Diversity at its best: bacterial taxis

Role of Dickeya dadantii 3937 chemoreceptors in the entry to Arabidopsis leaves through wounds

Chemotaxis enables bacteria to move towards an optimal environment in response to chemical signals. In the case of plant-pathogenic bacteria, chemotaxis allows pathogens to explore the plant surface for potential entry sites with the ultimate aim to prosper inside plant tissues and to cause disease. Chemoreceptors, which constitute the sensory core of the chemotaxis system, are usually transmembrane proteins which change their conformation when sensing chemicals in the periplasm and transduce the signal through a kinase pathway to the flagellar motor. In the particular case of the soft-rot pathogen Dickeya dadantii 3937, jasmonic acid released in a plant wound has been found to be a strong chemoattractant which drives pathogen entry into the plant apoplast. In order to identify candidate chemoreceptors sensing wound-derived plant compounds, we carried out a bioinformatics search of candidate chemoreceptors in the genome of Dickeya dadantii 3937. The study of the chemotactic response to several compounds and the analysis of the entry process to Arabidopsis leaves of 10 selected mutants in chemoreceptors allowed us to determine the implications of at least two of them (ABF-0020167 and ABF-0046680) in the chemotaxis-driven entry process through plant wounds. Our data suggest that ABF-0020167 and ABF-0046680 may be candidate receptors of jasmonic acid and xylose, respectively.

Salt Stress Induced Changes in the Exoproteome of the Halotolerant Bacterium Tistlia consotensis Deciphered by Proteogenomics

The ability of bacteria to adapt to external osmotic changes is fundamental for their survival. Halotolerant microorganisms, such as Tistlia consotensis, have to cope with continuous fluctuations in the salinity of their natural environments which require effective adaptation strategies against salt stress. Changes of extracellular protein profiles from Tistlia consotensis in conditions of low and high salinities were monitored by proteogenomics using a bacterial draft genome. At low salinity, we detected greater amounts of the HpnM protein which is involved in the biosynthesis of hopanoids. This may represent a novel, and previously unreported, strategy by halotolerant microorganisms to prevent the entry of water into the cell under conditions of low salinity. At high salinity, proteins associated with osmosensing, exclusion of Na+ and transport of compatible solutes, such as glycine betaine or proline are abundant. We also found that, probably in response to the high salt concentration, T. consotensis activated the synthesis of flagella and triggered a chemotactic response neither of which were observed at the salt concentration which is optimal for growth. Our study demonstrates that the exoproteome is an appropriate indicator of adaptive response of T. consotensis to changes in salinity because it allowed the identification of key proteins within its osmoadaptive mechanism that had not previously been detected in its cell proteome.

Mutualistic interaction between Salmonella enterica and Aspergillus niger and its effects on Zea mays colonization

Salmonella Typhimurium inhabits a variety of environments and is able to infect a broad range of hosts. Throughout its life cycle, some hosts can act as intermediates in the path to the infection of others. Aspergillus niger is a ubiquitous fungus that can often be found in soil or associated to plants and microbial consortia. Recently, S. Typhimurium was shown to establish biofilms on the hyphae of A. niger. In this work, we have found that this interaction is stable for weeks without a noticeable negative effect on either organism. Indeed, bacterial growth is promoted upon the establishment of the interaction. Moreover, bacterial biofilms protect the fungus from external insults such as the effects of the anti-fungal agent cycloheximide. Thus, the Salmonella-Aspergillus interaction can be defined as mutualistic. A tripartite gnotobiotic system involving the bacterium, the fungus and a plant revealed that co-colonization has a greater negative effect on plant growth than colonization by either organism in dividually. Strikingly, co-colonization also causes a reduction in plant invasion by S. Typhimurium. This work demonstrates that S. Typhimurium and A. niger establish a mutualistic interaction that alters bacterial colonization of plants and affects plant physiology.

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.

Bacterial chemoreceptors and chemoeffectors

Bacteria use chemotaxis signaling pathways to sense environmental changes. Escherichia coli chemotaxis system represents an ideal model that illustrates fundamental principles of biological signaling processes. Chemoreceptors are crucial signaling proteins that mediate taxis toward a wide range of chemoeffectors. Recently, in deep study of the biochemical and structural features of chemoreceptors, the organization of higher-order clusters in native cells, and the signal transduction mechanisms related to the on-off signal output provides us with general insights to understand how chemotaxis performs high sensitivity, precise adaptation, signal amplification, and wide dynamic range. Along with the increasing knowledge, bacterial chemoreceptors can be engineered to sense novel chemoeffectors, which has extensive applications in therapeutics and industry. Here we mainly review recent advances in the E. coli chemotaxis system involving structure and organization of chemoreceptors, discovery, design, and characterization of chemoeffectors, and signal recognition and transduction mechanisms. Possible strategies for changing the specificity of bacterial chemoreceptors to sense novel chemoeffectors are also discussed.

Variation in swimming speed of Escherichia coli in response to attractant

It is well known that Escherichia coli executes chemotactic motion in response to chemical cues by modulating the flagellar motor bias alone. However, previous studies have reported the possibility of variation in run speed in the presence of attractants although it is unclear whether bacteria can deliberately modulate their swimming speeds in response to environmental cues or if the motor speeds are hardwired. By studying the detailed motion of cells in a uniform concentration of glucose and its non-metabolizable analogue, we show that changing concentrations may be accompanied by variation in the swimming speed. For a fixed run duration, cells exposed to the attractants achieved a higher peak-swimming speed after a tumble compared with that in plain motility buffer. Our experiments using the mutant strain lacking the Trg sensor show no change in swimming speed with varying concentrations of the non-metabolizable analogue, suggesting that sensing may play a role in the observed variation of swimming speed.

Indole-3-acetic acid in plant-microbe interactions

Indole-3-acetic acid (IAA) is an important phytohormone with the capacity to control plant development in both beneficial and deleterious ways. The ability to synthesize IAA is an attribute that many bacteria including both plant growth-promoters and phytopathogens possess. There are three main pathways through which IAA is synthesized; the indole-3-pyruvic acid, indole-3-acetamide and indole-3-acetonitrile pathways. This chapter reviews the factors that effect the production of this phytohormone, the role of IAA in bacterial physiology and in plant-microbe interactions including phytostimulation and phytopathogenesis.

Purification, crystallization and preliminary crystallographic analysis of the ligand-binding regions of the PctA and PctB chemoreceptors from Pseudomonas aeruginosa in complex with amino acids

Pseudomonas aeruginosa is an opportunistic pathogen and one of the major model organisms for the study of chemotaxis. The bacterium harbours 26 genes encoding chemoreceptors, most of which have not been annotated with a function. The paralogous chemoreceptors PctA and PctB (Pseudomonas chemotactic transducer A and B) were found to mediate chemotaxis towards L-amino acids. However, the ligand spectrum of the receptors is quite different since the recombinant ligand-binding region (LBR) of PctA binds 17 different L-amino acids whereas that of PctB recognizes only five. To determine the molecular basis underlying this ligand specificity, PctA-LBR and PctB-LBR have been purified and crystals have been produced after pre-incubation with L-Ile and L-Arg, respectively. Initial crystallization conditions have been identified by the counter-diffusion method and X-ray data have been collected at 2.5 Å (PctA-LBR bound to L-Ile) and 3.14 Å (PctB-LBR bound to L-Arg) resolution. Crystals belonged to space groups P2(1)2(1)2(1) and P3(1)2(1), with unit-cell parameters a = 72.2, b = 78.5, c = 116.6 Å and a = b = 111.6, c = 117.4, respectively, for PctA-LBR and PctB-LBR. Molecular-replacement methods will be pursued for structural determination.

Functional characterization of the bacterial iac genes for degradation of the plant hormone indole-3-acetic acid

Pseudomonas putida 1290 is a model organism for the study of bacterial degradation of the plant hormone indole-3-acetic acid (IAA). This property is encoded by the iac gene cluster. Insertional inactivation and/or deletion of individual iac genes and heterologous expression of the gene cluster in Escherichia coli were combined with mass spectrometry to demonstrate that iac-based degradation of IAA is likely to involve 2-hydroxy-IAA, 3-hydroxy-2-oxo-IAA, and catechol as intermediates. The first gene of the cluster, iacA encodes for the first step in the pathway, and also can convert indole to indoxyl to produce the blue pigment indigo. Transcriptional profiling of iac genes in P. putida 1290 revealed that they were induced in the presence of IAA. Based on results with an iacR knockout, we propose that this gene codes for a repressor of iacA expression and that exposure to IAA relieves this repression. Transformation of P. putida KT2440 (which cannot degrade IAA) with the iac gene cluster conferred the ability to grow on IAA as a sole source of carbon and energy, but not the ability to chemotaxi towards IAA. We could show such tactic response for P. putida 1290, thus representing the first demonstration of bacterial chemotaxis towards IAA. We discuss the ecological significance of our findings, and specifically the following question: under what circumstances do bacteria with the ability to degrade, recognize, and move towards IAA have a selective advantage?

Paralogous chemoreceptors mediate chemotaxis towards protein amino acids and the non-protein amino acid gamma-aminobutyrate (GABA)

The paralogous receptors PctA, PctB and PctC of Pseudomonas aeruginosa were reported to mediate chemotaxis to amino acids, intermediates of amino acid metabolism and chlorinated hydrocarbons. We show that the recombinant ligand binding regions (LBRs) of PctA, PctB and PctC bind 17, 5 and 2 l-amino acids respectively. In addition, PctC-LBR recognized GABA but not any other structurally related compound. l-Gln, one of the three amino acids that is not recognized by PctA-LBR, was the most tightly binding ligand to PctB suggesting that PctB has evolved to mediate chemotaxis primarily towards l-Gln. Bacteria were efficiently attracted to l-Gln and GABA, but mutation of pctB and pctC, respectively, abolished chemoattraction. The physiological relevance of taxis towards GABA is proposed to reside in an interaction with plants. LBRs were predicted to adopt double PDC (PhoQ/DcuS/CitA) like structures and site-directed mutagenesis studies showed that ligands bind to the membrane-distal module. Analytical ultracentrifugation studies have shown that PctA-LBR and PctB-LBR are monomeric in the absence and presence of ligands, which is in contrast to the enterobacterial receptors that require sensor domain dimers for ligand recognition.

High specificity in CheR methyltransferase function: CheR2 of Pseudomonas putida is essential for chemotaxis, whereas CheR1 is involved in biofilm formation

Chemosensory pathways are a major signal transduction mechanism in bacteria. CheR methyltransferases catalyze the methylation of the cytosolic signaling domain of chemoreceptors and are among the core proteins of chemosensory cascades. These enzymes have primarily been studied Escherichia coli and Salmonella typhimurium, which possess a single CheR involved in chemotaxis. Many other bacteria possess multiple cheR genes. Because the sequences of chemoreceptor signaling domains are highly conserved, it remains to be established with what degree of specificity CheR paralogues exert their activity. We report here a comparative analysis of the three CheR paralogues of Pseudomonas putida. Isothermal titration calorimetry studies show that these paralogues bind the product of the methylation reaction, S-adenosylhomocysteine, with much higher affinity (KD of 0.14-2.2 μM) than the substrate S-adenosylmethionine (KD of 22-43 μM), which indicates product feedback inhibition. Product binding was particularly tight for CheR2. Analytical ultracentrifugation experiments demonstrate that CheR2 is monomeric in the absence and presence of S-adenosylmethionine or S-adenosylhomocysteine. Methylation assays show that CheR2, but not the other paralogues, methylates the McpS and McpT chemotaxis receptors. The mutant in CheR2 was deficient in chemotaxis, whereas mutation of CheR1 and CheR3 had either no or little effect on chemotaxis. In contrast, biofilm formation of the CheR1 mutant was largely impaired but not affected in the other mutants. We conclude that CheR2 forms part of a chemotaxis pathway, and CheR1 forms part of a chemosensory route that controls biofilm formation. Data suggest that CheR methyltransferases act with high specificity on their cognate chemoreceptors.

Bacterial microsystems and microrobots

Microorganisms and specifically motile bacteria have been recently added to the list of micro-actuators typically considered for the implementation of microsystems and microrobots. Such trend has been motivated by the fact these microorganisms are self-powered actuators with overall sizes at the lower end of the micrometer range and which have proven to be extremely effective in low Reynolds number hydrodynamic regime of usually less than 10(-2). Furthermore, the various sensors or taxes in bacteria influencing their movements can also be exploited to perform tasks that were previously considered only for futuristic artificial microrobots. Bacterial implementations and related issues are not only reviewed, but this paper also proposes many techniques and approaches that can be considered as building blocks for the implementations of more sophisticated microsystems and microrobots.

The Pseudomonas putida HskA hybrid sensor kinase controls the composition of the electron transport chain

Sensor kinases play a key role in sensing and responding to environmental and physiological signals in bacteria. In this study we characterized a previously unknown orphan hybrid sensor kinase from Pseudomonas putida, which is conserved in several Pseudomonads. Inactivation of the gene coding for this sensor kinase, which we have named HskA, modified the expression of at least 85 genes in cells growing in a complete medium. HskA showed a strong influence on the composition of the electron transport chain. In cells growing exponentially in a complete medium, the absence of HskA led to a significant reduction in the expression of the genes coding for the bc1 complex and for the CIO and Cbb3-1 terminal oxidases. In stationary phase cells, however, lack of HskA caused a higher expression of the Cyo terminal oxidase and a lower expression of the Aa3 terminal oxidase. The HskA polypeptide shows two PAS (signal-sensing) domains, a transmitter domain containing the invariant phosphorylatable histidine and an ATP binding site, and a receiver domain containing the conserved aspartate capable of transphosphorylation, but lacks an Hpt module. It is therefore a hybrid sensor kinase. Phosphorylation assays showed that purified HskA undergoes autophosphorylation in the presence of ATP.

Transcriptome profiling of Xanthomonas campestris pv. campestris grown in minimal medium MMX and rich medium NYG

Xanthomonas campestris pathovar campestris (Xcc) is the causal agent of black rot disease in cruciferous plants worldwide. Although the complete genomes of several Xcc strains have been determined, the gene expression and regulation mechanisms in this pathogen are far from clear. In this work, transcriptome profiling of Xcc 8004 grown in MMX medium (minimal medium for Xanthomonas campestris) and NYG medium (peptone yeast glycerol medium) were investigated by RNA-Seq. Using the Illumina HiSeq 2000 platform, a total of 26,514,630 reads (90 nt in average) were generated, of which 15,708,478 reads mapped uniquely to coding regions of Xcc 8004 genome. Of the 4273 annotated protein-coding genes of Xcc 8004, 629 were found differentially expressed in Xcc grown in MMX and NYG. Of the differentially expressed genes, 495 were up-regulated and 134 were down-regulated in MMX. The MMX-induced genes are mainly involved in amino acid metabolism, transport systems, atypical condition adaptation and pathogenicity, especially the type III secretion system, while the MMX-repressed genes are mainly involved in chemotaxis and degradation of small molecules. The global transcriptome analyzes of Xcc 8004 grown in MMX and NYG might facilitate the gene functional characterization of this phytopathogenic bacterium.

Signal balancing by the CetABC and CetZ chemoreceptors controls energy taxis in Campylobacter jejuni

The coupling of environmental sensing to flagella-mediated directed motility allows bacteria to move to optimum environments for growth and survival, either by sensing external stimuli (chemotaxis) or monitoring internal metabolic status (energy taxis). Sensing is mediated by transducer-like proteins (Tlp), either located in the membrane or in the cytoplasm, which commonly influence motility via the CheA-CheY chemotaxis pathway. In this study we have investigated the role of PAS-domain-containing intracellular Tlp-sensors in energy taxis of the food-borne pathogen Campylobacter jejuni, using plate- and tube-based assays utilising the conversion of the redox indicator dyes triphenyl tetrazolium chloride (TTC) and resazurin. Inactivation of the genes encoding the Campylobacter Energy Taxis system (CetA (Tlp9) and CetB (Aer2)) in C. jejuni strain NCTC 11168 resulted in reduced taxis. Inactivation of the cj1191c gene, encoding the CetB homolog CetC (Aer1), did not affect taxis per se, but the cetC gene complemented a cetB mutant in trans, indicating that CetC can form a functional signal transduction complex with CetA in the absence of CetB. Inactivation of both CetB and CetC resulted in greatly reduced taxis confirming the role of CetC in energy taxis. Inactivation of the cj1110c gene, encoding Tlp8 (CetZ), a cytoplasmic sensor with two PAS-domains, resulted in increased taxis, a phenotype opposite to that of CetAB. Inactivation of the cheA gene resulted in the same overall phenotype as the cetAB mutant in both wild-type and cetZ backgrounds, suggesting that both systems use the CheA system for signal transduction. Absence of both CetAB and CetZ resulted in the cetAB taxis phenotype, suggesting that CetZ is subordinate to CetAB. In conclusion, we present evidence that C. jejuni balances the input from two counteracting PAS-domain-containing sensory systems to position itself for optimal usage of energy resources.

Chemoreceptor VfcA mediates amino acid chemotaxis in Vibrio fischeri

Flagellar motility and chemotaxis by Vibrio fischeri are important behaviors mediating the colonization of its mutualistic host, the Hawaiian bobtail squid. However, none of the 43 putative methyl-accepting chemotaxis proteins (MCPs) encoded in the V. fischeri genome has been previously characterized. Using both an available transposon mutant collection and directed mutagenesis, we isolated mutants for 19 of these genes, and screened them for altered chemotaxis to six previously identified chemoattractants. Only one mutant was defective in responding to any of the tested compounds; the disrupted gene was thus named vfcA (Vibrio fischeri chemoreceptor A; locus tag VF_0777). In soft-agar plates, mutants disrupted in vfcA did not exhibit the serine-sensing chemotactic ring, and the pattern of migration in the mutant was not affected by the addition of exogenous serine. Using a capillary chemotaxis assay, we showed that, unlike wild-type V. fischeri, the vfcA mutant did not undergo chemotaxis toward serine and that expression of vfcA on a plasmid in the mutant was sufficient to restore the behavior. In addition to serine, we demonstrated that alanine, cysteine, and threonine are strong attractants for wild-type V. fischeri and that the attraction is also mediated by VfcA. This study thus provides the first insights into how V. fischeri integrates information from one of its 43 MCPs to respond to environmental stimuli.

Tactic responses to pollutants and their potential to increase biodegradation efficiency

A significant number of bacterial strains are able to use toxic aromatic hydrocarbons as carbon and energy sources. In a number of cases, the evolution of the corresponding degradation pathway was accompanied by the evolution of tactic behaviours either towards or away from these toxic carbon sources. Reports are reviewed which show that a chemoattraction to heterogeneously distributed aromatic pollutants increases the bioavailability of these compounds and their biodegradation efficiency. An extreme form of chemoattraction towards aromatic pollutants, termed 'hyperchemotaxis', was described for Pseudomonas putida DOT-T1E, which is based on the action of the plasmid-encoded McpT chemoreceptor. Cells with this phenotype were found of being able to approach and of establishing contact with undiluted crude oil samples. Although close McpT homologues are found on other degradation plasmids, the sequence of their ligand-binding domains does not share significant similarity with that of NahY, the other characterized chemoreceptor for aromatic hydrocarbons. This may suggest the existence of at least two families of chemoreceptors for aromatic pollutants. The use of receptor chimers comprising the ligand-binding region of McpT for biosensing purposes is discussed.

Swimming behaviour of the multicellular magnetotactic prokaryote 'Candidatus Magnetoglobus multicellularis' under applied magnetic fields and ultraviolet light

Magnetotactic bacteria move by rotating their flagella and concomitantly are aligned to magnetic fields because they present magnetosomes, which are intracellular organelles composed by membrane-bound magnetic crystals. This results in magnetotaxis, which is swimming along magnetic field lines. Magnetotactic bacteria are morphologically diverse, including cocci, rods, spirilla and multicellular forms known as magnetotactic multicellular prokaryotes (MMPs). 'Candidatus Magnetoglobus multicellularis' is presently the best known MMP. Here we describe the helical trajectories performed by these microorganisms as they swim forward, as well as their response to UV light. We measured the radius of the trajectory, time period and translational velocity (velocity along the helix axis), which enabled the calculation of other trajectory parameters such as pitch, tangential velocity (velocity along the helix path), angular frequency, and theta angle (the angle between the helix path and the helix axis). The data revealed that 'Ca. M. multicellularis' swims along elongated helical trajectories with diameters approaching the diameter of the microorganism. In addition, we observed that 'Ca. M. multicellularis' responds to UV laser pulses by swimming backwards, returning to forward swimming several seconds after the UV laser pulse. UV light from a fluorescence microscope showed a similar effect. Thus, phototaxis is used in addition to magnetotaxis in this microorganism.

Genes encoding Cher-TPR fusion proteins are predominantly found in gene clusters encoding chemosensory pathways with alternative cellular functions

Chemosensory pathways correspond to major signal transduction mechanisms and can be classified into the functional families flagellum-mediated taxis, type four pili-mediated taxis or pathways with alternative cellular functions (ACF). CheR methyltransferases are core enzymes in all of these families. CheR proteins fused to tetratricopeptide repeat (TPR) domains have been reported and we present an analysis of this uncharacterized family. We show that CheR-TPRs are widely distributed in GRAM-negative but almost absent from GRAM-positive bacteria. Most strains contain a single CheR-TPR and its abundance does not correlate with the number of chemoreceptors. The TPR domain fused to CheR is comparatively short and frequently composed of 2 repeats. The majority of CheR-TPR genes were found in gene clusters that harbor multidomain response regulators in which the REC domain is fused to different output domains like HK, GGDEF, EAL, HPT, AAA, PAS, GAF, additional REC, HTH, phosphatase or combinations thereof. The response regulator architectures coincide with those reported for the ACF family of pathways. Since the presence of multidomain response regulators is a distinctive feature of this pathway family, we conclude that CheR-TPR proteins form part of ACF type pathways. The diversity of response regulator output domains suggests that the ACF pathways form a superfamily which regroups many different regulatory mechanisms, in which all CheR-TPR proteins appear to participate. In the second part we characterize WspC of Pseudomonas putida, a representative example of CheR-TPR. The affinities of WspC-Pp for S-adenosylmethionine and S-adenosylhomocysteine were comparable to those of prototypal CheR, indicating that WspC-Pp activity is in analogy to prototypal CheRs controlled by product feed-back inhibition. The removal of the TPR domain did not impact significantly on the binding constants and consequently not on the product feed-back inhibition. WspC-Pp was found to be monomeric, which rules out a role of the TPR domain in self-association.

Genes for carbon metabolism and the ToxA virulence factor in Pseudomonas aeruginosa are regulated through molecular interactions of PtxR and PtxS

Homologs of the transcriptional regulator PtxS are omnipresent in Pseudomonas, whereas PtxR homologues are exclusively found in human pathogenic Pseudomonas species. In all Pseudomonas sp., PtxS with 2-ketogluconate is the regulator of the gluconate degradation pathway and controls expression from its own promoter and also from the P(gad) and P(kgu) for the catabolic operons. There is evidence that PtxS and PtxR play a central role in the regulation of exotoxin A expression, a relevant primary virulence factor of Pseudomonas aeruginosa. We show using DNaseI-footprint analysis that in P. aeruginosa PtxR binds to the -35 region of the P(toxA) promoter in front of the exotoxin A gene, whereas PtxS does not bind to this promoter. Bioinformatic and DNaseI-footprint analysis identified a PtxR binding site in the P(kgu) and P(gad) promoters that overlaps the -35 region, while the PtxS operator site is located 50 bp downstream from the PtxR site. In vitro, PtxS recognises PtxR with nanomolar affinity, but this interaction does not occur in the presence of 2-ketogluconate, the specific effector of PtxS. DNAaseI footprint assays of P(kgu) and P(gad) promoters with PtxS and PtxR showed a strong region of hyper-reactivity between both regulator binding sites, indicative of DNA distortion when both proteins are bound; however in the presence of 2-ketogluconate no protection was observed. We conclude that PtxS modulates PtxR activity in response to 2-ketogluconate by complex formation in solution in the case of the P(toxA) promoter, or via the formation of a DNA loop as in the regulation of gluconate catabolic genes. Data suggest two different mechanisms of control exerted by the same regulator.

Chemotaxis to furan compounds by furan-degrading Pseudomonas strains

Two Pseudomonas strains known to utilize furan derivatives were shown to respond chemotactically to furfural, 5-hydroxymethylfurfural, furfuryl alcohol, and 2-furoic acid. In addition, a LysR-family regulatory protein known to regulate furan metabolic genes was found to be involved in regulating the chemotactic response.

Three types of taxis used in the response of Acidovorax sp. strain JS42 to 2-nitrotoluene

Acidovorax sp. strain JS42 is able to utilize 2-nitrotoluene (2NT) as its sole carbon, nitrogen, and energy source. We report here that strain JS42 is chemotactic to 2NT and that the response is increased when cells are grown on compounds such as 2NT that are known to induce the first step of 2NT degradation. Assays with JS42 mutants unable to oxidize 2NT showed that the first step of 2NT metabolism was required for the induced response, but not for a portion of the constitutive response, indicating that 2NT itself is an attractant. The 2NT metabolite nitrite was shown to be a strong attractant for strain JS42, and sufficient nitrite was produced during the taxis assay to account for a large part of the induced response. A mutant with an inactivated ntdY gene, which is located adjacent to the 2NT degradation genes and codes for a putative methyl-accepting chemotaxis protein, showed a defect in taxis toward 2NT that may involve a reduced response to nitrite. Responses of a mutant defective for the energy-taxis receptor, Aer, indicated that a functional aer gene is required for a substantial part of the wild-type induced response to 2NT. In summary, strain JS42 utilizes three types of taxis to sense and respond to 2NT: constitutive 2NT-specific chemotaxis to directly sense 2NT, metabolism-dependent nitrite-specific chemotaxis that may be mediated by NtdY, and energy taxis mediated by Aer.

Excitable behavior can explain the "ping-pong" mode of communication between cells using the same chemoattractant

Here we elucidate a paradox: how a single chemoattractant-receptor system in two individuals is used for communication despite the seeming inevitability of self-excitation. In the filamentous fungus Neurospora crassa, genetically identical cells that produce the same chemoattractant fuse via the homing of individual cell protrusions toward each other. This is achieved via a recently described "ping-pong" pulsatile communication. Using a generic activator-inhibitor model of excitable behavior, we demonstrate that the pulse exchange can be fully understood in terms of two excitable systems locked into a stable oscillatory pattern of mutual excitation. The most puzzling properties of this communication are the sudden onset of oscillations with final amplitude, and the absence of seemingly inevitable self-excitation. We show that these properties result directly from both the excitability threshold and refractory period characteristic of excitable systems. Our model suggests possible molecular mechanisms for the ping-pong communication.

Protein dynamics and mechanisms controlling the rotational behaviour of the bacterial flagellar motor

The proteins that make up the bacterial flagellar rotary motor have recently been shown to be more dynamic than previously thought, with some key proteins exchanging with pools of proteins in the membrane/cytoplasm. It has also become clear that in addition to simply switching in response to chemosensory signals, the rotation of the bacterial flagellar motor can be slowed or stopped, using a clutch or a brake, by signals from metabolism and growth state.

Laboratory research aimed at closing the gaps in microbial bioremediation

The industrial revolution, the first agricultural 'green revolution', and the development of antibiotics and therapeutic chemicals have brought significant and undeniable benefits to the human race. However, these advances demand high levels of energy, exploit natural resources and create large amounts of waste that creates an environmental burden for our planet. The pollution rate and character of many of the pollutants results in a rapid deterioration of the environment. Bioremediation functions to isolate and select microorganisms that operate under aerobic and anoxic conditions to remove these harmful pollutants. Current 'omics' technologies allow the exploitation of the catabolic potential of microbes without the need to cultivate them. Synthetic microbiology builds new catabolic pathways to remove recalcitrant pollutants from the environment.