Chemotaxis of Salmonella typhimurium to amino acids and some sugars

Bacterial amino acid chemotaxis: a widespread strategy with multiple physiological and ecological roles

Chemotaxis is the directed, flagellum-based movement of bacteria in chemoeffector gradients. Bacteria respond chemotactically to a wide range of chemoeffectors, including amino, organic, and fatty acids, sugars, polyamines, quaternary amines, purines, pyrimidines, aromatic hydrocarbons, oxygen, inorganic ions, or polysaccharides. Most frequent are chemotactic responses to amino acids (AAs), which were observed in numerous bacteria regardless of their phylogeny and lifestyle. Mostly chemoattraction responses are observed, although a number of bacteria are repelled from certain AAs. Chemoattraction is associated with the important metabolic value of AAs as growth substrates or building blocks of proteins. However, additional studies revealed that AAs are also sensed as environmental cues. Many chemoreceptors are specific for AAs, and signaling is typically initiated by direct ligand binding to their four-helix bundle or dCache ligand-binding domains. Frequently, bacteria possess multiple AA-responsive chemoreceptors that at times possess complementary AA ligand spectra. The identification of sequence motifs in the binding sites at dCache_1 domains has permitted to define an AA-specific family of dCache_1AA chemoreceptors. In addition, AAs are among the ligands recognized by broad ligand range chemoreceptors, and evidence was obtained for chemoreceptor activation by the binding of AA-loaded solute-binding proteins. The biological significance of AA chemotaxis is very ample including in biofilm formation, root and seed colonization by beneficial bacteria, plant entry of phytopathogens, colonization of the intestine, or different virulence-related features in human/animal pathogens. This review provides insights that may be helpful for the study of AA chemotaxis in other uncharacterized bacteria.

The mannose phosphotransferase system (Man-PTS) - Mannose transporter and receptor for bacteriocins and bacteriophages

Mannose transporters constitute a superfamily (Man-PTS) of the Phosphoenolpyruvate Carbohydrate Phosphotransferase System (PTS). The membrane complexes are homotrimers of protomers consisting of two subunits, IIC and IID. The two subunits without recognizable sequence similarity assume the same fold, and in the protomer are structurally related by a two fold pseudosymmetry axis parallel to membrane-plane (Liu et al. (2019) Cell Research 29 680). Two reentrant loops and two transmembrane helices of each subunit together form the N-terminal transport domain. Two three-helix bundles, one of each subunit, form the scaffold domain. The protomer is stabilized by a helix swap between these bundles. The two C-terminal helices of IIC mediate the interprotomer contacts. PTS occur in bacteria and archaea but not in eukaryotes. Man-PTS are abundant in Gram-positive bacteria living on carbohydrate rich mucosal surfaces. A subgroup of IICIID complexes serve as receptors for class IIa bacteriocins and as channel for the penetration of bacteriophage lambda DNA across the inner membrane. Some Man-PTS are associated with host-pathogen and -symbiont processes.

Spirochete Flagella and Motility

Spirochetes can be distinguished from other flagellated bacteria by their long, thin, spiral (or wavy) cell bodies and endoflagella that reside within the periplasmic space, designated as periplasmic flagella (PFs). Some members of the spirochetes are pathogenic, including the causative agents of syphilis, Lyme disease, swine dysentery, and leptospirosis. Furthermore, their unique morphologies have attracted attention of structural biologists; however, the underlying physics of viscoelasticity-dependent spirochetal motility is a longstanding mystery. Elucidating the molecular basis of spirochetal invasion and interaction with hosts, resulting in the appearance of symptoms or the generation of asymptomatic reservoirs, will lead to a deeper understanding of host-pathogen relationships and the development of antimicrobials. Moreover, the mechanism of propulsion in fluids or on surfaces by the rotation of PFs within the narrow periplasmic space could be a designing base for an autonomously driving micro-robot with high efficiency. This review describes diverse morphology and motility observed among the spirochetes and further summarizes the current knowledge on their mechanisms and relations to pathogenicity, mainly from the standpoint of experimental biophysics.

Salmonella Typhimurium is Attracted to Egg Yolk and Repelled by Albumen

Salmonella Typhimurium is the causative agent of non-typhoidal, foodborne salmonellosis. Contamination of hen eggs by the bacterium is a common source of S. Typhimurium infection. S. Typhimurium is peritrichous, and flagellum-dependent motility and chemotaxis are believed to facilitate egg contamination despite the presence of many antimicrobial egg components. We performed motility and chemotaxis assays to demonstrate that S. Typhimurium cells are attracted to egg yolks and are repelled by albumen. The bacterial flagellar motor shows bidirectional rotation, and counterclockwise-biased rotation allows cells to swim smoothly. A rotation assay for a single flagellum showed that, in comparison with thin albumen, the thick albumen more strongly affected the directional bias of the flagellar rotation, resulting in a remarkable suppression of the migration distance. Nevertheless, the S. Typhimurium cells retained positive chemotaxis toward the yolk in the presence of the albumens, suggesting that motility facilitates the growth of S. Typhimurium and survival in eggs.

New approach to distinguishing chemoattractants, chemorepellents and catabolised chemoeffectors for Campylobacter jejuni

Chemotactic behaviour is an important part of the lifestyle of motile bacteria and enables cells to respond to various environmental stimuli. The Hard Agar Plug (HAP) method is used to study the chemotactic behaviour of bacteria, including the fastidious microaerophile Campylobacter jejuni, an intestinal pathogen of humans. However, the traditional HAP assay is not quantitative, is unsuitable for chemotaxis observation over short time periods and for the investigation of repellent taxis, and is prone to false-positive and -negative results. Here we report an accurate, rapid, and quantitative HAP-based chemotaxis assay, tHAP, for the investigation of bacterial chemotactic responses. The critical component of the new assay is the addition of triphenyltetrazolium chloride (TTC). Enzymatic reduction of TTC to TFP-Red (1, 3, 5-Triphenylformazan) enables colourimetric detection of actively metabolising bacterial cells. Quantitative assessment of chemotaxis is achieved by colourimetric measurement or viability count over a period of 10 min to 3 h. Using the tHAP assay, we observed the dose-responsive chemotactic motility of C. jejuni cells along different concentrations of attractants aspartate and serine. Importantly, we have also designed a competitive tHAP assay to differentiate between repellents and attractants and to identify chemoeffectors that do not activate metabolism.

IMPORTANCE

The modified tHAP assay described here enables the exploration of the chemoresponse of Campylobacter jejuni towards chemorepellents, and catabolizable and non-catabolizable chemoattractants.

Identification of Specific Ligands for Sensory Receptors by Small-Molecule Ligand Arrays and Surface Plasmon Resonance

Ligand-receptor interactions triggering signal transduction components of many sensory pathways, remain elusive due to paucity of high-throughput screening methods. Here we describe our use of small molecule microarrays comprising of small glycans, amino and organic acids, salts, and other known chemoeffectors, for screening of ligands specific to various sensory receptors, followed by surface plasmon resonance to verify the veracity and to determine the affinity constants of the interactions. This methodology allows for rapid and identification of the direct ligand binding between the sensory receptors and their specific ligands.

Identification of Ligand-Receptor Interactions: Ligand Molecular Arrays, SPR and NMR Methodologies

Despite many years of research into bacterial chemotaxis, the only well characterized system to date is that of E. coli. Even for E. coli, the direct ligand binding had been fully characterized only for aspartate and serene receptors Tar and Tsr. In 30 years since, no other direct receptor-ligand interaction had been described for bacteria, until the characterization of the C. jejuni aspartate and multiligand receptors (Hartley-Tassell et al. Mol Microbiol 75:710-730, 2010). While signal transduction components of many sensory pathways have now been characterized, ligand-receptor interactions remain elusive due to paucity of high-throughput screening methods. Here, we describe the use of microarray screening we developed to identify ligands, surface plasmon resonance, and saturation transfer difference nuclear magnetic resonance (STD-NMR) we used to verify the hits and to determine the affinity constants of the interactions, allowing for more targeted verification of ligands with traditional chemotaxis and in vivo assays described in Chapter 13 .

Characterization of Leptospiral Chemoreceptors Using a Microscopic Agar Drop Assay

Bacterial chemotaxis is induced by sensing chemical stimuli via chemoreceptors embedded in the cytoplasmic membrane, enabling the cells to migrate toward nutrients or away from toxins. The chemoreceptors of Escherichia coli and Salmonella spp. have been well studied and are functionally classified on the basis of detectable substrates. The spirochete Leptospira possesses more than ten chemoreceptors and shows attractive or repellent responses against some sugars, amino acids, and fatty acids. However, the roles of these chemoreceptors have not been investigated. In this study, we conducted a chemotaxis assay called microscopic agar drop assay in combination with competition experiments, determining whether two kinds of attractants are recognized by the same type of chemoreceptor in the saprophytic Leptospira strain, Leptospira biflexa. Analyzing the competition effect observed between several pairs of chemicals, we found that L. biflexa senses sugars via chemoreceptors different from those that sense amino acids and fatty acids.

Mannose-Binding Lectin Inhibits the Motility of Pathogenic Salmonella by Affecting the Driving Forces of Motility and the Chemotactic Response

Mannose-binding lectin (MBL) is a key pattern recognition molecule in the lectin pathway of the complement system, an important component of innate immunity. MBL functions as an opsonin which enhances the sequential immune process such as phagocytosis. We here report an inhibitory effect of MBL on the motility of pathogenic bacteria, which occurs by affecting the energy source required for motility and the signaling pathway of chemotaxis. When Salmonella cells were treated with a physiological concentration of MBL, their motile fraction and free-swimming speed decreased. Rotation assays of a single flagellum showed that the flagellar rotation rate was significantly reduced by the addition of MBL. Measurements of the intracellular pH and membrane potential revealed that MBL affected a driving force for the Salmonella flagellum, the electrochemical potential difference of protons. We also found that MBL treatment increased the reversal frequency of Salmonella flagellar rotation, which interfered with the relative positive chemotaxis toward an attractive substrate. We thus propose that the motility inhibition effect of MBL may be secondarily involved in the attack against pathogens, potentially facilitating the primary role of MBL in the complement system.

A putative porin gene of Burkholderia sp. NK8 involved in chemotaxis toward β-ketoadipate

Burkholderia sp. NK8 can utilize 3-chlorobenzoate (3CB) as a sole source of carbon because it has a megaplasmid (pNK8) that carries the gene cluster (tfdT-CDEF) encoding chlorocatechol-degrading enzymes. The expression of tfdT-CDEF is induced by 3CB. In this study, we found that NK8 cells were attracted to 3CB and its degradation products, 3- and 4-chlorocatechol, and β-ketoadipate. Capillary assays revealed that a pNK8-eliminated strain (NK82) was defective in chemotaxis toward β-ketoadipate. The introduction of a plasmid carrying a putative outer membrane porin gene, which we name ompNK8, into strain NK82 restored chemotaxis toward β-ketoadipate. RT-PCR analyses demonstrated that the transcription of the ompNK8 gene was enhanced in the presence of 3CB.

Analysis of the chemotactic behaviour of Leptospira using microscopic agar-drop assay

Chemotaxis allows bacterial cells to migrate towards or away from chemical compounds. In the present study, we developed a microscopic agar-drop assay (MAA) to investigate the chemotactic behaviour of a coiled spirochete, Leptospira biflexa. An agar drop containing a putative attractant or repellent was placed around the centre of a flow chamber and the behaviour of free-swimming cells was analysed under a microscope. MAA showed that L. biflexa cells gradually accumulated around an agar drop that contained an attractant such as glucose. Leptospira cells often spin without migration by transformation of their cell body. The frequency at which cells showed no net displacement decreased with a higher glucose concentration, suggesting that sensing an attractive chemical allows these cells to swim more smoothly. Investigation of the chemotactic behaviour of these cells in response to different types of sugars showed that fructose and mannitol induced negative chemotactic responses, whereas xylose and lactose were non-chemotactic for L. biflexa. The MAA developed in this study can be used to investigate other chemoattractants and repellents.

The relation of signal transduction to the sensitivity and dynamic range of bacterial chemotaxis

Complex networks of interacting molecular components of living cells are responsible for many important processes, such as signal processing and transduction. An important challenge is to understand how the individual properties of these molecular interactions and biochemical transformations determine the system-level properties of biological functions. Here, we address the issue of the accuracy of signal transduction performed by a bacterial chemotaxis system. The chemotaxis sensitivity of bacteria to a chemoattractant gradient has been measured experimentally from bacterial aggregation in a chemoattractant-containing capillary. The observed precision of the chemotaxis depended on environmental conditions such as the concentration and molecular makeup of the chemoattractant. In a quantitative model, we derived the chemotactic response function, which is essential to describing the signal transduction process involved in bacterial chemotaxis. In the presence of a gradient, an analytical solution is derived that reveals connections between the chemotaxis sensitivity and the characteristics of the signaling system, such as reaction rates. These biochemical parameters are integrated into two system-level parameters: one characterizes the efficiency of gradient sensing, and the other is related to the dynamic range of chemotaxis. Thus, our approach explains how a particular signal transduction property affects the system-level performance of bacterial chemotaxis. We further show that the two parameters can be derived from published experimental data from a capillary assay, which successfully characterizes the performance of bacterial chemotaxis.

Salmonella chemoreceptors McpB and McpC mediate a repellent response to L-cystine: a potential mechanism to avoid oxidative conditions

Chemoreceptors McpB and McpC in Salmonella enterica have been reported to promote chemotaxis in LB motility-plate assays. Of the chemicals tested as potential effectors of these receptors, the only response was towards L-cysteine and its oxidized form, L-cystine. Although enhanced radial migration in plates suggested positive chemotaxis to both amino acids, capillary assays failed to show an attractant response to either, in cells expressing only these two chemoreceptors. In vivo fluorescence resonance energy transfer (FRET) measurements of kinase activity revealed that in wild-type bacteria, cysteine and cystine are chemoeffectors of opposing sign, the reduced form being a chemoattractant and the oxidized form a repellent. The attractant response to cysteine was mediated primarily by Tsr, as reported earlier for Escherichia coli. The repellent response to cystine was mediated by McpB/C. Adaptive recovery upon cystine exposure required the methyl-transferase/-esterase pair, CheR/CheB, but restoration of kinase activity was never complete (i.e. imperfect adaptation). We provide a plausible explanation for the attractant-like responses to both cystine and cysteine in motility plates, and speculate that the opposing signs of response to this redox pair might afford Salmonella a mechanism to gauge and avoid oxidative environments.

Ligand specificity determined by differentially arranged common ligand-binding residues in bacterial amino acid chemoreceptors Tsr and Tar

Escherichia coli has closely related amino acid chemoreceptors with distinct ligand specificity, Tar for l-aspartate and Tsr for l-serine. Crystallography of the ligand-binding domain of Tar identified the residues interacting with aspartate, most of which are conserved in Tsr. However, swapping of the nonconserved residues between Tsr and Tar did not change ligand specificity. Analyses with chimeric receptors led us to hypothesize that distinct three-dimensional arrangements of the conserved ligand-binding residues are responsible for ligand specificity. To test this hypothesis, the structures of the apo- and serine-binding forms of the ligand-binding domain of Tsr were determined at 1.95 and 2.5 Å resolutions, respectively. Some of the Tsr residues are arranged differently from the corresponding aspartate-binding residues of Tar to form a high affinity serine-binding pocket. The ligand-binding pocket of Tsr was surrounded by negatively charged residues, which presumably exclude negatively charged aspartate molecules. We propose that all these Tsr- and Tar-specific features contribute to specific recognition of serine and aspartate with the arrangement of the side chain of residue 68 (Asn in Tsr and Ser in Tar) being the most critical.

Evolution of response dynamics underlying bacterial chemotaxis

Background

The ability to predict the function and structure of complex molecular mechanisms underlying cellular behaviour is one of the main aims of systems biology. To achieve it, we need to understand the evolutionary routes leading to a specific response dynamics that can underlie a given function and how biophysical and environmental factors affect which route is taken. Here, we apply such an evolutionary approach to the bacterial chemotaxis pathway, which is documented to display considerable complexity and diversity.

Results

We construct evolutionarily accessible response dynamics starting from a linear response to absolute levels of attractant, to those observed in current-day Escherichia coli. We explicitly consider bacterial movement as a two-state process composed of non-instantaneous tumbling and swimming modes. We find that a linear response to attractant results in significant chemotaxis when sensitivity to attractant is low and when time spent tumbling is large. More importantly, such linear response is optimal in a regime where signalling has low sensitivity. As sensitivity increases, an adaptive response as seen in Escherichia coli becomes optimal and leads to 'perfect' chemotaxis with a low tumbling time. We find that as tumbling time decreases and sensitivity increases, there exist a parameter regime where the chemotaxis performance of the linear and adaptive responses overlap, suggesting that evolution of chemotaxis responses might provide an example for the principle of functional change in structural continuity.

Conclusions

Our findings explain several results from diverse bacteria and lead to testable predictions regarding chemotaxis responses evolved in bacteria living under different biophysical constraints and with specific motility machinery. Further, they shed light on the potential evolutionary paths for the evolution of complex behaviours from simpler ones in incremental fashion.

Chemotactic Behavior of Azotobacter vinelandii

Chemotaxis was exhibited by Azotobacter vinelandii motile cells. Exposure of cells to sudden increases in attractant concentration suppressed the frequency of tumbling and resulted in smooth swimming. Cells responded chemotactically to a chemical gradient produced during metabolism. Motility occurred over a temperature range of 25 to 37 degrees C with an optimum pH range of between pH 7.0 and 8.0. The average speed of motile cells was determined to be 74 mum/s or 37 body lengths per s. The speed of cells appeared to increase as a function of attractant concentration. Chemotactic systems for fructose, glucose, xylitol, and mannitol were inducible. A. vinelandii exhibited chemotaxis for a number of compounds, including hexoses, hexitols, pentitols, pentoses, disaccharides, and amino sugars. We conclude from these studies that A. vinelandii exhibits a temporal chemotactic sensing system.

Isolation and characterization of chemotaxis mutants of the Lyme disease Spirochete Borrelia burgdorferi using allelic exchange mutagenesis, flow cytometry, and cell tracking

Constructing mutants by targeted gene inactivation is more difficult in the Lyme disease organism, Borrelia burgdorferi, than in many other species of bacteria. The B. burgdorferi genome is fragmented, with a large linear genome and 21 linear and circular plasmids. Some of these small linear and circular plasmids are often lost during laboratory propagation, and the loss of specific plasmids can have a significant impact on virulence. In addition to the unusual structure of the B. burgdorferi genome, the presence of an active restriction-modification system impedes genetic transformation. Furthermore, B. burgdorferi is relatively slow growing, with a 7- to 12-h generation time, requiring weeks to obtain single colonies. The beginning part of this chapter details the procedure in targeting specific B. burgdorferi genes by allelic exchange mutagenesis. Our laboratory is especially interested in constructing and analyzing B. burgdorferi chemotaxis and motility mutants. Characterization of these mutants with respect to chemotaxis and swimming behavior is more difficult than for many other bacterial species. We have developed swarm plate and modified capillary tube assays for assessing chemotaxis. In the modified capillary tube chemotaxis assay, flow cytometry is used to rapidly enumerate cells that accumulate in the capillary tubes containing attractants. To assess the swimming behavior and velocity of B. burgdorferi wild-type and mutant cells, we use a commercially available cell tracker referred to as "Volocity." The latter part of this chapter presents protocols for performing swarm plate and modified capillary tube assays, as well as cell motion analysis. It should be possible to adapt these procedures to study other spirochete species, as well as other species of bacteria, especially those that have long generation times.

Differential interaction of Salmonella enterica serovars with lettuce cultivars and plant-microbe factors influencing the colonization efficiency

The availability of knowledge of the route of infection and critical plant and microbe factors influencing the colonization efficiency of plants by human pathogenic bacteria is essential for the design of preventive strategies to maintain safe food. This research describes the differential interaction of human pathogenic Salmonella enterica with commercially available lettuce cultivars. The prevalence and degree of endophytic colonization of axenically grown lettuce by the S. enterica serovars revealed a significant serovar-cultivar interaction for the degree of colonization (S. enterica CFUs per g leaf), but not for the prevalence. The evaluated S. enterica serovars were each able to colonize soil-grown lettuce epiphytically, but only S. enterica serovar Dublin was able to colonize the plants also endophytically. The number of S. enterica CFU per g of lettuce was negatively correlated to the species richness of the surface sterilized lettuce cultivars. A negative trend was observed for cultivars Cancan and Nelly, but not for cultivar Tamburo. Chemotaxis experiments revealed that S. enterica serovars actively move toward root exudates of lettuce cultivar Tamburo. Subsequent micro-array analysis identified genes of S. enterica serovar Typhimurium that were activated by the root exudates of cultivar Tamburo. A sugar-like carbon source was correlated with chemotaxis, while also pathogenicity-related genes were induced in presence of the root exudates. The latter revealed that S. enterica is conditioned for host cell attachment during chemotaxis by these root exudates. Finally, a tentative route of infection is described that includes plant-microbe factors, herewith enabling further design of preventive strategies.

How phosphotransferase system-related protein phosphorylation regulates carbohydrate metabolism in bacteria

The phosphoenolpyruvate(PEP):carbohydrate phosphotransferase system (PTS) is found only in bacteria, where it catalyzes the transport and phosphorylation of numerous monosaccharides, disaccharides, amino sugars, polyols, and other sugar derivatives. To carry out its catalytic function in sugar transport and phosphorylation, the PTS uses PEP as an energy source and phosphoryl donor. The phosphoryl group of PEP is usually transferred via four distinct proteins (domains) to the transported sugar bound to the respective membrane component(s) (EIIC and EIID) of the PTS. The organization of the PTS as a four-step phosphoryl transfer system, in which all P derivatives exhibit similar energy (phosphorylation occurs at histidyl or cysteyl residues), is surprising, as a single protein (or domain) coupling energy transfer and sugar phosphorylation would be sufficient for PTS function. A possible explanation for the complexity of the PTS was provided by the discovery that the PTS also carries out numerous regulatory functions. Depending on their phosphorylation state, the four proteins (domains) forming the PTS phosphorylation cascade (EI, HPr, EIIA, and EIIB) can phosphorylate or interact with numerous non-PTS proteins and thereby regulate their activity. In addition, in certain bacteria, one of the PTS components (HPr) is phosphorylated by ATP at a seryl residue, which increases the complexity of PTS-mediated regulation. In this review, we try to summarize the known protein phosphorylation-related regulatory functions of the PTS. As we shall see, the PTS regulation network not only controls carbohydrate uptake and metabolism but also interferes with the utilization of nitrogen and phosphorus and the virulence of certain pathogens.

Identification of specific chemoattractants and genetic complementation of a Borrelia burgdorferi chemotaxis mutant: flow cytometry-based capillary tube chemotaxis assay

Measuring the chemotactic response of Borrelia burgdorferi, the bacterial species that causes Lyme disease, is relatively more difficult than measuring that of other bacteria. Because these spirochetes have long generation times, enumerating cells that swim up a capillary tube containing an attractant by using colony counts is impractical. Furthermore, direct counts with a Petroff-Hausser chamber is problematic, as this method has a low throughput and necessitates a high cell density; the latter can lead to misinterpretation of results when assaying for specific attractants. Only rabbit serum and tick saliva have been reported to be chemoattractants for B. burgdorferi. These complex biological mixtures are limited in their utility for studying chemotaxis on a molecular level. Here we present a modified capillary tube chemotaxis assay for B. burgdorferi that enumerates cells by flow cytometry. Initial studies identified N-acetylglucosamine as a chemoattractant. The assay was then optimized with respect to cell concentration, incubation time, motility buffer composition, and growth phase. Besides N-acetylglucosamine, glucosamine, glucosamine dimers (chitosan), glutamate, and glucose also elicited significant chemoattractant responses, although the response obtained with glucose was weak and variable. Serine and glycine were nonchemotactic. To further validate and to exploit the use of this assay, a previously described nonchemotactic cheA2 mutant was shown to be nonchemotactic by this assay; it also regained the wild-type phenotype when complemented in trans. This is the first report that identifies specific chemical attractants for B. burgdorferi and the use of flow cytometry for spirochete enumeration. The method should also be useful for assaying chemotaxis for other slow-growing prokaryotic species and in specific environments in nature.

Genetic identification of chemotactic transducers for amino acids in Pseudomonas aeruginosa

Two chemotactic transducer genes (termed pctB and pctC) and an open reading frame (orf1) were found in the pctA-flanking region which was previously identified as a chemotactic transducer gene in Pseudomonas aeruginosa. The pctB and pctC genes encode predicted polypeptides of 629 and 632 amino acids, respectively. Overall, PctB and PctC had 81 and 75% amino acid identities with PctA, respectively. A null mutant strain PCT2, which contained a deletion in the entire pctC, orf1, pctA and pctB genes, did not show chemotaxis towards all 20 commonly occurring L-amino acids. This mutant strain also failed to respond to amino acid catabolites (cadaverine, 4-aminobutyrate and putrescine) that are strong attractants for the wild-type strain PAO1. To study the role of each gene product in L-amino acid taxis, plasmids harbouring the pctC, orf1, pctA, or pctB genes were constructed and introduced into strain PCT2 by transformation. The orf1 gene did not complement the defect in chemotaxis of strain PCT2. The pctA gene restored the ability of strain PCT2 to respond to 18 L-amino acids, suggesting that PctA plays a major role in detecting L-amino acids in P. aeruginosa. The pctB and pctC genes complemented the defect in chemotaxis to only seven (Ala, Arg, Glu, Lys, Met, Tyr, Gln) and two (His, Pro) L-amino acids, respectively.

Phosphoenolpyruvate:carbohydrate phosphotransferase systems of bacteria

Numerous gram-negative and gram-positive bacteria take up carbohydrates through the phosphoenolpyruvate (PEP):carbohydrate phosphotransferase system (PTS). This system transports and phosphorylates carbohydrates at the expense of PEP and is the subject of this review. The PTS consists of two general proteins, enzyme I and HPr, and a number of carbohydrate-specific enzymes, the enzymes II. PTS proteins are phosphoproteins in which the phospho group is attached to either a histidine residue or, in a number of cases, a cysteine residue. After phosphorylation of enzyme I by PEP, the phospho group is transferred to HPr. The enzymes II are required for the transport of the carbohydrates across the membrane and the transfer of the phospho group from phospho-HPr to the carbohydrates. Biochemical, structural, and molecular genetic studies have shown that the various enzymes II have the same basic structure. Each enzyme II consists of domains for specific functions, e.g., binding of the carbohydrate or phosphorylation. Each enzyme II complex can consist of one to four different polypeptides. The enzymes II can be placed into at least four classes on the basis of sequence similarity. The genetics of the PTS is complex, and the expression of PTS proteins is intricately regulated because of the central roles of these proteins in nutrient acquisition. In addition to classical induction-repression mechanisms involving repressor and activator proteins, other types of regulation, such as antitermination, have been observed in some PTSs. Apart from their role in carbohydrate transport, PTS proteins are involved in chemotaxis toward PTS carbohydrates. Furthermore, the IIAGlc protein, part of the glucose-specific PTS, is a central regulatory protein which in its nonphosphorylated form can bind to and inhibit several non-PTS uptake systems and thus prevent entry of inducers. In its phosphorylated form, P-IIAGlc is involved in the activation of adenylate cyclase and thus in the regulation of gene expression. By sensing the presence of PTS carbohydrates in the medium and adjusting the phosphorylation state of IIAGlc, cells can adapt quickly to changing conditions in the environment. In gram-positive bacteria, it has been demonstrated that HPr can be phosphorylated by ATP on a serine residue and this modification may perform a regulatory function.

Relationships between C4 dicarboxylic acid transport and chemotaxis in Rhizobium meliloti

The relationship between chemotaxis and transport of C4 dicarboxylic acids was analyzed with Rhizobium meliloti dct mutants defective in one or all of the genes required for dicarboxylic acid transport. Succinate, malate, and fumarate were moderately potent chemoattractants for wild-type R. meliloti and appeared to share a common chemoreceptor. While dicarboxylate transport is inducible, taxis to succinate was shown to be constitutive. Mutations in the dctA and dctB genes both resulted in the reduction, but not elimination, of chemotactic responses to succinate, indicating that transport via DctA or chemosensing via DctB is not essential for C4 dicarboxylate taxis, although they appear to contribute to it. Mutations in dctD and rpoN genes did not affect taxis to succinate. Aspartate, which is also transported by the dicarboxylate transport system, elicited strong chemotactic responses via a chemoreceptor distinct from the succinate-malate-fumarate receptor. Taxis to aspartate was unaltered in dctA and dctB mutants but was considerably reduced in both dctD and rpoN mutants, indicating that aspartate taxis is strongly dependent on elements responsible for transcriptional activation of dctA. Methylation and methanol release experiments failed to show a significant increase in methyl esterification of R. meliloti proteins in response to any of the attractants tested.

Chemotaxis to chitin oligosaccharides by Vibrio furnissii, a chitinivorous marine bacterium

We have reported that Vibrio furnissii, a chitinivorous marine bacterium, expresses a complex apparatus for adhesion/deadhesion to chitin analogues (1). In the present studies, we show that this organism exhibits a chemotactic response (swarming) to chitin oligosaccharides at concentrations as low as 10 microM. In contrast, V. furnissii exhibits slight to no chemotaxis to other utilizable compounds (glycerol, lactate, amino acids), with the exception of L-glutamic acid. V. furnissii may lack the tar (aspartate) receptor of Escherichia coli.

L-glutamate-induced membrane hyperpolarization and behavioural responses in Paramecium tetraurelia

Paramecium tetraurelia is attracted to L-glutamic acid concentrations of 10(-9) M to 10(-4) M in a behavioural assay. Electrophysiological studies show that P. tetaurelia responds to L-glutamate application with hyperpolarization. This response is transient, even in the continued presence of the stimulus. The concentration dependence of the membrane potential response is similar to that of the behavioural responses, although the threshold concentration of L-glutamate required for hyperpolarization is three orders of magnitude lower than for attraction. The membrane potential response to L-glutamate persists following artificial deciliation of P. tetraurelia. While application of L-glutamate to P. tetraurelia invariably elicits a hyperpolarization, withdrawal of the stimulus frequently results in a second transient membrane response, in the form of either a hyperpolarization or a depolarization. It is suggested that these 'off-responses' may have a significant role in maintaining a behavioural response to L-glutamate.

Evidence against direct involvement of cyclic GMP or cyclic AMP in bacterial chemotactic signaling

Defects in phosphotransferase chemotaxis in cya and cpd mutants previously cited as evidence of a cyclic GMP or cyclic AMP intermediate in signal transduction were not reproduced in a study of chemotaxis in Escherichia coli and Salmonella typhimurium. In cya mutants, which lack adenylate cyclase, the addition of cyclic AMP was required for synthesis of proteins that were necessary for phosphotransferase transport and chemotaxis. However, the induced cells retained normal phosphotransferase chemotaxis after cyclic AMP was removed. Phosphotransferase chemotaxis was normal in a cpd mutant of S. typhimurium that has elevated levels of cyclic GMP and cyclic AMP. S. typhimurium crr mutants are deficient in enzyme III glucose, which is a component of the glucose transport system, and a regulator of adenylate cyclase. After preincubation with cyclic AMP, the crr mutants were deficient in enzyme II glucose-mediated transport and chemotaxis, but other chemotactic responses were normal. It is concluded that cyclic GMP does not determine the frequency of tumbling and is probably not a component of the transduction pathway. The only known role of cyclic AMP is in the synthesis of some proteins that are subject to catabolite repression.

Involvement of lactose enzyme II of the phosphotransferase system in rapid expulsion of free galactosides from Streptococcus pyogenes

Streptococcus pyogenes accumulated thiomethyl-beta-galactoside as the 6-phosphate ester due to the action of the phosphoenolpyruvate:lactose phosphotransferase system. Subsequent addition of glucose resulted in rapid efflux of the free galactoside after intracellular dephosphorylation (inducer expulsion). Efflux was shown to occur in the apparent absence of the galactose permease, but was inhibited by substrate analogs of the lactose enzyme II and could not be demonstrated in a mutant of S. lactis ML3 which lacked this enzyme. The results suggest that the enzymes II of the phosphotransferase system can catalyze the rapid efflux of free sugar under appropriate physiological conditions.

Chemotactic response of Escherichia coli to chemically synthesized amino acids

In Escherichia coli, seven of the commonly occurring amino acids are strong attractants: L-aspartate, L-serine, L-glutamate, L-alanine, L-asparagine, glycine, and L-cysteine, in order of decreasing effectiveness. The chemotactic response to each amino acid attractant is mediated by either methyl-accepting chemotaxis protein I or II, but not by both. Seven of the commonly occurring amino acids are repellents. This work was carried out with chemically synthesized amino acids.

Plasmid-directed synthesis of enzymes required for D-mannitol transport and utilization in Escherichia coli

A transformant Escherichia coli colony bank [Clarke, L. & Carbon, J. (1976) Cell 9, 91-99] has been screened for hybrid ColE1 plasmids carrying the genes for D-mannitol utilization. Two of the plasmids, pLC11-7 and pLC15-48, were shown to contain the mannitol operon, which includes the structural genes for the mannitol-specific enzyme II of the phosphotransferase system and mannitol-1-phosphate dehydrogenase. One E. coli strain harboring plasmid pLC15-48 overproduced mannitol-1-phosphate dehydrogenase activity 4- to 5-fold. However, there was no corresponding increase in mannitol enzyme II activity. Plasmid pLC15-48 was shown to direct the synthesis of two polypeptides in E. coli minicells in the presence of cyclic AMP and mannitol. The larger (Mr = 60,000) was membrane bound and was specifically precipitated by antibody directed against purified mannitol-specific enzyme II. The smaller (Mr = 40,000) was soluble and had an electrophoretic mobility indistinguishable from that of the major component in a partially purified mannitol-1-phosphate dehydrogenase preparation. These data are consistent with previous genetic studies of the mannitol locus and confirm an independent conclusion [Jacobson, G. R., Lee, C. A. & Saier, M. H., Jr. (1979) J. Biol. Chem. 254, 249-252] that mannitol enzyme II consists of a single type of polypeptide chain that has a Mr of 60,000. The plasmid pLC15-48 DNA was characterized by mapping of restriction endonuclease cleavage sites.

The phosphoenolpyruvate-dependent carbohydrate: phosphotransferase system enzymes II as chemoreceptors in chemotaxis of Escherichia coli K 12

In Escherichia coli K12, eight substrate-specific, membrane-bound enzymes II of the PEP-dependent carbohydrate: phosphotransferase system (PTS), specific for hexoses, hexosamines and hexitols, have been characterised in a series of isogenic and constitutive strains. In such mutants, lacking all but one enzyme II, the transport and vectorial phosphorylation activities as well as the chemotactical response in capillary tube assays have been compared. According to the data obtained, all enzymes II not only are directly involved in the transport and vectorial phosphorylation of their substrates, but they have also a primary role as the chemoreceptors for these substrates: (1) Metabolism of the attractant beyond the phosphorylation step is not a pre-requisite to eliciting positive chemotaxis. (2) Mutants, having only one enzyme II react in the capillary tube assay only to substrates of this enzyme II, but not to substrates of the missing enzymes II. This holds for enzymes II consisting of one membrane-bound protein as well as for systems containing a soluble factor III (FIII). (3) The substrate specificities or affinities, whether tested by transport and chemotaxis assays in vivo or by phosphorylation tests in vitro, are in correspondence. (4) The activities of enzymes II, regulated in a complex way at the level of enzyme synthesis and activity and tested as above, are also in agreement, (5) Mutants lacking the soluble proteins enzyme I or HPr of the PTS no longer respond chemotactically to any substrate taken up and phosphorylated by enzymes II. It is concluded that in PTS enzymes II some functions required for transport and chemotaxis are identical. It is suggested furthermore, that the alternation of intrinsic membrane-bound proteins between a phosphorylated and a dephosphorylated state, rather than binding of the substrate to the enzyme II, is the decisive stimulus in the chemotaxis toward carbohydrates taken up by these transport systems.

Genetic dissection of catalytic activities of the Salmonella typhimurium mannitol enzyme II

Approximately 60 mutants of Salmonella typhimurium were isolated which exhibited altered levels of the activities of the mannitol enzyme II. The mutants were grouped into six distinct categories based on their mannitol fermentation, transport, chemotaxis, and phosphorylation activities.

Effect of temperature on Pseudomonas fluorescens chemotaxis

The effects of temperature and attractants on chemotaxis in psychrotrophic Pseudomonas fluorescens were examined using the Adler capillary assay technique. Several organic acids, amino acids, and uronic acids were shown to be attractants, whereas glucose and its oxidation products, gluconate and 2-ketogluconate, elicited no detectable response. Chemotaxis toward many attractants was dependent on prior growth of the microorganism with these compounds. However, the organic acids, malate and succinate, caused strong chemotactic responses regardless of the carbon source used for growth of the bacteria. The temperature at which the cells were grown (30 or 5 degrees C) had no significant detectable effect on chemotaxis to the above attractants. The temperature at which the cells were assayed appeared to affect the rate but the extent of the chemotactic response, nor the concentration response curves. The ratios of the rate of accumulation of cells to the attractant malate were approximately 2, 4, and 1 at 30, 17, and 5 degrees C, respectively. Strong chemotactic responses were observed with cells assayed at temperatures approaching 0 degree C and appeared to be functional over a broad temperature range of 3 to 35 degrees C.

Catalytic activities associated with the enzymes II of the bacterial phosphotransferase system

The phosphotransferase system (PTS) in Escherichia coli is a multifunctional, multicomponent enzyme system. Its primary functions deal with carbon source acquisition, while its secondary functions are concerned with the regulation of bacterial physiology. The primary functions of the system include 1) extracellular detection, 2) unidirectional and exchange transmembrane transport, and 3) phosphoenolpyruvate-dependent and sugar phosphate-dependent phosphorylation of the sugar substrates of the system. The secondary functions include 1) regulation of the activities of adenylate cyclase and various non-PTS permeases and 2) regulation of the induced synthesis of several PTS enzymes. Both the primary and secondary functions appear to be elicited by the binding of a sugar substrate to an Enzyme II complex. One of these integral transmembrane enzymes, the mannitol Enzyme II (IImtl), has been solubilized with detergent, purified to homogeneity, and reconstituted in an artificial membrane system. The molecular weight of this protein, IImtl, is 60,000 daltons. It possesses an extracellular sugar binding site and distinct intracellular combining sites for sugar phosphate and phospho-HPr. An essential sulfhydryl group and an antibody combining site are localized to the cytoplasmic surface of the enzyme, while a dextran combining site is localized to the external surface. Preliminary experiments suggest that the different functions of the Enzyme IImtl can be dissected by genetic and biochemical techniques. These studies emphasize the functional complexity of the PTS and its integral membrane protein constituents.

Response to a metal ion-citrate complex in bacterial sensing

Salmonella typhimurium responds chemotactically to gradients of divalent cations in the presence of citrate ions. The actual chemoeffector is the citrate-metal ion complex, which acts as an attractant. Citrate (which is also a chemoeffector for Salmonella) and the citrate-metal ion complex are recognized by different receptors. The response of Salmonells, which can transport citrate through its membrane, is quite different than that of Escherichia coli, which cannot.

Chemotaxis of Salmonella typhimurium toward citrate

Salmonella, but not Escherichia coli, was attracted to citrate, a distinction that is understandable in view of the inability of E. coli to transport tricarboxylic acids. The Salmonella response to citrate and to two previously described attractants, aspartate and malate, was mutually noncompetitive. Citrate taxis different from citrate uptake in that it did not require Na+, was constitutive, and was not repressible by glucose.