 18 proteinogenic amino acids as well as 8 non-proteinogenic amino acids. PacB has a ligand preference for the three branched chain amino acids L-leucine, L-valine and L-isoleucine. PacA detects L-proline next to several quaternary amines. The third chemoreceptor, PacC, is an ortholog of E. coli Tsr and the only one of the 36 P. atrosepticum chemoreceptors that is encoded in the cluster of chemosensory pathway genes. Surprisingly, in contrast to Tsr, which primarily senses serine, PacC recognizes aspartate as the major chemoeffector but not serine. Our results demonstrate that bacteria use various strategies to sense a wide range of amino acids and that it takes more than one chemoreceptor to achieve this goal.

Bacterial envelope stress responses: Essential adaptors and attractive targets

Millions of deaths a year across the globe are linked to antimicrobial resistant infections. The need to develop new treatments and repurpose of existing antibiotics grows more pressing as the growing antimicrobial resistance pandemic advances. In this review article, we propose that envelope stress responses, the signaling pathways bacteria use to recognize and adapt to damage to the most vulnerable outer compartments of the microbial cell, are attractive targets. Envelope stress responses (ESRs) support colonization and infection by responding to a plethora of toxic envelope stresses encountered throughout the body; they have been co-opted into virulence networks where they work like global positioning systems to coordinate adhesion, invasion, microbial warfare, and biofilm formation. We highlight progress in the development of therapeutic strategies that target ESR signaling proteins and adaptive networks and posit that further characterization of the molecular mechanisms governing these essential niche adaptation machineries will be important for sparking new therapeutic approaches aimed at short-circuiting bacterial adaptation.

Discovery of a New Chemoeffector for Escherichia coli Chemoreceptor Tsr and Identification of a Molecular Mechanism of Repellent Sensing

Motile bacteria use chemotaxis to search for nutrients and escape from harmful chemicals. While the sensing mechanisms for chemical attractants are well established, the molecular details of chemorepellent detection are poorly understood. Here, by using combined computational and experimental approaches to screen potential chemoeffectors for the Escherichia coli chemoreceptor Tsr, we identified a specific chemorepellent, 1-aminocyclohexanecarboxylic acid (ACHC). Our study strongly suggests that ACHC directly binds to the periplasmic sensory domain of Tsr and competes with l-serine, the amino acid attractant of Tsr. We further characterized the binding features of l-serine, ACHC, and l-leucine (a natural repellent that binds Tsr) and found that Asn68 plays a key role in mediating chemotactic response. Mutating Asn68 to Ala inverted the response to l-leucine from a repellent to an attractant. Our study provides important insights into the molecular mechanisms of ligand sensing via bacterial chemoreceptors.

A bacterial chemoreceptor that mediates chemotaxis to two different plant hormones

Indole-3-acetic acid (IAA) is the main naturally occurring auxin and is produced by organisms of all kingdoms of life. In addition to the regulation of plant growth and development, IAA plays an important role in the interaction between plants and growth-promoting and phytopathogenic bacteria by regulating bacterial gene expression and physiology. We show here that an IAA metabolizing plant-associated Pseudomonas putida isolate exhibits chemotaxis to IAA that is independent of auxin metabolism. We found that IAA chemotaxis is based on the activity of the PcpI chemoreceptor and heterologous expression of pcpI conferred IAA taxis to different environmental and human pathogenic isolates of the Pseudomonas genus. Using ligand screening, microcalorimetry and quantitative chemotaxis assays, we found that PcpI failed to bind IAA directly, but recognized and mediated chemoattractions to various aromatic compounds, including the phytohormone salicylic acid. The expression of pcpI and its role in the interactions with plants was also investigated. PcpI extends the range of central signal molecules recognized by chemoreceptors. To our knowledge, this is the first report on a bacterial receptor that responds to two different phytohormones. Our study reinforces the multifunctional role of IAA and salicylic acid as intra- and inter-kingdom signal molecules.

Eukaryotic catecholamine hormones influence the chemotactic control of Vibrio campbellii by binding to the coupling protein CheW

Host-emitted stress hormones significantly influence the growth and behavior of various bacterial species; however, their cellular targets have so far remained elusive. Here, we used customized probes and quantitative proteomics to identify the target of epinephrine and the α-adrenoceptor agonist phenylephrine in live cells of the aquatic pathogen Vibrio campbellii. Consequently, we have discovered the coupling protein CheW, which is in the center of the chemotaxis signaling network, as a target of both molecules. We not only demonstrate direct ligand binding to CheW but also elucidate how this affects chemotactic control. These findings are pivotal for further research on hormone-specific effects on bacterial behavior.

In addition to their well-known role as stress-associated catecholamine hormones in animals and humans, epinephrine (EPI) and norepinephrine (NE) act as interkingdom signals between eukaryotic hosts and bacteria. However, the molecular basis of their effects on bacteria is not well understood. In initial phenotypic studies utilizing Vibrio campbellii as a model organism, we characterized the bipartite mode of action of catecholamines, which consists of promotion of growth under iron limitation and enhanced colony expansion on soft agar. In order to identify the molecular targets of the hormones, we designed and synthesized tailored probes for chemical proteomic studies. As the catechol group in EPI and NE acts as an iron chelator and is prone to form a reactive quinone moiety, we devised a photoprobe based on the adrenergic agonist phenylephrine (PE), which solely influenced colony expansion. Using this probe, we identified CheW, located at the core of the chemotaxis signaling network, as a major target. In vitro studies confirmed that EPI, NE, PE, and labetalol, a clinically applied antagonist, bind to purified CheW with affinity constants in the submicromolar range. In line with these findings, exposure of V. campbellii to these adrenergic agonists affects the chemotactic control of the bacterium. This study highlights an effect of eukaryotic signaling molecules on bacterial motility.

Chemotaxis of the Human Pathogen Pseudomonas aeruginosa to the Neurotransmitter Acetylcholine

Acetylcholine is a central biological signal molecule present in all kingdoms of life. In humans, acetylcholine is the primary neurotransmitter of the peripheral nervous system; it mediates signal transmission at neuromuscular junctions. Here, we show that the opportunistic human pathogen Pseudomonas aeruginosa exhibits chemoattraction toward acetylcholine over a concentration range of 1 μM to 100 mM. The maximal magnitude of the response was superior to that of many other P. aeruginosa chemoeffectors. We demonstrate that this chemoattraction is mediated by the PctD (PA4633) chemoreceptor. Using microcalorimetry, we show that the PctD ligand-binding domain (LBD) binds acetylcholine with a equilibrium dissociation constant (K_D) of 23 μM. It also binds choline and with lower affinity betaine. Highly sensitive responses to acetylcholine and choline, and less sensitive responses to betaine and l-carnitine, were observed in Escherichia coli expressing a chimeric receptor comprising the PctD-LBD fused to the Tar chemoreceptor signaling domain. We also identified the PacA (ECA_RS10935) chemoreceptor of the phytopathogen Pectobacterium atrosepticum, which binds choline and betaine but fails to recognize acetylcholine. To identify the molecular determinants for acetylcholine recognition, we report high-resolution structures of PctD-LBD (with bound acetylcholine and choline) and PacA-LBD (with bound betaine). We identified an amino acid motif in PctD-LBD that interacts with the acetylcholine tail. This motif is absent in PacA-LBD. Significant acetylcholine chemotaxis was also detected in the plant pathogens Agrobacterium tumefaciens and Dickeya solani. To the best of our knowledge, this is the first report of acetylcholine chemotaxis and extends the range of host signals perceived by bacterial chemoreceptors.

Identification of chemoreceptor proteins for amino acids involved in host plant infection in Pseudomonas syringae pv. tabaci 6605

Chemotaxis is crucial for Pseudomonas syringae pv. tabaci (Pta) 6605 to evoke disease in tobacco plants. Pta6605 harbors more than fifty genes for methyl-accepting chemotaxis proteins (mcp), but almost all are functionally uncharacterized. Previously we identified a dCache_1 type MCP in Pta6605 that mediates chemotaxis to γ-aminobutyric acid, called McpG. In this study, we characterized four more dCache_1 type MCPs, three of which, PscA, PscB, and PscC2, are responsible for sensing amino acids. Using a capillary chemotaxis assay, we observed that PscA, PscB, and PscC2 mutant strains had reduced chemotaxis to most amino acids, indicating that PscA and PscB mediate chemotaxis to 14 amino acids, while PscC2 has a slightly narrower ligand recognition, mediating chemotaxis to 12 amino acids. Other cellular functions were also affected in ΔpscB and ΔpscC2: swarming motility was reduced, and biofilm formation was increased. Furthermore, ΔpscB and ΔpscC2 but not ΔpscA had reduced virulence in the host tobacco plant. On the other hand, ΔpscC1 was defective in motility and did not even respond to yeast extract and was unable to cause disease. These findings supported the idea that the chemosensory pathway correlated with virulence-related phenotypes. Amino acids are abundant in tobacco apoplast; having multiple MCPs appears to support the invasion of Pta6605 into the plant.

Multiple functions of flagellar motility and chemotaxis in bacterial physiology

Most swimming bacteria are capable of following gradients of nutrients, signaling molecules and other environmental factors that affect bacterial physiology. This tactic behavior became one of the most-studied model systems for signal transduction and quantitative biology, and underlying molecular mechanisms are well characterized in Escherichia coli and several other model bacteria. In this review, we focus primarily on less understood aspect of bacterial chemotaxis, namely its physiological relevance for individual bacterial cells and for bacterial populations. As evident from multiple recent studies, even for the same bacterial species flagellar motility and chemotaxis might serve multiple roles, depending on the physiological and environmental conditions. Among these, finding sources of nutrients and more generally locating niches that are optimal for growth appear to be one of the major functions of bacterial chemotaxis, which could explain many chemoeffector preferences as well as flagellar gene regulation. Chemotaxis might also generally enhance efficiency of environmental colonization by motile bacteria, which involves intricate interplay between individual and collective behaviors and trade-offs between growth and motility. Finally, motility and chemotaxis play multiple roles in collective behaviors of bacteria including swarming, biofilm formation and autoaggregation, as well as in their interactions with animal and plant hosts.

Epinephrine affects gene expression levels and has a complex effect on biofilm formation in M icrococcus luteus strain C01 isolated from human skin

In this study, the effect of epinephrine on the biofilm formation of Micrococcus luteus C01 isolated from human skin was investigated in depth for the first time. This hormone has a complex effect on biofilms in various systems. In a system with polytetrafluoroethylene (PTFE) cubes, treatment with epinephrine at a physiological concentration of 4.9 × 10^-9 M increased the total amount of 72-h biofilm biomass stained with crystal violet and increased the metabolic activity of biofilms, but at higher and lower concentrations, the treatment had no significant effect. On glass fiber filters, treatment with the hormone decreased the number of colony forming units (CFUs) and changed the aggregation but did not affect the metabolic activity of biofilm cells. In glass bottom plates examined by confocal microscopy, epinephrine notably inhibited the growth of biofilms. RNA-seq analysis and RT-PCR demonstrated reproducible upregulation of genes encoding Fe-S cluster assembly factors and cyanide detoxification sulfurtransferase, whereas genes encoding the co-chaperone GroES, the LysE superfamily of lysine exporters, short-chain alcohol dehydrogenase and the potential c-di-GMP phosphotransferase were downregulated. Our results suggest that epinephrine may stimulate matrix synthesis in M. luteus biofilms, thereby increasing the activity of NAD(H) oxidoreductases. Potential c-di-GMP pathway proteins are essential in these processes.

Mechanosensitive recruitment of stator units promotes binding of the response regulator CheY-P to the flagellar motor

Reversible switching of the bacterial flagellar motor between clockwise (CW) and counterclockwise (CCW) rotation is necessary for chemotaxis, which enables cells to swim towards favorable chemical habitats. Increase in the viscous resistance to the rotation of the motor (mechanical load) inhibits switching. However, cells must maintain homeostasis in switching to navigate within environments of different viscosities. The mechanism by which the cell maintains optimal chemotactic function under varying loads is not understood. Here, we show that the flagellar motor allosterically controls the binding affinity of the chemotaxis response regulator, CheY-P, to the flagellar switch complex by modulating the mechanical forces acting on the rotor. Mechanosensitive CheY-P binding compensates for the load-induced loss of switching by precisely adapting the switch response to a mechanical stimulus. The interplay between mechanical forces and CheY-P binding tunes the chemotactic function to match the load. This adaptive response of the chemotaxis output to mechanical stimuli resembles the proprioceptive feedback in the neuromuscular systems of insects and vertebrates.

A catalogue of signal molecules that interact with sensor kinases, chemoreceptors and transcriptional regulators

Bacteria have evolved many different signal transduction systems that sense signals and generate a variety of responses. Generally, most abundant are transcriptional regulators, sensor histidine kinases and chemoreceptors. Typically, these systems recognize their signal molecules with dedicated ligand-binding domains (LBDs), which, in turn, generate a molecular stimulus that modulates the activity of the output module. There are an enormous number of different LBDs that recognize a similarly diverse set of signals. To give a global perspective of the signals that interact with transcriptional regulators, sensor kinases and chemoreceptors, we manually retrieved information on the protein-ligand interaction from about 1,200 publications and 3D structures. The resulting 811 proteins were classified according to the Pfam family into 127 groups. These data permit a delineation of the signal profiles of individual LBD families as well as distinguishing between families that recognize signals in a promiscuous manner and those that possess a well-defined ligand range. A major bottleneck in the field is the fact that the signal input of many signaling systems is unknown. The signal repertoire reported here will help the scientific community design experimental strategies to identify the signaling molecules for uncharacterised sensor proteins.

Distribution of fluoroquinolones in the two aqueous compartments of Escherichia coli

The double-layered cell envelope of Gram-negative bacteria and active drug efflux present a formidable barrier for antimicrobial compounds to penetrate. Fluoroquinolones are among the few classes of antimicrobials that are clinically useful in the treatment of Gram-negative bacterial infection. Previous studies on fluoroquinolone accumulation measured total bacteria associated compounds, rather than the cytoplasmic accumulation. Fluoroquinolones target the type II topoisomerases in the cytoplasm. Thus, the cytoplasmic accumulation is expected to be more relevant to the potency of the drugs. Here, we fractionated and measured the concentration of nine fluoroquinolone compounds in the periplasm and the cytoplasm of two strains of E. coli cells, a parent strain and its isogenic efflux-deficient tolC knockout strain. The potency of the drugs was determined using the minimum inhibitory concentration (MIC) assay. We found that all fluoroquinolones tested accumulated at much higher concentrations in the periplasm than in the cytoplasm. The periplasmic concentrations were 2–15 folds higher than the cytoplasmic concentration, while the actual distribution ratio varies drastically among the compounds tested. Good correlation between the MIC and the cytoplasmic accumulation, but not whole cell accumulation, was observed using a pair of isogenic wild type and drug-efflux deficient strains.

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Fluoroquinolones accumulate to high concentration in the periplasm of E. coli

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Periplasmic concentrations are higher than the cytoplasmic and exterior media concentration.

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Good correlation was observed for the cytoplasmic drug concentration and the antimicrobial potency.

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Disruption of active efflux has a larger impact on the periplasmic than the cytoplasmic accumulation.

Oral Treatments With Probiotics and Live Salmonella Vaccine Induce Unique Changes in Gut Neurochemicals and Microbiome in Chickens

Cross-talk between the gut microbiota and neurochemicals affects health and well-being of animals. However, little is known about this interaction in chickens despite their importance in food production. Probiotics and live Salmonella vaccines are microbial products commonly given orally to layer pullets to improve health and ensure food safety. This study's objective was to determine how these oral treatments, individually or in combination, would impact the gut environment of chickens. White Leghorn chicks were either non-treated (CON) or orally given probiotics (PRO), a recombinant attenuated Salmonella vaccine (RASV; VAX), or both (P+V). Birds were fed with probiotics daily beginning at 1-day-old and orally immunized with RASV at 4-days-old and boosted 2 weeks post-primary vaccination. At 5 weeks, ceca content, ceca tissues, and small intestinal scrapings (SISs) were collected from ten birds/group post-euthanasia for analyses. Catecholamine, but not serotonergic, metabolism was affected by treatments. Dopamine metabolism, indicated by L-DOPA and DOPAC levels, were increased in P+V birds versus CON and PRO birds. Based on 16S sequencing, beta diversity was more similar among vaccinated birds versus birds given probiotics, suggesting live Salmonella vaccination has a major selective pressure on microbial diversity. Abundances of Akkermansia muciniphila and Enterobacteriaceae positively correlated with levels of tyrosine and norepinephrine, respectively. Both enumeration and 16S sequencing, determined that PRO exhibited the greatest levels of Enterobacteriaceae in the ceca and feces, which was associated with greater IgA production against E. coli virulence factors as tested by ELISA. In summary, we demonstrate that using probiotics alone versus in combination with a live vaccine has major implications in catecholamine production and the microbiota of layer pullets. Additionally, unique correlations between changes in some neurochemicals and specific bacteria have been shown.

Molecular Mechanism for Attractant Signaling to DHMA by E. coli Tsr

The attractant chemotaxis response of Escherichia coli to norepinephrine requires that it be converted to 3,4-dihydroxymandelic acid (DHMA) by the monoamine oxidase TynA and the aromatic aldehyde dehydrogenase FeaB. DHMA is sensed by the serine chemoreceptor Tsr, and the attractant response requires that at least one subunit of the periplasmic domain of the Tsr homodimer (pTsr) has an intact serine-binding site. DHMA that is generated in vivo by E. coli is expected to be a racemic mixture of the (R) and (S) enantiomers, so it has been unclear whether one or both chiral forms are active. Here, we used a combination of state-of-the-art tools in molecular docking and simulations, including an in-house simulation-based docking protocol, to investigate the binding properties of (R)-DHMA and (S)-DHMA to E. coli pTsr. Our studies computationally predicted that (R)-DHMA should promote a stronger attractant response than (S)-DHMA because of a consistently greater-magnitude piston-like pushdown of the pTsr α-helix 4 toward the membrane upon binding of (R)-DHMA than upon binding of (S)-DHMA. This displacement is caused primarily by interaction of DHMA with Tsr residue Thr156, which has been shown by genetic studies to be critical for the attractant response to L-serine and DHMA. These findings led us to separate the two chiral species and test their effectiveness as chemoattractants. Both the tethered cell and motility migration coefficient assays validated the prediction that (R)-DHMA is a stronger attractant than (S)-DHMA. Our study demonstrates that refined computational docking and simulation studies combined with experiments can be used to investigate situations in which subtle differences between ligands may lead to diverse chemotactic responses.

Cell-cell communication, chemotaxis and recruitment in Vibrio parahaemolyticus

Motile bacteria are proficient at finding optimal environments for colonization. Often, they use chemotaxis to sense nutrient availability and dangerous concentrations of toxic chemicals. For many bacteria, the repertoire of chemoreceptors is large, suggesting they possess a broad palate with respect to sensing. However, knowledge of the molecules detected by chemotaxis signal transduction systems is limited. Some bacteria, like Vibrio parahaemolyticus, are social and swarm in groups on surfaces. This marine bacterium and human pathogen secretes the S signal autoinducer, which cues degradation of intracellular c-di-GMP leading to transcription of the swarming program. Here, we report that the S signal also directs motility at a behavioral level by serving as a chemoattractant. The data demonstrate that V. parahaemolyticus senses the S signal using SscL and SscS, homologous methyl-accepting chemotaxis proteins. SscL is required by planktonic bacteria for S signal chemotaxis. SscS plays a role during swarming, and mutants lacking this chemoreceptor swarm faster and produce colonies with more deeply branched swarming fronts than the wild type or the sscL mutant. Other Vibrio species can swim toward the S signal, suggesting a recruitment role for this cell-cell communication molecule in the context of polymicrobial marine communities.

Mechanism and impact of catecholamine conversion by Vibrio cholerae

Bacterial pathogens are influenced by signaling molecules including the catecholamines adrenaline and noradrenaline which are host-derived hormones and neurotransmitters. Adrenaline and noradrenaline modulate growth, motility and virulence of bacteria. We show that adrenaline is converted by the pathogen Vibrio cholerae to adrenochrome in the course of respiration, and demonstrate that superoxide produced by the respiratory, Na⁺ - translocating NADH:quinone oxidoreductase (NQR) acts as electron acceptor in the oxidative conversion of adrenaline to adrenochrome. Adrenochrome stimulates growth of V. cholerae, and triggers specific responses in V. cholerae and in immune cells. We performed a quantitative proteome analysis of V. cholerae grown in minimal medium with glucose as carbon source without catecholamines, or with adrenaline, noradrenaline or adrenochrome. Significant regulation of proteins participating in iron transport and iron homeostasis, in energy metabolism, and in signaling was observed upon exposure to adrenaline, noradrenaline or adrenochrome. On the host side, adrenochrome inhibited lipopolysaccharide-triggered formation of TNF-α by THP-1 monocytes, though to a lesser extent than adrenaline. It is proposed that adrenochrome produced from adrenaline by respiring V. cholerae functions as effector molecule in pathogen-host interaction.

High-Affinity Chemotaxis to Histamine Mediated by the TlpQ Chemoreceptor of the Human Pathogen Pseudomonas aeruginosa

Genome analyses indicate that many bacteria possess an elevated number of chemoreceptors, suggesting that these species are able to perform chemotaxis to a wide variety of compounds. The scientific community is now only beginning to explore this diversity and to elucidate the corresponding physiological relevance. The discovery of histamine chemotaxis in the human pathogen Pseudomonas aeruginosa provides insight into tactic movements that occur within the host. Since histamine is released in response to bacterial pathogens, histamine chemotaxis may permit bacterial migration and accumulation at infection sites, potentially modulating, in turn, quorum-sensing-mediated processes and the expression of virulence genes. As a consequence, the modulation of histamine chemotaxis by signal analogues may result in alterations of the bacterial virulence. As the first report of bacterial histamine chemotaxis, this study lays the foundation for the exploration of the physiological relevance of histamine chemotaxis and its role in pathogenicity.

Histamine is a key biological signaling molecule. It acts as a neurotransmitter in the central and peripheral nervous systems and coordinates local inflammatory responses by modulating the activity of different immune cells. During inflammatory processes, including bacterial infections, neutrophils stimulate the production and release of histamine. Here, we report that the opportunistic human pathogen Pseudomonas aeruginosa exhibits chemotaxis toward histamine. This chemotactic response is mediated by the concerted action of the TlpQ, PctA, and PctC chemoreceptors, which display differing sensitivities to histamine. Low concentrations of histamine were sufficient to activate TlpQ, which binds histamine with an affinity of 639 nM. To explore this binding, we resolved the high-resolution structure of the TlpQ ligand binding domain in complex with histamine. It has an unusually large dCACHE domain and binds histamine through a highly negatively charged pocket at its membrane distal module. Chemotaxis to histamine may play a role in the virulence of P. aeruginosa by recruiting cells at the infection site and consequently modulating the expression of quorum-sensing-dependent virulence genes. TlpQ is the first bacterial histamine receptor to be described and greatly differs from human histamine receptors, indicating that eukaryotes and bacteria have pursued different strategies for histamine recognition.

The effect of bacterial chemotaxis on host infection and pathogenicity

Chemotaxis enables microorganisms to move according to chemical gradients. Although this process requires substantial cellular energy, it also affords key physiological benefits, including enhanced access to growth substrates. Another important implication of chemotaxis is that it also plays an important role in infection and disease, as chemotaxis signalling pathways are broadly distributed across a variety of pathogenic bacteria. Furthermore, current research indicates that chemotaxis is essential for the initial stages of infection in different human, animal and plant pathogens. This review focuses on recent findings that have identified specific bacterial chemoreceptors and corresponding chemoeffectors associated with pathogenicity. Pathogenicity-related chemoeffectors are either host and niche-specific signals or intermediates of the host general metabolism. Plant pathogens were found to contain an elevated number of chemotaxis signalling genes and functional studies demonstrate that these genes are critical for their ability to enter the host. The expanding body of knowledge of the mechanisms underlying chemotaxis in pathogens provides a foundation for the development of new therapeutic strategies capable of blocking infection and preventing disease by interfering with chemotactic signalling pathways.

Genomic organization and role of SPI-13 in nutritional fitness of Salmonella

Salmonella pathogenicity island 13 (SPI-13) contributes to the virulence of Salmonella. The majority of the SPI-13 genes encode proteins putatively involved in bacterial metabolism, however, their functions largely remain uncharacterized. It is currently unknown if SPI-13 contributes to metabolic fitness of Salmonella and, if so, what are the metabolic substrates for the protein encoded by genes within SPI-13. We employed Phenotype Microarray (Biolog, USA) to compare the metabolic properties of SPI-13 deficient mutant (ΔSPI-13) and the WT parent strain of non-typhoidal Salmonella enterica sub sp. enterica serovar Enteritidis (S. Enteritidis). The results of Phenotype Microarray revealed that SPI-13 is required for efficient utilization of two micronutrients, namely, d-glucuronic acid (DGA) and tyramine (TYR), as sole sources of carbon and/or nitrogen. By systematic deletion of the individual gene(s), we identified specific genes within SPI-13 that are required for efficient utilization of DGA (SEN2977-80) and TYR (SEN2967 and SEN2971-72) as sole nutrient sources. The results show that SPI-13 mediated DGA and TYR metabolic pathways afford nutritional fitness to S. Enteritidis. Comparative genomics analysis of the SPI-13 locus from 247 Salmonella strains belonging to 57 different serovars revealed that SPI-13 genes specifically involved in the metabolism of DGA and TYR are highly conserved in Salmonella enterica. Because DGA and TYR are naturally present as metabolic byproducts in the gastrointestinal tract and other host tissues, we propose a metabolic model that shows that the role of SPI-13 mediated DGA and TYR metabolism in the nutritional fitness of Salmonella is likely linked to nutritional virulence of this pathogen.

Chemotaxis of Escherichia coli to major hormones and polyamines present in human gut

The microorganisms in the gastrointestinal (GI) tract can influence the metabolism, immunity, and behavior of animal hosts. Increasing evidence suggests that communication between the host and the microbiome also occurs in the opposite direction, with hormones and other host-secreted compounds being sensed by microorganisms. Here, we addressed one key aspect of the host–microbe communication by studying chemotaxis of a model commensal bacterium, Escherichia coli, to several compounds present abundantly in the GI tract, namely catecholamines, thyroid hormones, and polyamines. Our results show that E. coli reacts to five out of ten analyzed chemicals, sensing melatonin, and spermidine as chemorepellents and showing mixed responses to dopamine, norepinephrine and 3,4-dihydroxymandelic acid. The strongest repellent response was observed for the polyamine spermidine, and we demonstrate that this response involves the low-abundance chemoreceptor Trg and the periplasmic binding protein PotD of the spermidine uptake system. The chemotactic effects of the tested compounds apparently correlate with their influence on growth and their stability in the GI tract, pointing to the specificity of the observed behavior. We hypothesize that the repellent responses observed at high concentrations of chemoeffective compounds might enable bacteria to avoid harmful levels of hormones and polyamines in the gut and, more generally, antimicrobial activities of the mucous layer.

Symposium review: Microbial endocrinology-Why the integration of microbes, epithelial cells, and neurochemical signals in the digestive tract matters to ruminant health

The union of microbiology and neurobiology, which has been termed microbial endocrinology, is defined as the study of the ability of microorganisms to produce and respond to neurochemicals that originate either within the microorganisms themselves or within the host they inhabit. It serves as the basis for an evolutionarily derived method of communication between a host and its microbiota. Mechanisms elucidated by microbial endocrinology give new insight into the ways the microbiota can affect host stress, metabolic efficiency, resistance to disease, and other factors that may prove relevant to the dairy industry.

Cellular Stoichiometry of Methyl-Accepting Chemotaxis Proteins in Sinorhizobium meliloti

The chemosensory system in Sinorhizobium meliloti has several important deviations from the widely studied enterobacterial paradigm. To better understand the differences between the two systems and how they are optimally tuned, we determined the cellular stoichiometry of the methyl-accepting chemotaxis proteins (MCPs) and the histidine kinase CheA in S. meliloti Quantitative immunoblotting was used to determine the total amount of MCPs and CheA per cell in S. meliloti The MCPs are present in the cell in high abundance (McpV), low abundance (IcpA, McpU, McpX, and McpW), and very low abundance (McpY and McpZ), whereas McpT was below the detection limit. The approximate cellular ratio of these three receptor groups is 300:30:1. The chemoreceptor-to-CheA ratio is 23.5:1, highly similar to that seen in Bacillus subtilis (23:1) and about 10 times higher than that in Escherichia coli (3.4:1). Different from E. coli, the high-abundance receptors in S. meliloti are lacking the carboxy-terminal NWETF pentapeptide that binds the CheR methyltransferase and CheB methylesterase. Using transcriptional lacZ fusions, we showed that chemoreceptors are positively controlled by the master regulators of motility, VisNR and Rem. In addition, FlbT, a class IIA transcriptional regulator of flagellins, also positively regulates the expression of most chemoreceptors except for McpT and McpY, identifying chemoreceptors as class III genes. Taken together, these results demonstrate that the chemosensory complex and the adaptation system in S. meliloti deviates significantly from the established enterobacterial paradigm but shares some similarities with B. subtilisIMPORTANCE The symbiotic soil bacterium Sinorhizobium meliloti is of great agricultural importance because of its nitrogen-fixing properties, which enhances growth of its plant symbiont, alfalfa. Chemotaxis provides a competitive advantage for bacteria to sense their environment and interact with their eukaryotic hosts. For a better understanding of the role of chemotaxis in these processes, detailed knowledge on the regulation and composition of the chemosensory machinery is essential. Here, we show that chemoreceptor gene expression in S. meliloti is controlled through the main transcriptional regulators of motility. Chemoreceptor abundance is much lower in S. meliloti than in Escherichia coli and Bacillus subtilis Moreover, the chemoreceptor-to-kinase CheA ratio is different from that of E. coli but similar to that of B. subtilis.

A Static Microfluidic Device for Investigating the Chemotaxis Response to Stable, Non-linear Gradients

Microfluidic technology allows fast and precise measurement of chemotaxis responses to both attractant and repellent signals. One of the major drawbacks of current microfluidic chemotaxis assays is the presence of bacterial cells within the concentration gradient flow field, which has the potential for flow effects masking the chemotaxis response. This chapter describes a new microfluidic device for producing stable concentration gradients and measuring the response of cells to the gradient without exposing them to any flow. Unlike other methods described in the literature, this method is capable of producing gradients of any shape, almost instantaneously, allowing the measurement of time-dependent response of cells to a variety of signals.

Molecular Modeling of Chemoreceptor:Ligand Interactions

Docking algorithms have been widely used to elucidate ligand:receptor interactions that are important in biological function. Here, we introduce an in-house developed docking-refinement protocol that combines the following innovative features. (1) The use of multiple short molecular dynamics (MD) docking simulations, with residues within the binding pocket of the receptor unconstrained, so that the binding modes of the ligand in the binding pocket may be exhaustively examined. (2) The initial positioning of the ligand within the binding pocket based on complementary shape, and the use of both harmonic and quartic spherical potentials to constrain the ligand in the binding pocket during multiple short docking simulations. (3) The selection of the most probable binding modes generated by the short docking simulations using interaction energy calculations, as well as the subsequent application of all-atom MD simulations and physical-chemistry based free energy calculations to elucidate the most favorable binding mode of the ligand in complex with the receptor. In this chapter, we provide step-by-step instructions on how to computationally investigate the binding of small-molecule ligands to protein receptors by examining as control and test cases, respectively, the binding of L-serine and R-3,4-dihydroxymandelic acid (R-DHMA) to the Escherichia coli chemoreceptor Tsr. Similar computational strategies can be used for the molecular modeling of a series of ligand:protein receptor interactions.

Conversion of Norepinephrine to 3,4-Dihdroxymandelic Acid in Escherichia coli Requires the QseBC Quorum-Sensing System and the FeaR Transcription Factor

The detection of norepinephrine (NE) as a chemoattractant by Escherichia coli strain K-12 requires the combined action of the TynA monoamine oxidase and the FeaB aromatic aldehyde dehydrogenase. The role of these enzymes is to convert NE into 3,4-dihydroxymandelic acid (DHMA), which is a potent chemoattractant sensed by the Tsr chemoreceptor. These two enzymes must be induced by prior exposure to NE, and cells that are exposed to NE for the first time initially show minimal chemotaxis toward it. The induction of TynA and FeaB requires the QseC quorum-sensing histidine kinase, and the signaling cascade requires new protein synthesis. Here, we demonstrate that the cognate response regulator for QseC, the transcription factor QseB, is also required for induction. The related quorum-sensing kinase QseE appears not to be part of the signaling pathway, but its cognate response regulator, QseF, which is also a substrate for phosphotransfer from QseC, plays a nonessential role. The promoter of the feaR gene, which encodes a transcription factor that has been shown to be essential for the expression of tynA and feaB, has two predicted QseB-binding sites. One of these sites appears to be in an appropriate position to stimulate transcription from the P1 promoter of the feaR gene. This study unites two well-known pathways: one for expression of genes regulated by catecholamines (QseBC) and one for expression of genes required for metabolism of aromatic amines (FeaR, TynA, and FeaB). This cross talk allows E. coli to convert the host-derived and chemotactically inert NE into the potent bacterial chemoattractant DHMA.IMPORTANCE The chemotaxis of E. coli K-12 to norepinephrine (NE) requires the conversion of NE to 3,4-dihydroxymandleic acid (DHMA), and DHMA is both an attractant and inducer of virulence gene expression for a pathogenic enterohemorrhagic E. coli (EHEC) strain. The induction of virulence by DHMA and NE requires QseC. The results described here show that the cognate response regulator for QseC, QseB, is also required for conversion of NE into DHMA. Production of DHMA requires induction of a pathway involved in the metabolism of aromatic amines. Thus, the QseBC sensory system provides a direct link between virulence and chemotaxis, suggesting that chemotaxis to host signaling molecules may require that those molecules are first metabolized by bacterial enzymes to generate the actual chemoattractant.

Sensory Repertoire of Bacterial Chemoreceptors

Chemoreceptors in bacteria detect a variety of signals and feed this information into chemosensory pathways that represent a major mode of signal transduction. The five chemoreceptors from Escherichia coli have served as traditional models in the study of this protein family. Genome analyses revealed that many bacteria contain much larger numbers of chemoreceptors with broader sensory capabilities. Chemoreceptors differ in topology, sensing mode, cellular location, and, above all, the type of ligand binding domain (LBD). Here, we highlight LBD diversity using well-established and emerging model organisms as well as genomic surveys. Nearly a hundred different types of protein domains that are found in chemoreceptor sequences are known or predicted LBDs, but only a few of them are ubiquitous. LBDs of the same class recognize different ligands, and conversely, the same ligand can be recognized by structurally different LBDs; however, recent studies began to reveal common characteristics in signal-LBD relationships. Although signals can stimulate chemoreceptors in a variety of different ways, diverse LBDs appear to employ a universal transmembrane signaling mechanism. Current and future studies aim to establish relationships between LBD types, the nature of signals that they recognize, and the mechanisms of signal recognition and transduction.

The Norepinephrine Metabolite 3,4-Dihydroxymandelic Acid Is Produced by the Commensal Microbiota and Promotes Chemotaxis and Virulence Gene Expression in Enterohemorrhagic Escherichia coli

Enterohemorrhagic Escherichia coli (EHEC) is a commonly occurring foodborne pathogen responsible for numerous multistate outbreaks in the United States. It is known to infect the host gastrointestinal tract, specifically, in locations associated with lymphoid tissue. These niches serve as sources of enteric neurotransmitters, such as epinephrine and norepinephrine, that are known to increase virulence in several pathogens, including enterohemorrhagic E. coli The mechanisms that allow pathogens to target these niches are poorly understood. We previously reported that 3,4-dihydroxymandelic acid (DHMA), a metabolite of norepinephrine produced by E. coli, is a chemoattractant for the nonpathogenic E. coli RP437 strain. Here we report that DHMA is also a chemoattractant for EHEC. In addition, DHMA induces the expression of EHEC virulence genes and increases attachment to intestinal epithelial cells in vitro in a QseC-dependent manner. We also show that DHMA is present in murine gut fecal contents and that its production requires the presence of the commensal microbiota. On the basis of its ability to both attract and induce virulence gene expression in EHEC, we propose that DHMA acts as a molecular beacon to target pathogens to their preferred sites of infection in vivo.

Involvement of Two-Component Signaling on Bacterial Motility and Biofilm Development

Two-component signaling is a specialized mechanism that bacteria use to respond to changes in their environment. Nonpathogenic strains of Escherichia coli K-12 harbor 30 histidine kinases and 32 response regulators, which form a network of regulation that integrates many other global regulators that do not follow the two-component signaling mechanism, as well as signals from central metabolism. The output of this network is a multitude of phenotypic changes in response to changes in the environment. Among these phenotypic changes, many two-component systems control motility and/or the formation of biofilm, sessile communities of bacteria that form on surfaces. Motility is the first reversible attachment phase of biofilm development, followed by a so-called swim or stick switch toward surface organelles that aid in the subsequent phases. In the mature biofilm, motility heterogeneity is generated by a combination of evolutionary and gene regulatory events.

Mutations in the β-Subunit of the RNA Polymerase Impair the Surface-Associated Motility and Virulence of Acinetobacter baumannii

Acinetobacter baumannii is a major cause of antibiotic-resistant nosocomial infections worldwide. In this study, several rifampin-resistant spontaneous mutants obtained from the A. baumannii ATCC 17978 strain that differed in their point mutations in the rpoB gene, encoding the β-subunit of the RNA polymerase, were isolated. All the mutants harboring amino acid substitutions in position 522 or 540 of the RpoB protein were impaired in surface-associated motility and had attenuated virulence in the fertility model of Caenorhabditis elegans The transcriptional profile of these mutants included six downregulated genes encoding proteins homologous to transporters and metabolic enzymes widespread among A. baumannii clinical isolates. The construction of knockout mutants in each of the six downregulated genes revealed a significant reduction in the surface-associated motility and virulence of four of them in the A. baumannii ATCC 17978 strain, as well as in the virulent clinical isolate MAR002. Taken together, our results provide strong evidence of the connection between motility and virulence in this multiresistant nosocomial pathogen.

Microbial Endocrinology in the Pathogenesis of Infectious Disease

Microbial endocrinology represents the intersection of two seemingly disparate fields, microbiology and neurobiology, and is based on the shared presence of neurochemicals that are exactly the same in host as well as in the microorganism. The ability of microorganisms to not only respond to, but also produce, many of the same neurochemicals that are produced by the host, such as during periods of stress, has led to the introduction of this evolutionary-based mechanism which has a role in the pathogenesis of infectious disease. The consideration of microbial endocrinology-based mechanisms has demonstrated, for example, that the prevalent use of catecholamine-based synthetic drugs in the clinical setting contributes to the formation of biofilms in indwelling medical devices. Production of neurochemicals by microorganisms most often employs the same biosynthetic pathways as those utilized by the host, indicating that acquisition of host neurochemical-based signaling system in the host may have been acquired due to lateral gene transfer from microorganisms. That both host and microorganism produce and respond to the very same neurochemicals means that there is bidirectionality contained with the theoretical underpinnings of microbial endocrinology. This can be seen in the role of microbial endocrinology in the microbiota-gut-brain axis and its relevance to infectious disease. Such shared pathways argue for a role of microorganism-neurochemical interactions in infectious disease.

Salmonella Typhimurium and Multidirectional Communication in the Gut

The mammalian digestive tract is home to trillions of microbes, including bacteria, archaea, protozoa, fungi, and viruses. In monogastric mammals the stomach and small intestine harbor diverse bacterial populations but are typically less populated than the colon. The gut bacterial community (microbiota hereafter) varies widely among different host species and individuals within a species. It is influenced by season of the year, age of the host, stress and disease. Ideally, the host and microbiota benefit each other. The host provides nutrients to the microbiota and the microbiota assists the host with digestion and nutrient metabolism. The resident microbiota competes with pathogens for space and nutrients and, through this competition, protects the host in a phenomenon called colonization resistance. The microbiota participates in development of the host immune system, particularly regulation of autoimmunity and mucosal immune response. The microbiota also shapes gut–brain communication and host responses to stress; and, indeed, the microbiota is a newly recognized endocrine organ within mammalian hosts. Salmonella enterica serovar Typhimurium (S. Typhimurium hereafter) is a food-borne pathogen which adapts to and alters the gastrointestinal (GI) environment. In the GI tract, S. Typhimurium competes with the microbiota for nutrients and overcomes colonization resistance to establish infection. To do this, S. Typhimurium uses multiple defense mechanisms to resist environmental stressors, like the acidic pH of the stomach, and virulence mechanisms which allow it to invade the intestinal epithelium and disseminate throughout the host. To coordinate gene expression and disrupt signaling within the microbiota and between host and microbiota, S. Typhimurium employs its own chemical signaling and may regulate host hormone metabolism. This review will discuss the multidirectional interaction between S. Typhimurium, host and microbiota as well as mechanisms that allow S. Typhimurium to succeed in the gut.

Stochastic modelling suggests that an elevated superoxide anion - hydrogen peroxide ratio can drive extravascular phagocyte transmigration by lamellipodium formation

Chemotaxis, integrates diverse intra- and inter-cellular molecular processes into a purposeful patho-physiological response; the operatic rules of which, remain speculative. Here, I surmise, that superoxide anion induced directional motility, in a responding cell, results from a quasi pathway between the stimulus, surrounding interstitium, and its biochemical repertoire. The epochal event in the mounting of an inflammatory response, is the extravascular transmigration of a phagocyte competent cell towards the site of injury, secondary to the development of a lamellipodium. This stochastic-to-markovian process conversion, is initiated by the cytosolic-ROS of the damaged cell, but is maintained by the inverse association of a de novo generated pool of self-sustaining superoxide anions and sub-critical hydrogen peroxide levels. Whilst, the exponential rise of O2(.-) is secondary to the focal accumulation of higher order lipid raft-Rac1/2-actin oligomers; O2(.-) mediated inactivation and redistribution of ECSOD, accounts for the minimal concentration of H2O2 that the phagocyte experiences. The net result of this reciprocal association between ROS/ RNS members, is the prolonged perturbation and remodeling of the cytoskeleton and plasma membrane, a prelude to chemotactic migration. The manuscript also describes the significance of stochastic modeling, in the testing of plausible molecular hypotheses of observable phenomena in complex biological systems.

Chemodetection and Destruction of Host Urea Allows Helicobacter pylori to Locate the Epithelium

The gastric pathogen Helicobacter pylori interacts intimately with the gastric mucosa to avoid the microbicidal acid in the stomach lumen. The cues H. pylori senses to locate and colonize the gastric epithelium have not been well defined. We show that metabolites emanating from human gastric organoids rapidly attract H. pylori. This response is largely controlled by the bacterial chemoreceptor TlpB, and the main attractant emanating from epithelia is urea. Our previous structural analyses show that TlpB binds urea with high affinity. Here we demonstrate that this tight binding controls highly sensitive responses, allowing detection of urea concentrations as low as 50 nM. Attraction to urea requires that H. pylori urease simultaneously destroys the signal. We propose that H. pylori has evolved a sensitive urea chemodetection and destruction system that allows the bacterium to dynamically and locally modify the host environment to locate the epithelium.

Lessons in Fundamental Mechanisms and Diverse Adaptations from the 2015 Bacterial Locomotion and Signal Transduction Meeting

In response to rapid changes in their environment, bacteria control a number of processes, including motility, cell division, biofilm formation, and virulence. Research presented in January 2015 at the biennial Bacterial Locomotion and Signal Transduction (BLAST) meeting in Tucson, AZ, illustrates the elegant complexity of the nanoarrays, nanomachines, and networks of interacting proteins that mediate such processes. Studies employing an array of biophysical, genetic, cell biology, and mathematical methods are providing an increasingly detailed understanding of the mechanisms of these systems within well-studied bacteria. Furthermore, comparisons of these processes in diverse bacterial species are providing insight into novel regulatory and functional mechanisms. This review summarizes research presented at the BLAST meeting on these fundamental mechanisms and diverse adaptations, including findings of importance for applications involving bacteria of medical or agricultural relevance.

Specific gamma-aminobutyrate chemotaxis in pseudomonads with different lifestyle

The PctC chemoreceptor of Pseudomonas aeruginosa mediates chemotaxis with high specificity to gamma-aminobutyric acid (GABA). This compound is present everywhere in nature and has multiple functions, including being a human neurotransmitter or plant signaling compound. Because P. aeruginosa is ubiquitously distributed in nature and able to infect and colonize different hosts, the physiological relevance of GABA taxis is unclear, but it has been suggested that bacterial attraction to neurotransmitters may enhance virulence. We report the identification of McpG as a specific GABA chemoreceptor in non-pathogenic Pseudomonas putida KT2440. As with PctC, GABA was found to bind McpG tightly. The analysis of chimeras comprising the PctC and McpG ligand-binding domains fused to the Tar signaling domain showed very high GABA sensitivities. We also show that PctC inactivation does not alter virulence in Caenorhabditis elegans. Significant amounts of GABA were detected in tomato root exudates, and deletion of mcpG reduced root colonization that requires chemotaxis through agar. The C. elegans data and the detection of a GABA receptor in non-pathogenic species indicate that GABA taxis may not be related to virulence in animal systems but may be of importance in the context of colonization and infection of plant roots by soil-dwelling pseudomonads.

Relation between chemotaxis and consumption of amino acids in bacteria

Chemotaxis enables bacteria to navigate chemical gradients in their environment, accumulating toward high concentrations of attractants and avoiding high concentrations of repellents. Although finding nutrients is likely to be an important function of bacterial chemotaxis, not all characterized attractants are nutrients. Moreover, even for potential nutrients, the exact relation between the metabolic value of chemicals and their efficiency as chemoattractants has not been systematically explored. Here we compare the chemotactic response of amino acids with their use by bacteria for two well‐established models of chemotactic behavior, Escherichia coli and Bacillus subtilis. We demonstrate that in E. coli chemotaxis toward amino acids indeed strongly correlates with their utilization. However, no such correlation is observed for B. subtilis, suggesting that in this case, the amino acids are not followed because of their nutritional value but rather as environmental cues.

Signaling and sensory adaptation in Escherichia coli chemoreceptors: 2015 update

Motile Escherichia coli cells track gradients of attractant and repellent chemicals in their environment with transmembrane chemoreceptor proteins. These receptors operate in cooperative arrays to produce large changes in the activity of a signaling kinase, CheA, in response to small changes in chemoeffector concentration. Recent research has provided a much deeper understanding of the structure and function of core receptor signaling complexes and the architecture of higher-order receptor arrays, which, in turn, has led to new insights into the molecular signaling mechanisms of chemoreceptor networks. Current evidence supports a new view of receptor signaling in which stimulus information travels within receptor molecules through shifts in the dynamic properties of adjoining structural elements rather than through a few discrete conformational states.