Pilus-mediated adhesion of Neisseria meningitidis is negatively controlled by the pilus-retraction machinery

Variable disruption of epithelial monolayers by Neisseria meningitidis carriage isolates of the hypervirulent MenW cc11 and MenY cc23 lineages

Colonization of mucosal tissues by Neisseria meningitidis requires adhesion mediated by the type IV pilus and multiple outer-membrane proteins. Penetration of the mucosa and invasion of epithelial cells are thought to contribute to host persistence and invasive disease. Using Calu-3 cell monolayers grown at an air-liquid interface, we examined adhesion, invasion and monolayer disruption by carriage isolates of two clonal complexes of N. meningitidis. Carriage isolates of both the serogroup Y cc23 and the hypervirulent serogroup W cc11 lineages exhibited high levels of cellular adhesion, and a variable disruption phenotype across independent isolates. Inactivation of the gene encoding the main pilus sub-unit in multiple cc11 isolates abrogated both adhesive capacity and ability to disrupt epithelial monolayers. Contrastingly, inactivation of the phase-variable opa or nadA genes reduced adhesion and invasion, but not disruption of monolayer integrity. Adherence of tissue-disruptive meningococci correlated with loss of staining for the tight junction protein, occludin. Intriguingly, in a pilus-negative strain background, we observed compensatory ON switching of opa genes, which facilitated continued adhesion. We conclude that disruption of epithelial monolayers occurs in multiple meningococcal lineages but can vary during carriage and is intimately linked to pilus-mediated adhesion.

Type IV pilus retraction enables sustained bacteremia and plays a key role in the outcome of meningococcal sepsis in a humanized mouse model

Neisseria meningitidis (the meningococcus) remains a major cause of bacterial meningitis and fatal sepsis. This commensal bacterium of the human nasopharynx can cause invasive diseases when it leaves its niche and reaches the bloodstream. Blood-borne meningococci have the ability to adhere to human endothelial cells and rapidly colonize microvessels. This crucial step enables dissemination into tissues and promotes deregulated inflammation and coagulation, leading to extensive necrotic purpura in the most severe cases. Adhesion to blood vessels relies on type IV pili (TFP). These long filamentous structures are highly dynamic as they can rapidly elongate and retract by the antagonistic action of two ATPases, PilF and PilT. However, the consequences of TFP dynamics on the pathophysiology and the outcome of meningococcal sepsis in vivo have been poorly studied. Here, we show that human graft microvessels are replicative niches for meningococci, that seed the bloodstream and promote sustained bacteremia and lethality in a humanized mouse model. Intriguingly, although pilus-retraction deficient N. meningitidis strain (ΔpilT) efficiently colonizes human graft tissue, this mutant did not promote sustained bacteremia nor induce mouse lethality. This effect was not due to a decreased inflammatory response, nor defects in bacterial clearance by the innate immune system. Rather, TFP-retraction was necessary to promote the release of TFP-dependent contacts between bacteria and, in turn, the detachment from colonized microvessels. The resulting sustained bacteremia was directly correlated with lethality. Altogether, these results demonstrate that pilus retraction plays a key role in the occurrence and outcome of meningococcal sepsis by supporting sustained bacteremia. These findings open new perspectives on the role of circulating bacteria in the pathological alterations leading to lethal sepsis.

Discovery of a New Neisseria gonorrhoeae Type IV Pilus Assembly Factor, TfpC

Most bacterial species express one or more extracellular organelles called pili/fimbriae that are required for many properties of each bacterial cell. The Neisseria gonorrhoeae type IV pilus is a major virulence and colonization factor for the sexually transmitted infection gonorrhea. We have discovered a new protein of Neisseria gonorrhoeae called TfpC that is required to maintain type IV pili on the bacterial cell surface. There are similar proteins found in other members of the Neisseria genus and many other bacterial species important for human health.

Neisseria gonorrhoeae relies on type IV pili (T4p) to promote colonization of their human host and to cause the sexually transmitted infection gonorrhea. This organelle cycles through a process of extension and retraction back into the bacterial cell. Through a genetic screen, we identified the NGO0783 locus of N. gonorrhoeae strain FA1090 as containing a gene encoding a protein required to stabilize the type IV pilus in its extended, nonretracted conformation. We have named the gene tfpC and the protein TfpC. Deletion of tfpC produces a nonpiliated colony morphology, and immuno-transmission electron microscopy confirms that the pili are lost in the ΔtfpC mutant, although there is some pilin detected near the bacterial cell surface. A copy of the tfpC gene expressed from a lac promoter restores pilus expression and related phenotypes. A ΔtfpC mutant shows reduced levels of pilin protein, but complementation with a tfpC gene restored pilin to normal levels. Bioinformatic searches show that there are orthologues in numerous bacterial species, but not all type IV pilin-expressing bacteria contain orthologous genes. Coevolution and nuclear magnetic resonance (NMR) analysis indicates that TfpC contains an N-terminal transmembrane helix, a substantial extended/unstructured region, and a highly charged C-terminal coiled-coil domain.

Attenuation of the Type IV Pilus Retraction Motor Influences Neisseria gonorrhoeae Social and Infection Behavior

Retraction of the type IV pilus (Tfp) mediates DNA uptake, motility, and social and infection behavior in a wide variety of prokaryotes. To date, investigations into Tfp retraction-dependent activities have used a mutant deleted of PilT, the ATPase motor protein that causes the pilus fiber to retract. ΔpilT cells are nontransformable, nonmotile, and cannot aggregate into microcolonies. We tested the hypothesis that these retraction-dependent activities are sensitive to the strength of PilT enzymatic activity by using the pathogen Neisseria gonorrhoeae as a model. We constructed an N. gonorrhoeae mutant with an amino acid substitution in the PilT Walker B box (a substitution of cysteine for leucine at position 201, encoded by pilT_L201C). Purified PilT_L201C forms a native hexamer, but mutant hexamers hydrolyze ATP at half the maximal rate. N. gonorrhoeae pilT_L201C cells produce Tfp fibers, crawl at the same speed as the wild-type (wt) parent, and are equally transformable. However, the social behavior of pilT_L201C cells is intermediate between the behaviors of wt and ΔpilT cells. The infection behavior of pilT_L201C is also defective, due to its failure to activate the epidermal growth factor receptor (EGFR)-heparin-binding EGF-like growth factor (HB-EGF) pathway. Our study indicates that pilus retraction, per se, is not sufficient for N. gonorrhoeae microcolony formation or infectivity; rather, these activities are sensitive to the strength of PilT enzymatic activity. We discuss the implications of these findings for Neisseria pathogenesis in the context of mechanobiology.

Type IV pili are fibers expressed on the surface of many bacteria. Neisseria gonorrhoeae cells crawl, take up DNA, and communicate with each other and with human cells by retracting these fibers. Here, we show that an N. gonorrhoeae mutant expressing an enzymatically weakened type IV pilus retraction motor still crawls and takes up DNA normally. However, mutant cells exhibit abnormal social behavior, and they are less infective because they fail to activate the epidermal growth factor receptor. Our study shows that N. gonorrhoeae social and infection behaviors are sensitive to the strength of the retraction motor enzyme.

Calcium-Enhanced Twitching Motility in Xylella fastidiosa Is Linked to a Single PilY1 Homolog

The plant-pathogenic bacterium Xylella fastidiosa is restricted to the xylem vessel environment, where mineral nutrients are transported through the plant host; therefore, changes in the concentrations of these elements likely impact the growth and virulence of this bacterium. Twitching motility, dependent on type IV pili (TFP), is required for movement against the transpiration stream that results in basipetal colonization. We previously demonstrated that calcium (Ca) increases the motility of X. fastidiosa, although the mechanism was unknown. PilY1 is a TFP structural protein recently shown to bind Ca and to regulate twitching and adhesion in bacterial pathogens of humans. Sequence analysis identified three pilY1 homologs in X. fastidiosa (PD0023, PD0502, and PD1611), one of which (PD1611) contains a Ca-binding motif. Separate deletions of PD0023 and PD1611 resulted in mutants that still showed twitching motility and were not impaired in attachment or biofilm formation. However, the response of increased twitching at higher Ca concentrations was lost in the pilY1-1611 mutant. Ca does not modulate the expression of any of the X. fastidiosa PilY1 homologs, although it increases the expression of the retraction ATPase pilT during active movement. The evidence presented here suggests functional differences between the PilY1 homologs, which may provide X. fastidiosa with an adaptive advantage in environments with high Ca concentrations, such as xylem sap.

Identification of a novel transcriptional regulator involved in pilC1 regulation in Neisseria meningitidis

Type IV pili are crucial for the virulence of Neisseria meningitidis. PilC proteins belong to the complex protein machinery required for pili biosynthesis. The expression of the pilC1 gene is known to be induced during host cell contact and to be tightly controlled through four promoters, two transcription factors and a two-component signal transduction system. By screening of an insertional-mutant library, we identified a novel regulatory protein, i.e. NMA1805, involved in the pilC1 complex regulation. Increased transcription of gene NMA1805 was shown to result in augmented expression of the pilC1 gene, whereas abrogated expression of gene NMA1805 was associated with an absence of pilC1 induction upon contact with host cells. Moreover, we demonstrated that the NMA1805 gene displayed two promoters. The NMA1805 regulatory protein was evidenced to interact with one of them.

A two-component system is required for colonization of host cells by meningococcus

In order to adapt to changing environments, bacteria have evolved two-component systems (TCSs) that are able to sense and respond to environmental stimuli. The signal perception relies on a sensor protein whose activation allows rapid adaptation through transcriptional regulation achieved by the regulatory protein. The ability to adhere to and grow on the surface of human host cells is an absolute requirement for many pathogens, including Neisseria meningitidis, in order to colonize new hosts and to disseminate inside their host. Among the four TCSs encoded in the meningococcus genome, only the PhoQ (MisS)/PhoP (MisR) system has been shown to constitute a functional signal transduction circuit. To investigate the involvement of this TCS in the adaptation process requisite for host cell colonization, we have tested the ability to grow on host cells of a mutant inactivated for the sensor of the TCS. Our results demonstrate the involvement of the TCS in the adaptation of the meningococcus to growth on host cells. We show that the expression of the PhoQ (MisS)/PhoP (MisR) TCS is cell-contact controlled. Furthermore, this TCS controls the regulation of a group of genes, the REP2 regulon, previously shown to be cell-contact regulated and to encode functions crucial for the adaptation of the bacterium to host cell colonization. Thus, we provide evidence that one of the four TCSs existing in N. meningitidis contributes to the adaptation of the pathogen to growth on host cells.

Pilin regulation in the pilT mutant of Neisseria gonorrhoeae strain MS11

The ATPase protein PilT mediates retraction of type IV pili (Tfp). Tfp retraction of Neisseria gonorrhoeae causes many signal transduction events and changes in gene expression in infected epithelial cells. To find out whether a pilT mutation and lack of Tfp retraction, respectively, lead also to gene regulation in bacteria, we performed microarrays comparing the transcriptional profiles of the N. gonorrhoeae parent strain MS11 and its isogenic pilT mutant during growth in vitro. A loss-of-function-mutation in pilT led to altered transcript levels of 63 ORFs. Levels of pilE transcripts and its deduced protein, the major Tfp subunit pilin, were increased most markedly by a mutation in pilT. Further studies revealed that pilE expression was also controlled by two other genes encoding Tfp biogenesis proteins, pilD and pilF. Our studies strongly suggest that pilE expression is a finely tuned process.

Role of HrpA in biofilm formation of Neisseria meningitidis and regulation of the hrpBAS transcripts

Two-partner secretion systems of gram-negative organisms are utilized in adherence, invasion, and biofilm formation. The HrpAB proteins of Neisseria meningitidis are members of a two-partner secretion system, and HrpA is established as being important to adherence and intracellular escape. This study set out to determine the expression pattern of members of the hrpBAS putative operon and to find a functional role for the HrpA protein. The upregulation of these genes was found in situations of anaerobiosis and cell contact. These observations prompted the study of the function of HrpA in biofilms on human bronchial epithelial cells. HrpA mutants in encapsulated and unencapsulated NMB strains demonstrated biofilm growth equivalent to that of the wild-type strain at 6 h but a decreased ability to form biofilms at 48 h. Biofilms formed by hrpA mutants for 48 h on collagen-coated coverslips demonstrated significant reductions compared to those of wild-type strains. Taken together, these observations imply a role for HrpA in the biofilm structure. Further analysis demonstrated the presence of HrpA on the surface of the bacterium.

Twitching motility is essential for virulence in Dichelobacter nodosus

Type IV fimbriae are essential virulence factors of Dichelobacter nodosus, the principal causative agent of ovine foot rot. The fimA fimbrial subunit gene is required for virulence, but fimA mutants exhibit several phenotypic changes and it is not certain if the effects on virulence result from the loss of type IV fimbria-mediated twitching motility, cell adherence, or reduced protease secretion. We showed that mutation of either the pilT or pilU gene eliminated the ability to carry out twitching motility. However, the pilT mutants displayed decreased adhesion to epithelial cells and reduced protease secretion, whereas the pilU mutants had wild-type levels of extracellular protease secretion and adherence. These data provided evidence that PilT is required for the type IV fimbria-dependent protease secretion pathway in D. nodosus. It was postulated that sufficient fimbrial retraction must occur in the pilU mutants to allow protease secretion, but not twitching motility, to take place. Although no cell movement was detected in a pilU mutant of D. nodosus, aberrant motion was detected in an equivalent mutant of Pseudomonas aeruginosa. These observations explain how in D. nodosus protease secretion can occur in a pilU mutant but not in a pilT mutant. In addition, virulence studies with sheep showed that both the pilT and pilU mutants were avirulent, providing evidence that mutation of the type IV fimbrial system affects virulence by eliminating twitching motility, not by altering cell adherence or protease secretion.

Twitching motility is essential for endophytic rice colonization by the N2-fixing endophyte Azoarcus sp. strain BH72

Azoarcus sp. strain BH72, as an endophyte of grasses, depends on successful host colonization. Type IV pili are essential for mediating the initial interaction with rice roots. In the genome sequence analysis, the pilT gene was identified, which encodes for a putative type IV pilus retraction protein. PilT of Azoarcus sp. BH72 shares high similarity to PilT of the human pathogen Pseudomonas aeruginosa PAO1 (77% amino acid sequence identity) and contains a predicted nucleotide-binding motif. To gain more insights into the role of the type IV pili in the colonization process of Azoarcus spp., we constructed an insertional mutant of pilT and a deletion mutant of pilA, the major structural component of the pilus structure. The pilT mutant, as the pilin deletion mutant deltapilA, was abolished in twitching motility. Western blot analyses and electron microscopy studies demonstrated an enhanced piliation of the Azoarcus pilT mutant strain compared with the wild type, indicating that, indeed, PilT has a role in pilus retraction. Studies on rice root colonization in gnotobiotic cultures revealed that the establishment of microcolonies on the root surface was strongly reduced in the deltapilA mutant, whereas the surface colonization was reduced by only 50% in the nontwitching pilT mutant. However, endophytic colonization of rice roots was strongly reduced in both mutants. These results demonstrate that the retractile force mediated by PilT is not essential for the bacterial colonization of the plant surface, but that twitching motility is necessary for invasion of and establishment inside the plant. Thus, a novel determinant for endophytic interactions with grasses was identified.