The number of Neisseria meningitidis type IV pili determines host cell interaction

Adhesion pilus retraction powers twitching motility in the thermoacidophilic crenarchaeon Sulfolobus acidocaldarius

Type IV pili are filamentous appendages found in most bacteria and archaea, where they can support functions such as surface adhesion, DNA uptake, aggregation, and motility. In most bacteria, PilT-family ATPases disassemble adhesion pili, causing them to rapidly retract and produce twitching motility, important for surface colonization. As archaea do not possess PilT homologs, it was thought that archaeal pili cannot retract and that archaea do not exhibit twitching motility. Here, we use live-cell imaging, automated cell tracking, fluorescence imaging, and genetic manipulation to show that the hyperthermophilic archaeon Sulfolobus acidocaldarius exhibits twitching motility, driven by retractable adhesion (Aap) pili, under physiologically relevant conditions (75 °C, pH 2). Aap pili are thus capable of retraction in the absence of a PilT homolog, suggesting that the ancestral type IV pili in the last universal common ancestor (LUCA) were capable of retraction.

Type IV pili of Enterobacteriaceae species

Type IV pili (T4Ps) are surface filaments widely distributed among bacteria and archaea. T4Ps are involved in many cellular functions and contribute to virulence in some species of bacteria. Due to the diversity of T4Ps, different properties have been observed for homologous proteins that make up T4Ps in various organisms. In this review, we highlight the essential components of T4Ps, their functions, and similarities to related systems. We emphasize the unique T4Ps of enteric pathogens within the Enterobacteriaceae family, which includes pathogenic strains of Escherichia coli and Salmonella. These include the bundle-forming pilus (BFP) of enteropathogenic E. coli (EPEC), longus (Lng) and colonization factor III (CFA/III) of enterotoxigenic E. coli (ETEC), T4P of Salmonella enterica serovar Typhi, Colonization Factor Citrobacter (CFC) of Citrobacter rodentium, T4P of Yersinia pseudotuberculosis, a ubiquitous T4P that was characterized in enterohemorrhagic E. coli (EHEC), and the R64 plasmid thin pilus. Finally, we highlight areas for further study.

Oscillation of type IV pili regulated by the circadian clock in cyanobacterium Synechococcus elongatus PCC7942

Type IV pili (TFP) are known to be functionally related to cell motilities and natural transformation in many bacteria. However, the molecular and ecological functions of the TFP have rarely been reported for photosynthetic cyanobacteria. Here, by labeling pili in model cyanobacterium Synechococcus elongatus PCC 7942 (Syn7942), we have quantitatively characterized the TFP and its driven twitching motility in situ at the single-cell level. We found an oscillating pattern of TFP in accordance with the light and dark periods during light-dark cycles, which is correlated positively to the oscillating pattern of the natural transformation efficiency. We further showed that the internal circadian clock plays an important role in regulating the oscillating pattern of TFP, which is also supported by evidences at the molecular level by tracking the expression of 16 TFP-related genes. This study adds a detailed picture toward the gap between TFP and its relations to circadian regulations in Syn7942.

Genetic variants linked to the phenotypic outcome of invasive disease and carriage of Neisseria meningitidis

Neisseria meningitidis can be a human commensal in the upper respiratory tract but is also capable of causing invasive diseases such as meningococcal meningitis and septicaemia. No specific genetic markers have been detected to distinguish carriage from disease isolates. The aim here was to find genetic traits that could be linked to phenotypic outcomes associated with carriage versus invasive N. meningitidis disease through a bacterial genome-wide association study (GWAS). In this study, invasive N. meningitidis isolates collected in Sweden (n=103) and carriage isolates collected at Örebro University, Sweden (n=213) 2018-2019 were analysed. The GWAS analysis, treeWAS, was applied to single-nucleotide polymorphisms (SNPs), genes and k-mers. One gene and one non-synonymous SNP were associated with invasive disease and seven genes and one non-synonymous SNP were associated with carriage isolates. The gene associated with invasive disease encodes a phage transposase (NEIS1048), and the associated invasive SNP glmU S373C encodes the enzyme N-acetylglucosamine 1-phosphate (GlcNAC 1-P) uridyltransferase. Of the genes associated with carriage isolates, a gene variant of porB encoding PorB class 3, the genes pilE/pilS and tspB have known functions. The SNP associated with carriage was fkbp D33N, encoding a FK506-binding protein (FKBP). K-mers from PilS, tbpB and tspB were found to be associated with carriage, while k-mers from mtrD and tbpA were associated with invasiveness. In the genes fkbp, glmU, PilC and pilE, k-mers were found that were associated with both carriage and invasive isolates, indicating that specific variations within these genes could play a role in invasiveness. The data presented here highlight genetic traits that are significantly associated with invasive or carriage N. meningitidis across the species population. These traits could prove essential to our understanding of the pathogenicity of N. meningitidis and could help to identify future vaccine targets.

Feature architecture aware phylogenetic profiling indicates a functional diversification of type IVa pili in the nosocomial pathogen Acinetobacter baumannii

The Gram-negative bacterial pathogen Acinetobacter baumannii is a major cause of hospital-acquired opportunistic infections. The increasing spread of pan-drug resistant strains makes A. baumannii top-ranking among the ESKAPE pathogens for which novel routes of treatment are urgently needed. Comparative genomics approaches have successfully identified genetic changes coinciding with the emergence of pathogenicity in Acinetobacter. Genes that are prevalent both in pathogenic and a-pathogenic Acinetobacter species were not considered ignoring that virulence factors may emerge by the modification of evolutionarily old and widespread proteins. Here, we increased the resolution of comparative genomics analyses to also include lineage-specific changes in protein feature architectures. Using type IVa pili (T4aP) as an example, we show that three pilus components, among them the pilus tip adhesin ComC, vary in their Pfam domain annotation within the genus Acinetobacter. In most pathogenic Acinetobacter isolates, ComC displays a von Willebrand Factor type A domain harboring a finger-like protrusion, and we provide experimental evidence that this finger conveys virulence-related functions in A. baumannii. All three genes are part of an evolutionary cassette, which has been replaced at least twice during A. baumannii diversification. The resulting strain-specific differences in T4aP layout suggests differences in the way how individual strains interact with their host. Our study underpins the hypothesis that A. baumannii uses T4aP for host infection as it was shown previously for other pathogens. It also indicates that many more functional complexes may exist whose precise functions have been adjusted by modifying individual components on the domain level.

The PilT retraction ATPase promotes both extension and retraction of the MSHA type IVa pilus in Vibrio cholerae

Diverse bacterial species use type IVa pili (T4aP) to interact with their environments. The dynamic extension and retraction of T4aP is critical for their function, but the mechanisms that regulate this dynamic activity remain poorly understood. T4aP are typically extended via the activity of a dedicated extension motor ATPase and retracted via the action of an antagonistic retraction motor ATPase called PilT. These motors are generally functionally independent, and loss of PilT commonly results in T4aP hyperpiliation due to undeterred pilus extension. However, for the mannose-sensitive hemagglutinin (MSHA) T4aP of Vibrio cholerae, the loss of PilT unexpectedly results in a loss of surface piliation. Here, we employ a combination of genetic and cell biological approaches to dissect the underlying mechanism. Our results demonstrate that PilT is necessary for MSHA pilus extension in addition to its well-established role in promoting MSHA pilus retraction. Through a suppressor screen, we also provide genetic evidence that the MshA major pilin impacts pilus extension. Together, these findings contribute to our understanding of the factors that regulate pilus extension and describe a previously uncharacterized function for the PilT motor ATPase.

Direct visualization of sequence-specific DNA binding by gonococcal type IV pili

Neisseria gonorrhoeae, the causative agent of gonorrhoea, is a major burden on global healthcare systems, with an estimated ~80-90 million new global cases annually. This burden is exacerbated by increasing levels of antimicrobial resistance, which has greatly limited viable antimicrobial therapies. Decreasing gonococcal drug susceptibility has been driven largely by accumulation of chromosomal resistance determinants, which can be acquired through natural transformation, whereby DNA in the extracellular milieu is imported into cells and incorporated into the genome by homologous recombination. N. gonorrhoeae possesses a specialized system for DNA uptake, which strongly biases transformation in favour of DNA from closely related bacteria by recognizing a 10-12 bp DNA uptake sequence (DUS) motif, which is highly overrepresented in their chromosomal DNA. This process relies on numerous proteins, including the DUS-specific receptor ComP, which assemble retractile protein filaments termed type IV pili (T4P) extending from the cell surface, and one model for neisserial DNA uptake proposes that these filaments bind DNA in a DUS-dependent manner before retracting to transport DNA into the periplasm. However, conflicting evidence indicates that elongated pilus filaments may not have such a direct role in DNA binding uptake as this model suggests. Here, we quantitatively measured DNA binding to gonococcal T4P fibres by directly visualizing binding complexes with confocal fluorescence microscopy in order to confirm the sequence-specific, comP-dependent DNA binding capacity of elongated T4P fibres. This supports the idea that pilus filaments could be responsible for initially capturing DNA in the first step of sequence-specific DNA uptake.

The differential expression of PilY1 proteins by the HsfBA phosphorelay allows twitching motility in the absence of exopolysaccharides

Type Four Pili (T4P) are extracellular appendages mediating several bacterial functions such as motility, biofilm formation and infection. The ability to adhere to substrates is essential for all these functions. In Myxococcus xanthus, during twitching motility, the binding of polar T4P to exopolysaccharides (EPS), induces pilus retraction and the forward cell movement. EPS are produced, secreted and weakly associated to the M. xanthus cell surface or deposited on the substrate. In this study, a genetic screen allowed us to identify two factors involved in EPS-independent T4P-dependent twitching motility: the PilY1.1 protein and the HsfBA phosphorelay. Transcriptomic analyses show that HsfBA differentially regulates the expression of PilY1 proteins and that the down-regulation of pilY1.1 together with the accumulation of its homologue pilY1.3, allows twitching motility in the absence of EPS. The genetic and bioinformatic dissection of the PilY1.1 domains shows that PilY1.1 might be a bi-functional protein with a role in priming T4P extension mediated by its conserved N-terminal domain and roles in EPS-dependent motility mediated by an N-terminal DUF4114 domain activated upon binding to Ca²⁺. We speculate that the differential transcriptional regulation of PilY1 homologs by HsfBA in response to unknown signals, might allow accessorizing T4P tips with different modules allowing twitching motility in the presence of alternative substrates and environmental conditions.

In the motile bacterium Myxococcus xanthus, T4P mediate twitching motility by binding to the sugar moiety of the extracellular matrix deposited on the neighboring cells or on the substrate. The binding of T4P to these sugars also termed exopolysaccharides (EPS) stimulates the pilus fiber retraction pulling the cell forwards. In this study, we performed a series of genetic analyses leading to the discovery that M. xanthus cells can move in the absence of EPS if two conditions are fulfilled: the pilY1.1 gene is down-regulated and the PilY1.3 protein is accumulated on pili. RNAseq, qRT-PCR and gel retardation assays show that the differential accumulation of PilY1 proteins is under the control of the HsfBA phosphorelay, which up-regulates the expression of pilY1.1 and down-regulates that of a homologue, pilY1.3. We also found that PilY1.1 has a domain at the N terminus probably requiring Ca²⁺ to be active in EPS-dependent motility, and a conserved domain at the C terminus essential for T4P assembly. Conversely, PilY1.3 contains a Von Willebrand factor A (VWA) domain and is potentially involved in the binding to proteins. We speculate that thanks to the HsfBA regulation, T4P can be equipped with different PilY1 homologues to allow twitching motility in the presence of different substrates.

The Host-Pathogen Interactions and Epicellular Lifestyle of Neisseria meningitidis

Neisseria meningitidis is a gram-negative diplococcus and a transient commensal of the human nasopharynx. It shares and competes for this niche with a number of other Neisseria species including N. lactamica, N. cinerea and N. mucosa. Unlike these other members of the genus, N. meningitidis may become invasive, crossing the epithelium of the nasopharynx and entering the bloodstream, where it rapidly proliferates causing a syndrome known as Invasive Meningococcal Disease (IMD). IMD progresses rapidly to cause septic shock and meningitis and is often fatal despite aggressive antibiotic therapy. While many of the ways in which meningococci survive in the host environment have been well studied, recent insights into the interactions between N. meningitidis and the epithelial, serum, and endothelial environments have expanded our understanding of how IMD develops. This review seeks to incorporate recent work into the established model of pathogenesis. In particular, we focus on the competition that N. meningitidis faces in the nasopharynx from other Neisseria species, and how the genetic diversity of the meningococcus contributes to the wide range of inflammatory and pathogenic potentials observed among different lineages.

The Role of Minor Pilins in Assembly and Function of the Competence Pilus of Streptococcus pneumoniae

The remarkable genomic plasticity of Streptococcus pneumoniae largely depends on its ability to undergo natural genetic transformation. To take up extracellular DNA, S. pneumoniae assembles competence pili composed of the major pilin ComGC. In addition to ComGC, four minor pilins ComGD, E, F, and G are expressed during bacterial competence, but their role in pilus biogenesis and transformation is unknown. Here, using a combination of protein-protein interaction assays we show that all four proteins can directly interact with each other. Pneumococcal ComGG stabilizes the minor pilin ComGD and ComGF and can interact with and stabilize the major pilin ComGC, thus, deletion of ComGG abolishes competence pilus assembly. We further demonstrate that minor pilins are present in sheared pili fractions and find ComGF to be incorporated along the competence pilus by immunofluorescence and electron microscopy. Finally, mutants of the invariant Glu5 residue (E5), ComGD_E5A or ComGE_E5A, but not ComGF_E5A, were severely impaired in pilus formation and function. Together, our results suggest that ComGG, lacking E5, is essential for competence pilus assembly and function, and plays a central role in connecting the pneumococcal minor pilins to ComGC.

Type IV Pili: Dynamic Bacterial Nanomachines

Bacteria and archaea rely on appendages called type IV pili (T4P) to participate in diverse behaviors including surface sensing, biofilm formation, virulence, protein secretion, and motility across surfaces. T4P are broadly distributed fibers that dynamically extend and retract, and this dynamic activity is essential for their function in broad processes. Despite the essentiality of dynamics in T4P function, little is known about the role of these dynamics and molecular mechanisms controlling them. Recent advances in microscopy have yielded insight into the role of T4P dynamics in their diverse functions and recent structural work has expanded what is known about the inner workings of the T4P motor. This review discusses recent progress in understanding the function, regulation, and mechanisms of T4P dynamics.

The minor pilin PilV provides a conserved adhesion site throughout the antigenically variable meningococcal type IV pilus

Neisseria meningitidis utilizes type IV pili (T4P) to adhere to and colonize host endothelial cells, a process at the heart of meningococcal invasive diseases leading to meningitis and sepsis. T4P are polymers of an antigenically variable major pilin building block, PilE, plus several core minor pilins that initiate pilus assembly and are thought to be located at the pilus tip. Adhesion of N. meningitidis to human endothelial cells requires both PilE and a conserved noncore minor pilin PilV, but the localization of PilV and its precise role in this process remains to be clarified. Here, we show that both PilE and PilV promote adhesion to endothelial vessels in vivo. The substantial adhesion defect observed for pilV mutants suggests it is the main adhesin. Consistent with this observation, superresolution microscopy showed the abundant distribution of PilV throughout the pilus. We determined the crystal structure of PilV and modeled it within the pilus filament. The small size of PilV causes it to be recessed relative to adjacent PilE subunits, which are dominated by a prominent hypervariable loop. Nonetheless, we identified a conserved surface-exposed adhesive loop on PilV by alanine scanning mutagenesis. Critically, antibodies directed against PilV inhibit N. meningitidis colonization of human skin grafts. These findings explain how N. meningitidis T4P undergo antigenic variation to evade the humoral immune response while maintaining their adhesive function and establish the potential of this highly conserved minor pilin as a vaccine and therapeutic target for the prevention and treatment of N. meningitidis infections.

Fresh Extension of Vibrio cholerae Competence Type IV Pili Predisposes Them for Motor-Independent Retraction

Bacteria utilize dynamic appendages, called type IV pili (T4P), to interact with their environment and mediate a wide variety of functions. Pilus extension is mediated by an extension ATPase motor, commonly called PilB, in all T4P. Pilus retraction, however, can occur with the aid of an ATPase motor or in the absence of a retraction motor. While much effort has been devoted to studying motor-dependent retraction, the mechanism and regulation of motor-independent retraction remain poorly characterized. We have previously demonstrated that Vibrio cholerae competence T4P undergo motor-independent retraction in the absence of the dedicated retraction ATPases PilT and PilU. Here, we utilize this model system to characterize the factors that influence motor-independent retraction. We find that freshly extended pili frequently undergo motor-independent retraction, but if these pili fail to retract immediately, they remain statically extended on the cell surface. Importantly, we show that these static pili can still undergo motor-dependent retraction via tightly regulated ectopic expression of PilT, suggesting that these T4P are not broken but simply cannot undergo motor-independent retraction. Through additional genetic and biophysical characterization of pili, we suggest that pilus filaments undergo conformational changes during dynamic extension and retraction. We propose that only some conformations, like those adopted by freshly extended pili, are capable of undergoing motor-independent retraction. Together, these data highlight the versatile mechanisms that regulate T4P dynamic activity and provide additional support for the long-standing hypothesis that motor-independent retraction occurs via spontaneous depolymerization. IMPORTANCE Extracellular pilus fibers are critical to the virulence and persistence of many pathogenic bacteria. A crucial function for most pili is the dynamic ability to extend and retract from the cell surface. Inhibiting this dynamic pilus activity represents an attractive approach for therapeutic interventions; however, a detailed mechanistic understanding of this process is currently lacking. Here, we use the competence pilus of Vibrio cholerae to study how pili retract in the absence of dedicated retraction motors. Our results reveal a novel regulatory mechanism of pilus retraction that is an inherent property of the pilus filament. Thus, understanding the conformational changes that pili adopt under different conditions may be critical for the development of novel therapeutics that aim to target the dynamic activity of these structures.

Bacterial Vivisection: How Fluorescence-Based Imaging Techniques Shed a Light on the Inner Workings of Bacteria

The rise in fluorescence-based imaging techniques over the past 3 decades has improved the ability of researchers to scrutinize live cell biology at increased spatial and temporal resolution. In microbiology, these real-time vivisections structurally changed the view on the bacterial cell away from the "watery bag of enzymes" paradigm toward the perspective that these organisms are as complex as their eukaryotic counterparts. Capitalizing on the enormous potential of (time-lapse) fluorescence microscopy and the ever-extending pallet of corresponding probes, initial breakthroughs were made in unraveling the localization of proteins and monitoring real-time gene expression. However, later it became clear that the potential of this technique extends much further, paving the way for a focus-shift from observing single events within bacterial cells or populations to obtaining a more global picture at the intra- and intercellular level. In this review, we outline the current state of the art in fluorescence-based vivisection of bacteria and provide an overview of important case studies to exemplify how to use or combine different strategies to gain detailed information on the cell's physiology. The manuscript therefore consists of two separate (but interconnected) parts that can be read and consulted individually. The first part focuses on the fluorescent probe pallet and provides a perspective on modern methodologies for microscopy using these tools. The second section of the review takes the reader on a tour through the bacterial cell from cytoplasm to outer shell, describing strategies and methods to highlight architectural features and overall dynamics within cells.

PilY1 and minor pilins form a complex priming the type IVa pilus in Myxococcus xanthus

Type IVa pili are ubiquitous and versatile bacterial cell surface filaments that undergo cycles of extension, adhesion and retraction powered by the cell-envelope spanning type IVa pilus machine (T4aPM). The overall architecture of the T4aPM and the location of 10 conserved core proteins within this architecture have been elucidated. Here, using genetics, cell biology, proteomics and cryo-electron tomography, we demonstrate that the PilY1 protein and four minor pilins, which are widely conserved in T4aP systems, are essential for pilus extension in Myxococcus xanthus and form a complex that is an integral part of the T4aPM. Moreover, these proteins are part of the extended pilus. Our data support a model whereby the PilY1/minor pilin complex functions as a priming complex in T4aPM for pilus extension, a tip complex in the extended pilus for adhesion, and a cork for terminating retraction to maintain a priming complex for the next round of extension.

Genetic incorporation of non-canonical amino acid photocrosslinkers in Neisseria meningitidis: New method provides insights into the physiological function of the function-unknown NMB1345 protein

Although whole-genome sequencing has provided novel insights into Neisseria meningitidis, many open reading frames have only been annotated as hypothetical proteins with unknown biological functions. Our previous genetic analyses revealed that the hypothetical protein, NMB1345, plays a crucial role in meningococcal infection in human brain microvascular endothelial cells; however, NMB1345 has no homology to any identified protein in databases and its physiological function could not be elucidated using pre-existing methods. Among the many biological technologies to examine transient protein-protein interaction in vivo, one of the developed methods is genetic code expansion with non-canonical amino acids (ncAAs) utilizing a pyrrolysyl-tRNA synthetase/tRNAPyl pair from Methanosarcina species: However, this method has never been applied to assign function-unknown proteins in pathogenic bacteria. In the present study, we developed a new method to genetically incorporate ncAAs-encoded photocrosslinking probes into N. meningitidis by utilizing a pyrrolysyl-tRNA synthetase/tRNAPyl pair and elucidated the biological function(s) of the NMB1345 protein. The results revealed that the NMB1345 protein directly interacts with PilE, a major component of meningococcal pili, and further physicochemical and genetic analyses showed that the interaction between the NMB1345 protein and PilE was important for both functional pilus formation and meningococcal infectious ability in N. meningitidis. The present study using this new methodology for N. meningitidis provides novel insights into meningococcal pathogenesis by assigning the function of a hypothetical protein.

Deep mutational scanning of the Neisseria meningitidis major pilin reveals the importance of pilus tip-mediated adhesion

Type IV pili (TFP) are multifunctional micrometer-long filaments expressed at the surface of many prokaryotes. In Neisseria meningitidis, TFP are crucial for virulence. Indeed, these homopolymers of the major pilin PilE mediate interbacterial aggregation and adhesion to host cells. However, the mechanisms behind these functions remain unclear. Here, we simultaneously determined regions of PilE involved in pilus display, auto-aggregation, and adhesion by using deep mutational scanning and started mining this extensive functional map. For auto-aggregation, pili must reach a minimum length to allow pilus-pilus interactions through an electropositive cluster of residues centered around Lys140. For adhesion, results point to a key role for the tip of the pilus. Accordingly, purified pili interacting with host cells initially bind via their tip-located major pilin and then along their length. Overall, these results identify functional domains of PilE and support a direct role of the major pilin in TFP-dependent aggregation and adhesion.

Contribution of σ70 and σN Factors to Expression of Class II pilE in Neisseria meningitidis

Neisseria meningitidis expresses multicomponent organelles called type four pili (Tfp), which are key virulence factors required for attachment to human cells during carriage and disease. Pilin (PilE) is the main component of Tfp, and N. meningitidis isolates either have a class I pilE locus and express pilins that undergo antigenic variation or have a class II pilE locus and express invariant pilins. The transcriptional regulation of class I pilE has been studied in both N. meningitidis and Neisseria gonorrhoeae, while the control of expression of class II pilE has been elucidated in the nonpathogenic species Neisseria elongata However, the factors that govern the regulation of the class II pilE gene in N. meningitidis are not known. In this work, we have bioinformatically and experimentally identified the class II pilE promoter. We confirmed the presence of conserved σ⁷⁰ and σ^N-dependent promoters upstream of pilE in a collection of meningococcal genomes and demonstrated that class II pilE expression initiates from the σ⁷⁰ family-dependent promoter. By deletion or overexpression of sigma factors, we showed that σ^N, σ^H, and σ^E do not affect class II pilin expression. These findings are consistent with a role of the housekeeping σ^D in expression of this important component of Tfp. Taken together, our data indicate that the σ-dependent network responsible for the expression of class II pilE has been selected to maintain pilE expression, consistent with the essential roles of Tfp in colonization and pathogenesis.IMPORTANCE The type four pilus (Tfp) of Neisseria meningitidis contributes to fundamental processes such as adhesion, transformation, and disease pathology. Meningococci express one of two distinct classes of Tfp (class I or class II), which can be distinguished antigenically or by the major subunit (pilE) locus and its genetic context. The factors that govern transcription of the class II pilE gene are not known, even though it is present in isolates that cause epidemic disease. Here we show that the transcription of class II pilE is maintained throughout growth and under different stress conditions and is driven by a σ⁷⁰-dependent promoter. This is distinct from Tfp regulation in nonpathogenic Neisseria spp. and may confer an advantage during host-cell interaction and infection.

Structure and Assembly of the Enterohemorrhagic Escherichia coli Type 4 Pilus

Bacterial type 4a pili are dynamic surface filaments that promote bacterial adherence, motility, and macromolecular transport. Their genes are highly conserved among enterobacteria and their expression in enterohemorrhagic Escherichia coli (EHEC) promotes adhesion to intestinal epithelia and pro-inflammatory signaling. To define the molecular basis of EHEC pilus assembly, we determined the structure of the periplasmic domain of its major subunit PpdD (PpdDp), a prototype of an enterobacterial pilin subfamily containing two disulfide bonds. The structure of PpdDp, determined by NMR, was then docked into the density envelope of purified EHEC pili obtained by cryoelectron microscopy (cryo-EM). Cryo-EM reconstruction of EHEC pili at ∼8 Å resolution revealed extremely high pilus flexibility correlating with a large extended region of the pilin stem. Systematic mutagenesis combined with functional and interaction analyses identified charged residues essential for pilus assembly. Structural information on exposed regions and interfaces between EHEC pilins is relevant for vaccine and drug discovery.

How bacterial cells and colonies move on solid substrates

Many bacteria rely on active cell appendages, such as type IV pili, to move over substrates and interact with neighboring cells. Here, we study the motion of individual cells and bacterial colonies, mediated by the collective interactions of multiple pili. It was shown experimentally that the substrate motility of Neisseria gonorrhoeae cells can be described as a persistent random walk with a persistence length that exceeds the mean pili length. Moreover, the persistence length increases for a higher number of pili per cell. With the help of a simple, tractable stochastic model, we test whether a tug of war without directional memory can explain the persistent motion of single Neisseria gonorrhoeae cells. While persistent motion of single cells indeed emerges naturally in the model, a tug of war alone is not capable of explaining the motility of microcolonies, which becomes weaker with increasing colony size. We suggest sliding friction between the microcolonies and the substrate as the missing ingredient. While such friction almost does not affect the general mechanism of single cell motility, it has a strong effect on colony motility. We validate the theoretical predictions by using a three-dimensional computational model that includes explicit details of the pili dynamics, force generation, and geometry of cells.

Structure and Properties of a Natural Competence-Associated Pilin Suggest a Unique Pilus Tip-Associated DNA Receptor

Natural competence is the term used to describe the uptake of "naked" extracellular DNA by bacteria; it plays a significant role in horizontal genetic exchange. It is associated with type IV pili, and specialized competence pili mediate DNA uptake. Here, we show that the crystal structure of a competence-associated protein from Thermus thermophilus, ComZ, consists of a type II secretion pseudopilin-like domain, with a large β-solenoid domain inserted into the β-sheet of the pilin-like fold. ComZ binds with high affinity to another competence-associated pilin, PilA2, which lies adjacent to the comZ gene in the genome. The crystal structure of PilA2 revealed a similar type II secretion pseudopilin-like fold, with a small subdomain; docking simulations predicted that PilA2 binds between the pseudopilin-like and β-solenoid domains of ComZ. Electrophoretic shift analysis and DNase protection studies were used to show that ComZ alone and the ComZ/PilA2 complex are able to bind DNA. Protection against reductive dimethylation was used in combination with mass spectrometry and site-directed mutagenesis to identify two lysine residues in ComZ which are involved in DNA binding. They are located between the two domains in ComZ, on the opposite side from the predicted PilA2 binding site. These results suggest a model in which PilA2 assists ComZ in forming the competence pilus tip and DNA binds to the side of the fiber. The results demonstrate how a type IV pilin can be adapted to a specific function by domain insertion and provide the first structural insights into a tip-located competence pilin.IMPORTANCE Thermus thermophilus is a thermophilic bacterium which is capable of natural transformation, the uptake of external DNA with high efficiency. DNA uptake is thought to be mediated by a competence-associated pilus, which binds the DNA substrate and mediates its transfer across the outer membrane and periplasm. Here, we describe the structural and functional analysis of two pilins which are known to be essential for DNA uptake, ComZ and PilA2. ComZ adopts an unusual structure, incorporating a large β-solenoid domain into the pilin structural framework. We argue on structural grounds that this structure cannot readily be accommodated into the competence pilus fiber unless it is at the tip. We also show that ComZ binds DNA and identify two lysine residues which appear to be important for DNA binding. These results suggest a model in which ComZ and PilA2 form a tip-associated DNA receptor which mediates DNA uptake.

Real-time microscopy and physical perturbation of bacterial pili using maleimide-conjugated molecules

Bacteria use surface-exposed, proteinaceous fibers called pili for diverse behaviors, including horizontal gene transfer, surface sensing, motility, and pathogenicity. Visualization of these filamentous nanomachines and their activity in live cells has proven challenging, largely due to their small size. Here, we describe a broadly applicable method for labeling and imaging pili and other surface-exposed nanomachines in live cells. This technique uses a combination of genetics and maleimide-based click chemistry in which a cysteine substitution is made in the major pilin subunit for subsequent labeling with thiol-reactive maleimide dyes. Large maleimide-conjugated molecules can also be used to physically interfere with the dynamic activity of filamentous nanomachines. We describe parameters for selecting cysteine substitution positions, optimized labeling conditions for epifluorescence imaging of pilus fibers, and methods for impeding pilus activity. After cysteine knock-in strains have been generated, this protocol can be completed within 30 min to a few hours, depending on the species and the experiment of choice. Visualization of extracellular nanomachines such as pili using this approach can provide a more comprehensive understanding of the role played by these structures in distinct bacterial behaviors.

The Many Faces of Bacterium-Endothelium Interactions during Systemic Infections

A wide variety of pathogens reach the circulatory system during viral, parasitic, fungal, and bacterial infections, causing clinically diverse pathologies. Such systemic infections are usually severe and frequently life-threatening despite intensive care, in particular during the age of antibiotic resistance. Because of its position at the interface between the blood and the rest of the organism, the endothelium plays a central role during these infections. Using several examples of systemic infections, we explore the diversity of interactions between pathogens and the endothelium. These examples reveal that bacterial pathogens target specific vascular beds and affect most aspects of endothelial cell biology, ranging from cellular junction stability to endothelial cell proliferation and inflammation.

Inhibitors of the Neisseria meningitidis PilF ATPase provoke type IV pilus disassembly

Despite the availability of antibiotics and vaccines, Neisseria meningitidis remains a major cause of meningitis and sepsis in humans. Due to its extracellular lifestyle, bacterial adhesion to host cells constitutes an attractive therapeutic target. Here, we present a high-throughput microscopy-based approach that allowed the identification of compounds able to decrease type IV pilus-mediated interaction of bacteria with endothelial cells in the absence of bacterial or host cell toxicity. Compounds specifically inhibit the PilF ATPase enzymatic activity that powers type IV pilus extension but remain inefficient on the ATPase that promotes pilus retraction, thus leading to rapid pilus disappearance from the bacterial surface and loss of pili-mediated functions. Structure activity relationship of the most active compound identifies specific moieties required for the activity of this compound and highlights its specificity. This study therefore provides compounds targeting pilus biogenesis, thereby inhibiting bacterial adhesion, and paves the way for a novel therapeutic option for meningococcal infections.

Autophagy as a common pathway in amyotrophic lateral sclerosis

Age-dependent neurodegenerative diseases are associated with a decline in protein quality control systems including autophagy. Amyotrophic lateral sclerosis (ALS) is a motor neuron degenerative disease of complex etiology with increasing connections to other neurodegenerative conditions such as frontotemporal dementia. Among the diverse genetic causes for ALS, a striking feature is the common connection to autophagy and its associated pathways. There is a recurring theme of protein misfolding as in other neurodegenerative diseases, but importantly there is a distinct common thread among ALS genes that connects them to the cascade of autophagy. However, the roles of autophagy in ALS remain enigmatic and it is still unclear whether activation or inhibition of autophagy would be a reliable avenue to ameliorate the disease. The main evidence that links autophagy to different genetic forms of ALS is discussed.

DNA Uptake by Type IV Filaments

Bacterial uptake of DNA through type IV filaments is an essential component of natural competence in numerous gram-positive and gram-negative species. Recent advances in the field have broadened our understanding of the structures used to take up extracellular DNA. Here, we review seminal experiments in the literature describing DNA binding by type IV pili, competence pili and the flp pili of Micrococcus luteus; collectively referred to here as type IV filaments. We compare the current state of the field on mechanisms of DNA uptake for these three appendage systems and describe the current mechanistic understanding of both DNA-binding and DNA-uptake by these versatile molecular machines.

Genus-Wide Comparative Genomics Analysis of Neisseria to Identify New Genes Associated with Pathogenicity and Niche Adaptation of Neisseria Pathogens

N. gonorrhoeae and N. meningitidis, the only two human pathogens of Neisseria, are closely related species. But the niches they survived in and their pathogenic characteristics are distinctly different. However, the genetic basis of these differences has not yet been fully elucidated. In this study, comparative genomics analysis was performed based on 15 N. gonorrhoeae, 75 N. meningitidis, and 7 nonpathogenic Neisseria genomes. Core-pangenome analysis found 1111 conserved gene families among them, and each of these species groups had opening pangenome. We found that 452, 78, and 319 gene families were unique in N. gonorrhoeae, N. meningitidis, and both of them, respectively. Those unique gene families were regarded as candidates that related to their pathogenicity and niche adaptation. The relationships among them have been partly verified by functional annotation analysis. But at least one-third genes for each gene set have not found the certain functional information. Simple sequence repeat (SSR), the basis of gene phase variation, was found abundant in the membrane or related genes of each unique gene set, which may facilitate their adaptation to variable host environments. Protein-protein interaction (PPI) analysis found at least five distinct PPI clusters in N. gonorrhoeae and four in N. meningitides, and 167 and 52 proteins with unknown function were contained within them, respectively.

Adhesion to nanofibers drives cell membrane remodeling through one-dimensional wetting

The shape of cellular membranes is highly regulated by a set of conserved mechanisms that can be manipulated by bacterial pathogens to infect cells. Remodeling of the plasma membrane of endothelial cells by the bacterium Neisseria meningitidis is thought to be essential during the blood phase of meningococcal infection, but the underlying mechanisms are unclear. Here we show that plasma membrane remodeling occurs independently of F-actin, along meningococcal type IV pili fibers, by a physical mechanism that we term 'one-dimensional' membrane wetting. We provide a theoretical model that describes the physical basis of one-dimensional wetting and show that this mechanism occurs in model membranes interacting with nanofibers, and in human cells interacting with extracellular matrix meshworks. We propose one-dimensional wetting as a new general principle driving the interaction of cells with their environment at the nanoscale that is diverted by meningococci during infection.

The CRISPR/Cas system in Neisseria meningitidis affects bacterial adhesion to human nasopharyngeal epithelial cells

Neisseria meningitidis, a commensal β-proteobacterium of the human nasopharynx, constitutes a worldwide leading cause of sepsis and epidemic meningitis. A recent genome-wide association study suggested an association of its type II-C CRISPR/Cas system with carriage and thus less invasive lineages. Here, we show that knock-out strains lacking the Cas9 protein are impaired in the adhesion to human nasopharyngeal cells which constitutes a central step in the pathogenesis of invasive meningococcal disease. Transcriptome sequencing data further suggest that meningococcal Cas9 does not affect the expression of surface adhesins but rather exerts its effect on cell adhesion in an indirect manner. Consequently, we speculate that the meningococcal CRISPR/Cas system exerts novel functions beyond its established role in defence against foreign DNA.

Intermittent Pili-Mediated Forces Fluidize Neisseria meningitidis Aggregates Promoting Vascular Colonization

Neisseria meningitidis, a bacterium responsible for meningitis and septicemia, proliferates and eventually fills the lumen of blood capillaries with multicellular aggregates. The impact of this aggregation process and its specific properties are unknown. We first show that aggregative properties are necessary for efficient infection and study their underlying physical mechanisms. Micropipette aspiration and single-cell tracking unravel unique features of an atypical fluidized phase, with single-cell diffusion exceeding that of isolated cells. A quantitative description of the bacterial pair interactions combined with active matter physics-based modeling show that this behavior relies on type IV pili active dynamics that mediate alternating phases of bacteria fast mutual approach, contact, and release. These peculiar fluid properties proved necessary to adjust to the geometry of capillaries upon bacterial proliferation. Intermittent attractive forces thus generate a fluidized phase that allows for efficient colonization of the blood capillary network during infection.

Molecular mechanisms involved in the interaction of Neisseria meningitidis with cells of the human blood-cerebrospinal fluid barrier

Neisseria meningitidis is one of the most common aetiological agents of bacterial meningitis, affecting predominantly children and young adults. The interaction of N. meningitidis with human endothelial cells lining blood vessels of the blood-cerebrospinal fluid barrier (B-CSFB) is critical for meningitis development. In recent decades, there has been a significant increase in understanding of the molecular mechanisms involved in the interaction of N. meningitidis with brain vascular cells. In this review, we will describe how N. meningitidis adheres to the brain vasculature, may enter inside these cells, hijack receptor signalling pathways and alter host-cell responses in order to traverse the B-CSFB.

Host cell-derived lactate functions as an effector molecule in Neisseria meningitidis microcolony dispersal

The development of meningococcal disease, caused by the human pathogen Neisseria meningitidis, is preceded by the colonization of the epithelial layer in the nasopharynx. After initial adhesion to host cells meningococci form aggregates, through pilus-pilus interactions, termed microcolonies from which the bacteria later detach. Dispersal from microcolonies enables access to new colonization sites and facilitates the crossing of the cell barrier; however, this process is poorly understood. In this study, we used live-cell imaging to investigate the process of N. meningitidis microcolony dispersal. We show that direct contact with host cells is not required for microcolony dispersal, instead accumulation of a host-derived effector molecule induces microcolony dispersal. By using a host-cell free approach, we demonstrated that lactate, secreted from host cells, initiate rapid dispersal of microcolonies. Interestingly, metabolic utilization of lactate by the bacteria was not required for induction of dispersal, suggesting that lactate plays a role as a signaling molecule. Furthermore, Neisseria gonorrhoeae microcolony dispersal could also be induced by lactate. These findings reveal a role of host-secreted lactate in microcolony dispersal and virulence of pathogenic Neisseria.

The human restricted pathogen Neisseria meningitidis is a major cause of bacterial meningitis and sepsis worldwide. Colonization of the mucosal layer in the upper respiratory tract is essential to establish invasive disease. The initial interaction with host cells is characterized by bacterial proliferation and adhesion as aggregates, called microcolonies. Detachment from microcolonies in the nasopharyngeal epithelium facilitates crossing of the cell barrier that can result in invasive disease, yet this process is poorly understood. Here we demonstrate that lactate, an abundant molecule in host mucosal environments, induces N. meningitidis microcolony dispersal. Interestingly, metabolic utilization of lactate by the bacteria was not required for the process, suggesting that lactate play a role as a signaling molecule in pathogenic Neisseria. We propose that the microcolony dispersal in pathogenic Neisseria is influenced by environmental concentrations of lactate. These findings will assist in better understanding the transition from asymptomatic carriage to invasive disease.

Multiscale modeling of bacterial colonies: how pili mediate the dynamics of single cells and cellular aggregates

Neisseria gonorrhoeae is the causative agent of one of the most common sexually transmitted diseases, gonorrhea. Over the past two decades there has been an alarming increase of reported gonorrhea cases where the bacteria were resistant to the most commonly used antibiotics thus prompting for alternative antimicrobial treatment strategies. The crucial step in this and many other bacterial infections is the formation of microcolonies, agglomerates consisting of up to several thousands of cells. The attachment and motility of cells on solid substrates as well as the cell-cell interactions are primarily mediated by type IV pili, long polymeric filaments protruding from the surface of cells. While the crucial role of pili in the assembly of microcolonies has been well recognized, the exact mechanisms of how they govern the formation and dynamics of microcolonies are still poorly understood. Here, we present a computational model of individual cells with explicit pili dynamics, force generation and pili-pili interactions. We employ the model to study a wide range of biological processes, such as the motility of individual cells on a surface, the heterogeneous cell motility within the large cell aggregates, and the merging dynamics and the self-assembly of microcolonies. The results of numerical simulations highlight the central role of pili generated forces in the formation of bacterial colonies and are in agreement with the available experimental observations. The model can quantify the behavior of multicellular bacterial colonies on biologically relevant temporal and spatial scales and can be easily adjusted to include the geometry and pili characteristics of various bacterial species. Ultimately, the combination of the microbiological experimental approach with the in silico model of bacterial colonies might provide new qualitative and quantitative insights on the development of bacterial infections and thus pave the way to new antimicrobial treatments.

The Vibrio cholerae Minor Pilin TcpB Initiates Assembly and Retraction of the Toxin-Coregulated Pilus

Type IV pilus (T4P) systems are complex molecular machines that polymerize major pilin proteins into thin filaments displayed on bacterial surfaces. Pilus functions require rapid extension and depolymerization of the pilus, powered by the assembly and retraction ATPases, respectively. A set of low abundance minor pilins influences pilus dynamics by unknown mechanisms. The Vibrio cholerae toxin-coregulated pilus (TCP) is among the simplest of the T4P systems, having a single minor pilin TcpB and lacking a retraction ATPase. Here we show that TcpB, like its homolog CofB, initiates pilus assembly. TcpB co-localizes with the pili but at extremely low levels, equivalent to one subunit per pilus. We used a micropillars assay to demonstrate that TCP are retractile despite the absence of a retraction ATPase, and that retraction relies on TcpB, as a V. cholerae tcpB Glu5Val mutant is fully piliated but does not induce micropillars movements. This mutant is impaired in TCP-mediated autoagglutination and TcpF secretion, consistent with retraction being required for these functions. We propose that TcpB initiates pilus retraction by incorporating into the growing pilus in a Glu5-dependent manner, which stalls assembly and triggers processive disassembly. These results provide a framework for understanding filament dynamics in more complex T4P systems and the closely related Type II secretion system.

Bacterial pathogens utilize a number of highly complex and sophisticated molecular systems to colonize their hosts and alter them, creating customized niches in which to reproduce. One such system is the Type IV pilus system, made up of dozens of proteins that form a macromolecular machine to polymerize small pilin proteins into long thin filaments that are displayed on the bacterial surface. These pili have a remarkable array of functions that rely on their ability to (i) adhere to many substrates, including host cell surfaces, pili from nearby bacteria, DNA and bacterial viruses (bacteriophage), and (ii) to depolymerize or retract, which pulls the bacteria along mucosal surfaces, pulls them close together in protective aggregates, and can even draw in substrates like DNA and bacteriophage for nutrition and genetic variation. For most Type IV pilus systems, retraction is an energy-driven process facilitated by a retraction ATPase. We show here that in the simplest of the Type IV pilus systems, the Vibrio cholerae toxin-coregulated pilus, a pilin-like protein initiates pilus retraction by what appears to be mechanical rather than enzymatic means. Our results provide a framework for understanding more complex Type IV pili and the related Type II secretion systems, which represent targets for novel highly specific antibiotics.

Structures of type IV pilins from Thermus thermophilus demonstrate similarities with type II secretion system pseudopilins

Type IV pilins are proteins which form polymers that extend from the surface of the bacterial cell; they are involved in mediating a wide variety of functions, including adhesion, motility and natural competence. Here we describe the determination of the crystal structures of three type IVa pilins proteins from the thermophile Thermus thermophilus. They form part of a cluster of pilus-like proteins within the genome; our results show that one, Tt1222, is very closely related to the main structural type IV pilin, PilA4. The other two, Tt1218 and Tt1219, also adopt canonical pilin-like folds but, interestingly, are most closely related to the structures of the type II secretion system pseudopilins, EpsI/GspI and XcpW/GspJ. GspI and GspJ have been shown to form a complex with another pseudopilin, GspK, and this heterotrimeric complex is known to play a key role in initiating assembly of a pseudopilus which is thought to drive the secretion process. The structural similarity of Tt1218 and Tt1219 to GspI and GspJ suggests that they might work in a similar way, to deliver functions associated with type IV pili in T. thermophilus, such as natural competence.

The Pilin N-terminal Domain Maintains Neisseria gonorrhoeae Transformation Competence during Pilus Phase Variation

The obligate human pathogen Neisseria gonorrhoeae is the sole aetiologic agent of the sexually transmitted infection, gonorrhea. Required for gonococcal infection, Type IV pili (Tfp) mediate many functions including adherence, twitching motility, defense against neutrophil killing, and natural transformation. Critical for immune escape, the gonococcal Tfp undergoes antigenic variation, a recombination event at the pilE locus that varies the surface exposed residues of the major pilus subunit PilE (pilin) in the pilus fiber. This programmed recombination system has the potential to produce thousands of pilin variants and can produce strains with unproductive pilin molecules that are completely unable to form Tfp. Saturating mutagenesis of the 3' third of the pilE gene identified 68 unique single nucleotide mutations that each resulted in an underpiliated colony morphology. Notably, all isolates, including those with undetectable levels of pilin protein and no observable surface-exposed pili, retained an intermediate level of transformation competence not exhibited in ΔpilE strains. Site-directed, nonsense mutations revealed that only the first 38 amino acids of the mature pilin N-terminus (the N-terminal domain or Ntd) are required for transformation competence, and microscopy, ELISAs and pilus purification demonstrate that extended Tfp are not required for competence. Transformation in strains producing only the pilin Ntd has the same genetic determinants as wild-type transformation. The Ntd corresponds to the alternative product of S-pilin cleavage, a specific proteolysis unique to pathogenic Neisseria. Mutation of the S-pilin cleavage site demonstrated that S-pilin cleavage mediated release of the Ntd is required for competence when a strain produces unproductive pilin molecules that cannot assemble into a Tfp through mutation or antigenic variation. We conclude that S-pilin cleavage evolved as a mechanism to maintain competence in nonpiliated antigenic variants and suggest there are alternate forms of the Tfp assembly apparatus that mediate various functions including transformation.

Neisseria meningitidis Polynucleotide Phosphorylase Affects Aggregation, Adhesion, and Virulence

Neisseria meningitidis autoaggregation is an important step during attachment to human cells. Aggregation is mediated by type IV pili and can be modulated by accessory pilus proteins, such as PilX, and posttranslational modifications of the major pilus subunit PilE. The mechanisms underlying the regulation of aggregation remain poorly characterized. Polynucleotide phosphorylase (PNPase) is a 3'-5' exonuclease that is involved in RNA turnover and the regulation of small RNAs. In this study, we biochemically confirm that NMC0710 is the N. meningitidis PNPase, and we characterize its role in N. meningitidis pathogenesis. We show that deletion of the gene encoding PNPase leads to hyperaggregation and increased adhesion to epithelial cells. The aggregation induced was found to be dependent on pili and to be mediated by excessive pilus bundling. PNPase expression was induced following bacterial attachment to human cells. Deletion of PNPase led to global transcriptional changes and the differential regulation of 469 genes. We also demonstrate that PNPase is required for full virulence in an in vivo model of N. meningitidis infection. The present study shows that PNPase negatively affects aggregation, adhesion, and virulence in N. meningitidis.

Homo-trimeric Structure of the Type IVb Minor Pilin CofB Suggests Mechanism of CFA/III Pilus Assembly in Human Enterotoxigenic Escherichia coli

In gram-negative bacteria, the assembly of type IV pilus (T4P) and the evolutionally related pseudopilus of type II secretion system involves specialized structural proteins called pilins and pseudopilins, respectively, and is dynamically regulated to promote bacterial pathogenesis. Previous studies have suggested that a structural "tip"-like hetero-complex formed through the interaction of at least three minor (pseudo) pilins plays an important role in this process, while some members of the pathogenic type IVb subfamily are known to have only one such minor pilin subunit whose function is still unknown. Here, we determined the crystal structure of the type IVb minor pilin CofB of colonization factor antigen/III from human enterotoxigenic Escherichia coli at 1.88-Å resolution. The crystal structure, in conjunction with physicochemical analysis in solution, reveals a symmetrical homo-trimeric arrangement distinct from the hetero-complexes of minor (pseudo) pilins observed in other T4P and type II secretion systems. Each CofB monomer adopts a unique three-domain architecture, in which the C-terminal β-sheet-rich lectin domain can effectively initiate trimer association of its pilin-like N-terminal domain through extensive hydrophobic interactions followed by domain swapping at the central hinge-like domain. Deletion of cofB produces a phenotype with no detectable pili formation on the cell surface, while molecular modeling indicates that the characteristic homo-trimeric structure of CofB is well situated at the pilus tip of colonization factor antigen/III formed by the major pilin CofA, suggesting a role for the minor pilin in the efficient initiation of T4P assembly.

Staying in Shape: the Impact of Cell Shape on Bacterial Survival in Diverse Environments

Bacteria display an abundance of cellular forms and can change shape during their life cycle. Many plausible models regarding the functional significance of cell morphology have emerged. A greater understanding of the genetic programs underpinning morphological variation in diverse bacterial groups, combined with assays of bacteria under conditions that mimic their varied natural environments, from flowing freshwater streams to diverse human body sites, provides new opportunities to probe the functional significance of cell shape. Here we explore shape diversity among bacteria, at the levels of cell geometry, size, and surface appendages (both placement and number), as it relates to survival in diverse environments. Cell shape in most bacteria is determined by the cell wall. A major challenge in this field has been deconvoluting the effects of differences in the chemical properties of the cell wall and the resulting cell shape perturbations on observed fitness changes. Still, such studies have begun to reveal the selective pressures that drive the diverse forms (or cell wall compositions) observed in mammalian pathogens and bacteria more generally, including efficient adherence to biotic and abiotic surfaces, survival under low-nutrient or stressful conditions, evasion of mammalian complement deposition, efficient dispersal through mucous barriers and tissues, and efficient nutrient acquisition.

Structural and functional studies of the Pseudomonas aeruginosa minor pilin, PilE

Many bacterial pathogens, including Pseudomonas aeruginosa, use type IVa pili (T4aP) for attachment and twitching motility. T4aP are composed primarily of major pilin subunits, which are repeatedly assembled and disassembled to mediate function. A group of pilin-like proteins, the minor pilins FimU and PilVWXE, prime pilus assembly and are incorporated into the pilus. We showed previously that minor pilin PilE depends on the putative priming subcomplex PilVWX and the non-pilin protein PilY1 for incorporation into pili, and that with FimU, PilE may couple the priming subcomplex to the major pilin PilA, allowing for efficient pilus assembly. Here we provide further support for this model, showing interaction of PilE with other minor pilins and the major pilin. A 1.25 Å crystal structure of PilEΔ1-28 shows a typical type IV pilin fold, demonstrating how it may be incorporated into the pilus. Despite limited sequence identity, PilE is structurally similar to Neisseria meningitidis minor pilins PilXNm and PilVNm, recently suggested via characterization of mCherry fusions to modulate pilus assembly from within the periplasm. A P. aeruginosa PilE-mCherry fusion failed to complement twitching motility or piliation of a pilE mutant. However, in a retraction-deficient strain where surface piliation depends solely on PilE, the fusion construct restored some surface piliation. PilE-mCherry was present in sheared surface fractions, suggesting that it was incorporated into pili. Together, these data provide evidence that PilE, the sole P. aeruginosa equivalent of PilXNm and PilVNm, likely connects a priming subcomplex to the major pilin, promoting efficient assembly of T4aP.

Crystal Structure of the Minor Pilin CofB, the Initiator of CFA/III Pilus Assembly in Enterotoxigenic Escherichia coli

Type IV pili are extracellular polymers of the major pilin subunit. These subunits are held together in the pilus filament by hydrophobic interactions among their N-terminal α-helices, which also anchor the pilin subunits in the inner membrane prior to pilus assembly. Type IV pilus assembly involves a conserved group of proteins that span the envelope of Gram-negative bacteria. Among these is a set of minor pilins, so named because they share their hydrophobic N-terminal polymerization/membrane anchor segment with the major pilins but are much less abundant. Minor pilins influence pilus assembly and retraction, but their precise functions are not well defined. The Type IV pilus systems of enterotoxigenic Escherichia coli and Vibrio cholerae are among the simplest of Type IV pilus systems and possess only a single minor pilin. Here we show that the enterotoxigenic E. coli minor pilins CofB and LngB are required for assembly of their respective Type IV pili, CFA/III and Longus. Low levels of the minor pilins are optimal for pilus assembly, and CofB can be detected in the pilus fraction. We solved the 2.0 Å crystal structure of N-terminally truncated CofB, revealing a pilin-like protein with an extended C-terminal region composed of two discrete domains connected by flexible linkers. The C-terminal region is required for CofB to initiate pilus assembly. We propose a model for CofB-initiated pilus assembly with implications for understanding filament growth in more complex Type IV pilus systems as well as the related Type II secretion system.

Oxygen governs gonococcal microcolony stability by enhancing the interaction force between type IV pili

The formation of small bacterial clusters, called microcolonies, is the first step towards the formation of bacterial biofilms. The human pathogen Neisseria gonorrhoeae requires type IV pili (T4P) for microcolony formation and for surface motility. Here, we investigated the effect of oxygen on the dynamics of microcolony formation. We found that an oxygen concentration exceeding 3 μM is required for formation and maintenance of microcolonies. Depletion of proton motive force triggers microcolony disassembly. Disassembly of microcolonies is actively driven by T4P retraction. Using laser tweezers we showed that under aerobic conditions T4P-T4P interaction forces exceed 50 pN. Under anaerobic conditions T4P-T4P interaction is severely inhibited. We conclude that oxygen is required for gonococcal microcolony formation by enhancing pilus-pilus interaction.

Fatal attraction: how bacterial adhesins affect host signaling and what we can learn from them

The ability of bacterial species to colonize and infect host organisms is critically dependent upon their capacity to adhere to cellular surfaces of the host. Adherence to cell surfaces is known to be essential for the activation and delivery of certain virulence factors, but can also directly affect host cell signaling to aid bacterial spread and survival. In this review we will discuss the recent advances in the field of bacterial adhesion, how we are beginning to unravel the effects adhesins have on host cell signaling, and how these changes aid the bacteria in terms of their survival and evasion of immune responses. Finally, we will highlight how the exploitation of bacterial adhesins may provide new therapeutic avenues for the treatment of a wide range of bacterial infections.

Exceptionally widespread nanomachines composed of type IV pilins: the prokaryotic Swiss Army knives

Prokaryotes have engineered sophisticated surface nanomachines that have allowed them to colonize Earth and thrive even in extreme environments. Filamentous machineries composed of type IV pilins, which are associated with an amazing array of properties ranging from motility to electric conductance, are arguably the most widespread since distinctive proteins dedicated to their biogenesis are found in most known species of prokaryotes. Several decades of investigations, starting with type IV pili and then a variety of related systems both in bacteria and archaea, have outlined common molecular and structural bases for these nanomachines. Using type IV pili as a paradigm, we will highlight in this review common aspects and key biological differences of this group of filamentous structures.

Using type IV pili as a paradigm, we review common genetic, structural and mechanistic features (many) as well as differences (few) of the exceptionally widespread and functionally versatile prokaryotic nano-machines composed of type IV pilins.

Neisseria meningitidis: pathogenesis and immunity

The recent advances in cellular microbiology, genomics, and immunology has opened new horizons in the understanding of meningococcal pathogenesis and in the definition of new prophylactic intervention. It is now clear that Neissera meningitidis has evolved a number of surface structures to mediate interaction with host cells and a number of mechanisms to subvert the immune system and escape complement-mediated killing. In this review we report the more recent findings on meningococcal adhesion and on the bacteria-complement interaction highlighting the redundancy of these mechanisms. An effective vaccine against meningococcus B, based on multiple antigens with different function, has been recently licensed. The antibodies induced by the 4CMenB vaccine could mediate bacterial killing by activating directly the classical complement pathway or, indirectly, by preventing binding of fH on the bacterial surface and interfering with colonization.

Pseudomonas aeruginosa minor pilins prime type IVa pilus assembly and promote surface display of the PilY1 adhesin

Type IV pili (T4P) contain hundreds of major subunits, but minor subunits are also required for assembly and function. Here we show that Pseudomonas aeruginosa minor pilins prime pilus assembly and traffic the pilus-associated adhesin and anti-retraction protein, PilY1, to the cell surface. PilV, PilW, and PilX require PilY1 for inclusion in surface pili and vice versa, suggestive of complex formation. PilE requires PilVWXY1 for inclusion, suggesting that it binds a novel interface created by two or more components. FimU is incorporated independently of the others and is proposed to couple the putative minor pilin-PilY1 complex to the major subunit. The production of small amounts of T4P by a mutant lacking the minor pilin operon was traced to expression of minor pseudopilins from the P. aeruginosa type II secretion (T2S) system, showing that under retraction-deficient conditions, T2S minor subunits can prime T4P assembly. Deletion of all minor subunits abrogated pilus assembly. In a strain lacking the minor pseudopilins, PilVWXY1 and either FimU or PilE comprised the minimal set of components required for pilus assembly. Supporting functional conservation of T2S and T4P minor components, our 1.4 Å crystal structure of FimU revealed striking architectural similarity to its T2S ortholog GspH, despite minimal sequence identity. We propose that PilVWXY1 form a priming complex for assembly and that PilE and FimU together stably couple the complex to the major subunit. Trafficking of the anti-retraction factor PilY1 to the cell surface allows for production of pili of sufficient length to support adherence and motility.