As prokaryotic models for multicellular development, Stigmatellaaurantiaca and Myxococcus xanthus share many similarities in terms of social behaviors, such as gliding motility. Our current understanding of myxobacterial grouped-cell motilities comes mainly from the research on M. xanthus, which shows that filamentous type IV pili (TFP), composed of type IV pilin (also called PilA protein) subunits, are the key apparatus for social motility (S-motility). However, little is known about the pilin protein in S. aurantiaca. We cloned and sequenced four genes (pilASa1~4) from S. aurantiaca DSM17044 that are homologous to pilAMx (pilA gene in M. xanthus DK1622). The homology and similarities among PilASa proteins and other myxobacterial homologues were systematically analyzed. To determine their potential biological functions, the four pilASa genes were expressed in M. xanthus DK10410 (ΔpilAMx), which did not restore S-motility on soft agar or EPS production to host cells. After further analysis of the motile behaviors in a methylcellulose solution, the M. xanthus strains were categorized into three types. YL6101, carrying pilASa1, and YL6104, carrying pilASa4, produced stable but unretractable surface pili; YL6102, carrying pilASa2, produced stable surface pili and exhibited reduced TFP-dependent motility in methylcellulose; YL6103, carrying pilASa3, produced unstable surface pili. Based on these findings, we propose that pilASa2 might be responsible for the type IV pilin production involved in group motility in S. aurantiaca DSM17044. After examining the developmental processes, it was suggested that the expression of PilASa4 protein might have positive effects on the fruiting body formation of M. xanthus DK10410 cells. Moreover, the formation of fruiting body in M. xanthus cells with stable exogenous TFPSa were compensated by mixing them with S. aurantiaca DSM17044 cells. Our results shed some light on the features and functions of type IV pilin homologues in S. aurantiaca.
Type IV pili (TFP) are membrane-anchored filaments with a number of important biological functions. In the model organism Myxococcus xanthus, TFP act as molecular engines that power social (S) motility through cycles of extension and retraction. TFP filaments consist of several thousand copies of a protein called PilA or pilin. PilA contains an N-terminal α-helix essential for TFP assembly and a C-terminal globular domain important for its activity. The role of the PilA sequence and its structure–function relationship in TFP-dependent S motility remain active areas of research. In this study, we identified an M. xanthus PilA mutant carrying an alanine to valine substitution at position 32 in the α-helix, which produced structurally intact but retraction-defective TFP. Characterization of this mutant and additional single-residue variants at this position in PilA demonstrated the critical role of alanine 32 in PilA stability, TFP assembly and retraction.
Gliding motility is observed in a large variety of phylogenetically unrelated bacteria. Gliding provides a means for microbes to travel in environments with a low water content, such as might be found in biofilms, microbial mats, and soil. Gliding is defined as the movement of a cell on a surface in the direction of the long axis of the cell. Because this definition is operational and not mechanistic, the underlying molecular motor(s) may be quite different in diverse microbes. In fact, studies on the gliding bacterium Myxococcus xanthus suggest that two independent gliding machineries, encoded by two multigene systems, operate in this microorganism. One machinery, which allows individual cells to glide on a surface, independent of whether the cells are moving alone or in groups, requires the function of the genes of the A-motility system. More than 37 A-motility genes are known to be required for this form of movement. Depending on an additional phenotype, these genes are divided into two subclasses, the agl and cgl genes. Videomicroscopic studies on gliding movement, as well as ultrastructural observations of two myxobacteria, suggest that the A-system motor may consist of multiple single motor elements that are arrayed along the entire cell body. Each motor element is proposed to be localized to the periplasmic space and to be anchored to the peptidoglycan layer. The force to glide which may be generated here is coupled to adhesion sites that move freely in the outer membrane. These adhesion sites provide a specific contact with the substratum. Based on single-cell observations, similar models have been proposed to operate in the unrelated gliding bacteria Flavobacterium johnsoniae (formerly Cytophaga johnsonae), Cytophaga strain U67, and Flexibacter polymorphus (a filamentous glider). Although this model has not been verified experimentally, M. xanthus seems to be the ideal organism with which to test it, given the genetic tools available. The second gliding motor in M. xanthus controls cell movement in groups (S-motility system). It is dependent on functional type IV pili and is operative only when cells are in close proximity to each other. Type IV pili are known to be involved in another mode of bacterial surface translocation, called twitching motility. S-motility may well represent a variation of twitching motility in M. xanthus. However, twitching differs from gliding since it involves cell movements that are jerky and abrupt and that lack the organization and smoothness observed in gliding. Components of this motor are encoded by genes of the S-system, which appear to be homologs of genes involved in the biosynthesis, assembly, and function of type IV pili in Pseudomonas aeruginosa and Neisseria gonorrhoeae. How type IV pili generate force in S-motility is currently unknown, but it is to be expected that ongoing physiological, genetic, and biochemical studies in M. xanthus, in conjunction with studies on twitching in P. aeruginosa and N. gonorrhoeae, will provide important insights into this microbial motor. The two motility systems of M. xanthus are affected to different degrees by the MglA protein, which shows similarity to a small GTPase. Bacterial chemotaxis-like sensory transduction systems control gliding motility in M. xanthus. The frz genes appear to regulate gliding movement of individual cells and movement by the S-motility system, suggesting that the two motors found in this bacterium can be regulated to result in coordinated multicellular movements. In contrast, the dif genes affect only S-system-dependent swarming.
Acinetobacter baumannii is a Gram-negative, opportunistic pathogen. Recently, multiple A. baumannii genomes have been sequenced; these data have led to the identification of many genes predicted to encode proteins required for the biogenesis of type IV pili (TFP). However, there is no experimental evidence demonstrating that A. baumannii strains actually produce functional TFP. Here, we demonstrated that A. baumannii strain M2 is naturally transformable and capable of twitching motility, two classical TFP-associated phenotypes. Strains were constructed with mutations in pilA, pilD, and pilT, genes whose products have been well characterized in other systems. These mutants were no longer naturally transformable and did not exhibit twitching motility. These TFP-associated phenotypes were restored when these mutations were complemented. More PilA was detected on the surface of the pilT mutant than the parental strain, and TFP were visualized on the pilT mutant by transmission electron microscopy. Thus, A. baumannii produces functional TFP and utilizes TFP for both natural transformation and twitching motility. Several investigators have hypothesized that TFP might be responsible, in part, for the flagellum-independent surface-associated motility exhibited by many A. baumannii clinical isolates. We demonstrated that surface-associated motility was not dependent on the products of the pilA, pilD, and pilT genes and, by correlation, TFP. The identification of functional TFP in A. baumannii lays the foundation for future work determining the role of TFP in models of virulence that partially recapitulate human disease.
IMPORTANCE Several investigators have documented the presence of genes predicted to encode proteins required for the biogenesis of TFP in many A. baumannii genomes. Furthermore, some have speculated that TFP may play a role in the unique surface-associated motility phenotype exhibited by many A. baumannii clinical isolates, yet there has been no experimental evidence to prove this. Unfortunately, progress in understanding the biology and virulence of A. baumannii has been slowed by the difficulty of constructing and complementing mutations in this species. Strain M2, a recently characterized clinical isolate, is amenable to genetic manipulation. We have established a reproducible system for the generation of marked and/or unmarked mutations using a modified recombineering strategy as well as a genetic complementation system utilizing a modified mini-Tn7 element in strain M2. Using this strategy, we demonstrated that strain M2 produces TFP and that TFP are not required for surface-associated motility exhibited by strain M2.
Several investigators have documented the presence of genes predicted to encode proteins required for the biogenesis of TFP in many A. baumannii genomes. Furthermore, some have speculated that TFP may play a role in the unique surface-associated motility phenotype exhibited by many A. baumannii clinical isolates, yet there has been no experimental evidence to prove this. Unfortunately, progress in understanding the biology and virulence of A. baumannii has been slowed by the difficulty of constructing and complementing mutations in this species. Strain M2, a recently characterized clinical isolate, is amenable to genetic manipulation. We have established a reproducible system for the generation of marked and/or unmarked mutations using a modified recombineering strategy as well as a genetic complementation system utilizing a modified mini-Tn7 element in strain M2. Using this strategy, we demonstrated that strain M2 produces TFP and that TFP are not required for surface-associated motility exhibited by strain M2.
The causative agent of gonorrhea, Neisseria gonorrhoeae, bears retractable filamentous appendages called type IV pili (Tfp). Tfp are used by many pathogenic and nonpathogenic bacteria to carry out a number of vital functions, including DNA uptake, twitching motility (crawling over surfaces), and attachment to host cells. In N. gonorrhoeae, Tfp binding to epithelial cells and the mechanical forces associated with this binding stimulate signaling cascades and gene expression that enhance infection. Retraction of a single Tfp filament generates forces of 50–100 piconewtons, but nothing is known, thus far, on the retraction force ability of multiple Tfp filaments, even though each bacterium expresses multiple Tfp and multiple bacteria interact during infection. We designed a micropillar assay system to measure Tfp retraction forces. This system consists of an array of force sensors made of elastic pillars that allow quantification of retraction forces from adherent N. gonorrhoeae bacteria. Electron microscopy and fluorescence microscopy were used in combination with this novel assay to assess the structures of Tfp. We show that Tfp can form bundles, which contain up to 8–10 Tfp filaments, that act as coordinated retractable units with forces up to 10 times greater than single filament retraction forces. Furthermore, single filament retraction forces are transient, whereas bundled filaments produce retraction forces that can be sustained. Alterations of noncovalent protein–protein interactions between Tfp can inhibit both bundle formation and high-amplitude retraction forces. Retraction forces build over time through the recruitment and bundling of multiple Tfp that pull cooperatively to generate forces in the nanonewton range. We propose that Tfp retraction can be synchronized through bundling, that Tfp bundle retraction can generate forces in the nanonewton range in vivo, and that such high forces could affect infection.
Type IV pili are filamentous appendages borne by a large number of pathogenic and nonpathogenic bacteria. They play crucial roles in basic microbial processes such as surface motility, virulence, and DNA exchange. Neisseria gonorrhoeae, the causative agent of gonorrhea, can extend and retract these long, thin threads—around 6 nm in diameter and up to 30 μm long—to explore and pull on the environment. The retraction of one N. gonorrhoeae pilus filament can exert forces of 50–100 piconewtons, or roughly 10,000 times the bacterium's bodyweight. The bacteria can exert those forces on human cells that they infect, and force has been shown to be an important parameter in their infectivity. We use a micropillar assay system to show that N. gonorrhoeae cells can exert even higher forces by forming bundles of 8–10 filaments that act as coordinated retractable units. The bacteria can thus achieve forces in the nanonewton range (or 100,000 times their bodyweight) making them the strongest microscale elements known to date. This study demonstrates the power and cooperativity of pilus nanomotors and opens new territories for the exploration of force-mediated bacteria–host-cell interactions.
To make its way into cells,Neisseria gonorrhoeae, the pathogen that causes gonorrhea, may exert nanonewton forces through cooperative bundling of pilus fibers.
Myxococcus xanthus is a gram-negative bacterium capable of complex developmental processes involving vegetative swarming and fruiting body formation. Social (S-) gliding motility, one of the two motility systems employed by M. xanthus, requires at least two cell surface structures: type IV pili (TFP) and extracellular polysaccharides (EPS). Extended TFP which are composed of thousands of copies of PilA retract upon binding to EPS and thereby pull the cell forward. TFP also act as external sensor to regulate EPS production. In this study, we generated a random PilA mutant library and identified one derivative, SW1066, which completely failed to undergo developmental processes. Detailed characterization revealed that SW1066 produced very little EPS but wild-type amounts of PilA. These mutated PilA subunits, however, are unable to assemble into functional TFP despite their ability to localize to the membrane. By preventing the mutated PilA of SW1066 to translocate from the cytoplasm to the membrane, fruiting body formation and EPS production was restored to the levels observed in mutant strains lacking PilA. This apparent connection between PilA membrane accumulation and reduction in surface EPS implies that specific cellular PilA localization are required to maintain the EPS level necessary to sustain normal S-motilityin M. xanthus.
Myxococcus xanthus; type four pili; PilA; extracellular polysaccharide
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.
Type IV pili (TFP) play central roles in the expression of many phenotypes including motility, multicellular behavior, sensitivity to bacteriophages, natural genetic transformation, and adherence. In Neisseria gonorrhoeae, these properties require ancillary proteins that act in conjunction with TFP expression and influence organelle dynamics. Here, the intrinsic contributions of the pilin protein itself to TFP dynamics and associated phenotypes were examined by expressing the Pseudomonas aeruginosa PilAPAK pilin subunit in N. gonorrhoeae. We show here that, although PilAPAK pilin can be readily assembled into TFP in this background, steady-state levels of purifiable fibers are dramatically reduced relative those of endogenous pili. This defect is due to aberrant TFP dynamics as it is suppressed in the absence of the PilT pilus retraction ATPase. Functionally, PilAPAK pilin complements gonococcal adherence for human epithelial cells but only in a pilT background, and this property remains dependent on the coexpression of both the PilC adhesin and the PilV pilin-like protein. Since P. aeruginosa pilin only moderately supports neisserial sequence-specific transformation despite its assembly proficiency, these results together suggest that PilAPAK pilin functions suboptimally in this environment. This appears to be due to diminished compatibility with resident proteins essential for TFP function and dynamics. Despite this, PilAPAK pili support retractile force generation in this background equivalent to that reported for endogenous pili. Furthermore, PilAPAK pili are both necessary and sufficient for bacteriophage PO4 binding, although the strain remains phage resistant. Together, these findings have significant implications for TFP biology in both N. gonorrhoeae and P. aeruginosa.
Biofilms are surface-attached multicellular communities. Using single-cell tracking microscopy, we showed that a pilY1 mutant of Pseudomonas aeruginosa is defective in early biofilm formation. We leveraged the observation that PilY1 protein levels increase on a surface to perform a genetic screen to identify mutants altered in surface-grown expression of this protein. Based on our genetic studies, we found that soon after initiating surface growth, cyclic AMP (cAMP) levels increase, dependent on PilJ, a chemoreceptor-like protein of the Pil-Chp complex, and the type IV pilus (TFP). cAMP and its receptor protein Vfr, together with the FimS-AlgR two-component system (TCS), upregulate the expression of PilY1 upon surface growth. FimS and PilJ interact, suggesting a mechanism by which Pil-Chp can regulate FimS function. The subsequent secretion of PilY1 is dependent on the TFP assembly system; thus, PilY1 is not deployed until the pilus is assembled, allowing an ordered signaling cascade. Cell surface-associated PilY1 in turn signals through the TFP alignment complex PilMNOP and the diguanylate cyclase SadC to activate downstream cyclic di-GMP (c-di-GMP) production, thereby repressing swarming motility. Overall, our data support a model whereby P. aeruginosa senses the surface through the Pil-Chp chemotaxis-like complex, TFP, and PilY1 to regulate cAMP and c-di-GMP production, thereby employing a hierarchical regulatory cascade of second messengers to coordinate its program of surface behaviors.
Biofilms are surface-attached multicellular communities. Here, we show that a stepwise regulatory circuit, involving ordered signaling via two different second messengers, is required for Pseudomonas aeruginosa to control early events in cell-surface interactions. We propose that our studies have uncovered a multilayered “surface-sensing” system that allows P. aeruginosa to effectively coordinate its surface-associated behaviors. Understanding how cells transition into the biofilm state on a surface may provide new approaches to prevent formation of these communities.
Nutrient or niche-based competition among bacteria is a widespread phenomenon in natural environment. Such inter-species interactions are often mediated by secreted soluble factors and/or direct cell–cell contact. As ubiquitous soil bacteria, Myxococcus species are able to produce a variety of bioactive secondary metabolites to inhibit the growth of other competing bacterial species. Meanwhile, Myxococcus sp. also exhibit sophisticated predatory behavior, an extreme form of competition that is often stimulated by close contact with prey cells and largely depends on the availability of solid surfaces. Myxococcus sp. can also be isolated from aquatic environments. However, studies focusing on the interaction between Myxococcus and other bacteria in such environments are still limited. In this study, using the well-studied M. xanthus DK1622 and E. coli as model interspecies interaction pair, we demonstrated that in a aqueous environment, Myxococcus xanthus was able to kill Escherichia coli in a cell contact-dependent manner, and that the observed contact dependent killing required the formation of co-aggregates between M. xanthus and E. coli cells. Further analysis revealed that exopolysaccharide (EPS), type IV pilus (TFP) and lipopolysaccharide (LPS) mutants of M. xanthus displayed various degrees of attenuation in E. coli killing, and it correlated well with the mutants' reduction in EPS production. In addition, M. xanthus showed differential binding ability to different bacteria, and bacterial strains unable to co-aggregate with M. xanthus can escape the killing, suggesting the specific nature of co-aggregation and the targeted killing of interacting bacteria. In conclusion, our results demonstrated EPS mediated, contact-dependent killing of E. coli by M. xanthus, a strategy that might facilitate the survival of this ubiquitous bacterium in aquatic environments.
M. xanthus; contact-dependent interaction; co-aggregation; aqueous environment
Myxococcus xanthus, a Gram-negative soil bacterium, undergoes multicellular development when nutrients become limiting. Aggregation, which is part of the developmental process, requires the surface motility of this organism. One component of M. xanthus motility, the social (S) gliding motility, enables the movement of cells in close physical proximity. Previous studies demonstrated that the cell-surface associated exopolysaccharide (EPS) is essential for S motility and the Dif proteins form a chemotaxis-like pathway that regulates EPS production in M. xanthus. DifA, a homologue of methyl-accepting chemotaxis proteins (MCPs) in the Dif system, is required for EPS production, S motility and development. In this study, a spontaneous extragenic suppressor of a difA deletion was isolated in order to identify additional regulators of EPS production. The suppressor mutation was found to be a single base-pair insertion in cheW7 at the che7 chemotaxis gene cluster. Further examination indicated that mutations in cheW7 may lead to the interaction of Mcp7 with DifC (CheW-like) and DifE (CheA-like) to reconstruct a functional pathway to regulate EPS production in the absence of DifA. In addition, the cheW7 mutation was found to partially suppress a pilA mutation in EPS production in a difA+ background. Further deletion of difA from the pilA cheW7 double mutant resulted in a triple mutant that produced wild-type levels of EPS, implying that DifA (MCP-like) and Mcp7 compete for interactions with DifC and DifE in the modulation of EPS production.
Myxococcus xanthus, a Gram-negative soil bacterium, undergoes multicellular development when nutrients become limiting. Aggregation, which is part of the developmental process, requires the surface motility of this organism. One component of M. xanthus motility, the social (S) gliding motility, enables the movement of cells in close physical proximity. Previous studies demonstrated that the cell surface-associated exopolysaccharide (EPS) is essential for S motility and that the Dif proteins form a chemotaxis-like pathway that regulates EPS production in M. xanthus. DifA, a homologue of methyl-accepting chemotaxis proteins (MCPs) in the Dif system, is required for EPS production, S motility and development. In this study, a spontaneous extragenic suppressor of a difA deletion was isolated in order to identify additional regulators of EPS production. The suppressor mutation was found to be a single base pair insertion in cheW7 at the che7 chemotaxis gene cluster. Further examination indicated that mutations in cheW7 may lead to the interaction of Mcp7 with DifC (CheW-like) and DifE (CheA-like) to reconstruct a functional pathway to regulate EPS production in the absence of DifA. In addition, the cheW7 mutation was found to partially suppress a pilA mutation in EPS production in a difA+ background. Further deletion of difA from the pilA cheW7 double mutant resulted in a triple mutant that produced wild-type levels of EPS, implying that DifA (MCP-like) and Mcp7 compete for interactions with DifC and DifE in the modulation of EPS production.
Pseudomonas aeruginosa, an important opportunistic pathogen of man, exploits numerous factors for initial attachment to the host, an event required to establish bacterial infection. In this paper, we rigorously explore the role of two major bacterial adhesins, type IV pili (Tfp) and flagella, in bacterial adherence to distinct host receptors at the apical (AP) and basolateral (BL) surfaces of polarized lung epithelial cells and induction of subsequent host signaling and pathogenic events. Using an isogenic mutant of P. aeruginosa that lacks flagella or utilizing beads coated with purified Tfp, we establish that Tfp are necessary and sufficient for maximal binding to host N-glycans at the AP surface of polarized epithelium. In contrast, experiments utilizing a P. aeruginosa isogenic mutant that lacks Tfp or using beads coated with purified flagella demonstrate that flagella are necessary and sufficient for maximal binding to heparan sulfate (HS) chains of heparan sulfate proteoglycans (HSPGs) at the BL surface of polarized epithelium. Using two different cell-free systems, we demonstrate that Tfp-coated beads show highest binding affinity to complex N-glycan chains coated onto plastic plates and preferentially aggregate with beads coated with N-glycans, but not with single sugars or HS. In contrast, flagella-coated beads bind to or aggregate preferentially with HS or HSPGs, but demonstrate little binding to N-glycans. We further show that Tfp-mediated binding to host N-glycans results in activation of phosphatidylinositol 3-kinase (PI3K)/Akt pathway and bacterial entry at the AP surface. At the BL surface, flagella-mediated binding to HS activates the epidermal growth factor receptor (EGFR), adaptor protein Shc, and PI3K/Akt, and induces bacterial entry. Remarkably, flagella-coated beads alone can activate EGFR and Shc. Together, this work provides new insights into the intricate interactions between P. aeruginosa and lung epithelium that may be potentially useful in the development of novel treatments for P. aeruginosa infections.
Pseudomonas aeruginosa is one of the most virulent nosocomial opportunistic pathogens that is associated with a broad spectrum of life-threatening infections. Antibiotic resistance is widespread and attributable mortality remains near 50%. Complex binding to epithelial cells is a key first step for this potent pathogen to unleash its armamentarium of virulence factors. Polarized epithelium has distinct apical (AP) and basolateral (BL) surface, composed of different glycosylated molecules, and P. aeruginosa can potentially employ different adhesins to bind to these receptors. Using isogenic mutants as well as in vitro cell-free assays, we demonstrate that bacterial type IV pili are necessary and sufficient to mediate AP interactions with N-glycans whereas bacterial flagella interact with heparan sulfate chains of proteoglycans on the BL surface. These interactions induce specific host signaling pathways that lead to subsequent pathogenic events, such as bacterial entry into host epithelium. Moreover, we show that flagella alone are sufficient to activate the epidermal growth factor receptor and the adaptor protein on the BL surface. These studies reveal new information about key players in the versatile interactions of P. aeruginosa with the host and provide appealing targets for blocking early binding steps essential for establishment of P. aeruginosa infections.
Type IV pili (TFP) and exopolysaccharides (EPS) are important components for social behaviors in Myxococcus xanthus, including gliding motility and fruiting body formation. Although specific interactions between TFP and EPS have been proposed, direct observations of these interactions under native condition have not yet been made. In this study, we found that a truncated PilA protein (PilACt) which only contains the C-terminal domain (amino acids 32-208) is sufficient for EPS binding in vitro. Furthermore, an enhanced green fluorescent protein (eGFP) and PilACt fusion protein was constructed and used to label the native EPS in M. xanthus. Under confocal laser scanning microscope, the eGFP-PilACt-bound fruiting bodies, trail structures and biofilms exhibited similar patterns as the wheat germ agglutinin lectin (WGA)-labeled EPS structures. This study showed that eGFP-PilACt fusion protein was able to efficiently label the EPS of M. xanthus and for the first time provided evidence for the direct interaction between the PilA protein and EPS under native conditions.
Type IV Pilin; Exopolysaccharides; Biofilm; Fruiting body; Confocal laser scanning microscopy; eGFP
Clostridium perfringens is an anaerobic, Gram-positive bacterium that causes a range of diseases in humans, including lethal gas gangrene. We have recently shown that strains of C. perfringens move across the surface of agar plates by a unique type IV pilus (TFP)-mediated social motility that had not been previously described. Based on sequence homology to pilins in Gram-negative bacteria, C. perfringens appears to have two pilin subunits, PilA1 and PilA2. Structural prediction analysis indicated PilA1 is similar to the pseudopilin found in Klebsiella oxytoca, while PilA2 is more similar to true pilins found in the Gram-negative pathogens Pseudomonas aeruginosa and Neisseria gonorrhoeae. Strains of N. gonorrhoeae that were genetically deficient in the native pilin, PilE, but supplemented with inducible expression of PilA1 and PilA2 of C. perfringens were constructed. Genetic competence, wild-type twitching motility, and attachment to human urogenital epithelial cells were not restored by expression of either pilin. However, attachment to mouse and rat myoblast (muscle) cell lines was observed with the N. gonorrhoeae strain expressing PilA2. Significantly, wild-type C. perfringens cells adhered to mouse myoblasts under anaerobic conditions, and adherence was 10-fold lower in a pilT mutant that lacked functional TFP. These findings implicate C. perfringens TFP in the ability of C. perfringens to adhere to and move along muscle fibers in vivo, which may provide a therapeutic approach to limiting this rapidly spreading and highly lethal infection.
Retroviral capsid recognition by Trim5 blocks productive infection. Rhesus macaques harbor three functionally distinct Trim5 alleles: Trim5αQ, Trim5αTFP and Trim5CypA. Despite the high degree of amino acid identity between Trim5αQ and Trim5αTFP alleles, the Q/TFP polymorphism results in the differential restriction of some primate lentiviruses, suggesting these alleles differ in how they engage these capsids. Simian immunodeficiency virus of rhesus macaques (SIVmac) evolved to resist all three alleles. Thus, SIVmac provides a unique opportunity to study a virus in the context of the Trim5 repertoire that drove its evolution in vivo. We exploited the evolved rhesus Trim5α resistance of this capsid to identify gain-of-sensitivity mutations that distinguish targets between the Trim5αQ and Trim5αTFP alleles. While both alleles recognize the capsid surface, Trim5αQ and Trim5αTFP alleles differed in their ability to restrict a panel of capsid chimeras and single amino acid substitutions. When mapped onto the structure of the SIVmac239 capsid N-terminal domain, single amino acid substitutions affecting both alleles mapped to the β-hairpin. Given that none of the substitutions affected Trim5αQ alone, and the fact that the β-hairpin is conserved among retroviral capsids, we propose that the β-hairpin is a molecular pattern widely exploited by Trim5α proteins. Mutations specifically affecting rhesus Trim5αTFP (without affecting Trim5αQ) surround a site of conservation unique to primate lentiviruses, overlapping the CPSF6 binding site. We believe targeting this site is an evolutionary innovation driven specifically by the emergence of primate lentiviruses in Africa during the last 12 million years. This modularity in targeting may be a general feature of Trim5 evolution, permitting different regions of the PRYSPRY domain to evolve independent interactions with capsid.
TRIM5α is an intrinsic immunity protein that blocks retrovirus infection through a specific interaction with the viral capsid. Uniquely among primates, rhesus macaques harbor three functionally distinct kinds of Trim5 alleles: rhTrim5αTFP, rhTrim5αQ and rhTrim5CypA. SIVmac239, a simian immunodeficiency virus that causes AIDS in rhesus macaques, is resistant to all three, whereas its relative, the human AIDS virus HIV-1, is inhibited by rhTrim5αTFP and rhTrim5αQ alleles. We exploited this difference between these two retroviruses to figure out how Trim5α proteins recognize viral capsids. By combining mutagenesis, structural biology and evolutionary data we determined that both rhTrim5αTFP and rhTrim5αQ recognize a conserved structure common to all retroviral capsids. However, we also found evidence suggesting that rhTrim5αTFP evolved to recognize an additional target that is specifically conserved among primate immunodeficiency viruses. Molecular evolutionary analysis indicates that this expanded function appeared in a common ancestor of modern African monkeys sometime between 9–12 million years ago, and that it thereafter continued to be modified by strong evolutionary pressure. Our results provide insight into the evolutionary flexibility of Trim5α-capsid interactions, and support the notion that viruses related to modern HIV and SIV have been present in Africa for millions of years.
Myxococcus xanthus cells harbor two motility machineries, type IV pili (Tfp) and the A-engine. During reversals, the two machineries switch polarity synchronously. We present a mechanism that synchronizes this polarity switching. We identify the required for motility response regulator (RomR) as essential for A-motility. RomR localizes in a bipolar, asymmetric pattern with a large cluster at the lagging cell pole. The large RomR cluster relocates to the new lagging pole in parallel with cell reversals. Dynamic RomR localization is essential for cell reversals, suggesting that RomR relocalization induces the polarity switching of the A-engine. The analysis of RomR mutants shows that the output domain targets RomR to the poles and the receiver domain is essential for dynamic localization. The small GTPase MglA establishes correct RomR polarity, and the Frz two-component system regulates dynamic RomR localization. FrzS localizes with Tfp at the leading pole and relocates in an Frz-dependent manner to the opposite pole during reversals; FrzS and RomR localize and oscillate independently. The Frz system synchronizes these oscillations and thus the synchronous polarity switching of the motility machineries.
A-motility; morphogenetic cell movements; Myxococcus xanthus; polarity; response regulator
Neisseria gonorrhoeae is the bacterium that causes gonorrhea, a major sexually transmitted disease and a significant cofactor for human immunodeficiency virus transmission. The retactile N. gonorrhoeae type IV pilus (Tfp) mediates twitching motility and attachment. Using live-cell microscopy, we reveal for the first time the dynamics of twitching motility by N. gonorrhoeae in its natural environment, human epithelial cells. Bacteria aggregate into microcolonies on the cell surface and induce a massive remodeling of the microvillus architecture. Surprisingly, the microcolonies are motile, and they fuse to form progressively larger structures that undergo rapid reorganization, suggesting that bacteria communicate with each other during infection. As reported, actin plaques form beneath microcolonies. Here, we show that cortical plaques comigrate with motile microcolonies. These activities are dependent on pilT, the Tfp retraction locus. Cultures infected with a pilT mutant have significantly higher numbers of apoptotic cells than cultures infected with the wild-type strain. Inducing pilT expression with isopropyl-β-d-thiogalactopyranoside partially rescues cells from infection-induced apoptosis, demonstrating that Tfp retraction is intrinsically cytoprotective for the host. Tfp-mediated attachment is therefore a continuum of microcolony motility and force stimulation of host cell signaling, leading to a cytoprotective effect.
Moraxella catarrhalis is a gram-negative mucosal pathogen of the human respiratory tract. Although little information is available regarding the initial steps of M. catarrhalis pathogenesis, this organism must be able to colonize the human mucosal surface in order to initiate an infection. Type IV pili (TFP), filamentous surface appendages primarily comprised of a single protein subunit termed pilin, play a crucial role in the initiation of disease by a wide range of bacteria. We previously identified the genes that encode the major proteins involved in the biosynthesis of M. catarrhalis TFP and determined that the TFP expressed by this organism are highly conserved and essential for natural transformation. We extended this initial study by investigating the contribution of TFP to the early stages of M. catarrhalis colonization. TFP-deficient M. catarrhalis bacteria exhibit diminished adherence to eukaryotic cells in vitro. Additionally, our studies demonstrate that M. catarrhalis cells form a mature biofilm in continuous-flow chambers and that biofilm formation is enhanced by TFP expression. The potential role of TFP in colonization by M. catarrhalis was further investigated using in vivo studies comparing the abilities of wild-type M. catarrhalis and an isogenic TFP mutant to colonize the nasopharynx of the chinchilla. These results suggest that the expression of TFP contributes to mucosal airway colonization. Furthermore, these data indicate that the chinchilla model of nasopharyngeal colonization provides an effective animal system for studying the early steps of M. catarrhalis pathogenesis.
Type IV pili (Tfp) of gram-negative species share many characteristics, including a common architecture and conserved biogenesis pathway. Much less is known about the regulation of Tfp expression in response to changing environmental conditions. We investigated the diversity of Tfp regulatory systems by searching for the molecular basis of the reported variable expression of the Tfp gene cluster of the pathogen Actinobacillus pleuropneumoniae. Despite the presence of an intact Tfp gene cluster consisting of four genes, apfABCD, no Tfp were formed under standard growth conditions. Sequence analysis of the predicted major subunit protein ApfA showed an atypical alanine residue at position −1 from the prepilin peptidase cleavage site in 42 strains. This alanine deviates from the consensus glycine at this position in Tfp from other species. Yet, cloning of the apfABCD genes under a constitutive promoter in A. pleuropneumoniae resulted in pilin and Tfp assembly. Tfp promoter-luxAB reporter gene fusions demonstrated that the Tfp promoter was intact but tightly regulated. Promoter activity varied with bacterial growth phase and was detected only when bacteria were grown in chemically defined medium. Infection experiments with cultured epithelial cells demonstrated that Tfp promoter activity was upregulated upon adherence of the pathogen to primary cultures of lung epithelial cells. Nonadherent bacteria in the culture supernatant exhibited virtually no promoter activity. A similar upregulation of Tfp promoter activity was observed in vivo during experimental infection of pigs. The host cell contact-induced and in vivo-upregulated Tfp promoter activity in A. pleuropneumoniae adds a new dimension to the diversity of Tfp regulation.
In Gram-negative bacteria, type IV pili (TFP) have long been known to play important roles in such diverse biological phenomena as surface adhesion, motility, and DNA transfer, with significant consequences for pathogenicity. More recently it became apparent that Gram-positive bacteria also express type IV pili; however, little is known about the diversity and abundance of these structures in Gram-positives. Computational tools for automated identification of type IV pilins are not currently available.
To assess TFP diversity in Gram-positive bacteria and facilitate pilin identification, we compiled a comprehensive list of putative Gram-positive pilins encoded by operons containing highly conserved pilus biosynthetic genes (pilB, pilC). A surprisingly large number of species were found to contain multiple TFP operons (pil, com and/or tad). The N-terminal sequences of predicted pilins were exploited to develop PilFind, a rule-based algorithm for genome-wide identification of otherwise poorly conserved type IV pilins in any species, regardless of their association with TFP biosynthetic operons (http://signalfind.org). Using PilFind to scan 53 Gram-positive genomes (encoding >187,000 proteins), we identified 286 candidate pilins, including 214 in operons containing TFP biosynthetic genes (TBG+ operons). Although trained on Gram-positive pilins, PilFind identified 55 of 58 manually curated Gram-negative pilins in TBG+ operons, as well as 53 additional pilin candidates in operons lacking biosynthetic genes in ten species (>38,000 proteins), including 27 of 29 experimentally verified pilins. False positive rates appear to be low, as PilFind predicted only four pilin candidates in eleven bacterial species (>13,000 proteins) lacking TFP biosynthetic genes.
We have shown that Gram-positive bacteria contain a highly diverse set of type IV pili. PilFind can be an invaluable tool to study bacterial cellular processes known to involve type IV pilus-like structures. Its use in combination with other currently available computational tools should improve the accuracy of predicting the subcellular localization of bacterial proteins.
Myxococcus xanthus cells move on a solid surface by gliding motility. Several genes required for gliding motility have been identified, including those of the A- and S-motility systems as well as the mgl and frz genes. However, the cellular defects in gliding movement in many of these mutants were unknown. We conducted quantitative, high-resolution single-cell motility assays and found that mutants defective in mglAB or in cglB, an A-motility gene, reversed the direction of gliding at frequencies which were more than 1 order of magnitude higher than that of wild type cells (2.9 min−1 for ΔmglAB mutants and 2.7 min−1 for cglB mutants, compared to 0.17 min−1 for wild-type cells). The average gliding speed of ΔmglAB mutant cells was 40% of that of wild-type cells (on average 1.9 μm/min for ΔmglAB mutants, compared to 4.4 μm/min for wild-type cells). The mglA-dependent reversals and gliding speeds were dependent on the level of intracellular MglA protein: mglB mutant cells, which contain only 15 to 20% of the wild-type level of MglA protein, glided with an average reversal frequency of about 1.8 min−1 and an average speed of 2.6 μm/min. These values range between those exhibited by wild-type cells and by ΔmglAB mutant cells. Epistasis analysis of frz mutants, which are defective in aggregation and in single-cell reversals, showed that a frzD mutation, but not a frzE mutation, partially suppressed the mglA phenotype. In contrast to mgl mutants, cglB mutant cells were able to move with wild-type speeds only when in close proximity to each other. However, under those conditions, these mutant cells were found to glide less often with those speeds. By analyzing double mutants, the high reversing movements and gliding speeds of cglB cells were found to be strictly dependent on type IV pili, encoded by S-motility genes, whereas the high-reversal pattern of mglAB cells was only partially reduced by a pilR mutation. These results suggest that the MglA protein is required for both control of reversal frequency and gliding speed and that in the absence of A motility, type IV pilus-dependent cell movement includes reversals at high frequency. Furthermore, mglAB mutants behave as if they were severely defective in A motility but only partially defective in S motility.
Myxococcus xanthus Social (S) motility occurs at high cell densities and is powered by the extension and retraction of Type IV pili which bind ligands normally found in matrix exopolysaccharides (EPS). Previous studies showed that FrzS, a protein required for S-motility, is organized in polar clusters that show pole-to-pole translocation as cells reverse their direction of movement. Since the leading cell pole is the site of both the major FrzS cluster and type IV pilus extension/retraction, it was suggested that FrzS might regulate S-motility by activating pili at the leading cell pole. Here, we show that FrzS regulates EPS production, rather than type IV pilus function. We found that the frzS phenotype is distinct from that of Type IV pilus mutants such as pilA and pilT, but indistinguishable from EPS mutants, such as epsZ. Indeed, frzS mutants can be rescued by the addition of purified EPS, 1% methylcellulose, or co-culturing with wildtype cells. Our data also indicate that the cell density requirement in S-motility is likely a function of the ability of cells to construct functional multicellular clusters surrounding an EPS core.
Bdellovibrio bacteriovorus invade Gram-negative bacteria in a predatory process requiring Type IV pili (T4P) at a single invasive pole, and also glide on surfaces to locate prey. Ras-like G-protein MglA, working with MglB and RomR in the deltaproteobacterium Myxococcus xanthus, regulates adventurous gliding and T4P-mediated social motility at both M. xanthus cell poles. Our bioinformatic analyses suggested that the GTPase activating protein (GAP)-encoding gene mglB was lost in Bdellovibrio, but critical residues for MglABd GTP-binding are conserved. Deletion of mglABd abolished prey-invasion, but not gliding, and reduced T4P formation. MglABd interacted with a previously uncharacterised tetratricopeptide repeat (TPR) domain protein Bd2492, which we show localises at the single invasive pole and is required for predation. Bd2492 and RomR also interacted with cyclic-di-GMP-binding receptor CdgA, required for rapid prey-invasion. Bd2492, RomRBd and CdgA localize to the invasive pole and may facilitate MglA-docking. Bd2492 was encoded from an operon encoding a TamAB-like secretion system. The TamA protein and RomR were found, by gene deletion tests, to be essential for viability in both predatory and non-predatory modes. Control proteins, which regulate bipolar T4P-mediated social motility in swarming groups of deltaproteobacteria, have adapted in evolution to regulate the anti-social process of unipolar prey-invasion in the “lone-hunter” Bdellovibrio. Thus GTP-binding proteins and cyclic-di-GMP inputs combine at a regulatory hub, turning on prey-invasion and allowing invasion and killing of bacterial pathogens and consequent predatory growth of Bdellovibrio.
Bacterial cell polarity control is important for maintaining asymmetry of polar components such as flagella and pili. Bdellovibrio bacteriovorus is a predatory deltaproteobacterium which attaches to, and invades, other bacteria using Type IV pili (T4P) extruded from the specialised, invasive, non-flagellar pole of the cell. It was not known how that invasive pole is specified and regulated. Here we discover that a regulatory protein-hub, including Ras-GTPase-like protein MglA and cyclic-di-GMP receptor-protein CdgA, control prey-invasion. In the deltaproteobacterium, Myxococcus xanthus, MglA, with MglB and RomR, was found by others to regulate switching of T4P in social ‘swarming’ surface motility by swapping the pole at which T4P are found. In contrast, in B. bacteriovorus MglA regulates the process of prey-invasion and RomR, which is required for surface motility regulation in Myxococcus, is essential for growth and viability in Bdellovibrio. During evolution, B. bacteriovorus has lost mglB, possibly as T4P-pole-switching is not required; pili are only required at the invasive pole. A previously unidentified tetratricopeptide repeat (TPR) protein interacts with MglA and is essential for prey-invasion. This regulatory protein hub allows prey-invasion, likely integrating cyclic-di-GMP signals, pilus assembly and TamAB secretion in B. bacteriovorus.
Francisella tularensis is a highly virulent intracellular human pathogen that is capable of rapid proliferation in the infected host. Mutants affected in intracellular survival and growth are highly attenuated which highlights the importance of the intracellular phase of the infection. Genomic analysis has revealed that Francisella encodes all genes required for expression of functional type IV pili (Tfp), and in this focused review we summarize recent findings regarding this system in the pathogenesis of tularemia. Tfp are dynamic adhesive structures that have been identified as major virulence determinants in several human pathogens, but it is not obvious what role these structures could have in an intracellular pathogen like Francisella. In the human pathogenic strains, genes required for secretion and assembly of Tfp and one pilin, PilA, have shown to be required for full virulence. Importantly, specific genetic differences have been identified between the different Francisella subspecies where in the most pathogenic type A variants all genes are intact while several Tfp genes are pseudogenes in the less pathogenic type B strains. This suggests that there has been a selection for expression of Tfp with different properties in the different subspecies. There is also a possibility that the genetic differences reflect adaptation to different environmental niches of the subspecies and plays a role in transmission of tularemia. This is also in line with recent findings where Tfp pilins are found to be glycosylated which could reflect a role for Tfp in the environment to promote survival and transmission. We are still far from understanding the role of Tfp in virulence and transmission of tularemia, but with the genomic information and genetic tools available we are in a good position to address these issues in the future.
Francisella tularensis; type IV pili; virulence; type II secretion