Requirements for conversion of the Na(+)-driven flagellar motor of Vibrio cholerae to the H(+)-driven motor of Escherichia coli

PAS Domain-Containing Chemoreceptors Influence the Signal Sensing and Intestinal Colonization of Vibrio cholerae

Bacterial chemotaxis is the phenomenon in which bacteria migrate toward a more favorable niche in response to chemical cues in the environment. The methyl-accepting chemotaxis proteins (MCPs) are the principal sensory receptors of the bacterial chemotaxis system. Aerotaxis is a special form of chemotaxis in which oxygen serves as the signaling molecule; the process is dependent on the aerotaxis receptors (Aer) containing the Per-Arnt-Sim (PAS) domain. Over 40 MCPs are annotated on the genome of Vibrio cholerae; however, little is known about their functions. We investigated six MCPs containing the PAS domain in V. cholerae El Tor C6706, namely aer2, aer3, aer4, aer5, aer6, and aer7. Deletion analyses of each aer homolog gene indicated that these Aer receptors are involved in aerotaxis, chemotaxis, biofilm formation, and intestinal colonization. Swarming motility assay indicated that the aer2 gene was responsible for sensing the oxygen gradient independent of the other five homologs. When bile salts and mucin were used as chemoattractants, each Aer receptor influenced the chemotaxis differently. Biofilm formation was enhanced by overexpression of the aer6 and aer7 genes. Moreover, deletion of the aer2 gene resulted in better bacterial colonization of the mutant in adult mice; however, virulence gene expression was unaffected. These data suggest distinct roles for different Aer homologs in V. cholerae physiology.

Dynamic Hybrid Flagellar Motors-Fuel Switch and More

Flagellar motors are intricate rotating nanomachines that are powered by transmembrane ion gradients. The stator complexes are the powerhouses of the flagellar motor: They convert a transmembrane ion gradient, mainly of H⁺ or Na⁺, into rotation of the helical flagellar filament. They are thus essential for motor function. The number of stators synchronously engaged in the motor is surprisingly dynamic and depends on the load and the environmental concentration of the corresponding coupling ion. Thus, the rotor-stator interactions determine an important part of the properties of the motor. Numerous bacteria have been identified as possessing more than one set of stators, and some species have been demonstrated to use these different stators in various configurations to modify motor functions by dynamic in-flight swapping. Here, we review knowledge of the properties, the functions, and the evolution of these hybrid motors and discuss questions that remain unsolved.

Achievements in bacterial flagellar research with focus on Vibrio species

In 1980's, the most genes involved in the bacterial flagellar function and formation had been isolated though many of their functions or roles were not clarified. Bacterial flagella are the primary locomotive organ and are not necessary for growing in vitro but are probably essential for living in natural condition and are involved in the pathogenicity. In vitro, the flagella-deficient strains can grow at rates similar to wild-type strains. More than 50 genes are responsible for flagellar function, and the flagellum is constructed by more than 20 structural proteins. The maintenance cost of flagellum is high as several genes are required for its development. The fact that it evolved as a motor organ even with such the high cost shows that the motility is indispensable to survive under the harsh environment of Earth. In this review, we focus on flagella-related research conducted by the authors for about 40 years and flagellar research focused on Vibrio spp. This article is protected by copyright. All rights reserved.

The Structure, Composition, and Role of Periplasmic Stator Scaffolds in Polar Bacterial Flagellar Motors

In the bacterial flagellar motor, the cell-wall-anchored stator uses an electrochemical gradient across the cytoplasmic membrane to generate a turning force that is applied to the rotor connected to the flagellar filament. Existing theoretical concepts for the stator function are based on the assumption that it anchors around the rotor perimeter by binding to peptidoglycan (P). The existence of another anchoring region on the motor itself has been speculated upon, but is yet to be supported by binding studies. Due to the recent advances in electron cryotomography, evidence has emerged that polar flagellar motors contain substantial proteinaceous periplasmic structures next to the stator, without which the stator does not assemble and the motor does not function. These structures have a morphology of disks, as is the case with Vibrio spp., or a round cage, as is the case with Helicobacter pylori. It is now recognized that such additional periplasmic components are a common feature of polar flagellar motors, which sustain higher torque and greater swimming speeds compared to peritrichous bacteria such as Escherichia coli and Salmonella enterica. This review summarizes the data available on the structure, composition, and role of the periplasmic scaffold in polar bacterial flagellar motors and discusses the new paradigm for how such motors assemble and function.

[Flagellar related genes and functions in Vibrio]

Bacteria can move or swim by flagella. On the other hand, the motile ability is not necessary to live at all. In laboratory, the flagella-deficient strains can grow just like the wild-type strains. The flagellum is assembled from more than 20 structural proteins and there are more than 50 genes including the structural genes to regulate or support the flagellar formation. The cost to construct the flagellum is so expensive. The fact that it evolved as a motor organ means even at such the large cost shows that the flagellum is essential for survival in natural condition. In this review, we would like to focus on the flagella-related researches conducted by the authors and the flagellar research on Vibrio spp.

Bacterial motility enhances adhesion to oil droplets

Adhesion of bacteria to liquid-liquid interfaces can play a role in the biodegradation of dispersed hydrocarbons and in biochemical and bioprocess engineering. Whereas thermodynamic factors underpinning adhesion are well studied, the role of bacterial activity on adhesion is less explored. Here, we show that bacterial motility enhances adhesion to surfactant-decorated oil droplets dispersed in artificial sea water. Motile Halomonas titanicae adhered to hexadecane droplets stabilized with dioctyl sodium sulfosuccinate (DOSS) more rapidly and at greater surface densities compared to nonmotile H. titanicae, whose flagellar motion was arrested through addition of a proton uncoupler. Increasing the concentration of DOSS reduced the surface density of both motile and nonmotile bacteria as a result of the reduced interfacial tension.

Coupling Ion Specificity of the Flagellar Stator Proteins MotA1/MotB1 of Paenibacillus sp. TCA20

The bacterial flagellar motor is a reversible rotary molecular nanomachine, which couples ion flux across the cytoplasmic membrane to torque generation. It comprises a rotor and multiple stator complexes, and each stator complex functions as an ion channel and determines the ion specificity of the motor. Although coupling ions for the motor rotation were presumed to be only monovalent cations, such as H⁺ and Na⁺, the stator complex MotA1/MotB1 of Paenibacillus sp. TCA20 (MotA1^TCA/MotB1^TCA) was reported to use divalent cations as coupling ions, such as Ca²⁺ and Mg²⁺. In this study, we initially aimed to measure the motor torque generated by MotA1^TCA/MotB1^TCA under the control of divalent cation motive force; however, we identified that the coupling ion of MotA1^TCAMotB1^TCA is very likely to be a monovalent ion. We engineered a series of functional chimeric stator proteins between MotB1^TCA and Escherichia coli MotB. E. coli ΔmotAB cells expressing MotA1^TCA and the chimeric MotB presented significant motility in the absence of divalent cations. Moreover, we confirmed that MotA1^TCA/MotB1^TCA in Bacillus subtilis ΔmotABΔmotPS cells generates torque without divalent cations. Based on two independent experimental results, we conclude that the MotA1^TCA/MotB1^TCA complex directly converts the energy released from monovalent cation flux to motor rotation.

Anti-virulence activity of polyphenolic fraction isolated from Kombucha against Vibrio cholerae

The use of traditional foods and beverages or their bioactive compounds as anti-virulence agents is a new alternative method to overcome the increased global emergence of antimicrobial resistance in enteric pathogens. In the present study, we investigated the anti-virulence activity of a polyphenolic fraction previously isolated from Kombucha, a 14-day fermented beverage of sugared black tea, against Vibrio cholerae O1. The isolated fraction was mainly composed of the polyphenols catechin and isorhamnetin. The fraction, the individual polyphenols and the combination of the individual polyphenols significantly inhibited bacterial swarming motility and expression of flagellar regulatory genes motY and flaC, even at sub-inhibitory concentrations. The polyphenolic compounds also decreased bacterial protease secretion and mucin penetration in vitro. In vivo study revealed that the polyphenolic fraction significantly inhibited V. cholerae induced fluid accumulation in the rabbit ileal loop model and intestinal colonization in suckling mice model. Therefore, the anti-virulence activity of the Kombucha polyphenolic fraction involved inhibition of motility and protease secretion of V. cholerae, thus preventing bacterial penetration through the mucin layer as well as fluid accumulation and bacterial colonization in the intestinal epithelial cells. The overall results implied that Kombucha might be considered as a potential alternative source of anti-virulence polyphenols against V. cholerae. To the best of our knowledge, this is the first report on the anti-virulence activity of Kombucha, mostly attributed to its polyphenolic content.

Amino acids as wetting agents: surface translocation by Porphyromonas gingivalis

Our understanding of how oral microbiota adapt in response to changes in their surroundings remains limited. This is particularly true of the slow-growing anaerobes that persist below the gum line. Here, we report that the oral anaerobe Porphyromonas gingivalis strain 381 can surface translocate when sandwiched between two surfaces. We show that during movement, this bacterium alters its metabolism, specifically side products of arginine utilization including citrulline and ornithine accumulated in the translocating cells; while arginine, N-acetyl-arginine, and the polyamine putrescine, which is produced from arginine were consumed. In addition, our results indicate that movement requires modification of the surrounding environment via proteolysis, cell dispersion, cell-on-cell rolling, and sub-diffusive cell-driven motility. We also show that production of fimbriae and fimbriae-associated proteins; as well as the regulation of contact-dependent growth inhibition genes, which are known to be involved in self-nonself discrimination, and the type IX secretion system are central to surface translocation. These studies provide a first glimpse into P. gingivalis motility and its relationship to ecological variables.

The role of conserved charged residues in the bidirectional rotation of the bacterial flagellar motor

Many bacteria rotate their flagella both counterclockwise (CCW) and clockwise (CW) to achieve swimming toward attractants or away from repellents. Highly conserved charged residues are important for that motility, which suggests that electrostatic interactions are crucial for the rotor-stator function. It remains unclear if those residues contribute equally to rotation in the CCW and CW directions. To address this uncertainty, in this study, we expressed chimeric rotors and stators from Vibrio alginolyticus and Escherichia coli in E. coli, and measured the rotational speed of each motor in both directions using a tethered-cell assay. In wild-type cells, the rotational speeds in both directions were equal, as demonstrated previously. Some charge-neutralizing residue replacements in the stator decreased the rotational speed in both directions to the same extent. However, mutations in two charged residues in the rotor decreased the rotational speed only in the CCW direction. Subsequent analysis and previous results suggest that these amino acid residues are involved in supporting the conformation of the rotor, which is important for proper torque generation in the CCW direction.

Mutations targeting the plug-domain of the Shewanella oneidensis proton-driven stator allow swimming at increased viscosity and under anaerobic conditions

Shewanella oneidensis MR-1 possesses two different stator units to drive flagellar rotation, the Na⁺ -dependent PomAB stator and the H⁺ -driven MotAB stator, the latter possibly acquired by lateral gene transfer. Although either stator can independently drive swimming through liquid, MotAB-driven motors cannot support efficient motility in structured environments or swimming under anaerobic conditions. Using ΔpomAB cells we isolated spontaneous mutants able to move through soft agar. We show that a mutation that alters the structure of the plug domain in MotB affects motor functions and allows cells to swim through media of increased viscosity and under anaerobic conditions. The number and exchange rates of the mutant stator around the rotor were not significantly different from wild-type stators, suggesting that the number of stators engaged is not the cause of increased swimming efficiency. The swimming speeds of planktonic mutant MotAB-driven cells was reduced, and overexpression of some of these stators caused reduced growth rates, implying that mutant stators not engaged with the rotor allow some proton leakage. The results suggest that the mutations in the MotB plug domain alter the proton interactions with the stator ion channel in a way that both increases torque output and allows swimming at decreased pmf values.

Functional genomics analysis of free fatty acid production under continuous phosphate limiting conditions

Free fatty acids (FFA) are an attractive platform chemical that serves as a functional intermediate in metabolic pathways for producing oleochemicals. Many groups have established strains of Escherichia coli capable of producing various chain-length mixtures of FFA by heterologous expression of acyl-ACP thioesterases. For example, high levels of dodecanoic acid are produced by an E. coli strain expressing the Umbellularia californica FatB2 thioesterase, BTE. Prior studies achieved high dodecanoic acid yields and productivities under phosphate-limiting media conditions. In an effort to understand the metabolic and physiological changes that led to increased FFA production, the transcriptome of this strain was assessed as a function of nutrient limitation and growth rate. FFA generation under phosphate limitation led to consistent changes in transporter expression, osmoregulation, and central metabolism. Guided by these results, targeted knockouts led to a further ~11 % in yield in FFA.

Rotation of Vibrio fischeri Flagella Produces Outer Membrane Vesicles That Induce Host Development

Using the squid-vibrio association, we aimed to characterize the mechanism through which Vibrio fischeri cells signal morphogenesis of the symbiotic light-emitting organ. The symbiont releases two cell envelope molecules, peptidoglycan (PG) and lipopolysaccharide (LPS) that, within 12 h of light organ colonization, act in synergy to trigger normal tissue development. Recent work has shown that outer membrane vesicles (OMVs) produced by V. fischeri are sufficient to induce PG-dependent morphogenesis; however, the mechanism(s) of OMV release by these bacteria has not been described. Like several genera of both beneficial and pathogenic bacteria, V. fischeri cells elaborate polar flagella that are enclosed by an extension of the outer membrane, whose function remains unclear. Here, we present evidence that along with the well-recognized phenomenon of blebbing from the cell's surface, rotation of this sheathed flagellum also results in the release of OMVs. In addition, we demonstrate that most of the development-inducing LPS is associated with these OMVs and that the presence of the outer membrane protein OmpU but not the LPS O antigen on these OMVs is important in triggering normal host development. These results also present insights into a possible new mechanism of LPS release by pathogens with sheathed flagella.

IMPORTANCE

Determining the function(s) of sheathed flagella in bacteria has been challenging, because no known mutation results only in the loss of this outer membrane-derived casing. Nevertheless, the presence of a sheathed flagellum in such host-associated genera as Vibrio, Helicobacter, and Brucella has led to several proposed functions, including physical protection of the flagella and masking of their immunogenic flagellins. Using the squid-vibrio light organ symbiosis, we demonstrate another role, that of V. fischeri cells require rotating flagella to induce apoptotic cell death within surface epithelium, which is a normal step in the organ's development. Further, we present evidence that this rotation releases apoptosis-triggering lipopolysaccharide in the form of outer membrane vesicles. Such release may also occur by pathogens but with different outcomes for the host.

Response of Vibrio cholerae to the Catecholamine Hormones Epinephrine and Norepinephrine

In Escherichia coli or Salmonella enterica, the stress-associated mammalian hormones epinephrine (E) and norepinephrine (NE) trigger a signaling cascade by interacting with the QseC sensor protein. Here we show that Vibrio cholerae, the causative agent of cholera, exhibits a specific response to E and NE. These catecholates (0.1 mM) enhanced the growth and swimming motility of V. cholerae strain O395 on soft agar in a medium containing calf serum, which simulated the environment within the host. During growth, the hormones were converted to degradation products, including adrenochrome formed by autooxidation with O2 or superoxide. In E. coli, the QseC sensor kinase, which detects the autoinducer AI-3, also senses E or NE. The genome of V. cholerae O395 comprises an open reading frame coding for a putative protein with 29% identity to E. coli QseC. Quantitative reverse transcriptase PCR (qRT-PCR) experiments revealed increased transcript levels of the qseC-like gene and of pomB, a gene encoding a structural component of the flagellar motor complex, under the influence of E or NE. Phentolamine blocks the response of E. coli QseC to E or NE. A V. cholerae mutant devoid of the qseC-like gene retained the phentolamine-sensitive motility in the presence of E, whereas NE-stimulated motility was no longer inhibited by phentolamine. Our study demonstrates that V. cholerae senses the stress hormones E and NE. A sensor related to the histidine kinase QseC from E. coli is identified and is proposed to participate in the sensing of NE.

IMPORTANCE

Vibrio cholerae is a Gram-negative bacterium that may cause cholera, a severe illness with high mortality due to acute dehydration caused by diarrhea and vomiting. Pathogenic V. cholerae strains possess virulence factors like the cholera toxin (CTX) and the toxin-coregulated pilus (TCP) produced in response to signals provided by the host. In pathogenic enterobacteria, the stress-associated hormones epinephrine (E) and norepinephrine (NE) of the human host act as signal molecules for the production of virulence factors and promote bacterial growth by the sequestration of iron from the host. Here we show that V. cholerae, like some enterobacteria, benefits from these stress hormones and possesses a sensor to recognize them.

Serine 26 in the PomB subunit of the flagellar motor is essential for hypermotility of Vibrio cholerae

Vibrio cholerae is motile by means of its single polar flagellum which is driven by the sodium-motive force. In the motor driving rotation of the flagellar filament, a stator complex consisting of subunits PomA and PomB converts the electrochemical sodium ion gradient into torque. Charged or polar residues within the membrane part of PomB could act as ligands for Na⁺, or stabilize a hydrogen bond network by interacting with water within the putative channel between PomA and PomB. By analyzing a large data set of individual tracks of swimming cells, we show that S26 located within the transmembrane helix of PomB is required to promote very fast swimming of V. cholerae. Loss of hypermotility was observed with the S26T variant of PomB at pH 7.0, but fast swimming was restored by decreasing the H⁺ concentration of the external medium. Our study identifies S26 as a second important residue besides D23 in the PomB channel. It is proposed that S26, together with D23 located in close proximity, is important to perturb the hydration shell of Na⁺ before its passage through a constriction within the stator channel.

Roles of the sodium-translocating NADH:quinone oxidoreductase (Na+-NQR) on vibrio cholerae metabolism, motility and osmotic stress resistance

The Na⁺ translocating NADH:quinone oxidoreductase (Na⁺-NQR) is a unique respiratory enzyme catalyzing the electron transfer from NADH to quinone coupled with the translocation of sodium ions across the membrane. Typically, Vibrio spp., including Vibrio cholerae, have this enzyme but lack the proton-pumping NADH:ubiquinone oxidoreductase (Complex I). Thus, Na⁺-NQR should significantly contribute to multiple aspects of V. cholerae physiology; however, no detailed characterization of this aspect has been reported so far. In this study, we broadly investigated the effects of loss of Na⁺-NQR on V. cholerae physiology by using Phenotype Microarray (Biolog), transcriptome and metabolomics analyses. We found that the V. cholerae ΔnqrA-F mutant showed multiple defects in metabolism detected by Phenotype Microarray. Transcriptome analysis revealed that the V. cholerae ΔnqrA-F mutant up-regulates 31 genes and down-regulates 55 genes in both early and mid-growth phases. The most up-regulated genes included the cadA and cadB genes, encoding a lysine decarboxylase and a lysine/cadaverine antiporter, respectively. Increased CadAB activity was further suggested by the metabolomics analysis. The down-regulated genes include sialic acid catabolism genes. Metabolomic analysis also suggested increased reductive pathway of TCA cycle and decreased purine metabolism in the V. cholerae ΔnqrA-F mutant. Lack of Na⁺-NQR did not affect any of the Na⁺ pumping-related phenotypes of V. cholerae suggesting that other secondary Na⁺ pump(s) can compensate for Na⁺ pumping activity of Na⁺-NQR. Overall, our study provides important insights into the contribution of Na⁺-NQR to V. cholerae physiology.

A model symbiosis reveals a role for sheathed-flagellum rotation in the release of immunogenic lipopolysaccharide

Bacterial flagella mediate host–microbe interactions through tissue tropism during colonization, as well as by activating immune responses. The flagellar shaft of some bacteria, including several human pathogens, is encased in a membranous sheath of unknown function. While it has been hypothesized that the sheath may allow these bacteria to evade host responses to the immunogenic flagellin subunit, this unusual structural feature has remained an enigma. Here we demonstrate that the rotation of the sheathed flagellum in both the mutualist Vibrio fischeri and the pathogen Vibrio cholerae promotes release of a potent bacteria-derived immunogen, lipopolysaccharide, found in the flagellar sheath. We further present a new role for the flagellar sheath in triggering, rather than circumventing, host immune responses in the model squid-vibrio symbiosis. Such an observation not only has implications for the study of bacterial pathogens with sheathed flagella, but also raises important biophysical questions of sheathed-flagellum function.

DOI:http://dx.doi.org/10.7554/eLife.01579.001

While a few of the bacteria that live in and on the bodies of humans and other animals are harmful and can cause disease, most others can offer benefits to their hosts. Many bacteria—including some important human pathogens—have tails called flagella that rotate to move the bacteria inside its host. However, the immune system can detect parts of these flagella and eliminate the pathogen.

Bacterial flagella are made from filaments of proteins, and some flagella are also enclosed by a sheath that is similar to the outer membrane that encloses certain bacteria. The function of this sheath is unclear, although some researchers have suggested that it might prevent the immune system from detecting the proteins in the flagellum. Now, by studying the interactions between the Hawaiian bobtail squid and a marine bacterium, Brennan et al. show that the sheath can actually alert the host that the bacteria are around.

The Hawaiian bobtail squid collects bioluminescent bacteria within a so-called ‘light organ’. This organ undergoes a number of developmental changes to house the bacteria, and the squid then uses the light from the bacteria to mask its own shadow, which helps it to avoid being detected by predators. Brennan et al. compared how wild-type bacteria and mutant bacteria that either had no flagella, or had flagella that did not rotate, interacted with young squid. Only bacteria with working flagella were able to trigger the normal development of the squid’s light organ, which suggests that the rotating flagella are releasing the signal that tells the squid that the beneficial bacteria are present.

Brennan et al. demonstrated that the rotation of sheathed flagella led to the release of a molecule called lipopolysaccharide. This molecule is known to activate the immune system in animals, and it is one of the bacterial signals that the squid responds to. Moreover, when the flagella of other bacteria with sheaths—such as those that cause cholera—are rotating, there is also an increase in the release of lipopolysaccharide. However, rotation of the flagella of bacteria without sheaths has no such effect. The next challenge will be to test the importance of this release of lipopolysaccharide from rotating flagella on the outcome of bacterial diseases of humans and other animals.

DOI:http://dx.doi.org/10.7554/eLife.01579.002

The function of the Na+-driven flagellum of Vibrio cholerae is determined by osmolality and pH

Vibrio cholerae is motile by its polar flagellum, which is driven by a Na(+)-conducting motor. The stators of the motor, composed of four PomA and two PomB subunits, provide access for Na(+) to the torque-generating unit of the motor. To characterize the Na(+) pathway formed by the PomAB complex, we studied the influence of chloride salts (chaotropic, Na(+), and K(+)) and pH on the motility of V. cholerae. Motility decreased at elevated pH but increased if a chaotropic chloride salt was added, which rules out a direct Na(+) and H(+) competition in the process of binding to the conserved PomB D23 residue. Cells expressing the PomB S26A/T or D42N variants lost motility at low Na(+) concentrations but regained motility in the presence of 170 mM chloride. Both PomA and PomB were modified by N,N'-dicyclohexylcarbodiimide (DCCD), indicating the presence of protonated carboxyl groups in the hydrophobic regions of the two proteins. Na(+) did not protect PomA and PomB from this modification. Our study shows that both osmolality and pH have an influence on the function of the flagellum from V. cholerae. We propose that D23, S26, and D42 of PomB are part of an ion-conducting pathway formed by the PomAB stator complex.

A quinazoline-2,4-diamino analog suppresses Vibrio cholerae flagellar motility by interacting with motor protein PomB and induces envelope stress

Vibrio cholerae strains of serogroups O1 and O139, the causative agents of the diarrheal illness cholera, express a single polar flagellum powered by sodium motive force and require motility to colonize and spread along the small intestine. In a previous study, we described a high-throughput assay for screening for small molecules that selectively inhibit bacterial motility and identified a family of quinazoline-2,4-diamino analogs (Q24DAs) that (i) paralyzed the sodium-driven polar flagellum of Vibrios and (ii) diminished cholera toxin secreted by El Tor biotype V. cholerae. In this study, we provide evidence that a Q24DA paralyzes the polar flagellum by interacting with the motor protein PomB. Inhibition of motility with the Q24DA enhanced the transcription of the cholera toxin genes in both biotypes. We also show that the Q24DA interacts with outer membrane protein OmpU and other porins to induce envelope stress and expression of the extracellular RNA polymerase sigma factor σ(E). We suggest that Q24DA-induced envelope stress could affect the correct folding, assembly, and secretion of pentameric cholera toxin in El Tor biotype V. cholerae independently of its effect on motility.

Specificity of motor components in the dual flagellar system of Shewanella putrefaciens CN-32

Bacterial flagellar motors are intricate nanomachines in which the stator units and rotor component FliM may be dynamically exchanged during function. Similar to other bacterial species, the gammaproteobacterium Shewanella putrefaciens CN-32 possesses a complete secondary flagellar system along with a corresponding stator unit. Expression of the secondary system occurs during planktonic growth in complex media and leads to the formation of a subpopulation with one or more additional flagella at random positions in addition to the primary polar system. We used physiological and phenotypic characterizations of defined mutants in concert with fluorescent microscopy on labelled components of the two different systems, the stator proteins PomB and MotB, the rotor components FliM(1) and FliM(2), and the auxiliary motor components MotX and MotY, to determine localization, function and dynamics of the proteins in the flagellar motors. The results demonstrate that the polar flagellum is driven by a Na(+)-dependent FliM(1)/PomAB/MotX/MotY flagellar motor while the secondary system is rotated by a H(+)-dependent FliM(2)/MotAB motor. The components were highly specific for their corresponding motor and are unlikely to be extensively swapped or shared between the two flagellar systems under planktonic conditions. The results have implications for both specificity and dynamics of flagellar motor components.

Step-wise loss of bacterial flagellar torsion confers progressive phagocytic evasion

Phagocytosis of bacteria by innate immune cells is a primary method of bacterial clearance during infection. However, the mechanisms by which the host cell recognizes bacteria and consequentially initiates phagocytosis are largely unclear. Previous studies of the bacterium Pseudomonas aeruginosa have indicated that bacterial flagella and flagellar motility play an important role in colonization of the host and, importantly, that loss of flagellar motility enables phagocytic evasion. Here we use molecular, cellular, and genetic methods to provide the first formal evidence that phagocytic cells recognize bacterial motility rather than flagella and initiate phagocytosis in response to this motility. We demonstrate that deletion of genes coding for the flagellar stator complex, which results in non-swimming bacteria that retain an initial flagellar structure, confers resistance to phagocytic binding and ingestion in several species of the gamma proteobacterial group of Gram-negative bacteria, indicative of a shared strategy for phagocytic evasion. Furthermore, we show for the first time that susceptibility to phagocytosis in swimming bacteria is proportional to mot gene function and, consequently, flagellar rotation since complementary genetically- and biochemically-modulated incremental decreases in flagellar motility result in corresponding and proportional phagocytic evasion. These findings identify that phagocytic cells respond to flagellar movement, which represents a novel mechanism for non-opsonized phagocytic recognition of pathogenic bacteria.

Flagella-driven bacterial motility, referred to as swimming, has been recognized for over 20 years to affect the ability of bacteria to infect and colonize a host. The common theme is that bacteria must be motile to colonize the host but must become non-motile to chronically persist; this has been observed in many pathogenic bacteria including species of Vibrio and Pseudomonas. Therefore it makes sense that the immune system would evolve mechanisms to exploit this virulence determinant of pathogenic bacteria. Here we present evidence that flagellar motility is recognized by innate immune cells as a phagocytic activation signal. We show that step-wise loss of flagellar motility confers a proportional ability to evade phagocytic engulfment, independent of the flagellum itself acting as a phagocytic activator. This is not due to motility- co-regulated secretions or compensatory genetic changes by the bacteria, but instead is due to a mechano-sensory response whereby phagocytic cells respond directly to flagellar motility. This represents a novel mechanism by which the innate immune system facilitates clearance of bacterial pathogens, and provides an explanation for how selective pressure may result in bacteria with down-regulated flagellar gene expression and motility as is observed in isolates taken from chronic infections.

A high-throughput screening assay for inhibitors of bacterial motility identifies a novel inhibitor of the Na+-driven flagellar motor and virulence gene expression in Vibrio cholerae

Numerous bacterial pathogens, particularly those that colonize fast-flow areas in the bladder and gastrointestinal tract, require motility to establish infection and spread beyond the initially colonized tissue. Vibrio cholerae strains of serogroups O1 and O139, the causative agents of the diarrheal illness cholera, express a single polar flagellum powered by sodium motive force and require motility to colonize and spread along the small intestine. Therefore, motility may be an attractive target for small molecules that can prevent and/or block the infective process. In this study, we describe a high-throughput screening (HTS) assay to identify small molecules that selectively inhibit bacterial motility. The HTS assay was used to screen an ∼8,000-compound structurally diverse chemical library for inhibitors of V. cholerae motility. The screen identified a group of quinazoline-2,4-diamino analogs that completely suppressed motility without affecting the growth rate in broth. A further study on the effects of one analog, designated Q24DA, showed that it induces a flagellated but nonmotile (Mot(-)) phenotype and is specific for the Na(+)-driven flagellar motor of pathogenic Vibrio species. A mutation conferring phenamil-resistant motility did not eliminate inhibition of motility by Q24DA. Q24DA diminished the expression of cholera toxin and toxin-coregulated pilus as well as biofilm formation and fluid secretion in the rabbit ileal loop model. Furthermore, treatment of V. cholerae with Q24DA impacted additional phenotypes linked to Na(+) bioenergetics, such as the function of the primary Na(+) pump, Nqr, and susceptibility to fluoroquinolones. The above results clearly show that the described HTS assay is capable of identifying small molecules that specifically block bacterial motility. New inhibitors such as Q24DA may be instrumental in probing the molecular architecture of the Na(+)-driven polar flagellar motor and in studying the role of motility in the expression of other virulence factors.

Structure of the flagellar motor protein complex PomAB: implications for the torque-generating conformation

The bacterial flagellar motor is driven by an ion flux through a channel called MotAB in Escherichia coli or Salmonella and PomAB in Vibrio alginolyticus. PomAB is composed of two transmembrane (TM) components, PomA and PomB, and converts a sodium ion flux to rotation of the flagellum. Its homolog, MotAB, utilizes protons instead of sodium ions. PomB/MotB has a peptidoglycan (PG)-binding motif in the periplasmic domain, allowing it to function as the stator by being anchored to the PG layer. To generate torque, PomAB/MotAB is thought to undergo a conformational change triggered by the ion flux and to interact directly with FliG, a component of the rotor. Here, we present the first three-dimensional structure of this torque-generating stator unit analyzed by electron microscopy. The structure of PomAB revealed two arm domains, which contain the PG-binding site, connected to a large base made of the TM and cytoplasmic domains. The arms lean downward to the membrane surface, likely representing a "plugged" conformation, which would prevent ions leaking through the channel. We propose a model for how PomAB units are placed around the flagellar basal body to function as torque generators.

Tuning the flagellar motor

Many bacteria are motile by means of flagella, semi-rigid helical filaments rotated at the filament's base and energized by proton or sodium-ion gradients. Torque is created between the two major components of the flagellar motor: the rotating switch complex and the cell-wall-associated stators, which are arranged in a dynamic ring-like structure. Being motile provides a survival advantage to many bacteria, and thus the flagellar motor should work optimally under a wide range of environmental conditions. Recent studies have demonstrated that numerous species possess a single flagellar system but have two or more individual stator systems that contribute differentially to flagellar rotation. This review describes recent findings on rotor–stator interactions, on the role of different stators, and on how stator selection could be regulated. An emerging model suggests that bacterial flagellar motors are dynamic and can be tuned by stator swapping in response to different environmental conditions.

Role of FlgT in anchoring the flagellum of Vibrio cholerae

Flagellar motility has long been regarded as an important virulence factor. In Vibrio cholerae, the single polar flagellum is essential for motility as well as for proper attachment and colonization. In this study, we demonstrate that the novel flagellar protein FlgT is involved in anchoring the flagellum to the V. cholerae cell. A screen for novel colonization factors by use of TnphoA mutagenesis identified flgT. An in-frame deletion of flgT established that FlgT is required for attachment, colonization, and motility. Transmission electron microscopy revealed that while the flgT mutant is capable of assembling a phenotypically normal flagellum, the flgT population is mostly aflagellate compared to the wild-type population. Further analyses indicated that the flagellum of the flgT mutant is released into the culture supernatant from the cell upon completion of assembly. Additionally, hook basal body complexes appear to be released along with the filament. These results indicate that FlgT functions to stabilize the flagellar apparatus at the pole of the cell.

Characterization of two outer membrane proteins, FlgO and FlgP, that influence vibrio cholerae motility

Vibrio cholerae is highly motile by the action of a single polar flagellum. The loss of motility reduces the infectivity of V. cholerae, demonstrating that motility is an important virulence factor. FlrC is the sigma-54-dependent positive regulator of flagellar genes. Recently, the genes VC2206 (flgP) and VC2207 (flgO) were identified as being regulated by FlrC via a microarray analysis of an flrC mutant (D. C. Morris, F. Peng, J. R. Barker, and K. E. Klose, J. Bacteriol. 190:231-239, 2008). FlgP is reported to be an outer membrane lipoprotein required for motility that functions as a colonization factor. The study reported here focuses on the characterization of flgO, the first gene in the flgOP operon. We show that FlgO and FlgP are important for motility, as strains with mutations in the flgOP genes have reduced motility phenotypes. The flgO and flgP mutant populations display fewer motile cells as well as reduced numbers of flagellated cells. The flagella produced by the flgO and flgP mutant strains are shorter in length than the wild-type flagella, which can be restored by inhibiting rotation of the flagellum. FlgO is an outer membrane protein that localizes throughout the membrane and not at the flagellar pole. Although FlgO and FlgP do not specifically localize to the flagellum, they are required for flagellar stability. Due to the nature of these motility defects, we established that the flagellum is not sufficient for adherence; rather, motility is the essential factor required for attachment and thus colonization by V. cholerae O1 of the classical biotype. This study reveals a novel mechanism for which the outer membrane proteins FlgO and FlgP function in motility to mediate flagellar stability and influence attachment and colonization.

Functional role of a conserved aspartic acid residue in the motor of the Na(+)-driven flagellum from Vibrio cholerae

The flagellar motor consists of a rotor and a stator and couples the flux of cations (H(+) or Na(+)) to the generation of the torque necessary to drive flagellum rotation. The inner membrane proteins PomA and PomB are stator components of the Na(+)-driven flagellar motor from Vibrio cholerae. Affinity-tagged variants of PomA and PomB were co-expressed in trans in the non-motile V. cholerae pomAB deletion strain to study the role of the conserved D23 in the transmembrane helix of PomB. At pH 9, the D23E variant restored motility to 100% of that observed with wild type PomB, whereas the D23N variant resulted in a non-motile phenotype, indicating that a carboxylic group at position 23 in PomB is important for flagellum rotation. Motility tests at decreasing pH revealed a pronounced decline of flagellar function with a motor complex containing the PomB-D23E variant. It is suggested that the protonation state of the glutamate residue at position 23 determines the performance of the flagellar motor by altering the affinity of Na(+) to PomB. The conserved aspartate residue in the transmembrane helix of PomB and its H(+)-dependent homologs might act as a ligand for the coupling cation in the flagellar motor.

MotX and MotY are required for flagellar rotation in Shewanella oneidensis MR-1

The single polar flagellum of Shewanella oneidensis MR-1 is powered by two different stator complexes, the sodium-dependent PomAB and the proton-driven MotAB. In addition, Shewanella harbors two genes with homology to motX and motY of Vibrio species. In Vibrio, the products of these genes are crucial for sodium-dependent flagellar rotation. Resequencing of S. oneidensis MR-1 motY revealed that the gene does not harbor an authentic frameshift as was originally reported. Mutational analysis demonstrated that both MotX and MotY are critical for flagellar rotation of S. oneidensis MR-1 for both sodium- and proton-dependent stator systems but do not affect assembly of the flagellar filament. Fluorescence tagging of MotX and MotY to mCherry revealed that both proteins localize to the flagellated cell pole depending on the presence of the basal flagellar structure. Functional localization of MotX requires MotY, whereas MotY localizes independently of MotX. In contrast to the case in Vibrio, neither protein is crucial for the recruitment of the PomAB or MotAB stator complexes to the flagellated cell pole, nor do they play a major role in the stator selection process. Thus, MotX and MotY are not exclusive features of sodium-dependent flagellar systems. Furthermore, MotX and MotY in Shewanella, and possibly also in other genera, must have functions beyond the recruitment of the stator complexes.

Two redundant sodium-driven stator motor proteins are involved in Aeromonas hydrophila polar flagellum rotation

Motility is an essential characteristic for mesophilic Aeromonas strains. We identified a new polar flagellum region (region 6) in the A. hydrophila AH-3 (serotype O34) chromosome that contained two additional polar stator genes, named pomA2 and pomB2. A. hydrophila PomA2 and PomB2 are highly homologous to other sodium-conducting polar flagellum stator motors as well as to the previously described A. hydrophila AH-3 PomA and PomB. pomAB and pomA2B2 were present in all the mesophilic Aeromonas strains tested and were independent of the strains' ability to produce lateral flagella. Unlike MotX, which is a stator protein that is essential for polar flagellum rotation, here we demonstrate that PomAB and PomA2B2 are redundant sets of proteins, as neither set on its own is essential for polar flagellum motility in either aqueous or high-viscosity environments. Both PomAB and PomA2B2 are sodium-coupled stator complexes, although PomA2B2 is more sensitive to low concentrations of sodium than PomAB. Furthermore, the level of transcription in aqueous and high-viscosity environments of pomA2B2 is reduced compared to that of pomAB. The A. hydrophila AH-3 polar flagellum is the first case described in which two redundant sodium-driven stator motor proteins (PomAB and PomA2B2) are found.

Genetic analysis of Vibrio cholerae monolayer formation reveals a key role for DeltaPsi in the transition to permanent attachment

A bacterial monolayer biofilm is a collection of cells attached to a surface but not to each other. Monolayer formation is initiated when a bacterial cell forms a transient attachment to a surface. While some transient attachments are broken, others transition into the permanent attachments that define a monolayer biofilm. In this work, we describe the results of a large-scale, microscopy-based genetic screen for Vibrio cholerae mutants that are defective in formation of a monolayer biofilm. This screen identified mutations that alter both transient and permanent attachment. Transient attachment was somewhat slower in the absence of flagellar motility. However, flagellar mutants eventually formed a robust monolayer. In contrast, in the absence of the flagellar motor, monolayer formation was severely impaired. A number of proteins that modulate the V. cholerae ion motive force were also found to affect the transition from transient to permanent attachment. Using chemicals that dissipate various components of the ion motive force, we discovered that dissipation of the membrane potential (DeltaPsi) completely blocks the transition from transient to permanent attachment. We propose that as a bacterium approaches a surface, the interaction of the flagellum with the surface leads to transient hyperpolarization of the bacterial cell membrane. This, in turn, initiates the transition to permanent attachment.

Bacterial flagellar motor

The bacterial flagellar motor is a reversible rotary nano-machine, about 45 nm in diameter, embedded in the bacterial cell envelope. It is powered by the flux of H+or Na+ions across the cytoplasmic membrane driven by an electrochemical gradient, the proton-motive force or the sodium-motive force. Each motor rotates a helical filament at several hundreds of revolutions per second (hertz). In many species, the motor switches direction stochastically, with the switching rates controlled by a network of sensory and signalling proteins. The bacterial flagellar motor was confirmed as a rotary motor in the early 1970s, the first direct observation of the function of a single molecular motor. However, because of the large size and complexity of the motor, much remains to be discovered, in particular, the structural details of the torque-generating mechanism. This review outlines what has been learned about the structure and function of the motor using a combination of genetics, single-molecule and biophysical techniques, with a focus on recent results and single-molecule techniques.

Insights into the stator assembly of the Vibrio flagellar motor from the crystal structure of MotY

Rotation of the sodium-driven polar flagellum of Vibrio alginolyticus requires four motor proteins: PomA, PomB, MotX, and MotY. PomA and PomB form a sodium-ion channel in the cytoplasmic membrane that functions as a stator complex to couple sodium-ion flux with torque generation. MotX and MotY are components of the T-ring, which is located beneath the P-ring of the polar flagellar basal body and is involved in incorporation of the PomA/PomB complex into the motor. Here, we describe the determination of the crystal structure of MotY at 2.9 A resolution. The structure shows two distinct domains: an N-terminal domain (MotY-N) and a C-terminal domain (MotY-C). MotY-N has a unique structure. MotY-C contains a putative peptidoglycan-binding motif that is remarkably similar to those of peptidoglycan-binding proteins, such as Pal and RmpM, but this region is disordered in MotY. Motility assay of cells producing either of the MotY-N and MotY-C fragments and subsequent biochemical analyses indicate that MotY-N is essential for association of the stator units around the rotor, whereas MotY-C stabilizes the association by binding to the peptidoglycan layer. Based on these observations, we propose a model for the mechanism of stator assembly around the rotor.

Roles of charged residues in the C-terminal region of PomA, a stator component of the Na+-driven flagellar motor

Bacterial flagellar motors use specific ion gradients to drive their rotation. It has been suggested that the electrostatic interactions between charged residues of the stator and rotor proteins are important for rotation in Escherichia coli. Mutational studies have indicated that the Na(+)-driven motor of Vibrio alginolyticus may incorporate interactions similar to those of the E. coli motor, but the other electrostatic interactions between the rotor and stator proteins may occur in the Na(+)-driven motor. Thus, we investigated the C-terminal charged residues of the stator protein, PomA, in the Na(+)-driven motor. Three of eight charge-reversing mutations, PomA(K203E), PomA(R215E), and PomA(D220K), did not confer motility either with the motor of V. alginolyticus or with the Na(+)-driven chimeric motor of E. coli. Overproduction of the R215E and D220K mutant proteins but not overproduction of the K203E mutant protein impaired the motility of wild-type V. alginolyticus. The R207E mutant conferred motility with the motor of V. alginolyticus but not with the chimeric motor of E. coli. The motility with the E211K and R232E mutants was similar to that with wild-type PomA in V. alginolyticus but was greatly reduced in E. coli. Suppressor analysis suggested that R215 may participate in PomA-PomA interactions or PomA intramolecular interactions to form the stator complex.

Ion-swimming speed variation of Vibrio cholerae cells

In the present work we report the variation in swimming speed of Vibrio cholerae with respect to the change in concentration of sodium ions in the medium. We have also studied the variation in swimming speed with respect to temperature. We find that the swimming speed initially shows a linear increase with the increase of the sodium ions in the medium and then plateaus. The range within which the swimming speed attains saturation is approximately the same at different temperatures.

Gene expression profile of Vibrio cholerae in the cold stress-induced viable but non-culturable state

Vibrio cholerae is an aetiological agent of cholera that inhabits marine and estuarine environments. It can survive harsh environments by entering the viable but non-culturable (VBNC) state, but the related changes in gene expression have not been described. Here, we experimentally induced the VBNC state in V. cholerae O1, by incubation in artificial seawater at 4 degrees C. Bacterial cells that were incubated for 70 days retained their membrane integrity and were pathogenic, colonizing the gut of iron-dextran-treated mice, even though they formed no colonies on tryptic soy agar (TSA) or TSA amended with pyruvate. We therefore used this stage of cells as the VBNC bacteria. We compared the global transcription pattern of the VBNC cells with that of stationary-phase cells grown in rich medium. A total of 100 genes were induced by more than fivefold in the VBNC state, and the modulated genes were mostly those responsible for cellular processes. Furthermore, real-time RT-PCR analysis verified the changes in the expression levels, showing that the VC0230 [iron(III) ABC transporter], VC1212 (polB), VC2132 (fliG) and VC2187 (flaC) mRNAs were increased in the non-culturable state. Thus, these genes may be suitable markers for the detection of VBNC V. cholerae. To our knowledge, this is the first report of a comprehensive transcriptome analysis of V. cholerae in the VBNC state. The significance of this gene expression profile compared with those of in vivo isolates and non-stressed bacteria (culturable in vitro) is its potential to provide information about the public health risk from dormant bacteria.

The Vibrio motor proteins, MotX and MotY, are associated with the basal body of Na-driven flagella and required for stator formation

The four motor proteins PomA, PomB, MotX and MotY, which are believed to be stator proteins, are essential for motility by the Na(+)-driven flagella of Vibrio alginolyticus. When we purified the flagellar basal bodies, MotX and MotY were detected in the basal body, which is the supramolecular complex comprised of the rotor and the bushing, but PomA and PomB were not. By antibody labelling, MotX and MotY were detected around the LP ring. These results indicate that MotX and MotY associate with the basal body. The basal body had a new ring structure beneath the LP ring, which was named the T ring. This structure was changed or lost in the basal body from a DeltamotX or DeltamotY strain. The T ring probably comprises MotX and MotY. In the absence of MotX or MotY, we demonstrated that PomA and PomB were not localized to a cell pole. From the above results, we suggest that MotX and MotY of the T ring are involved in the incorporation and/or stabilization of the PomA/PomB complex in the motor.

Roles of charged residues of rotor and stator in flagellar rotation: comparative study using H+-driven and Na+-driven motors in Escherichia coli

In Escherichia coli, rotation of the flagellar motor has been shown to depend upon electrostatic interactions between charged residues of the stator protein MotA and the rotor protein FliG. These charged residues are conserved in the Na+-driven polar flagellum of Vibrio alginolyticus, but mutational studies in V. alginolyticus suggested that they are relatively unimportant for motor rotation. The electrostatic interactions detected in E. coli therefore might not be a general feature of flagellar motors, or, alternatively, the V. alginolyticus motor might rely on similar interactions but incorporate additional features that make it more robust against mutation. Here, we have carried out a comparative study of chimeric motors that were resident in E. coli but engineered to use V. alginolyticus stator components, rotor components, or both. Charged residues in the V. alginolyticus rotor and stator proteins were found to be essential for motor rotation when the proteins functioned in the setting of the E. coli motor. Patterns of synergism and suppression in rotor/stator double mutants indicate that the V. alginolyticus proteins interact in essentially the same way as their counterparts in E. coli. The robustness of the rotor-stator interface in V. alginolyticus is in part due to the presence of additional charged residues in PomA but appears mainly due to other factors, because an E. coli motor using both rotor and stator components from V. alginolyticus remained sensitive to mutation. Motor function in V. alginolyticus may be enhanced by the proteins MotX and MotY.

Electron cryomicroscopic visualization of PomA/B stator units of the sodium-driven flagellar motor in liposomes

A motor protein complex of the bacterial flagellum, PomA/B from Vibrio alginolyticus, was reconstituted into liposomes and visualized by electron cryomicroscopy. PomA/B is a sodium channel, composed of two membrane proteins, PomA and PomB, and converts ion flux to the rotation of the flagellar motor. Escherichia coli and Salmonella have a homolog called MotA/B, which utilizes proton instead of sodium ion. PomB and MotB have a peptidoglycan-binding motif in their C-terminal region, and therefore PomA/B and MotA/B are regarded as the stator. Energy filtering electron cryomicroscopy enhanced the image contrast of the proteins reconstituted into liposomes and showed that two extramembrane domains with clearly different sizes stick out of the lipid bilayers on opposite sides. Image analysis combined with gold labeling and deletion of the peptidoglycan-binding motif revealed that the longer one, approximately 70 A long, is likely to correspond to the periplasmic domain, and the other, about half size, to the cytoplasmic domain.

Polar flagellum biogenesis in Aeromonas hydrophila

Mesophilic Aeromonas spp. constitutively express a single polar flagellum that helps the bacteria move to more favorable environments and is an important virulence and colonization factor. Certain strains can also produce multiple lateral flagella in semisolid media or over surfaces. We have previously reported 16 genes (flgN to flgL) that constitute region 1 of the Aeromonas hydrophila AH-3 polar flagellum biogenesis gene clusters. We identified 39 new polar flagellum genes distributed in four noncontiguous chromosome regions (regions 2 to 5). Region 2 contained six genes (flaA to maf-1), including a modification accessory factor gene (maf-1) that has not been previously reported and is thought to be involved in glycosylation of polar flagellum filament. Region 3 contained 29 genes (fliE to orf29), most of which are involved in flagellum basal body formation and chemotaxis. Region 4 contained a single gene involved in the motor stator formation (motX), and region 5 contained the three master regulatory genes for the A. hydrophila polar flagella (flrA to flrC). Mutations in the flaH, maf-1, fliM, flhA, fliA, and flrC genes, as well as the double mutant flaA flaB, all caused loss of polar flagella and reduction in adherence and biofilm formation. A defined mutation in the pomB stator gene did not affect polar flagellum motility, in contrast to the motX mutant, which was unable to swim even though it expressed a polar flagellum. Mutations in all of these genes did not affect lateral flagellum synthesis or swarming motility, showing that both A. hydrophila flagellum systems are entirely distinct.

Polar flagellum biogenesis in Aeromonas hydrophila

Mesophilic Aeromonas spp. constitutively express a single polar flagellum that helps the bacteria move to more favorable environments and is an important virulence and colonization factor. Certain strains can also produce multiple lateral flagella in semisolid media or over surfaces. We have previously reported 16 genes (flgN to flgL) that constitute region 1 of the Aeromonas hydrophila AH-3 polar flagellum biogenesis gene clusters. We identified 39 new polar flagellum genes distributed in four noncontiguous chromosome regions (regions 2 to 5). Region 2 contained six genes (flaA to maf-1), including a modification accessory factor gene (maf-1) that has not been previously reported and is thought to be involved in glycosylation of polar flagellum filament. Region 3 contained 29 genes (fliE to orf29), most of which are involved in flagellum basal body formation and chemotaxis. Region 4 contained a single gene involved in the motor stator formation (motX), and region 5 contained the three master regulatory genes for the A. hydrophila polar flagella (flrA to flrC). Mutations in the flaH, maf-1, fliM, flhA, fliA, and flrC genes, as well as the double mutant flaA flaB, all caused loss of polar flagella and reduction in adherence and biofilm formation. A defined mutation in the pomB stator gene did not affect polar flagellum motility, in contrast to the motX mutant, which was unable to swim even though it expressed a polar flagellum. Mutations in all of these genes did not affect lateral flagellum synthesis or swarming motility, showing that both A. hydrophila flagellum systems are entirely distinct.

Properties of motility in Bacillus subtilis powered by the H+-coupled MotAB flagellar stator, Na+-coupled MotPS or hybrid stators MotAS or MotPB

Bacillus subtilis has a single set of flagellar rotor proteins that interact with two distinct stator-force generators, the H+-coupled MotAB complex and the Na+-coupled MotPS complex, that energize rotation. Here, motility on soft agar plates and in liquid was assayed in wild-type B.subtilis and strains expressing only one stator, either MotAB, MotPS or hybrid MotAS or MotPB. The strains expressing MotAB or MotAS had an average of 11 flagella/cell while those expressing MotPS or MotPB had an average of seven flagella/cell, and a Mot-less double mutant had three to four flagella/cell. MotAB had a more dominant role in motility than MotPS under most conditions, but MotPS supported comparable motility to MotAB on malate-containing soft agar plating media at elevated pH and Na+. MotAB supported much faster swimming speeds in liquid than MotPS, MotAS or MotPB under all conditions, but a contribution of MotPS to wild-type swimming was discernible from differences in swimming speeds of wild-type and MotAB at elevated viscosity, pH and Na+. Swimming supported by MotPS and MotAS was stimulated by Na+ and elevated pH whereas the converse was true of MotAB and MotPB. This suggests that MotAS is Na+-coupled and MotPB is H+-coupled and that MotB and MotS are major determinants of ion-coupling. However, the swimming speed supported by MotPB, as well as MotPS and MotAS, was inhibited severely at Na+ concentrations above 300 mM whereas MotAB-dependent swimming was not. The presence of either the MotP or MotS component in the stator also conferred sensitivity to inhibition by an amiloride analogue. These observations suggest that MotP contributes to Na+-coupling and inhibition by Na+ channel inhibitors. Similarly, a role for MotA in H+-dependent stator properties is indicated by the larger effects of pH on the Na+-response of MotAS versus MotPS. Finally, optimal function at elevated viscosity was found only in MotPS and MotPB and is therefore conferred by MotP.

Components of the Legionella pneumophila flagellar regulon contribute to multiple virulence traits, including lysosome avoidance and macrophage death

Legionella pneumophila is a motile intracellular pathogen of macrophages and amoebae. When nutrients become scarce, the bacterium induces expression of transmission traits, some of which are dependent on the flagellar sigma factor FliA (sigma(28)). To test how particular components of the L. pneumophila flagellar regulon contribute to virulence, we compared a fliA mutant with strains whose flagellar construction is disrupted at various stages. We find that L. pneumophila requires FliA to avoid lysosomal degradation in murine bone marrow-derived macrophages (BMM), to regulate production of a melanin-like pigment, and to regulate binding to the dye crystal violet, whereas motility, flagellar secretion, and external flagella or flagellin are dispensable for these activities. Thus, in addition to flagellar genes, the FliA sigma factor regulates an effector(s) or regulator(s) that contributes to other transmissive traits, notably inhibition of phagosome maturation. Whether or not the microbes produced flagellin, all nonmotile L. pneumophila mutants bound BMM less efficiently than the wild type, resulting in poor infectivity and a loss of contact-dependent death of BMM. Therefore, bacterial motility increases contact with host cells during infection, but flagellin is not an adhesin. When BMM contact by each nonmotile strain was promoted by centrifugation, all the mutants bound BMM similarly, but only those microbes that synthesized flagellin induced BMM death. Thus, the flagellar regulon equips the aquatic pathogen L. pneumophila to coordinate motility with multiple traits vital to virulence.

A chimeric N-terminal Escherichia coli--C-terminal Rhodobacter sphaeroides FliG rotor protein supports bidirectional E. coli flagellar rotation and chemotaxis

Flagellate bacteria such as Escherichia coli and Salmonella enterica serovar Typhimurium typically express 5 to 12 flagellar filaments over their cell surface that rotate in clockwise (CW) and counterclockwise directions. These bacteria modulate their swimming direction towards favorable environments by biasing the direction of flagellar rotation in response to various stimuli. In contrast, Rhodobacter sphaeroides expresses a single subpolar flagellum that rotates only CW and responds tactically by a series of biased stops and starts. Rotor protein FliG transiently links the MotAB stators to the rotor, to power rotation and also has an essential function in flagellar export. In this study, we sought to determine whether the FliG protein confers directionality on flagellar motors by testing the functional properties of R. sphaeroides FliG and a chimeric FliG protein, EcRsFliG (N-terminal and central domains of E. coli FliG fused to an R. sphaeroides FliG C terminus), in an E. coli FliG null background. The EcRsFliG chimera supported flagellar synthesis and bidirectional rotation; bacteria swam and tumbled in a manner qualitatively similar to that of the wild type and showed chemotaxis to amino acids. Thus, the FliG C terminus alone does not confer the unidirectional stop-start character of the R. sphaeroides flagellar motor, and its conformation continues to support tactic, switch-protein interactions in a bidirectional motor, despite its evolutionary history in a bacterium with a unidirectional motor.

Deletion analysis of the carboxyl-terminal region of the PomB component of the vibrio alginolyticus polar flagellar motor

The stator of the sodium-driven flagellar motor of Vibrio alginolyticus is a membrane protein complex composed of four PomA and two PomB subunits. PomB has a peptidoglycan-binding motif in the C-terminal region. In this study, four kinds of PomB deletions in the C terminus were constructed. None of the deletion proteins restored motility of the DeltapomB strain. The PomA protein was coisolated with all of the PomB derivatives under detergent-solubilized conditions. Homotypic disulfide cross-linking of all of the deletion derivatives through naturally occurring Cys residues was detected. We conclude that the C-terminal region of PomB is essential for motor function but not for oligomerization of PomB with itself or PomA.

Evidence for two flagellar stators and their role in the motility of Pseudomonas aeruginosa

Pseudomonas aeruginosa is a ubiquitous bacterium capable of twitching, swimming, and swarming motility. In this study, we present evidence that P. aeruginosa has two flagellar stators, conserved in all pseudomonads as well as some other gram-negative bacteria. Either stator is sufficient for swimming, but both are necessary for swarming motility under most of the conditions tested, suggesting that these two stators may have different roles in these two types of motility.

Concerted effects of amino acid substitutions in conserved charged residues and other residues in the cytoplasmic domain of PomA, a stator component of Na+-driven flagella

PomA is a membrane protein that is one of the essential components of the sodium-driven flagellar motor in Vibrio species. The cytoplasmic charged residues of Escherichia coli MotA, which is a PomA homolog, are believed to be required for the interaction of MotA with the C-terminal region of FliG. It was previously shown that a PomA variant with neutral substitutions in the conserved charged residues (R88A, K89A, E96Q, E97Q, and E99Q; AAQQQ) was functional. In the present study, five other conserved charged residues were replaced with neutral amino acids in the AAQQQ PomA protein. These additional substitutions did not affect the function of PomA. However, strains expressing the AAQQQ PomA variant with either an L131F or a T132M substitution, neither of which affected motor function alone, exhibited a temperature-sensitive (TS) motility phenotype. The double substitutions R88A or E96Q together with L131F were sufficient for the TS phenotype. The motility of the PomA TS mutants immediately ceased upon a temperature shift from 20 to 42 degrees C and was restored to the original level approximately 10 min after the temperature was returned to 20 degrees C. It is believed that PomA forms a channel complex with PomB. The complex formation of TS PomA and PomB did not seem to be affected by temperature. Suppressor mutations of the TS phenotype were mapped in the cytoplasmic boundaries of the transmembrane segments of PomA. We suggest that the cytoplasmic surface of PomA is changed by the amino acid substitutions and that the interaction of this surface with the FliG C-terminal region is temperature sensitive.

The complex flagellar torque generator of Pseudomonas aeruginosa

Flagella act as semirigid helical propellers that are powered by reversible rotary motors. Two membrane proteins, MotA and MotB, function as a complex that acts as the stator and generates the torque that drives rotation. The genome sequence of Pseudomonas aeruginosa PAO1 contains dual sets of motA and motB genes, PA1460-PA1461 (motAB) and PA4954-PA4953 (motCD), as well as another gene, motY (PA3526), which is known to be required for motor function in some bacteria. Here, we show that these five genes contribute to motility. Loss of function of either motAB-like locus was dispensable for translocation in aqueous environments. However, swimming could be entirely eliminated by introduction of combinations of mutations in the two motAB-encoding regions. Mutation of both genes encoding the MotA homologs or MotB homologs was sufficient to abolish motility. Mutants carrying double mutations in nonequivalent genes (i.e., motA motD or motB motC) retained motility, indicating that noncognate components can function together. motY appears to be required for motAB function. The combination of motY and motCD mutations rendered the cells nonmotile. Loss of function of motAB, motY, or motAB motY produced similar phenotypes; although the swimming speed was only reduced to approximately 85% of the wild-type speed, translocation in semisolid motility agar and swarming on the surface of solidified agar were severely impeded. Thus, the flagellar motor of P. aeruginosa represents a more complex configuration than the configuration that has been studied in other bacteria, and it enables efficient movement under different circumstances.

Interaction of PomB with the third transmembrane segment of PomA in the Na+-driven polar flagellum of Vibrio alginolyticus

The marine bacterium Vibrio alginolyticus has four motor components, PomA, PomB, MotX, and MotY, responsible for its Na(+)-driven flagellar rotation. PomA and PomB are integral inner membrane proteins having four and one transmembrane segments (TMs), respectively, which are thought to form an ion channel complex. First, site-directed Cys mutagenesis was systematically performed from Asp-24 to Glu-41 of PomB, and the resulting mutant proteins were examined for susceptibility to a sulfhydryl reagent. Secondly, the Cys substitutions at the periplasmic boundaries of the PomB TM (Ser-38) and PomA TMs (Gly-23, Ser-34, Asp-170, and Ala-178) were combined. Cross-linked products were detected for the combination of PomB-S38C and PomA-D170C mutant proteins. The Cys substitutions in the periplasmic boundaries of PomA TM3 (from Met-169 to Asp-171) and the PomB TM (from Leu-37 to Ser-40) were combined to construct a series of double mutants. Most double mutations reduced the motility, whereas each single Cys substitution slightly affected it. Although the motility of the strain carrying PomA-D170C and PomB-S38C was significantly inhibited, it was recovered by reducing reagent. The strain with this combination showed a lower affinity for Na(+) than the wild-type combination. PomA-D148C and PomB-P16C, which are located at the cytoplasmic boundaries of PomA TM3 and the PomB TM, also formed the cross-linked product. From these lines of evidence, we infer that TM3 of PomA and the TM of PomB are in close proximity over their entire length and that cooperation between these two TMs is required for coupling of Na(+) conduction to flagellar rotation.

Isolation of Vibrio alginolyticus sodium-driven flagellar motor complex composed of PomA and PomB solubilized by sucrose monocaprate

The polar flagella ofVibrio alginolyticushave sodium-driven motors, and four membrane proteins, PomA, PomB, MotX and MotY, are essential for torque generation of the motor. PomA and PomB are believed to form a sodium-conducting channel. This paper reports the purification of the motor complex by using sucrose monocaprate, a non-ionic detergent, to solubilize the complex. Plasmid pKJ301, which encodes intact PomA, and PomB tagged with a C-terminal hexahistidine that does not interfere with PomB function, was constructed. The membrane fraction of cells transformed with pKJ301 was solubilized with sucrose monocaprate, and the solubilized materials were applied to a Ni-NTA column. The imidazole eluate contained both PomA and PomB, which were further purified by anion-exchange chromatography. Gel-filtration chromatography was used to investigate the apparent molecular size of the complex; the PomA/PomB complex was eluted as approx. 900 kDa and PomB alone was eluted as approx. 260 kDa. These findings suggest that the motor complex may have a larger structure than previously assumed.