Identification of a novel gene, fimV, involved in twitching motility in Pseudomonas aeruginosa

Unraveling DNA Triplex Assembly: Mass Spectrometric Investigation of Modified Triplex Forming Oligonucleotides for Enhanced Gene Targeting

Deoxyribonucleic acid triplexes have potential roles in a range of biological processes involving gene and transcriptional regulation. A major challenge in exploiting the formation of these higher-order structures to target genes in vivo is their low stability, which is dependent on many factors including the length and composition of bases in the sequence. Here, different DNA base modifications have been explored, primarily using native mass spectrometry, in efforts to enable stronger binding between the triplex forming oligonucleotide (TFO) and duplex target sites. These modifications can also be used to overcome pyrimidine interruptions in the duplex sequence in promoter regions of genomes, to expand triplex target sequences for antigene therapies. Using model sequences with a single pyrimidine interruption, triplex forming oligonucleotides containing locked nucleic acid base modifications were shown to have a higher triplex binding propensity than DNA-only and dSpacer-containing TFOs. However, the triplex forming ability of these systems was limited by the competitive formation of multiple higher order assemblies. Triplex forming sequences that correspond to specific gene targets from the Pseudomonas aeruginosa genome were also investigated, with LNA-containing TFOs the only variant able to form triplex using these sequences. This work indicates the advantages of utilizing synthetically modified TFOs to form triplex assemblies in vivo for potential therapeutic applications and highlights the advantages of native mass spectrometry for the study of their formation.

GGDEF domain as spatial on-switch for a phosphodiesterase by interaction with landmark protein HubP

In bacteria, the monopolar localization of enzymes and protein complexes can result in a bimodal distribution of enzyme activity between the dividing cells and heterogeneity of cellular behaviors. In Shewanella putrefaciens, the multidomain hybrid diguanylate cyclase/phosphodiesterase PdeB, which degrades the secondary messenger c-di-GMP, is located at the flagellated cell pole. Here, we show that direct interaction between the inactive diguanylate cyclase (GGDEF) domain of PdeB and the FimV domain of the polar landmark protein HubP is crucial for full function of PdeB as a phosphodiesterase. Thus, the GGDEF domain serves as a spatially controlled on-switch that effectively restricts PdeBs activity to the flagellated cell pole. PdeB regulates abundance and activity of at least two crucial surface-interaction factors, the BpfA surface-adhesion protein and the MSHA type IV pilus. The heterogeneity in c-di-GMP concentrations, generated by differences in abundance and timing of polar appearance of PdeB, orchestrates the population behavior with respect to cell-surface interaction and environmental spreading.

Surface Sensing and Adaptation in Bacteria

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

Pseudomonas aeruginosa is capable of natural transformation in biofilms

Natural transformation is a mechanism that enables competent bacteria to acquire naked, exogenous DNA from the environment. It is a key process that facilitates the dissemination of antibiotic resistance and virulence determinants throughout bacterial populations. Pseudomonas aeruginosa is an opportunistic Gram-negative pathogen that produces large quantities of extracellular DNA (eDNA) that is required for biofilm formation. P. aeruginosa has a remarkable level of genome plasticity and diversity that suggests a high degree of horizontal gene transfer and recombination but is thought to be incapable of natural transformation. Here we show that P. aeruginosa possesses homologues of all proteins known to be involved in natural transformation in other bacterial species. We found that P. aeruginosa in biofilms is competent for natural transformation of both genomic and plasmid DNA. Furthermore, we demonstrate that type-IV pili (T4P) facilitate but are not absolutely essential for natural transformation in P. aeruginosa.

Expression of Ralstonia solanacearum type III secretion system is dependent on a novel type 4 pili (T4P) assembly protein (TapV) but is T4P independent

Type IV pili (T4P) are virulence factors in various pathogenic bacteria of animals and plants that play important roles in twitching motility, swimming motility, biofilm formation, and adhesion to host cells. Here, we genetically characterized functional roles of a putative T4P assembly protein TapV (Rsc1986 in reference strain GMI1000) and its homologue Rsp0189, which shares 58% amino acid identity with TapV, in Ralstonia solanacearum. Deletion of tapV, but not rsp0189, resulted in significantly impaired twitching motility, swimming motility, and adhesion to tomato roots, which are consistent as phenotypes of the pilA mutant (a known R. solanacearum T4P-deficient mutant). However, unlike the pilA mutant, the tapV mutant produced more biofilm than the wild-type strain. Our gene expression studies revealed that TapV, but not Rsp0189, is important for expression of a type III secretion system (T3SS, a pathogenicity determinant of R. solanacearum) both in vitro and in planta, but it is T4P independent. We further revealed that TapV affected the T3SS expression via the PhcA-TapV-PrhG-HrpB pathway, consistent with previous reports that PhcA positively regulates expression of pilA and prhG. Moreover, deletion of tapV, but not rsp0189, significantly impaired the ability to migrate into and colonize xylem vessels of host plants, but there was no alteration in intercellular proliferation of R. solanacearum in tobacco leaves, which is similar to the pilA mutant. The tapV mutant showed significantly impaired virulence in host plants. This is the first report on the impact of T4P components on the T3SS, providing novel insights into our understanding of various biological functions of T4P and the complex regulatory pathway of T3SS in R. solanacearum.

c-di-GMP-related phenotypes are modulated by the interaction between a diguanylate cyclase and a polar hub protein

c-di-GMP is a major player in the switch between biofilm and motile lifestyles. Several bacteria exhibit a large number of c-di-GMP metabolizing proteins, thus a fine-tuning of this nucleotide levels may occur. It is hypothesized that some c-di-GMP metabolizing proteins would provide the global c-di-GMP levels inside the cell whereas others would maintain a localized pool, with the resulting c-di-GMP acting at the vicinity of its production. Although attractive, this hypothesis has yet to be demonstrated in Pseudomonas aeruginosa. We found that the diguanylate cyclase DgcP interacts with the cytosolic region of FimV, a polar peptidoglycan-binding protein involved in type IV pilus assembly. Moreover, DgcP is located at the cell poles in wild type cells but scattered in the cytoplasm of cells lacking FimV. Overexpression of dgcP leads to the classical phenotypes of high c-di-GMP levels (increased biofilm and impaired motilities) in the wild-type strain, but not in a ΔfimV background. Therefore, our findings suggest that DgcP activity is regulated by FimV. The polar localization of DgcP might contribute to a local c-di-GMP pool that can be sensed by other proteins at the cell pole, bringing to light a specialized function for a specific diguanylate cyclase.

In vitro genomic and proteomic evidence of a type IV pili-like structure in the fish pathogen Piscirickettsia salmonis

Piscirickettsia salmonis is an intracellular γ-proteobacteria and the etiological agent of piscirickettsiosis, which causes massive economic losses in the Chilean salmon industry. The type IV pili (T4P) play an important role in adherence to host cell surfaces and bacterial pathogenicity. T4P contains a variable number of components, as predicted in P. salmonis genomes. However, no studies have determined if P. salmonis possesses T4P. The aims of this investigation were to identify T4P components in the P. salmonis type strain LF-89T, evaluate respective transcript expressions, and analyze the main putative T4P proteins using bioinformatics and proteomic approaches. Two main clusters of P. salmonis T4P genes were found. Expression of the pilA gene was upregulated at 4 h post-infection (hpi), while pilQ was upregulated 4 days post-infection. At 16 hpi, pilB and pilD were strongly upregulated. The PilA amino acid sequence analysis showed a conserved N-terminal domain and sequence motifs critical for T4P biosynthesis. MudPIT analysis revealed PilA in the P. salmonis LF-89T proteome, and TEM showed pili-like filamentous structures on the P. salmonis surface. These results strongly suggest the presence of a T4P-like structure in P. salmonis.

Polar landmark protein HubP recruits flagella assembly protein FapA under glucose limitation in Vibrio vulnificus

How motile bacteria recognize their environment and decide whether to stay or navigate toward more favorable location is a fundamental issue in survival. The flagellum is an elaborate molecular device responsible for bacterial locomotion, and the flagellum-driven motility allows bacteria to move themselves to the appropriate location at the right time. Here, we identify the polar landmark protein HubP as a modulator of polar flagellation that recruits the flagellar assembly protein FapA to the old cell pole, thereby controlling its activity for the early events of flagellar assembly in Vibrio vulnificus. We show that dephosphorylated EIIA^Glc of the PEP-dependent sugar transporting phosphotransferase system sequesters FapA from HubP in response to glucose and hence inhibits FapA-mediated flagellation. Thus, flagellar assembly and motility is governed by spatiotemporal control of FapA, which is orchestrated by the competition between dephosphorylated EIIA^Glc and HubP, in the human pathogen V. vulnificus.

The Dynamic Structures of the Type IV Pilus

Type IV pilus (T4P)-like systems have been identified in almost every major phylum of prokaryotic life. They include the type IVa pilus (T4aP), type II secretion system (T2SS), type IVb pilus (T4bP), Tad/Flp pilus, Com pilus, and archaeal flagellum (archaellum). These systems are used for adhesion, natural competence, phage adsorption, folded-protein secretion, surface sensing, swimming motility, and twitching motility. The T4aP allows for all of these functions except swimming and is therefore a good model system for understanding T4P-like systems. Recent structural analyses have revolutionized our understanding of how the T4aP machinery assembles and functions. Here we review the structure and function of the T4aP.

Spatiotemporal control of FlgZ activity impacts Pseudomonas aeruginosa flagellar motility

The c-di-GMP-binding effector protein FlgZ has been demonstrated to control motility in the opportunistic pathogen Pseudomonas aeruginosa and it was suggested that c-di-GMP-bound FlgZ impedes motility via its interaction with the MotCD stator. To further understand how motility is downregulated in P. aeruginosa and to elucidate the general control mechanisms operating during bacterial growth, we examined the spatiotemporal activity of FlgZ. We re-annotated the P. aeruginosaflgZ open reading frame and demonstrated that FlgZ-mediated downregulation of motility is fine-tuned via three independent mechanisms. First, we found that flgZ gene is transcribed independently from flgMN in stationary growth phase to increase FlgZ protein levels in the cell. Second, FlgZ localizes to the cell pole upon c-di-GMP binding and third, we describe that FimV, a cell pole anchor protein, is involved in increasing the polar localized c-di-GMP bound FlgZ to inhibit both, swimming and swarming motility. Our results shed light on the complex dynamics and spatiotemporal control of c-di-GMP-dependent bacterial motility phenotypes and on how the polar anchor protein FimV, the motor brake FlgZ and the stator proteins function to repress flagella-driven swimming and swarming motility.

The GGDEF Domain of the Phosphodiesterase PdeB in Shewanella putrefaciens Mediates Recruitment by the Polar Landmark Protein HubP

Bacteria commonly exhibit a high degree of cellular organization and polarity which affect many vital processes such as replication, cell division, and motility. In Shewanella and other bacteria, HubP is a polar marker protein which is involved in proper chromosome segregation, placement of the chemotaxis system, and various aspects of pilus- and flagellum-mediated motility. Here, we show that HubP also recruits a transmembrane multidomain protein, PdeB, to the flagellated cell pole. PdeB is an active phosphodiesterase and degrades the second messenger c-di-GMP. In Shewanella putrefaciens, PdeB affects both the polar and the lateral flagellar systems at the level of function and/or transcription in response to environmental medium conditions. Mutant analysis on fluorescently labeled PdeB indicated that a diguanylate cyclase (GGDEF) domain in PdeB is strictly required for HubP-dependent localization. Bacterial two-hybrid and in vitro interaction studies on purified proteins strongly indicate that this GGDEF domain of PdeB directly interacts with the C-terminal FimV domain of HubP. Polar localization of PdeB occurs late during the cell cycle after cell division and separation and is not dependent on medium conditions. In vitro activity measurements did not reveal a difference in PdeB phosphodiesterase activities in the presence or absence of the HubP FimV domain. We hypothesize that recruitment of PdeB to the flagellated pole by HubP may create an asymmetry of c-di-GMP levels between mother and daughter cells and may assist in organization of c-di-GMP-dependent regulation within the cell.IMPORTANCE c-di-GMP-dependent signaling affects a range of processes in many bacterial species. Most bacteria harbor a plethora of proteins with domains which are potentially involved in synthesis and breakdown of c-di-GMP. A potential mechanism to elicit an appropriate c-di-GMP-dependent response is to organize the corresponding proteins in a spatiotemporal fashion. Here, we show that a major contributor to c-di-GMP levels and flagellum-mediated swimming in Shewanella, PdeB, is recruited to the flagellated cell pole by the polar marker protein HubP. Polar recruitment involves a direct interaction between HubP and a GGDEF domain in PdeB, demonstrating a novel mechanism of polar targeting by the widely conserved HubP/FimV polar marker.

Cyclic AMP-Independent Control of Twitching Motility in Pseudomonas aeruginosa

FimV is a Pseudomonas aeruginosa inner membrane hub protein that modulates levels of the second messenger, cyclic AMP (cAMP), through the activation of adenylate cyclase CyaB. Although type IVa pilus (T4aP)-dependent twitching motility is modulated by cAMP levels, mutants lacking FimV are twitching impaired, even when exogenous cAMP is provided. Here we further define FimV's cAMP-dependent and -independent regulation of twitching. We confirmed that the response regulator of the T4aP-associated Chp chemotaxis system, PilG, requires both FimV and the CyaB regulator, FimL, to activate CyaB. However, in cAMP-replete backgrounds-lacking the cAMP phosphodiesterase CpdA or the CheY-like protein PilH or expressing constitutively active CyaB-pilG and fimV mutants failed to twitch. Both cytoplasmic and periplasmic domains of FimV were important for its cAMP-dependent and -independent roles, while its septal peptidoglycan-targeting LysM motif was required only for twitching motility. Polar localization of the sensor kinase PilS, a key regulator of transcription of the major pilin, was FimV dependent. However, unlike its homologues in other species that localize flagellar system components, FimV was not required for swimming motility. These data provide further evidence to support FimV's role as a key hub protein that coordinates the polar localization and function of multiple structural and regulatory proteins involved in P. aeruginosa twitching motility.IMPORTANCEPseudomonas aeruginosa is a serious opportunistic pathogen. Type IVa pili (T4aP) are important for its virulence, because they mediate dissemination and invasion via twitching motility and are involved in surface sensing, which modulates pathogenicity via changes in cAMP levels. Here we show that the hub protein FimV and the response regulator of the Chp system, PilG, regulate twitching independently of their roles in the modulation of cAMP synthesis. These functions do not require the putative scaffold protein FimL, proposed to link PilG with FimV. PilG may regulate asymmetric functioning of the T4aP system to allow for directional movement, while FimV appears to localize both structural and regulatory elements-including the PilSR two-component system-to cell poles for optimal function.

Subunit vaccine candidates against Aeromonas salmonicida in rainbow trout Oncorhynchus mykiss

Aeromonas salmonicida subsp. salmonicida is the etiological agent of furunculosis and a major fish health problem in salmonid aquaculture worldwide. Injection vaccination with commercial mineral oil-adjuvanted bacterin vaccines has been partly successful in preventing the disease but in Danish rainbow trout (Oncorhynchus mykiss, Walbaum) aquaculture furunculosis outbreaks still occur. In this study we tested the efficacy of experimental subunit vaccines against A. salmonicida infection in rainbow trout. We utilized in silico screening of the proteome of A. salmonicida subsp. salmonicida strain A449 and identified potential protective protein antigens that were tested by in vivo challenge trial. A total of 14 proteins were recombinantly expressed in Escherichia coli and prepared in 3 different subunit vaccine combinations to immunize 3 groups of rainbow trout by intraperitoneal (i.p.) injection. The fish were exposed to virulent A. salmonicida 7 weeks after immunization. To assess the efficacy of the subunit vaccines we evaluated the immune response in fish after immunization and challenge infection by measuring the antibody levels and monitoring the survival of fish in different groups. The survival of fish at 3 weeks after challenge infection showed that all 3 groups of fish immunized with 3 different protein combinations exhibited significantly lower mortalities (17–30%) compared to the control groups (48% and 56%). The ELISA results revealed significantly elevated antibody levels in fish against several protein antigens, which in some cases were positively correlated to the survival.

The Type IVa Pilus Machinery Is Recruited to Sites of Future Cell Division

Type IVa pili (T4aP) are ubiquitous microbial appendages used for adherence, twitching motility, DNA uptake, and electron transfer. Many of these functions depend on dynamic assembly and disassembly of the pilus by a megadalton-sized, cell envelope-spanning protein complex located at the poles of rod-shaped bacteria. How the T4aP assembly complex becomes integrated into the cell envelope in the absence of dedicated peptidoglycan (PG) hydrolases is unknown. After ruling out the potential involvement of housekeeping PG hydrolases in the installation of the T4aP machinery in Pseudomonas aeruginosa, we discovered that key components of inner (PilMNOP) and outer (PilQ) membrane subcomplexes are recruited to future sites of cell division. Midcell recruitment of a fluorescently tagged alignment subcomplex component, mCherry-PilO, depended on PilQ secretin monomers—specifically, their N-terminal PG-binding AMIN domains. PilP, which connects PilO to PilQ, was required for recruitment, while PilM, which is structurally similar to divisome component FtsA, was not. Recruitment preceded secretin oligomerization in the outer membrane, as loss of the PilQ pilotin PilF had no effect on localization. These results were confirmed in cells chemically blocked for cell division prior to outer membrane invagination. The hub protein FimV and a component of the polar organelle coordinator complex—PocA—were independently required for midcell recruitment of PilO and PilQ. Together, these data suggest an integrated, energy-efficient strategy for the targeting and preinstallation—rather than retrofitting—of the T4aP system into nascent poles, without the need for dedicated PG-remodeling enzymes.

The peptidoglycan (PG) layer of bacterial cell envelopes has limited porosity, representing a physical barrier to the insertion of large protein complexes involved in secretion and motility. Many systems include dedicated PG hydrolase components that create space for their insertion, but the ubiquitous type IVa pilus (T4aP) system lacks such an enzyme. Instead, we found that components of the T4aP system are recruited to future sites of cell division, where they could be incorporated into the cell envelope during the formation of new poles, eliminating the need for PG hydrolases. Targeting depends on the presence of septal PG-binding motifs in specific components, as removal of those motifs causes delocalization. This preinstallation strategy for the T4aP assembly system would ensure that both daughter cells are poised to extrude pili from new poles as soon as they separate from one another.

HubP, a Polar Landmark Protein, Regulates Flagellar Number by Assisting in the Proper Polar Localization of FlhG in Vibrio alginolyticus

The marine bacterium Vibrio alginolyticus has a single polar flagellum, the number of which is regulated positively by FlhF and negatively by FlhG. FlhF is intrinsically localized at the cell pole, whereas FlhG is localized there through putative interactions with the polar landmark protein HubP. Here we focused on the role of HubP in the regulation of flagellar number in V. alginolyticus Deletion of hubP increased the flagellar number and completely disrupted the polar localization of FlhG. It was thought that the flagellar number is determined primarily by the absolute amount of FlhF localized at the cell pole. Here we found that deletion of hubP increased the flagellar number although it did not increase the polar amount of FlhF. We also found that FlhG overproduction did not reduce the polar localization of FlhF. These results show that the absolute amount of FlhF is not always the determinant of flagellar number. We speculate that cytoplasmic FlhG works as a quantitative regulator, controlling the amount of FlhF localized at the pole, and HubP-anchored polar FlhG works as a qualitative regulator, directly inhibiting the activity of polar FlhF. This regulation by FlhF, FlhG, and HubP might contribute to achieving optimal flagellar biogenesis at the cell pole in V. alginolyticus IMPORTANCE: For regulation of the flagellar number in marine Vibrio, two proteins, FlhF and FlhG, work as positive and negative regulators, respectively. In this study, we found that the polar landmark protein HubP is involved in the regulation of flagellar biogenesis. Deletion of hubP increased the number of flagella without increasing the amount of pole-localizing FlhF, indicating that the number of flagella is not determined solely by the absolute amount of pole-localizing FlhF, which is inconsistent with the previous model. We propose that cytoplasmic FlhG and HubP-anchored polar FlhG negatively regulate flagellar formation through two independent schemes.

The Conserved Tetratricopeptide Repeat-Containing C-Terminal Domain of Pseudomonas aeruginosa FimV Is Required for Its Cyclic AMP-Dependent and -Independent Functions

FimV is a Pseudomonas aeruginosa inner membrane protein that regulates intracellular cyclic AMP (cAMP) levels-and thus type IV pilus (T4P)-mediated twitching motility and type II secretion (T2S)-by activating the adenylate cyclase CyaB. Its cytoplasmic domain contains three predicted tetratricopeptide repeat (TPR) motifs separated by an unstructured region: two proximal to the inner membrane and one within the "FimV C-terminal domain," which is highly conserved across diverse homologs. Here, we present the crystal structure of the FimV C terminus, FimV861-919, containing a TPR motif decorated with solvent-exposed, charged side chains, plus a C-terminal capping helix. FimV689, a truncated form lacking this C-terminal motif, did not restore wild-type levels of twitching or surface piliation compared to the full-length protein. FimV689 failed to restore wild-type levels of the T4P motor ATPase PilU or T2S, suggesting that it was unable to activate cAMP synthesis. Bacterial two-hybrid analysis showed that TPR3 interacts directly with the CyaB activator, FimL. However, FimV689 failed to restore wild-type motility in a fimV mutant expressing a constitutively active CyaB (fimV cyaB-R456L), suggesting that the C-terminal motif is also involved in cAMP-independent functions of FimV. The data show that the highly conserved TPR-containing C-terminal domain of FimV is critical for its cAMP-dependent and -independent functions.

IMPORTANCE

FimV is important for twitching motility and cAMP-dependent virulence gene expression in P. aeruginosa FimV homologs have been identified in several human pathogens, and their functions are not limited to T4P expression. The C terminus of FimV is remarkably conserved among otherwise very diverse family members, but its role is unknown. We provide here biological evidence for the importance of the C-terminal domain in both cAMP-dependent (through FimL) and -independent functions of FimV. We present X-ray crystal structures of the conserved C-terminal domain and identify a consensus sequence for the C-terminal TPR within the conserved domain. Our data extend our knowledge of FimV's functionally important domains, and the structures and consensus sequences provide a foundation for studies of FimV and its homologs.

Colony-morphology screening uncovers a role for the Pseudomonas aeruginosa nitrogen-related phosphotransferase system in biofilm formation

Pseudomonas aeruginosa is an opportunistic human pathogen whose survival is aided by forming communities known as biofilms, in which cells are encased in a self-produced matrix. We devised a mutant screen based on colony morphology to identify additional genes with previously unappreciated roles in biofilm formation. Our screen, which identified most known biofilm-related genes, also uncovered PA14_16550 and PA14_69700, deletions of which abrogated and augmented biofilm formation respectively. We also identified ptsP, which encodes enzyme I of the nitrogen-regulated phosphotransferase (PTS(Ntr)) system, as being important for cyclic-di-GMP production and for biofilm formation. Further experiments showed that biofilm formation is hindered in the absence of phosphotransfer through the PTS(Ntr), but only in the presence of enzyme II (PtsN), the putative regulatory module of the PTS(Ntr). These results implicate unphosphorylated PtsN as a negative regulator of biofilm formation and establish one of the first known roles of the PTS(Ntr) in P. aeruginosa.

Modulation of Type III Secretion System in Pseudomonas aeruginosa: Involvement of the PA4857 Gene Product

Pseudomonas aeruginosa is an opportunistic pathogen that causes serious acute or chronic infections in humans. Acute infections typically involve the type III secretion systems (T3SSs) and bacterial motility, whereas chronic infections are often associated with biofilm formation and the type VI secretion system. To identify new genes required for pathogenesis, a transposon mutagenesis library was constructed and the gene PA4857, named tspR, was found to modulate T3SS gene expression. Deletion of P. aeruginosa tspR reduced the virulence in a mouse acute lung infection model and diminished cytotoxicity. Suppression of T3SS gene expression in the tspR mutant resulted from compromised translation of the T3SS master regulator ExsA. TspR negatively regulated two small RNAs, RsmY and RsmZ, which control RsmA. Our data demonstrated that defects in T3SS expression and biofilm formation in retS mutant could be partially restored by overexpression of tspR. Taken together, our results demonstrated that the newly identified retS-tspR pathway is coordinated with the retS-gacS system, which regulates the genes associated with acute and chronic infections and controls the lifestyle choice of P. aeruginosa.

The role of FlhF and HubP as polar landmark proteins in Shewanella putrefaciens CN-32

Spatiotemporal regulation of cell polarity plays a role in many fundamental processes in bacteria and often relies on 'landmark' proteins which recruit the corresponding clients to their designated position. Here, we explored the localization of two multi-protein complexes, the polar flagellar motor and the chemotaxis array, in Shewanella putrefaciens CN-32. We demonstrate that polar positioning of the flagellar system, but not of the chemotaxis system, depends on the GTPase FlhF. In contrast, the chemotaxis array is recruited by a transmembrane protein which we identified as the functional ortholog of Vibrio cholerae HubP. Mediated by its periplasmic N-terminal LysM domain, SpHubP exhibits an FlhF-independent localization pattern during cell cycle similar to its Vibrio counterpart and also has a role in proper chromosome segregation. In addition, while not affecting flagellar positioning, SpHubP is crucial for normal flagellar function and is involved in type IV pili-mediated twitching motility. We hypothesize that a group of HubP/FimV homologs, characterized by a rather conserved N-terminal periplasmic section required for polar targeting and a highly variable acidic cytoplasmic part, primarily mediating recruitment of client proteins, serves as polar markers in various bacterial species with respect to different cellular functions.

Biogenesis of Pseudomonas aeruginosa type IV pili and regulation of their function

Type IV pili (T4P) are bacterial virulence factors involved in a wide variety of functions including deoxyribonucleic acid uptake, surface attachment, biofilm formation and twitching motility. While T4P are common surface appendages, the systems that assemble them and the regulation of their function differ between species. Pseudomonas aeruginosa, Neisseria spp. and Myxococcus xanthus are common model systems used to study T4P biology. This review focuses on recent advances in P. aeruginosa T4P structural biology, and the regulatory pathways controlling T4P biogenesis and function.

Xanthomonas citri subsp. citri type IV Pilus is required for twitching motility, biofilm development, and adherence

Bacterial type IV pili (T4P) are long, flexible surface filaments that consist of helical polymers of mostly pilin subunits. Cycles of polymerization, attachment, and depolymerization mediate several pilus-dependent bacterial behaviors, including twitching motility, surface adhesion, pathogenicity, natural transformation, escape from immune system defense mechanisms, and biofilm formation. The Xanthomonas citri subsp. citri strain 306 genome codes for a large set of genes involved in T4P biogenesis and regulation and includes several pilin homologs. We show that X. citri subsp. citri can exhibit twitching motility in a manner similar to that observed in other bacteria such as Pseudomonas aeruginosa and Xylella fastidiosa and that this motility is abolished in Xanthomonas citri subsp. citri knockout strains in the genes coding for the major pilin subunit PilAXAC3241, the ATPases PilBXAC3239 and PilTXAC2924, and the T4P biogenesis regulators PilZXAC1133 and FimXXAC2398. Microscopy analyses were performed to compare patterns of bacterial migration in the wild-type and knockout strains and we observed that the formation of mushroom-like structures in X. citri subsp. citri biofilm requires a functional T4P. Finally, infection of X. citri subsp. citri cells by the bacteriophage (ΦXacm4-11 is T4P dependent. The results of this study improve our understanding of how T4P influence Xanthomonas motility, biofilm formation, and susceptibility to phage infection.

Peptidoglycan-binding protein TsaP functions in surface assembly of type IV pili

Type IV pili (T4P) are ubiquitous and versatile bacterial cell surface structures involved in adhesion to host cells, biofilm formation, motility, and DNA uptake. In Gram-negative bacteria, T4P pass the outer membrane (OM) through the large, oligomeric, ring-shaped secretin complex. In the β-proteobacterium Neisseria gonorrhoeae, the native PilQ secretin ring embedded in OM sheets is surrounded by an additional peripheral structure, consisting of a peripheral ring and seven extending spikes. To unravel proteins important for formation of this additional structure, we identified proteins that are present with PilQ in the OM. One such protein, which we name T4P secretin-associated protein (TsaP), was identified as a phylogenetically widely conserved component of the secretin complex that co-occurs with genes for T4P in Gram-negative bacteria. TsaP contains an N-terminal carbohydrate-binding lysin motif (LysM) domain and a C-terminal domain of unknown function. In N. gonorrhoeae, lack of TsaP results in the formation of membrane protrusions containing multiple T4P, concomitant with reduced formation of surface-exposed T4P. Lack of TsaP did not affect the oligomeric state of PilQ, but resulted in loss of the peripheral structure around the PilQ secretin. TsaP binds peptidoglycan and associates strongly with the OM in a PilQ-dependent manner. In the δ-proteobacterium Myxococcus xanthus, TsaP is also important for surface assembly of T4P, and it accumulates and localizes in a PilQ-dependent manner to the cell poles. Our results show that TsaP is a novel protein associated with T4P function and suggest that TsaP functions to anchor the secretin complex to the peptidoglycan.

Mucin inhibits Pseudomonas aeruginosa biofilm formation by significantly enhancing twitching motility

In Pseudomonas aeruginosa, type IV pili (TFP)-dependent twitching motility is required for development of surface-attached biofilm (SABF), yet excessive twitching motility is detrimental once SABF is established. In this study, we show that mucin significantly enhanced twitching motility and decreased SABF formation in strain PAO1 and other P. aeruginosa strains in a concentration-dependent manner. Mucin also disrupted partially established SABF. Our analyses revealed that mucin increased the amount of surface pilin and enhanced transcription of the pilin structural gene pilA. Mucin failed to enhance twitching motility in P. aeruginosa mutants defective in genes within the pilin biogenesis operons pilGHI/pilJK-chpA-E. Furthermore, mucin did not enhance twitching motility nor reduce biofilm development by chelating iron. We also examined the role of the virulence factor regulator Vfr in the effect of mucin. In the presence or absence of mucin, PAOΔvfr produced a significantly reduced SABF. However, mucin partially complemented the twitching motility defect of PAOΔvfr. These results suggest that mucin interferes with SABF formation at specific concentrations by enhancing TFP synthesis and twitching motility, that this effect, which is iron-independent, requires functional Vfr, and only part of the Vfr-dependent effect of mucin on SABF development occurs through twitching motility.

The biology of Neisseria adhesins

Members of the genus Neisseria include pathogens causing important human diseases such as meningitis, septicaemia, gonorrhoea and pelvic inflammatory disease syndrome. Neisseriae are found on the exposed epithelia of the upper respiratory tract and the urogenital tract. Colonisation of these exposed epithelia is dependent on a repertoire of diverse bacterial molecules, extending not only from the surface of the bacteria but also found within the outer membrane. During invasive disease, pathogenic Neisseriae also interact with immune effector cells, vascular endothelia and the meninges. Neisseria adhesion involves the interplay of these multiple surface factors and in this review we discuss the structure and function of these important molecules and the nature of the host cell receptors and mechanisms involved in their recognition. We also describe the current status for recently identified Neisseria adhesins. Understanding the biology of Neisseria adhesins has an impact not only on the development of new vaccines but also in revealing fundamental knowledge about human biology.

A multidomain hub anchors the chromosome segregation and chemotactic machinery to the bacterial pole

The cell poles constitute key subcellular domains that are often critical for motility, chemotaxis, and chromosome segregation in rod-shaped bacteria. However, in nearly all rods, the processes that underlie the formation, recognition, and perpetuation of the polar domains are largely unknown. Here, in Vibrio cholerae, we identified HubP (hub of the pole), a polar transmembrane protein conserved in all vibrios, that anchors three ParA-like ATPases to the cell poles and, through them, controls polar localization of the chromosome origin, the chemotactic machinery, and the flagellum. In the absence of HubP, oriCI is not targeted to the cell poles, chemotaxis is impaired, and a small but increased fraction of cells produces multiple, rather than single, flagella. Distinct cytoplasmic domains within HubP are required for polar targeting of the three ATPases, while a periplasmic portion of HubP is required for its localization. HubP partially relocalizes from the poles to the mid-cell prior to cell division, thereby enabling perpetuation of the polar domain in future daughter cells. Thus, a single polar hub is instrumental for establishing polar identity and organization.

Basis for the essentiality of H-NS family members in Pseudomonas aeruginosa

Members of the histone-like nucleoid-structuring (H-NS) family of proteins have been shown to play important roles in silencing gene expression and in nucleoid compaction. In Pseudomonas aeruginosa, the two H-NS family members MvaT and MvaU are thought to bind the same AT-rich regions of the chromosome and function coordinately to control a common set of genes. Here we present evidence that the loss of both MvaT and MvaU cannot be tolerated because it results in the production of Pf4 phage that superinfect and kill cells or inhibit their growth. Using a ClpXP-based protein depletion system in combination with transposon mutagenesis, we identify mutants of P. aeruginosa that can tolerate the depletion of MvaT in an ΔmvaU mutant background. Many of these mutants contain insertions in genes encoding components, assembly factors, or regulators of type IV pili or contain insertions in genes of the prophage Pf4. We demonstrate that cells that no longer produce type IV pili or that no longer produce the replicative form of the Pf4 genome can tolerate the loss of both MvaT and MvaU. Furthermore, we show that the loss of both MvaT and MvaU results in an increase in expression of Pf4 genes and that cells that cannot produce type IV pili are resistant to infection by Pf4 phage. Our findings suggest that type IV pili are the receptors for Pf4 phage and that the essential activities of MvaT and MvaU are to repress the expression of Pf4 genes.

Mechanisms of meningococcal colonisation

Despite advances against infectious diseases over the past century, Neisseria meningitidis remains a major causative agent of meningitis and septicaemia worldwide. Its adaptation for survival in the human nasopharynx makes the meningococcus a highly successful commensal bacterium. Recent progress has been made in understanding the mechanisms that enable neisserial colonisation, in terms of the role of type IV pili, the impact of other adhesins, biofilm formation, nutrient acquisition and resistance to host immune defences. Refinements in cell-based and in vivo models will lead to improved understanding of the colonisation process, and hopefully to more effective vaccines and therapeutic strategies.

Coordinated, long-range, solid substrate movement of the purple photosynthetic bacterium Rhodobacter capsulatus

The long-range movement of Rhodobacter capsulatus cells in the glass-agar interstitial region of borosilicate Petri plates was found to be due to a subset of the cells inoculated into plates. The macroscopic appearance of plates indicated that a small group of cells moved in a coordinated manner to form a visible satellite cluster of cells. Satellite clusters were initially separated from the point of inoculation by the absence of visible cell density, but after 20 to 24 hours this space was colonized by cells apparently shed from a group of cells moving away from the point of inoculation. Cell movements consisted of flagellum-independent and flagellum-dependent motility contributions. Flagellum-independent movement occurred at an early stage, such that satellite clusters formed after 12 to 24 hours. Subsequently, after 24 to 32 hours, a flagellum-dependent dispersal of cells became visible, extending laterally outward from a line of flagellum-independent motility. These modes of taxis were found in several environmental isolates and in a variety of mutants, including a strain deficient in the production of the R. capsulatus acyl-homoserine lactone quorum-sensing signal. Although there was great variability in the direction of movement in illuminated plates, cells were predisposed to move toward broad spectrum white light. This predisposition was increased by the use of square plates, and a statistical analysis indicated that R. capsulatus is capable of genuine phototaxis. Therefore, the variability in the direction of cell movement was attributed to optical effects on light waves passing through the plate material and agar medium.

Role of fimV in type II secretion system-dependent protein secretion of Pseudomonas aeruginosa on solid medium

Although classical type II secretion systems (T2SSs) are widely present in Gram-negative bacteria, atypical T2SSs can be found in some species. In Pseudomonas aeruginosa, in addition to the classical T2SS Xcp, it was reported that two genes, xphA and xqhA, located outside the xcp locus were organized in an operon (PaQa) which encodes the orphan PaQa subunit. This subunit is able to associate with other components of the classical Xcp machinery to form a functional hybrid T2SS. In the present study, using a transcriptional lacZ fusion, we found that the PaQa operon was more efficiently expressed (i) on solid LB agar than in liquid LB medium, (ii) at 25 °C than at 37 °C and (iii) at an early stage of growth. These results suggested an adaptation of the hybrid system to particular environmental conditions. Transposon mutagenesis led to the finding that vfr and fimV genes are required for optimal expression of the orphan PaQa operon in the defined growth conditions used. Using an original culturing device designed to monitor secretion on solid medium, the ring-plate system, we found that T2SS-dependent secretion of exoproteins, namely the elastase LasB, was affected in a fimV deletion mutant. Our findings led to the discovery of an interplay between FimV and the global regulator Vfr triggering the modulation of the level of Vfr and consequently the modulation of T2SS-dependent secretion on solid medium.

Architecture of the type II secretion and type IV pilus machineries

Motility and protein secretion are key processes contributing to bacterial virulence. A wealth of phylogenetic, biochemical and structural evidence support the hypothesis that the widely distributed type IV pilus (T4P) system, involved in twitching motility, and the type II secretion (T2S) system, involved in exoprotein release, are descended from a common progenitor. Both are composed of dedicated but dynamic assemblages, which have been proposed to function through alternate polymerization and depolymerization or degradation of pilin-like subunits. While ongoing studies aimed at understanding the details of assembly and function of these systems are leading to new insights, there are still large knowledge gaps with respect to several fundamental aspects of their biology, including the localization and stoichiometry of critical assembly components, and the nature of their interactions. This article highlights recent advances in understanding the architectures of the T4P and T2S systems, and the organization of their inner and outer membrane components. As structural data accumulates, it is becoming increasingly apparent that even components with little-to-no sequence similarity have similar folds, further supporting the idea that both systems function by a similar mechanism.

The peptidoglycan-binding protein FimV promotes assembly of the Pseudomonas aeruginosa type IV pilus secretin

The Pseudomonas aeruginosa inner membrane protein FimV is among several proteins of unknown function required for type IV pilus-mediated twitching motility, arising from extension and retraction of pili from their site of assembly in the inner membrane. The pili transit the periplasm and peptidoglycan (PG) layer, ultimately exiting the cell through the PilQ secretin. Although fimV mutants are nonmotile, they are susceptible to killing by pilus-specific bacteriophage, a hallmark of retractable surface pili. Here we show that levels of recoverable surface pili were markedly decreased in fimV pilT retraction-deficient mutants compared with levels in the pilT control, demonstrating that FimV acts at the level of pilus assembly. Levels of inner membrane assembly subcomplex proteins PilM/N/O/P were decreased in fimV mutants, but supplementation of these components in trans did not restore pilus assembly or motility. Loss of FimV dramatically reduced the levels of the PilQ secretin multimer through which pili exit the cell, in part due to decreased levels of PilQ monomers, while PilF pilotin levels were unchanged. Expression of pilQ in trans in the wild type or fimV mutants increased total PilQ monomer levels but did not alter secretin multimer levels or motility. PG pulldown assays showed that the N terminus of FimV bound PG in a LysM motif-dependent manner, and a mutant with an in-frame chromosomal deletion of the LysM motif had reduced motility, secretin levels, and surface piliation. Together, our data show that FimV's role in pilus assembly is to promote secretin formation and that this function depends upon its PG-binding domain.

The role of fimV and the importance of its tandem repeat copy number in twitching motility, pigment production, and morphology in Legionella pneumophila

Twitching motility, a flagella-independent type of translocation of bacteria over moist surfaces, requires an array of proteins, including FimV. To investigate the role of this protein in twitching motility in Legionella pneumophila, we have generated a knockout mutant of fimV and characterized its phenotypic effects. In addition to a major reduction in twitching motility, deletion of the fimV gene caused a number of other phenotypic effects including decreased protective pigment formation, and it also affected cell morphology. Since fimV contains a variable number of tandem repeats, which can vary according to the origin of a given strain, we have examined the importance of this variability found within the coding region of this gene. By complementing the knockout strain with constructs containing a different number of this tandem repeat, we have been able to also show that repeat copy number is important in the functioning of this gene.

The Pseudomonas aeruginosa Chp chemosensory system regulates intracellular cAMP levels by modulating adenylate cyclase activity

Multiple virulence systems in the opportunistic pathogen Pseudomonas aeruginosa are regulated by the second messenger signalling molecule adenosine 3', 5'-cyclic monophosphate (cAMP). Production of cAMP by the putative adenylate cyclase enzyme CyaB represents a critical control point for virulence gene regulation. To identify regulators of CyaB, we screened a transposon insertion library for mutants with reduced intracellular cAMP. The majority of insertions resulting in reduced cAMP mapped to the Chp gene cluster encoding a putative chemotaxis-like chemosensory system. Further genetic analysis of the Chp system revealed that it has both positive and negative effects on intracellular cAMP and that it regulates cAMP levels by modulating CyaB activity. The Chp system was previously implicated in the production and function of type IV pili (TFP). Given that cAMP and the cAMP-dependent transcriptional regulator Vfr control TFP biogenesis gene expression, we explored the relationship between cAMP, the Chp system and TFP regulation. We discovered that the Chp system controls TFP production through modulation of cAMP while control of TFP-dependent twitching motility is cAMP-independent. Overall, our data define a novel function for a chemotaxis-like system in controlling cAMP production and establish a regulatory link between the Chp system, TFP and other cAMP-dependent virulence systems.

Response of Pseudomonas aeruginosa PAO1 to low shear modelled microgravity involves AlgU regulation

As a ubiquitous environmental organism that is occasionally part of the human flora, Pseudomonas aeruginosa could pose a health hazard for the immunocompromised astronauts during long-term missions. Therefore, insights into the behaviour of P. aeruginosa under spaceflight conditions were gained using two spaceflight-analogue culture systems: the rotating wall vessel (RWV) and the random position machine (RPM). Microarray analysis of P. aeruginosa PAO1 grown in the low shear modelled microgravity (LSMMG) environment of the RWV, compared with the normal gravity control (NG), revealed an apparent regulatory role for the alternative sigma factor AlgU (RpoE-like). Accordingly, P. aeruginosa cultured in LSMMG exhibited increased alginate production and upregulation of AlgU-controlled transcripts, including those encoding stress-related proteins. The LSMMG increased heat and oxidative stress resistance and caused a decrease in the oxygen transfer rate of the culture. This study also showed the involvement of the RNA-binding protein Hfq in the LSMMG response, consistent with its previously identified role in the Salmonella LSMMG and spaceflight response. The global transcriptional response of P. aeruginosa grown in the RPM was highly similar to that in NG. Fluid mixing was assessed in both systems and is believed to be a pivotal factor contributing to transcriptional differences between RWV- and RPM-grown P. aeruginosa. This study represents the first step towards the identification of virulence mechanisms of P. aeruginosa activated in response to spaceflight-analogue conditions, and could direct future research regarding the risk assessment and prevention of Pseudomonas infections during spaceflight and in immunocompromised patients.

Single-residue changes in the C-terminal disulfide-bonded loop of the Pseudomonas aeruginosa type IV pilin influence pilus assembly and twitching motility

PilA, the major pilin subunit of Pseudomonas aeruginosa type IV pili (T4P), is a principal structural component. PilA has a conserved C-terminal disulfide-bonded loop (DSL) that has been implicated as the pilus adhesinotope. Structural studies have suggested that DSL is involved in intersubunit interactions within the pilus fiber. PilA mutants with single-residue substitutions, insertions, or deletions in the DSL were tested for pilin stability, pilus assembly, and T4P function. Mutation of either Cys residue of the DSL resulted in pilins that were unable to assemble into fibers. Ala replacements of the intervening residues had a range of effects on assembly or function, as measured by changes in surface pilus expression and twitching motility. Modification of the C-terminal P-X-X-C type II beta-turn motif, which is one of the few highly conserved features in pilins across various species, caused profound defects in assembly and twitching motility. Expression of pilins with suspected assembly defects in a pilA pilT double mutant unable to retract T4P allowed us to verify which subunits were physically unable to assemble. Use of two different PilA antibodies showed that the DSL may be an immunodominant epitope in intact pili compared with pilin monomers. Sequence diversity of the type IVa pilins likely reflects an evolutionary compromise between retention of function and antigenic variation. The consequences of DSL sequence changes should be evaluated in the intact protein since it is technically feasible to generate DSL-mimetic peptides with mutations that will not appear in the natural repertoire due to their deleterious effects on assembly.

Twitching motility in Legionella pneumophila

Twitching motility is a form of bacterial translocation over solid or semi-solid surfaces mediated by the extension, tethering, and subsequent retraction of type IV pili. These pili are also known to be involved in virulence, biofilm formation, formation of fruiting bodies, horizontal gene transfer, and protein secretion. We have characterized the presence of twitching motility on agar plates in Legionella pneumophila, the etiological agent of Legionnaires' disease. By examining twitching motility zones, we have demonstrated that twitching motility was dependent on agar thickness/concentration, the chemical composition of the media, the presence of charcoal and cysteine, proximity to other bacteria, and temperature. A knockout mutant of the pilus subunit, pilE, exhibited a total loss of twitching motility at 37 degrees C, but not at 27 degrees C, suggesting either the existence of a compensating pilus subunit or of another twitching motility system in this organism.

Intragenic tandem repeat variation between Legionella pneumophila strains

Background

Bacterial genomes harbour a large number of tandem repeats, yet the possible phenotypic effects of those found within the coding region of genes are only beginning to be examined. Evidence exists from other organisms that these repeats can be involved in the evolution of new genes, gene regulation, adaptation, resistance to environmental stresses, and avoidance of the immune system.

Results

In this study, we have investigated the presence and variability in copy number of intragenic tandemly repeated sequences in the genome of Legionella pneumophila, the etiological agent of a severe pneumonia known as Legionnaires' disease. Within the genome of the Philadelphia strain, we have identified 26 intragenic tandem repeat sequences using conservative selection criteria. Of these, seven were "polymorphic" in terms of repeat copy number between a large number of L. pneumophila serogroup 1 strains. These strains were collected from a wide variety of environments and patients in several geographical regions. Within this panel of strains, all but one of these seven genes exhibited statistically different patterns in repeat copy number between samples from different origins (environmental, clinical, and hot springs).

Conclusion

These results support the hypothesis that intragenic tandem repeats could play a role in virulence and adaptation to different environments. While tandem repeats are an increasingly popular focus of molecular typing studies in prokaryotes, including in L. pneumophila, this study is the first examining the difference in tandem repeat distribution as a function of clinical or environmental origin.

Novel proteins that modulate type IV pilus retraction dynamics in Pseudomonas aeruginosa

Pseudomonas aeruginosa uses type IV pili to colonize various materials and for surface-associated twitching motility. We previously identified five phylogenetically distinct alleles of pilA in P. aeruginosa, four of which occur in genetic cassettes with specific accessory genes (J. V. Kus, E. Tullis, D. G. Cvitkovitch, and L. L. Burrows, Microbiology 150:1315-1326, 2004). Each of the five pilin alleles, with and without its associated pilin accessory gene, was used to complement a group II PAO1 pilA mutant. Expression of group I or IV pilA genes restored twitching motility to the same extent as the PAO1 group II pilin. In contrast, poor twitching resulted from complementation with group III or group V pilA genes but increased significantly when the cognate tfpY or tfpZ accessory genes were cointroduced. The enhanced motility was linked to an increase in recoverable surface pili and not to alterations in total pilin pools. Expression of the group III or V pilins in a PAO1 pilA-pilT double mutant yielded large amounts of surface pili, regardless of the presence of the accessory genes. Therefore, poor piliation in the absence of the TfpY and TfpZ accessory proteins results from a net increase in PilT-mediated retraction. Similar phenotypes were observed for tfpY single and tfpY-pilT double knockout mutants of group III strain PA14. A PilAV-TfpY chimera produced few surface pili, showing that the accessory proteins are specific for their cognate pilin. The genetic linkage between specific pilin and accessory genes may be evolutionarily conserved because the accessory proteins increase pilus expression on the cell surface, thereby enhancing function.

T-cell stimulating protein A (TspA) of Neisseria meningitidis is required for optimal adhesion to human cells

T-cell stimulating protein A (TspA) is an immunogenic, T-cell and B-cell stimulating protein of Neisseria meningitidis. Sequence similarity between TspA and FimV, a Pseudomonas aeruginosa protein involved in twitching motility, suggested a link between TspA and type IV pili (Tfp). To determine the role of TspA an isogenic deletion mutant was created. Loss of TspA did not affect twitching motility or piliation indicating that there are functional differences between TspA and FimV. Mutation of tspA led to a significant reduction in adhesion of meningococci to meningothelial and HEp-2 cells, which was not due to a lack of transcription of adjacent genes or pilC1. Other Tfp-mediated phenotypes (i.e. auto-aggregation and transformation competence) were not altered. Our results indicate that the role of TspA in adhesion is unlikely to be directly linked to the function of Tfp. TspA was expressed by all N. meningitidis and Neisseria polysaccharea strains examined but not by Neisseria gonorrhoeae or Neisseria lactamica, although sequences with homology to tspA were present in their genomes. In summary, TspA is a highly conserved antigen that is required for optimal adhesion of meningococci to human cells.

Pleiotropic role of quorum-sensing autoinducer 2 in Photorhabdus luminescens

Bacterial virulence is an integrative process that may involve quorum sensing. In this work, we compared by global expression profiling the wild-type entomopathogenic Photorhabdus luminescens subsp. laumondii TT01 to a luxS-deficient mutant unable to synthesize the type 2 quorum-sensing inducer AI-2. AI-2 was shown to regulate more than 300 targets involved in most compartments and metabolic pathways of the cell. AI-2 is located high in the hierarchy, as it controls the expression of several transcriptional regulators. The regulatory effect of AI-2 appeared to be dose dependent. The luxS-deficient strain exhibited decreased biofilm formation and increased type IV/V pilus-dependent twitching motility. AI-2 activated its own synthesis and transport. It also modulated bioluminescence by regulating the synthesis of spermidine. AI-2 was further shown to increase oxidative stress resistance, which is necessary to overcome part of the innate immune response of the host insect involving reactive oxygen species. Finally, we showed that the luxS-deficient strain had attenuated virulence against the lepidopteran Spodoptera littoralis. We concluded that AI-2 is involved mainly in early steps of insect invasion in P. luminescens.

Weapons of mass retraction

Twitching motility is a unique form of bacterial propulsion on solid surfaces associated with cycles of extension, tethering and retraction of type IV pili (T4P). Although investigations over the last two decades in a number of species have identified the majority of the genes involved in this process, we are still learning how these pili are assembled and the mechanics by which bacteria use T4P to drag themselves from one place to another. Among the puzzles that remain to be solved is the mechanism by which hydrolysis of ATP is coupled to pilus assembly and disassembly, and how the cell envelope structure is modified to accommodate the passage of the pilus through the periplasm. Unravelling of these and other enigmas in the T4P system will not only teach us more about these important colonization and virulence factors, but also help us to understand related processes such as type II secretion, which relies on a set of proteins homologous to those in the T4P system, and bacterial conjugation, involving retractable pili belonging to the F-like subgroup of the type IV secretion family. This review focuses on recent discoveries relating to the assembly and function of T4P in generation of twitching motility.

tonB3 is required for normal twitching motility and extracellular assembly of type IV pili

Three mutants with Tn5-B21 insertion in tonB3 (PA0406) of Pseudomonas aeruginosa exhibited defective twitching motility and reduced assembly of extracellular pili. These defects could be complemented with wild-type tonB3.

The truA gene of Pseudomonas aeruginosa is required for the expression of type III secretory genes

Invasive strains of Pseudomonas aeruginosa can cause rapid host cell apoptosis by injecting the type III effector molecule ExoS. A transposon insertional mutant bank of P. aeruginosa was screened to identify P. aeruginosa genes that contribute to the ability of the bacteria to trigger host cell apoptosis. Several isolated mutants had disruptions in the fimV gene. A fimV mutant was unable to induce the expression of exoS, exoT and exsA genes under type III inducing conditions, thus exhibiting a defect in type III protein secretion. Furthermore, this mutant was defective in twitching motility, although type IV pili were present on the bacterial surface. Complementation by a fimV-containing cosmid clone restored both phenotypes to the wild-type levels. However, expression of the type III genes in the fimV mutant was not restored by the introduction of a fimV gene alone, although it restored the twitching motility. A gene downstream of fimV, encoding a tRNA pseudouridine synthase (truA) homologue, was able to complement the type III gene expression defect of the fimV mutant. Thus fimV and truA form an operon and fimV mutation has a polar effect on truA. Indeed, a truA mutant is defective in type III gene expression while its twitching motility is unaffected, and a truA clone is able to complement the type III secretion defect. Pseudouridination of tRNAs is important for tRNA structure, thereby improving the fidelity of protein synthesis and helping to maintain the proper reading frame; thus the results imply that truA controls tRNAs that are critical for the translation of type III genes or their regulators.

FimX, a multidomain protein connecting environmental signals to twitching motility in Pseudomonas aeruginosa

Twitching motility is a form of surface translocation mediated by the extension, tethering, and retraction of type IV pili. Three independent Tn5-B21 mutations of Pseudomonas aeruginosa with reduced twitching motility were identified in a new locus which encodes a predicted protein of unknown function annotated PA4959 in the P. aeruginosa genome sequence. Complementation of these mutants with the wild-type PA4959 gene, which we designated fimX, restored normal twitching motility. fimX mutants were found to express normal levels of pilin and remained sensitive to pilus-specific bacteriophages, but they exhibited very low levels of surface pili, suggesting that normal pilus function was impaired. The fimX gene product has a molecular weight of 76,000 and contains four predicted domains that are commonly found in signal transduction proteins: a putative response regulator (CheY-like) domain, a PAS-PAC domain (commonly involved in environmental sensing), and DUF1 (or GGDEF) and DUF2 (or EAL) domains, which are thought to be involved in cyclic di-GMP metabolism. Red fluorescent protein fusion experiments showed that FimX is located at one pole of the cell via sequences adjacent to its CheY-like domain. Twitching motility in fimX mutants was found to respond relatively normally to a range of environmental factors but could not be stimulated by tryptone and mucin. These data suggest that fimX is involved in the regulation of twitching motility in response to environmental cues.

Type IV pili and twitching motility

Twitching motility is a flagella-independent form of bacterial translocation over moist surfaces. It occurs by the extension, tethering, and then retraction of polar type IV pili, which operate in a manner similar to a grappling hook. Twitching motility is equivalent to social gliding motility in Myxococcus xanthus and is important in host colonization by a wide range of plant and animal pathogens, as well as in the formation of biofilms and fruiting bodies. The biogenesis and function of type IV pili is controlled by a large number of genes, almost 40 of which have been identified in Pseudomonas aeruginosa. A number of genes required for pili assembly are homologous to genes involved in type II protein secretion and competence for DNA uptake, suggesting that these systems share a common architecture. Twitching motility is also controlled by a range of signal transduction systems, including two-component sensor-regulators and a complex chemosensory system.

Bacterial gliding motility: multiple mechanisms for cell movement over surfaces

The mechanisms responsible for bacterial gliding motility have been a mystery for almost 200 years. Gliding bacteria move actively over surfaces by a process that does not involve flagella. Gliding bacteria are phylogenetically diverse and are abundant in many environments. Recent results indicate that more than one mechanism is needed to explain all forms of bacterial gliding motility. Myxococcus xanthus "social gliding motility" and Synechocystis gliding are similar to bacterial "twitching motility" and rely on type IV pilus extension and retraction for cell movement. In contrast, gliding of filamentous cyanobacteria, mycoplasmas, members of the Cytophaga-Flavobacterium group, and "adventurous gliding" of M. xanthus do not appear to involve pili. The mechanisms of movement employed by these bacteria are still a matter of speculation. Genetic, biochemical, ultrastructural, and behavioral studies are providing insight into the machineries employed by these diverse bacteria that enable them to glide over surfaces.