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

Type IV pilus retraction is required for Neisseria musculi colonization and persistence in a natural mouse model of infection

IMPORTANCE

We describe the importance of Type IV pilus retraction to colonization and persistence by a mouse commensal Neisseria, N. musculi, in its native host. Our findings have implications for the role of Tfp retraction in mediating interactions of human-adapted pathogenic and commensal Neisseria with their human host due to the relatedness of these species.

Sphingosine kinase 1/S1P receptor signaling axis is essential for cellular uptake of Neisseria meningitidis in brain endothelial cells

Invasion of brain endothelial cells (BECs) is central to the pathogenicity of Neisseria meningitidis infection. Here, we established a key role for the bioactive sphingolipid sphingosine-1-phosphate (S1P) and S1P receptor (S1PR) 2 in the uptake process. Quantitative sphingolipidome analyses of BECs infected with N. meningitidis revealed elevated S1P levels, which could be attributed to enhanced expression of the enzyme sphingosine kinase 1 and its activity. Increased activity was dependent on the interaction of meningococcal type IV pilus with the endothelial receptor CD147. Concurrently, infection led to increased expression of the S1PR2. Blocking S1PR2 signaling impaired epidermal growth factor receptor (EGFR) phosphorylation, which has been shown to be involved in cytoskeletal remodeling and bacterial endocytosis. Strikingly, targeting S1PR1 or S1PR3 also interfered with bacterial uptake. Collectively, our data support a critical role of the SphK/S1P/S1PR axis in the invasion of N. meningitidis into BECs, defining a potential target for adjuvant therapy.

Vertebrate and Invertebrate Animal and New In Vitro Models for Studying Neisseria Biology

The history of Neisseria research has involved the use of a wide variety of vertebrate and invertebrate animal models, from insects to humans. In this review, we itemise these models and describe how they have made significant contributions to understanding the pathophysiology of Neisseria infections and to the development and testing of vaccines and antimicrobials. We also look ahead, briefly, to their potential replacement by complex in vitro cellular models.

Implantation of engineered human microvasculature to study human infectious diseases in mouse models

Animal models for studying human pathogens are crucially lacking. We describe the implantation in mice of engineered human mature microvasculature consisting of endothelial and perivascular cells embedded in collagen hydrogel that allows investigation of pathogen interactions with the endothelium, including in vivo functional studies. Using Neisseria meningitidis as a paradigm of human-restricted infection, we demonstrated the strength and opportunities associated with the use of this approach.

Type IV Pili Are a Critical Virulence Factor in Clinical Isolates of Paenibacillus thiaminolyticus

Hydrocephalus, the leading indication for childhood neurosurgery worldwide, is particularly prevalent in low- and middle-income countries. Hydrocephalus preceded by an infection, or postinfectious hydrocephalus, accounts for up to 60% of hydrocephalus in these areas. Since many children with hydrocephalus suffer poor long-term outcomes despite surgical intervention, prevention of hydrocephalus remains paramount. Our previous studies implicated a novel bacterial pathogen, Paenibacillus thiaminolyticus, as a causal agent of neonatal sepsis and postinfectious hydrocephalus in Uganda. Here, we report the isolation of three P. thiaminolyticus strains, Mbale, Mbale2, and Mbale3, from patients with postinfectious hydrocephalus. We constructed complete genome assemblies of the clinical isolates as well as the nonpathogenic P. thiaminolyticus reference strain and performed comparative genomic and proteomic analyses to identify potential virulence factors. All three isolates carry a unique beta-lactamase gene, and two of the three isolates exhibit resistance in culture to the beta-lactam antibiotics penicillin and ampicillin. In addition, a cluster of genes carried on a mobile genetic element that encodes a putative type IV pilus operon is present in all three clinical isolates but absent in the reference strain. CRISPR-mediated deletion of the gene cluster substantially reduced the virulence of the Mbale strain in mice. Comparative proteogenomic analysis identified various additional potential virulence factors likely acquired on mobile genetic elements in the virulent strains. These results provide insight into the emergence of virulence in P. thiaminolyticus and suggest avenues for the diagnosis and treatment of this novel bacterial pathogen. IMPORTANCE Postinfectious hydrocephalus, a devastating sequela of neonatal infection, is associated with increased childhood mortality and morbidity. A novel bacterial pathogen, Paenibacillus thiaminolyticus, is highly associated with postinfectious hydrocephalus in an African cohort. Whole-genome sequencing, RNA sequencing, and proteomics of clinical isolates and a reference strain in combination with CRISPR editing identified type IV pili as a critical virulence factor for P. thiaminolyticus infection. Acquisition of a type IV pilus-encoding mobile genetic element critically contributed to converting a nonpathogenic strain of P. thiaminolyticus into a pathogen capable of causing devastating diseases. Given the widespread presence of type IV pilus in pathogens, the presence of the type IV pilus operon could serve as a diagnostic and therapeutic target in P. thiaminolyticus and related bacteria.

Prevalence of Type IV Pili-Mediated Twitching Motility in Streptococcus sanguinis Strains and Its Impact on Biofilm Formation and Host Adherence

Type IV pili (Tfp) are known to mediate several biological activities, including surface-dependent twitching motility. Although a pil gene cluster for Tfp biosynthesis is found in all sequenced Streptococcus sanguinis strains, Tfp-mediated twitching motility is less commonly detected. Upon examining 81 clinical strains, 39 strains generated twitching zones on blood agar plates (BAP), while 27 strains displayed twitching on Todd-Hewitt (TH) agar. Although BAP appears to be more suitable for the development of twitching zones, 5 strains exhibited twitching motility only on TH agar, indicating that twitching motility is not only strain specific but also sensitive to growth media. Furthermore, different twitching phenotypes were observed in strains expressing comparable levels of pilT, encoding the retraction ATPase, suggesting that the twitching phenotype on agar plates is regulated by multiple factors. By using a PilT-null and a pilin protein-null derivative (CHW02) of twitching-active S. sanguinis CGMH010, we found that Tfp retraction was essential for biofilm stability. Further, biofilm growth was amplified in CHW02 in the absence of shearing force, indicating that S. sanguinis may utilize other ligands for biofilm formation in the absence of Tfp. Similar to SK36, Tfp from CGMH010 were required for colonization of host cells, but PilT only marginally affected adherence and only in the twitching-active strain. Taken together, the results suggest that Tfp participates in host cell adherence and that Tfp retraction facilitates biofilm stability. IMPORTANCE Although the gene clusters encoding Tfp are commonly present in Streptococcus sanguinis, not all strains express surface-dependent twitching motility on agar surfaces. Regardless of whether the Tfp could drive motility, Tfp can serve as a ligand for the colonization of host cells. Though many S. sanguinis strains lack twitching activity, motility can enhance biofilm stability in a twitching-active strain; thus, perhaps motility provides little or no advantage to the survival of bacteria within dental plaque. Rather, Tfp retraction could provide additional advantages for the bacteria to establish infections outside the oral cavity.

N-terminal signal peptides facilitate the engineering of PVC complex as a potent protein delivery system

Extracellular contractile injection systems (eCISs) are widespread bacterial nanomachines that resemble T4 phage tail. As a typical eCIS, Photorhabdus virulence cassette (PVC) was proposed to inject toxins into eukaryotic cells by puncturing the cell membrane from outside. This makes it an ideal tool for protein delivery in biomedical research. However, how to manipulate this nanocomplex as a molecular syringe is still undetermined. Here, we identify that one group of N-terminal signal peptide (SP) sequences are crucial for the effector loading into the inner tube of PVC complex. By application of genetic operation, cryo-electron microscopy, in vitro translocation assays, and animal experiments, we show that, under the guidance of the SP, numerous prokaryotic and eukaryotic proteins can be loaded into PVC to exert their functions across cell membranes. We therefore might customize PVC as a potent protein delivery nanosyringe for biotherapy by selecting cargo proteins in a broad spectrum, regardless of their species, sizes, and charges.

Mechanical Activation of the β2-Adrenergic Receptor by Meningococcus: A Historical and Future Perspective Analysis of How a Bacterial Probe Can Reveal Signalling Pathways in Endothelial Cells, and a Unique Mode of Receptor Activation Involving Its N-Terminal Glycan Chains

More than 12 years have passed since the seminal observation that meningococcus, a pathogen causing epidemic meningitis in humans, occasionally associated with infectious vasculitis and septic shock, can promote the translocation of β-arrestins to the cell surface beneath bacterial colonies. The cellular receptor used by the pathogen to induce signalling in host cells and allowing it to open endothelial cell junctions and reach meninges was unknown. The involvement of β-arrestins, which are scaffolding proteins regulating G protein coupled receptor signalling and function, incited us to specifically investigate this class of receptors. In this perspective article we will summarize the events leading to the discovery that the β₂-adrenergic receptor is the receptor that initiates the signalling cascades induced by meningococcus in host cells. This receptor, however, cannot mediate cell infection on its own. It needs to be pre-associated with an "early" adhesion receptor, CD147, within a hetero-oligomeric complex, stabilized by the cytoskeletal protein α-actinin 4. It then required several years to understand how the pathogen actually activates the signalling receptor. Once bound to the N-terminal glycans of the β₂-adrenergic receptor, meningococcus provides a mechanical stimulation that induces the biased activation of β-arrestin-mediated signalling pathways. This activating mechanical stimulus can be reproduced in the absence of any pathogen by applying equivalent forces on receptor glycans. Mechanical activation of the β₂-adrenergic receptor might have a physiological role in signalling events promoted in the context of cell-to-cell interaction.

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.

Meningococcus, this famous unknown

Neisseria meningitidis (meningococcus) is a Gram-negative bacterium responsible for two devastating forms of invasive diseases: purpura fulminans and meningitis. Since the first description of the epidemic nature of the illness at the dawn of the nineteenth century, the scientific knowledge of meningococcal infection has increased greatly. Major advances have been made in the management of the disease with the advent of antimicrobial therapy and the implementation of meningococcal vaccines. More recently, an extensive knowledge has been accumulated on meningococcal interaction with its human host, revealing key processes involved in disease progression and new promising therapeutic approaches.