Blood-brain barrier invasion by group B Streptococcus depends upon proper cell-surface anchoring of lipoteichoic acid

Lipoteichoic acid anchor triggers Mincle to drive protective immunity against invasive group A Streptococcus infection

Group A Streptococcus (GAS) is a Gram-positive bacterial pathogen that causes a range of diseases, including fatal invasive infections. However, the mechanisms by which the innate immune system recognizes GAS are not well understood. We herein report that the C-type lectin receptor macrophage inducible C-type lectin (Mincle) recognizes GAS and initiates antibacterial immunity. Gene expression analysis of myeloid cells upon GAS stimulation revealed the contribution of the caspase recruitment domain-containing protein 9 (CARD9) pathway to the antibacterial responses. Among receptors signaling through CARD9, Mincle induced the production of inflammatory cytokines, inducible nitric oxide synthase, and reactive oxygen species upon recognition of the anchor of lipoteichoic acid, monoglucosyldiacylglycerol (MGDG), produced by GAS. Upon GAS infection, Mincle-deficient mice exhibited impaired production of proinflammatory cytokines, severe bacteremia, and rapid lethality. GAS also possesses another Mincle ligand, diglucosyldiacylglycerol; however, this glycolipid interfered with MGDG-induced activation. These results indicate that Mincle plays a central role in protective immunity against acute GAS infection.

The plasminogen binding protein PbsP is required for brain invasion by hypervirulent CC17 Group B streptococci

Streptococcus agalactiae (Group B Streptococcus or GBS) is a frequent cause of serious disease in newborns and adults. Epidemiological evidence indicates a strong association between GBS strains belonging to the hypervirulent CC17 clonal complex and the occurrence of meningitis in neonates. We investigate here the role of PbsP, a cell wall plasminogen binding protein, in colonization of the central nervous system by CC17 GBS. Deletion of pbsP selectively impaired the ability of the CC17 strain BM110 to colonize the mouse brain after intravenous challenge, despite its unchanged capacity to persist at high levels in the blood and to invade the kidneys. Moreover, immunization with a recombinant form of PbsP considerably reduced brain infection and lethality. In vitro, pbsP deletion markedly decreased plasmin-dependent transmigration of BM110 through brain microvascular endothelial cells. Although PbsP was modestly expressed in bacteria grown under standard laboratory conditions, pbsP expression was markedly upregulated during in vivo infection or upon contact with cultured brain endothelial cells. Collectively, our studies indicate that PbsP is a highly conserved Plg binding adhesin, which is functionally important for invasion of the central nervous system by the hypervirulent CC17 GBS. Moreover, this antigen is a promising candidate for inclusion in a universal GBS vaccine.

Mechanisms of Blood Brain Barrier Disruption by Different Types of Bacteria, and Bacterial-Host Interactions Facilitate the Bacterial Pathogen Invading the Brain

This review aims to elucidate the different mechanisms of blood brain barrier (BBB) disruption that may occur due to invasion by different types of bacteria, as well as to show the bacteria-host interactions that assist the bacterial pathogen in invading the brain. For example, platelet-activating factor receptor (PAFR) is responsible for brain invasion during the adhesion of pneumococci to brain endothelial cells, which might lead to brain invasion. Additionally, the major adhesin of the pneumococcal pilus-1, RrgA is able to bind the BBB endothelial receptors: polymeric immunoglobulin receptor (pIgR) and platelet endothelial cell adhesion molecule (PECAM-1), thus leading to invasion of the brain. Moreover, Streptococcus pneumoniae choline binding protein A (CbpA) targets the common carboxy-terminal domain of the laminin receptor (LR) establishing initial contact with brain endothelium that might result in BBB invasion. Furthermore, BBB disruption may occur by S. pneumoniae penetration through increasing in pro-inflammatory markers and endothelial permeability. In contrast, adhesion, invasion, and translocation through or between endothelial cells can be done by S. pneumoniae without any disruption to the vascular endothelium, upon BBB penetration. Internalins (InlA and InlB) of Listeria monocytogenes interact with its cellular receptors E-cadherin and mesenchymal-epithelial transition (MET) to facilitate invading the brain. L. monocytogenes species activate NF-κB in endothelial cells, encouraging the expression of P- and E-selectin, intercellular adhesion molecule 1 (ICAM-1), and Vascular cell adhesion protein 1 (VCAM-1), as well as IL-6 and IL-8 and monocyte chemoattractant protein-1 (MCP-1), all these markers assist in BBB disruption. Bacillus anthracis species interrupt both adherens junctions (AJs) and tight junctions (TJs), leading to BBB disruption. Brain microvascular endothelial cells (BMECs) permeability and BBB disruption are induced via interendothelial junction proteins reduction as well as up-regulation of IL-1α, IL-1β, IL-6, TNF-α, MCP-1, macrophage inflammatory proteins-1 alpha (MIP1α) markers in Staphylococcus aureus species. Streptococcus agalactiae or Group B Streptococcus toxins (GBS) enhance IL-8 and ICAM-1 as well as nitric oxide (NO) production from endothelial cells via the expression of inducible nitric oxide synthase (iNOS) enhancement, resulting in BBB disruption. While Gram-negative bacteria, Haemophilus influenza OmpP2 is able to target the common carboxy-terminal domain of LR to start initial interaction with brain endothelium, then invade the brain. H. influenza type b (HiB), can induce BBB permeability through TJ disruption. LR and PAFR binding sites have been recognized as common routes of CNS entrance by Neisseria meningitidis. N. meningitidis species also initiate binding to BMECs and induces AJs deformation, as well as inducing specific cleavage of the TJ component occludin through the release of host MMP-8. Escherichia coli bind to BMECs through LR, resulting in IL-6 and IL-8 release and iNOS production, as well as resulting in disassembly of TJs between endothelial cells, facilitating BBB disruption. Therefore, obtaining knowledge of BBB disruption by different types of bacterial species will provide a picture of how the bacteria enter the central nervous system (CNS) which might support the discovery of therapeutic strategies for each bacteria to control and manage infection.

Characterization of a Two-Component System Transcriptional Regulator, LtdR, That Impacts Group B Streptococcal Colonization and Disease

Streptococcus agalactiae (group B Streptococcus [GBS]) is often a commensal bacterium that colonizes healthy adults asymptomatically and is a frequent inhabitant of the vaginal tract in women. However, in immunocompromised individuals, particularly the newborn, GBS may transition to an invasive pathogen and cause serious disease. Despite the use of the currently recommended intrapartum antibiotic prophylaxis for GBS-positive mothers, GBS remains a leading cause of neonatal septicemia and meningitis. To adapt to the various host environments encountered during its disease cycle, GBS possesses multiple two-component regulatory systems (TCSs). Here we investigated the contribution of a transcriptional regulator containing a LytTR domain, LtdR, to GBS pathogenesis. Disruption of the ltdR gene in the GBS chromosome resulted in a significant increase in bacterial invasion into human cerebral microvascular endothelial cells (hCMEC) in vitro as well as the greater penetration of the blood-brain barrier (BBB) and the development of meningitis in vivo Correspondingly, infection of hCMEC with the ΔltdR mutant resulted in increased secretion of the proinflammatory cytokines interleukin-8 (IL-8), CXCL-1, and IL-6. Further, using a mouse model of GBS vaginal colonization, we observed that the ΔltdR mutant was cleared more readily from the vaginal tract and also that infection with the ΔltdR mutant resulted in increased cytokine production from human vaginal epithelial cells. RNA sequencing revealed global transcriptional differences between the ΔltdR mutant and the parental wild-type GBS strain. These results suggest that LtdR regulates many bacterial processes that can influence GBS-host interactions to promote both bacterial persistence and disease progression.

Group B streptococcus exploits vaginal epithelial exfoliation for ascending infection

Thirteen percent of pregnancies result in preterm birth or stillbirth, accounting for fifteen million preterm births and three and a half million deaths annually. A significant cause of these adverse pregnancy outcomes is in utero infection by vaginal microorganisms. To establish an in utero infection, vaginal microbes enter the uterus by ascending infection; however, the mechanisms by which this occurs are unknown. Using both in vitro and murine models of vaginal colonization and ascending infection, we demonstrate how a vaginal microbe, group B streptococcus (GBS), which is frequently associated with adverse pregnancy outcomes, uses vaginal exfoliation for ascending infection. GBS induces vaginal epithelial exfoliation by activation of integrin and β-catenin signaling. However, exfoliation did not diminish GBS vaginal colonization as reported for other vaginal microbes. Rather, vaginal exfoliation increased bacterial dissemination and ascending GBS infection, and abrogation of exfoliation reduced ascending infection and improved pregnancy outcomes. Thus, for some vaginal bacteria, exfoliation promotes ascending infection rather than preventing colonization. Our study provides insight into mechanisms of ascending infection by vaginal microbes.

A cytosol derived factor of Group B streptococcus prevent its invasion into human epithelial cells

Group B streptococcus (GBS) or Streptococcus agalactiae, is an opportunistic pathogen causing a wide range of infections like pneumonia, sepsis, and meningitis in newborn, pregnant women and adults. While this bacterium has adapted well to asymptomatic colonization of adult humans, it still remains a potentially devastating pathogen to susceptible infants. Advances in molecular techniques and refinement of in vitro and in vivo model systems have elucidated key elements of the pathogenic process, from initial attachment to the maternal vaginal epithelium to penetration of the newborn blood-brain barrier. Still, the formidable array of GBS virulence factors makes this bacterium at the forefront of neonatal pathogens. The involvement of bacterial components in the host-pathogen interaction of GBS pathogenesis and its related diseases is not clearly understood. In this study we demonstrated the role of a 39 kDa factor from GBS which plays an important role in the process of its invasion. We found a homogeneous 39 kDa factor from the cytosol of GBS after following a combination of sequential purification steps involving molecular sieving and ion exchange chromatography using ACTA-FPLC system. Its N-terminal sequence showed a homology with xenobiotic response element type transcriptional regulator protein, a 40 kDa protein of Streptococcus. This factor leads to inhibition of GBS invasion in HeLa and A549 cells. This protein also showed sensitivity and specific cross reactivity with the antibodies raised against it in New Zealand white rabbits by western immunoblotting. This inhibitory factor was further confirmed tolerant for its cytotoxicity. These results add a novel aspect to bacterial pathogenesis where bacteria's own intracellular protein component can act as a potential therapeutic candidate by decreasing the severity of disease thus promoting its invasion inhibition.

Modeling Group B Streptococcus and Blood-Brain Barrier Interaction by Using Induced Pluripotent Stem Cell-Derived Brain Endothelial Cells

Bacterial meningitis is a serious infection of the central nervous system (CNS) that occurs after bacteria interact with and penetrate the blood-brain barrier (BBB). The BBB is comprised of highly specialized brain microvascular endothelial cells (BMECs) that function to separate the circulation from the CNS and act as a formidable barrier for toxins and pathogens. Certain bacteria, such as Streptococcus agalactiae (group B Streptococcus [GBS]), possess the ability to interact with and penetrate the BBB to cause meningitis. Modeling bacterial interaction with the BBB in vitro has been limited to primary and immortalized BMEC culture. While useful, these cells often do not retain BBB-like properties, and human primary cells have limited availability. Recently, a human induced pluripotent stem cell (iPSC)-derived BMEC model has been established that is readily renewable and retains key BBB phenotypes. Here, we sought to evaluate whether the iPSC-derived BMECs were appropriate for modeling bacterial interaction with the BBB. Using GBS as a model meningeal pathogen, we demonstrate that wild-type GBS adhered to, invaded, and activated the iPSC-derived BMECs, while GBS mutants known to have diminished BBB interaction were attenuated in the iPSC-derived model. Furthermore, bacterial infection resulted in the disruption of tight junction components ZO-1, occludin, and claudin-5. Thus, we show for the first time that the iPSC-derived BBB model can be utilized to study BBB interaction with a bacterial CNS pathogen. IMPORTANCE Here for the first time, human iPSC-derived BMECs were used to model bacterial interaction with the BBB. Unlike models previously used to study these interactions, iPSC-derived BMECs possess robust BBB properties, such as the expression of complex tight junctions that are key components for the investigation of bacterial effects on the BBB. Here, we demonstrated that GBS interacts with the iPSC-derived BMECs and specifically disrupts these tight junctions. Thus, using this BBB model may allow researchers to uncover novel mechanisms of BBB disruption during meningitis that are inaccessible to immortalized or primary cell models that lack substantial tight junctions.

Quantitative assessment of the blood-brain barrier opening caused by Streptococcus agalactiae hyaluronidase in a BALB/c mouse model

Streptococcus agalactiae is a pathogen causing meningitis in animals and humans. However, little is known about the entry of S. agalactiae into brain tissue. In this study, we developed a BALB/c mouse model based on the intravenous injection of β-galactosidase-positive Escherichia coli M5 as an indicator of blood-brain barrier (BBB) opening. Under physiological conditions, the BBB is impermeable to E. coli M5. In pathological conditions caused by S. agalactiae, E. coli M5 is capable of penetrating the brain through a disrupted BBB. The level of BBB opening can be assessed by quantitative measurement of E. coli M5 loads per gram of brain tissue. Further, we used the model to evaluate the role of S. agalactiae hyaluronidase in BBB opening. The inactivation of hylB gene encoding a hyaluronidase, HylB, resulted in significantly decreased E. coli M5 colonization, and the intravenous injection of purified HylB protein induced BBB opening in a dose-dependent manner. This finding verified the direct role of HylB in BBB invasion and traversal, and further demonstrated the practicability of the in vivo mouse model established in this study. This model will help to understand the S. agalactiae-host interactions that are involved in this bacterial traversal of the BBB and to develop efficacious strategies to prevent central nervous system infections.

Strains of Group B streptococci from septic patients induce platelet activation via Toll-like Receptor 2

Group B Streptococcus (GBS) causes life-threatening bacterial sepsis, especially in newborns and pregnant women. Patients suffering from sepsis often display low platelet counts, characterized by thrombocytopenia, because of platelet activation. In the present study, the roles of six GBS strains from septic patients in platelet aggregation, as well as the underlying mechanisms, were investigated. Incubation of platelets with three of the strains induced platelet aggregation, increased the secretion of cellular adhesin molecule CD62P and activation of GPIIb/IIIa. Furthermore, the GBS strains that induced platelet activation also caused an increase in the expression of Toll-like receptor (TLR) 2 in platelets. Pre-incubation of platelets with anti-TLR2 monoclonal antibody, but not anti-TLR4 monoclonal antibody, inhibited these functional responses induced by GBS. TLR2 stimulation also activated the phosphoinositide 3-kinase (PI3-K)/Akt signalling pathway in platelets, and inhibition of PI3-K significantly reduced GBS-induced platelet responses. Our results indicate that three of the GBS strains from the septic patients can trigger platelet activation by interacting with platelets, which involves the elevation of platelet TLR2 expression.

Perinatal Group B Streptococcal Infections: Virulence Factors, Immunity, and Prevention Strategies

Group B streptococcus (GBS) or Streptococcus agalactiae is a β-hemolytic, Gram-positive bacterium that is a leading cause of neonatal infections. GBS commonly colonizes the lower gastrointestinal and genital tracts and, during pregnancy, neonates are at risk of infection. Although intrapartum antibiotic prophylaxis during labor and delivery has decreased the incidence of early-onset neonatal infection, these measures do not prevent ascending infection that can occur earlier in pregnancy leading to preterm births, stillbirths, or late-onset neonatal infections. Prevention of GBS infection in pregnancy is complex and is likely influenced by multiple factors, including pathogenicity, host factors, vaginal microbiome, false-negative screening, and/or changes in antibiotic resistance. A deeper understanding of the mechanisms of GBS infections during pregnancy will facilitate the development of novel therapeutics and vaccines. Here, we summarize and discuss important advancements in our understanding of GBS vaginal colonization, ascending infection, and preterm birth.

Human milk oligosaccharides inhibit growth of group B Streptococcus

Streptococcus agalactiae (group B Streptococcus, GBS) is a leading cause of invasive bacterial infections in newborns, typically acquired vertically during childbirth secondary to maternal vaginal colonization. Human milk oligosaccharides (HMOs) have important nutritional and biological activities that guide the development of the immune system of the infant and shape the composition of normal gut microbiota. In this manner, HMOs help protect against pathogen colonization and reduce the risk of infection. In the course of our studies of HMO-microbial interactions, we unexpectedly uncovered a novel HMO property to directly inhibit the growth of GBS independent of host immunity. By separating different HMO fractions through multidimensional chromatography, we found the bacteriostatic activity to be confined to specific non-sialylated HMOs and synergistic with a number of conventional antibiotic agents. Phenotypic screening of a GBS transposon insertion library identified a mutation within a GBS-specific gene encoding a putative glycosyltransferase that confers resistance to HMOs, suggesting that HMOs may function as an alternative substrate to modify a GBS component in a manner that impairs growth kinetics. Our study uncovers a unique antibacterial role for HMOs against a leading neonatal pathogen and expands the potential therapeutic utility of these versatile molecules.

A journey into the brain: insight into how bacterial pathogens cross blood-brain barriers

The blood-brain barrier, which is one of the tightest barriers in the body, protects the brain from insults, such as infections. Indeed, only a few of the numerous blood-borne bacteria can cross the blood-brain barrier to cause meningitis. In this Review, we focus on invasive extracellular pathogens, such as Neisseria meningitidis, Streptococcus pneumoniae, group B Streptococcus and Escherichia coli, to review the obstacles that bacteria have to overcome in order to invade the meninges from the bloodstream, and the specific skills they have developed to bypass the blood-brain barrier. The medical importance of understanding how these barriers can be circumvented is underlined by the fact that we need to improve drug delivery into the brain.

Community-acquired bacterial meningitis

Meningitis is an inflammation of the meninges and subarachnoid space that can also involve the brain cortex and parenchyma. It can be acquired spontaneously in the community - community-acquired bacterial meningitis - or in the hospital as a complication of invasive procedures or head trauma (nosocomial bacterial meningitis). Despite advances in treatment and vaccinations, community-acquired bacterial meningitis remains one of the most important infectious diseases worldwide. Streptococcus pneumoniae and Neisseria meningitidis are the most common causative bacteria and are associated with high mortality and morbidity; vaccines targeting these organisms, which have designs similar to the successful vaccine that targets Haemophilus influenzae type b meningitis, are now being used in many routine vaccination programmes. Experimental and genetic association studies have increased our knowledge about the pathogenesis of bacterial meningitis. Early antibiotic treatment improves the outcome, but the growing emergence of drug resistance as well as shifts in the distribution of serotypes and groups are fuelling further development of new vaccines and treatment strategies. Corticosteroids were found to be beneficial in high-income countries depending on the bacterial species. Further improvements in the outcome are likely to come from dampening the host inflammatory response and implementing preventive measures, especially the development of new vaccines.

Cell wall glycopolymers of Firmicutes and their role as nonprotein adhesins

Cell wall glycopolymers (CWGs) of gram-positive bacteria have gained increasing interest with respect to their role in colonization and infection. In most gram-positive pathogens they constitute a large fraction of the cell wall biomass and represent major cell envelope determinants. Depending on their chemical structure they modulate interaction with complement factors and play roles in immune evasion or serve as nonprotein adhesins that mediate, especially under dynamic conditions, attachment to different host cell types. In particular, covalently peptidoglycan-attached CWGs that extend well above the cell wall seem to interact with glyco-receptors on host cell surfaces. For example, in the case of Staphylococcus aureus, the cell wall-attached teichoic acid (WTA) has been identified as a major CWG adhesin. A recent report indicates that a type-F scavenger receptor, termed SR-F1 (SREC-I), is the predominant WTA receptor in the nasal cavity and that WTA-SREC-I interaction plays an important role in S. aureus nasal colonization. Therefore, understanding the role of CWGs in complex processes that mediate colonization and infection will allow novel insights into the mechanisms of host-microbiota interaction.

Comprehensive identification and profiling of Nile tilapia (Oreochromis niloticus) microRNAs response to Streptococcus agalactiae infection through high-throughput sequencing

MicroRNAs are a kind of small non-coding RNAs that participate in various biological processes. Deregulated microRNA expression is associated with several types of diseases. Tilapia (Oreochromis niloticus) is an important commercial fish species in China. To identify miRNAs and investigate immune-related miRNAs of O. niloticus, we applied high-throughput sequencing technology to identify and analyze miRNAs from tilapia infected with Streptococcus agalactiae at a timescale of 72 h divided into six different time points. The results showed that a total of 3009 tilapia miRNAs were identified, including in 1121 miRNAs which have homologues in the currently available databases and 1878 novel miRNAs. The expression levels of 218 tilapia miRNAs were significantly altered at 6 h-72 h post-bacterial infection (pi), and these miRNAs were therefore classified as differentially expressed tilapia miRNAs. For the 1121 differentially expressed tilapia miRNAs target 41961 genes. GO and KEGG enrichment analysis revealed that some target genes of tilapia miRNAs were grouped mainly into the categories of apoptotic process, signal pathway, and immune response. This is the first report of comprehensive identification of O. niloticus miRNAs being differentially regulated in spleen in normal conditions relating to S. agalactiae infection. This work provides an opportunity for further understanding of the molecular mechanisms of miRNA regulation in O. niloticus host-pathogen interactions.

Cellular Microbiology of Mycoplasma canis

Mycoplasma canis can infect many mammalian hosts but is best known as a commensal or opportunistic pathogen of dogs. The unexpected presence of M. canis in brains of dogs with idiopathic meningoencephalitis prompted new in vitro studies to help fill the void of basic knowledge about the organism's candidate virulence factors, the host responses that it elicits, and its potential roles in pathogenesis. Secretion of reactive oxygen species and sialidase varied quantitatively (P < 0.01) among strains of M. canis isolated from canine brain tissue or mucosal surfaces. All strains colonized the surface of canine MDCK epithelial and DH82 histiocyte cells and murine C8-D1A astrocytes. Transit through MDCK and DH82 cells was demonstrated by gentamicin protection assays and three-dimensional immunofluorescence imaging. Strains further varied (P < 0.01) in the extents to which they influenced the secretion of tumor necrosis factor alpha (TNF-α) and the neuroendocrine regulatory peptide endothelin-1 by DH82 cells. Inoculation with M. canis also decreased major histocompatibility complex class II (MHC-II) antigen expression by DH82 cells (P < 0.01), while secretion of gamma interferon (IFN-γ), interleukin-6 (IL-6), interleukin-10 (IL-10), and complement factor H was unaffected. The basis for differences in the responses elicited by these strains was not obvious in their genome sequences. No acute cytopathic effects on any homogeneous cell line, or consistent patterns of M. canis polyvalent antigen distribution in canine meningoencephalitis case brain tissues, were apparent. Thus, while it is not likely a primary neuropathogen, M. canis has the capacity to influence meningoencephalitis through complex interactions within the multicellular and neurochemical in vivo milieu.

The essential genome of Streptococcus agalactiae

BACKGROUND

Next-generation sequencing of transposon-genome junctions from a saturated bacterial mutant library (Tn-seq) is a powerful tool that permits genome-wide determination of the contribution of genes to fitness of the organism under a wide range of experimental conditions. We report development, testing, and results from a Tn-seq system for use in Streptococcus agalactiae (group B Streptococcus; GBS), an important cause of neonatal sepsis.

METHODS

Our method uses a Himar1 mini-transposon that inserts at genomic TA dinucleotide sites, delivered to GBS on a temperature-sensitive plasmid that is subsequently cured from the bacterial population. In order to establish the GBS essential genome, we performed Tn-seq on DNA collected from three independent mutant libraries-with at least 135,000 mutants per library-at serial 24 h time points after outgrowth in rich media.

RESULTS

After statistical analysis of transposon insertion density and distribution, we identified 13.5 % of genes as essential and 1.2 % as critical, with high levels of reproducibility. Essential and critical genes are enriched for fundamental cellular housekeeping functions, such as acyl-tRNA biosynthesis, nucleotide metabolism, and glycolysis. We further validated our system by comparing fitness assignments of homologous genes in GBS and a close bacterial relative, Streptococcus pyogenes, which demonstrated 93 % concordance. Finally, we used our fitness assignments to identify signal transduction pathway components predicted to be essential or critical in GBS.

CONCLUSIONS

We believe that our baseline fitness assignments will be a valuable tool for GBS researchers and that our system has the potential to reveal key pathogenesis gene networks and potential therapeutic/preventative targets.

Identification of CiaR Regulated Genes That Promote Group B Streptococcal Virulence and Interaction with Brain Endothelial Cells

Group B Streptococcus (GBS) is a major causative agent of neonatal meningitis due to its ability to efficiently cross the blood-brain barrier (BBB) and enter the central nervous system (CNS). It has been demonstrated that GBS can invade human brain microvascular endothelial cells (hBMEC), a primary component of the BBB; however, the mechanism of intracellular survival and trafficking is unclear. We previously identified a two component regulatory system, CiaR/H, which promotes GBS intracellular survival in hBMEC. Here we show that a GBS strain deficient in the response regulator, CiaR, localized more frequently with Rab5, Rab7 and LAMP1 positive vesicles. Further, lysosomes isolated from hBMEC contained fewer viable bacteria following initial infection with the ΔciaR mutant compared to the WT strain. To characterize the contribution of CiaR-regulated genes, we constructed isogenic mutant strains lacking the two most down-regulated genes in the CiaR-deficient mutant, SAN_2180 and SAN_0039. These genes contributed to bacterial uptake and intracellular survival. Furthermore, competition experiments in mice showed that WT GBS had a significant survival advantage over the Δ2180 and Δ0039 mutants in the bloodstream and brain.

Molecular Cloning and Expression Analysis of IgD in Nile Tilapia (Oreochromis niloticus) in Response to Streptococcus agalactiae Stimulus

IgD is considered to be a recently-evolved Ig and a puzzling molecule, being previously found in all vertebrate taxa, except for birds. Although IgD likely plays an important role in vertebrate immune responses, the function of IgD in Nile tilapia (Oreochromis niloticus) is virtually unknown. In the present study, a membrane form of IgD (mIgD) heavy chains were cloned from the GIFT strain of Nile tilapia (designated On-mIgD). The On-mIgD heavy chain’s cDNA is composed of 3347 bp with a 31 bp of 5′-UTR, 3015 bp open reading frame (ORF) and 301 bp 3′-UTR, encoding a polypeptide of 1004 amino acids (GenBank accession no: KF530821). Phylogenetic analysis revealed that On-mIgD heavy chains showed the highest similarity to Siniperca chuatsi. Quantitative real-time PCR (qRT-PCR) analysis showed that On-mIgD expression occurred predominately in head kidney, thymus, spleen, and kidney. After Streptococcus agalactiae infection, transcripts of On-mIgD increased and reached its peak at 48 h in the head kidney and thymus, and 72 h in the spleen, respectively. Taken together, these results collectively indicated that IgD could possibly have a key role to play in the immune response when bacterial infections in Nile tilapia.

Molecular characterization and expression of CD2 in Nile tilapia (Oreochromis niloticus) in response to Streptococcus agalactiae stimulus

The cluster of differentiation 2 (CD2), functioning as a cell adhesion and costimulatory molecule, plays a crucial role in T-cell activation. In this paper, the CD2 gene of Nile tilapia, Oreochromis niloticus (designated as On-CD2) was cloned and its expression pattern under the stimulation of Streptococcus agalactiae was investigated. Sequence analysis showed On-CD2 protein consists of two extracellular Ig-like domains, a transmembrane region, and a long proline-rich cytoplasmic tail, which is a hallmark of CD2, and several important structural characteristics required for T-cell activation were detected in the deduced amino acid sequence of On-CD2. In healthy tilapia, the On-CD2 transcripts were mainly detected in the head kidney, spleen, blood and thymus. Moreover, there was a clear time-dependent expression pattern of On-CD2 after immunized by formalin-inactivated S. agalactiae and the expression reached the highest level at 12 h in the brain and head kidney, 48 h in the spleen, and 72 h in the thymus, respectively. This is the first report on the expression of CD2 induced by bacteria vaccination in teleosts. These findings indicated that On-CD2 may play an important role in the immune response to intracellular bacteria in Nile tilapia.

Host-pathogen interactions in bacterial meningitis

Bacterial meningitis is a devastating disease occurring worldwide with up to half of the survivors left with permanent neurological sequelae. Due to intrinsic properties of the meningeal pathogens and the host responses they induce, infection can cause relatively specific lesions and clinical syndromes that result from interference with the function of the affected nervous system tissue. Pathogenesis is based on complex host–pathogen interactions, some of which are specific for certain bacteria, whereas others are shared among different pathogens. In this review, we summarize the recent progress made in understanding the molecular and cellular events involved in these interactions. We focus on selected major pathogens, Streptococcus pneumonia, S. agalactiae (Group B Streptococcus), Neisseria meningitidis, and Escherichia coli K1, and also include a neglected zoonotic pathogen, Streptococcus suis. These neuroinvasive pathogens represent common themes of host–pathogen interactions, such as colonization and invasion of mucosal barriers, survival in the blood stream, entry into the central nervous system by translocation of the blood–brain and blood–cerebrospinal fluid barrier, and induction of meningeal inflammation, affecting pia mater, the arachnoid and subarachnoid spaces.

Functional characterization and molecular modelling of FnFgBP, a surface protein from Streptococcus agalactiae

GBS1263 (FnFgBP) exhibits dual-ligand (fibronectin and fibrinogen) binding property. Molecular modeling of FnFgBP is suggestive of a unique ligand binding mechanism.

Beta-Lactamase Repressor BlaI Modulates Staphylococcus aureus Cathelicidin Antimicrobial Peptide Resistance and Virulence

BlaI is a repressor of BlaZ, the beta-lactamase responsible for penicillin resistance in Staphylococcus aureus. Through screening a transposon library in S. aureus Newman for susceptibility to cathelicidin antimicrobial peptide, we discovered BlaI as a novel cathelicidin resistance factor. Additionally, through integrational mutagenesis in S. aureus Newman and MRSA Sanger 252 strains, we confirmed the role of BlaI in resistance to human and murine cathelidicin and showed that it contributes to virulence in human whole blood and murine infection models. We further demonstrated that BlaI could be a target for innate immune-based antimicrobial therapies; by removing BlaI through subinhibitory concentrations of 6-aminopenicillanic acid, we were able to sensitize S. aureus to LL-37 killing.

Group B streptococcal infection and activation of human astrocytes

Background

Streptococcus agalactiae (Group B Streptococcus, GBS) is the leading cause of life-threatening meningitis in human newborns in industrialized countries. Meningitis results from neonatal infection that occurs when GBS leaves the bloodstream (bacteremia), crosses the blood-brain barrier (BBB), and enters the central nervous system (CNS), where the bacteria contact the meninges. Although GBS is known to invade the BBB, subsequent interaction with astrocytes that physically associate with brain endothelium has not been well studied.

Methodology/Principal Findings

We hypothesize that human astrocytes play a unique role in GBS infection and contribute to the development of meningitis. To address this, we used a well- characterized human fetal astrocyte cell line, SVG-A, and examined GBS infection in vitro. We observed that all GBS strains of representative clinically dominant serotypes (Ia, Ib, III, and V) were able to adhere to and invade astrocytes. Cellular invasion was dependent on host actin cytoskeleton rearrangements, and was specific to GBS as Streptococcus gordonii failed to enter astrocytes. Analysis of isogenic mutant GBS strains deficient in various cell surface organelles showed that anchored LTA, serine-rich repeat protein (Srr1) and fibronectin binding (SfbA) proteins all contribute to host cell internalization. Wild-type GBS also displayed an ability to persist and survive within an intracellular compartment for at least 12 h following invasion. Moreover, GBS infection resulted in increased astrocyte transcription of interleukin (IL)-1β, IL-6 and VEGF.

Conclusions/Significance

This study has further characterized the interaction of GBS with human astrocytes, and has identified the importance of specific virulence factors in these interactions. Understanding the role of astrocytes during GBS infection will provide important information regarding BBB disruption and the development of neonatal meningitis.

Bacterial induction of Snail1 contributes to blood-brain barrier disruption

Bacterial meningitis is a serious infection of the CNS that results when blood-borne bacteria are able to cross the blood-brain barrier (BBB). Group B Streptococcus (GBS) is the leading cause of neonatal meningitis; however, the molecular mechanisms that regulate bacterial BBB disruption and penetration are not well understood. Here, we found that infection of human brain microvascular endothelial cells (hBMECs) with GBS and other meningeal pathogens results in the induction of host transcriptional repressor Snail1, which impedes expression of tight junction genes. Moreover, GBS infection also induced Snail1 expression in murine and zebrafish models. Tight junction components ZO-1, claudin 5, and occludin were decreased at both the transcript and protein levels in hBMECs following GBS infection, and this repression was dependent on Snail1 induction. Bacteria-independent Snail1 expression was sufficient to facilitate tight junction disruption, promoting BBB permeability to allow bacterial passage. GBS induction of Snail1 expression was dependent on the ERK1/2/MAPK signaling cascade and bacterial cell wall components. Finally, overexpression of a dominant-negative Snail1 homolog in zebrafish elevated transcription of tight junction protein-encoding genes and increased zebrafish survival in response to GBS challenge. Taken together, our data support a Snail1-dependent mechanism of BBB disruption and penetration by meningeal pathogens.

Molecular and functional characterization of CD59 from Nile tilapia (Oreochromis niloticus) involved in the immune response to Streptococcus agalactiae

CD59, the major inhibitor of membrane attack complex, plays a crucial role in regulation of complement activation. In this paper, a CD59 gene of Nile tilapia, Oreochromis niloticus (designated as On-CD59) was cloned and its expression pattern under the stimulation of Streptococcus agalactiae was investigated. Sequence analysis showed main structural features required for complement-inhibitory activity were detected in the deduced amino acid sequence of On-CD59. In healthy Nile tilapia, the On-CD59 transcripts could be detected in all the examined tissues, with the most abundant expression in the brain. When immunized with inactivated S. agalactiae, there was a clear time-dependent expression pattern of On-CD59 in the skin, brain, head kidney, thymus and spleen, with quite different kinetic expressions. The assays for the complement-inhibitory activity suggested that recombinant On-CD59 protein had a species-selective inhibition of complement. Moreover, our works showed that recombinant On-CD59 protein may possess both binding activities to PGN and LTA and inhibiting activity of S. agalactiae. These findings indicated that On-CD59 may play important roles in the immune response to S. agalactiae in Nile tilapia.

Streptococcus agalactiae infection in zebrafish larvae

Streptococcus agalactiae (Group B Streptococcus, GBS) is an encapsulated, Gram-positive bacterium that is a leading cause of neonatal pneumonia, sepsis and meningitis, and an emerging aquaculture pathogen. The zebrafish (Danio rerio) is a genetically tractable model vertebrate that has been used to analyze the pathogenesis of both aquatic and human bacterial pathogens. We have developed a larval zebrafish model of GBS infection to study bacterial and host factors that contribute to disease progression. GBS infection resulted in dose dependent larval death, and GBS serotype III, ST-17 strain was observed as the most virulent. Virulence was dependent on the presence of the GBS capsule, surface anchored lipoteichoic acid (LTA) and toxin production, as infection with GBS mutants lacking these factors resulted in little to no mortality. Additionally, interleukin-1β (il1b) and CXCL-8 (cxcl8a) were significantly induced following GBS infection compared to controls. We also visualized GBS outside the brain vasculature, suggesting GBS penetration into the brain during the course of infection. Our data demonstrate that zebrafish larvae are a valuable model organism to study GBS pathogenesis.

The sensor histidine kinase RgfC affects group B streptococcal virulence factor expression independent of its response regulator RgfA

Group B streptococci (GBS; Streptococcus agalactiae) are beta-hemolytic, Gram-positive bacteria that are common asymptomatic colonizers of healthy adults. However, these opportunistic bacteria also cause invasive infections in human newborns and in certain adult populations. To adapt to the various environments encountered during its disease cycle, GBS encodes a number of two-component signaling systems. Previous studies have indicated that the TCS comprising the sensor histidine kinase RgfC and the response regulator RgfA mediate GBS binding to extracellular matrix components, such as fibrinogen. However, in certain GBS clinical isolates, a point mutation in rgfA results in premature truncation of the response regulator. The truncated RgfA protein lacks the C-terminal DNA binding domain necessary for promoter binding and gene regulation. Here, we show that deletion of rgfC in GBS strains lacking a functional RgfA increased systemic infection. Furthermore, infection with the rgfC mutant increased induction of proinflammatory signaling pathways in vivo. Phosphoproteomic analysis revealed that 19 phosphopeptides corresponding to 12 proteins were differentially phosphorylated at aspartate, cysteine, serine, threonine, or tyrosine residues in the rgfC mutant. This included aspartate phosphorylation of a tyrosine kinase, CpsD, and a transcriptional regulator. Consistent with this observation, microarray analysis of the rgfC mutant indicated that >200 genes showed altered expression compared to the isogenic wild-type strain and included transcriptional regulators, transporters, and genes previously associated with GBS pathogenesis. Our observations suggest that in the absence of RgfA, nonspecific RgfC signaling affects the expression of virulence factors and GBS pathogenesis.

The role of autophagy during group B Streptococcus infection of blood-brain barrier endothelium

Bacterial meningitis occurs when bloodborne pathogens invade and penetrate the blood-brain barrier (BBB), provoking inflammation and disease. Group B Streptococcus (GBS), the leading cause of neonatal meningitis, can enter human brain microvascular endothelial cells (hBMECs), but the host response to intracellular GBS has not been characterized. Here we sought to determine whether antibacterial autophagy, which involves selective recognition of intracellular organisms and their targeting to autophagosomes for degradation, is activated in BBB endothelium during bacterial infection. GBS infection resulted in increased punctate distribution of GFP-microtubule-associated protein 1 light chain 3 (LC3) and increased levels of endogenous LC3-II and p62 turnover, two hallmark indicators of active autophagic flux. Infection with GBS mutants revealed that bacterial invasion and the GBS pore-forming β-hemolysin/cytolysin (β-h/c) trigger autophagic activation. Cell-free bacterial extracts containing β-h/c activity induced LC3-II conversion, identifying this toxin as a principal provocative factor for autophagy activation. These results were confirmed in vivo using a mouse model of GBS meningitis as infection with WT GBS induced autophagy in brain tissue more frequently than a β-h/c-deficient mutant. Elimination of autophagy using Atg5-deficient fibroblasts or siRNA-mediated impairment of autophagy in hBMECs led to increased recovery of intracellular GBS. However, electron microscopy revealed that GBS was rarely found within double membrane autophagic structures even though we observed GBS-LC3 co-localization. These results suggest that although autophagy may act as a BBB cellular defense mechanism in response to invading and toxin-producing bacteria, GBS may actively thwart the autophagic pathway.

Pathogens penetrating the central nervous system: infection pathways and the cellular and molecular mechanisms of invasion

The brain is well protected against microbial invasion by cellular barriers, such as the blood-brain barrier (BBB) and the blood-cerebrospinal fluid barrier (BCSFB). In addition, cells within the central nervous system (CNS) are capable of producing an immune response against invading pathogens. Nonetheless, a range of pathogenic microbes make their way to the CNS, and the resulting infections can cause significant morbidity and mortality. Bacteria, amoebae, fungi, and viruses are capable of CNS invasion, with the latter using axonal transport as a common route of infection. In this review, we compare the mechanisms by which bacterial pathogens reach the CNS and infect the brain. In particular, we focus on recent data regarding mechanisms of bacterial translocation from the nasal mucosa to the brain, which represents a little explored pathway of bacterial invasion but has been proposed as being particularly important in explaining how infection with Burkholderia pseudomallei can result in melioidosis encephalomyelitis.

Association and virulence gene expression vary among serotype III group B streptococcus isolates following exposure to decidual and lung epithelial cells

Group B Streptococcus (GBS) causes severe disease in neonates, the elderly, and immunocompromised individuals. GBS species are highly diverse and can be classified by serotype and multilocus sequence typing. Sequence type 17 (ST-17) strains cause invasive neonatal disease more frequently than strains of other STs. Attachment and invasion of host cells are key steps in GBS pathogenesis. We investigated whether four serotype III strains representing ST-17 (two strains), ST-19, and ST-23 differ in their abilities to attach to and invade both decidual cells and lung epithelial cells. Virulence gene expression following host cell association and exposure to amnion cells was also tested. The ST-17 strains differed in their abilities to attach to and invade decidual cells, whereas there were no differences with lung epithelial cells. The ST-19 and ST-23 strains, however, attached to and invaded decidual cells less than both ST-17 strains. Although the ST-23 strain attached to lung epithelial cells better than ST-17 and -19 strains, none of the strains effectively invaded the lung epithelial cells. Notably, the association with host cells resulted in the differential expression of several virulence genes relative to basal expression levels. Similar expression patterns of some genes were observed regardless of cell type used. Collectively, these results show that GBS strains differ in their abilities to attach to distinct host cell types and express key virulence genes that are relevant to the disease process. Enhancing our understanding of pathogenic mechanisms could aid in the identification of novel therapeutic targets or vaccine candidates that could potentially decrease morbidity and mortality associated with neonatal infections.

Invasion of the central nervous system by Cryptococcus neoformans requires a secreted fungal metalloprotease

Cryptococcus spp. cause life-threatening fungal infection of the central nervous system (CNS), predominantly in patients with a compromised immune system. Why Cryptococcus neoformans has this remarkable tropism for the CNS is not clear. Recent research on cerebral pathogenesis of C. neoformans revealed a predominantly transcellular migration of cryptococci across the brain endothelium; however, the identities of key fungal virulence factors that function specifically to invade the CNS remain unresolved. Here we found that a novel, secreted metalloprotease (Mpr1) that we identified in the extracellular proteome of C. neoformans (CnMpr1) is required for establishing fungal disease in the CNS. Mpr1 belongs to a poorly characterized M36 class of fungalysins that are expressed in only some fungal species. A strain of C. neoformans lacking the gene encoding Mpr1 (mpr1Δ) failed to breach the endothelium in an in vitro model of the human blood-brain barrier (BBB). A mammalian host infected with the mpr1Δ null strain demonstrated significant improvement in survival due to a reduced brain fungal burden and lacked the brain pathology commonly associated with cryptococcal disease. The in vivo studies further indicate that Mpr1 is not required for fungal dissemination and Mpr1 likely targets the brain endothelium specifically. Remarkably, the sole expression of CnMPR1 in Saccharomyces cerevisiae resulted in a robust migration of yeast cells across the brain endothelium, demonstrating Mpr1's specific activity in breaching the BBB and suggesting that Mpr1 may function independently of the hyaluronic acid-CD44 pathway. This distinct role for Mpr1 may develop into innovative treatment options and facilitate a brain-specific drug delivery platform.

IMPORTANCE

Cryptococcus neoformans is a medically relevant fungal pathogen causing significant morbidity and mortality, particularly in immunocompromised individuals. An intriguing feature is its strong neurotropism, and consequently the hallmark of cryptococcal disease is a brain infection, cryptococcal meningoencephalitis. For C. neoformans to penetrate the central nervous system (CNS), it first breaches the blood-brain barrier via a transcellular pathway; however, the identities of fungal factors required for this transmigration remain largely unknown. In an effort to identify extracellular fungal proteins that could mediate interactions with the brain endothelium, we undertook a proteomic analysis of the extracellular proteome and identified a secreted metalloprotease (Mpr1) belonging to the M36 class of fungalysins. Here we found that Mpr1 promotes migration of C. neoformans across the brain endothelium and into the CNS by facilitating attachment of cryptococci to the endothelium surface, thus underscoring the critical role of M36 proteases in fungal pathogenesis.

Analysis of two-component systems in group B Streptococcus shows that RgfAC and the novel FspSR modulate virulence and bacterial fitness

Group B Streptococcus (GBS), in the transition from commensal organisms to pathogens, will encounter diverse host environments and, thus, require coordinated control of the transcriptional responses to these changes. This work was aimed at better understanding the role of two-component signal transduction systems (TCS) in GBS pathophysiology through a systematic screening procedure. We first performed a complete inventory and sensory mechanism classification of all putative GBS TCS by genomic analysis. Five TCS were further investigated by the generation of knockout strains, and in vitro transcriptome analysis identified genes regulated by these systems, ranging from 0.1% to 3% of the genome. Interestingly, two sugar phosphotransferase systems appeared to be differentially regulated in the TCS-16 knockout strain (TCS loci were numbered in order of their appearance on the chromosome), suggesting an involvement in monitoring carbon source availability. High-throughput analysis of bacterial growth on different carbon sources showed that TCS-16 was necessary for the growth of GBS on fructose-6-phosphate. Additional transcriptional analysis provided further evidence for a stimulus-response circuit where extracellular fructose-6-phosphate leads to autoinduction of TCS-16, with concomitant dramatic upregulation of the adjacent operon, which encodes a phosphotransferase system. The TCS-16-deficient strain exhibited decreased persistence in a model of vaginal colonization. All mutant strains were also characterized in a murine model of systemic infection, and inactivation of TCS-17 (also known as RgfAC) resulted in hypervirulence. Our data suggest a role for the previously unknown TCS-16, here named FspSR, in bacterial fitness and carbon metabolism during host colonization, and the data also provide experimental evidence for TCS-17/RgfAC involvement in virulence.

Two-component systems have been evolved by bacteria to detect environmental changes, and they play key roles in pathogenicity. A comprehensive analysis of TCS in GBS has not been performed previously. In this work, we classify 21 TCS and present evidence for the involvement of two specific TCS in GBS virulence and colonization in vivo. Although pinpointing specific TCS stimuli is notoriously difficult, we used a combination of techniques to identify two systems with different effects on GBS pathogenesis. For one of the systems, we propose that fructose-6-phosphate, a metabolite in glycolysis, is sufficient to induce a regulatory response involving a sugar transport system. Our catalogue and classification of TCS may guide further studies into the role of TCS in GBS pathogenicity and biology.

Differential localization of LTA synthesis proteins and their interaction with the cell division machinery in Staphylococcus aureus

Lipoteichoic acid (LTA) is an important cell wall component of Gram-positive bacteria. In Staphylococcus aureus it consists of a polyglycerolphosphate-chain that is retained within the membrane via a glycolipid. Using an immunofluorescence approach, we show here that the LTA polymer is not surface exposed in S. aureus, as it can only be detected after digestion of the peptidoglycan layer. S. aureus mutants lacking LTA are enlarged and show aberrant positioning of septa, suggesting a link between LTA synthesis and the cell division process. Using a bacterial two-hybrid approach, we show that the three key LTA synthesis proteins, YpfP and LtaA, involved in glycolipid production, and LtaS, required for LTA backbone synthesis, interact with one another. All three proteins also interacted with numerous cell division and peptidoglycan synthesis proteins, suggesting the formation of a multi-enzyme complex and providing further evidence for the co-ordination of these processes. When assessed by fluorescence microscopy, YpfP and LtaA fluorescent protein fusions localized to the membrane while the LtaS enzyme accumulated at the cell division site. These data support a model whereby LTA backbone synthesis proceeds in S. aureus at the division site in co-ordination with cell division, while glycolipid synthesis takes place throughout the membrane.

Identification of a group B streptococcal fibronectin binding protein, SfbA, that contributes to invasion of brain endothelium and development of meningitis

Group B Streptococcus (GBS) is currently the leading cause of neonatal meningitis. This is due to its ability to survive and multiply in the bloodstream and interact with specialized human brain microvascular endothelial cells (hBMEC), which constitute the blood-brain barrier (BBB). The exact mechanism(s) of GBS-BBB penetration is still largely unknown. We and others have shown that GBS interacts with components of the extracellular matrix. In this study, we demonstrate that GBS of representative serotypes binds immobilized and cell surface fibronectin and identify a putative fibronectin binding protein, streptococcal fibronectin binding protein A (SfbA). Allelic replacement of sfbA in the GBS chromosome resulted in a significant decrease in ability to bind fibronection and invade hBMEC compared with the wild-type (WT) parental strain. Expression of SfbA in the noninvasive strain Lactococcus lactis was sufficient to promote fibronectin binding and hBMEC invasion. Furthermore, the addition of an antifibronectin antibody or an RGD peptide that blocks fibronectin binding to integrins significantly reduced invasion of the WT but not the sfbA-deficient mutant strain, demonstrating the importance of an SfbA-fibronectin-integrin interaction for GBS cellular invasion. Using a murine model of GBS meningitis, we also observed that WT GBS penetrated the brain and established meningitis more frequently than did the ΔsfbA mutant strain. Our data suggest that GBS SfbA plays an important role in bacterial interaction with BBB endothelium and the pathogenesis of streptococcal meningitis.

Group B streptococcal serine-rich repeat proteins promote interaction with fibrinogen and vaginal colonization

Group B streptococcus (GBS) can cause severe disease in susceptible hosts, including newborns, pregnant women, and the elderly. GBS serine-rich repeat (Srr) surface glycoproteins are important adhesins/invasins in multiple host tissues, including the vagina. However, exact molecular mechanisms contributing to their importance in colonization are unknown. We have recently determined that Srr proteins contain a fibrinogen-binding region (BR) and hypothesize that Srr-mediated fibrinogen binding may contribute to GBS cervicovaginal colonization. In this study, we observed that fibrinogen enhanced wild-type GBS attachment to cervical and vaginal epithelium, and that this was dependent on Srr1. Moreover, purified Srr1-BR peptide bound directly to host cells, and peptide administration in vivo reduced GBS recovery from the vaginal tract. Furthermore, a GBS mutant strain lacking only the Srr1 "latching" domain exhibited decreased adherence in vitro and decreased persistence in a mouse model of GBS vaginal colonization, suggesting the importance of Srr-fibrinogen interactions in the female reproductive tract.

Altered lipid composition in Streptococcus pneumoniae cpoA mutants

BACKGROUND

Penicillin-resistance in Streptococcus pneumoniae is mainly due to alterations in genes encoding the target enzymes for beta-lactams, the penicillin-binding proteins (PBPs). However, non-PBP genes are altered in beta-lactam-resistant laboratory mutants and confer decreased susceptibility to beta-lactam antibiotics. Two piperacillin resistant laboratory mutants of Streptococcus pneumoniae R6 contain mutations in the putative glycosyltransferase gene cpoA. The CpoA gene is part of an operon including another putative glycosyltransferase gene spr0982, both of which being homologous to glycolipid synthases present in other Gram-positive bacteria.

RESULTS

We now show that the cpoA mutants as well as a cpoA deletion mutant are defective in the synthesis of galactosyl-glucosyl-diacylglycerol (GalGlcDAG) in vivo consistent with the in vitro function of CpoA as α-GalGlcDAG synthase as shown previously. In addition, the proportion of phosphatidylglycerol increased relative to cardiolipin in cpoA mutants. Moreover, cpoA mutants are more susceptible to acidic stress, have an increased requirement for Mg(2+) at low pH, reveal a higher resistance to lysis inducing conditions and are hypersensitive to bacitracin.

CONCLUSIONS

The data show that deficiency of the major glycolipid GalGlcDAG causes a pleitotropic phenotype of cpoA mutant cells consistent with severe membrane alterations. We suggest that the cpoA mutations selected with piperacillin are directed against the lytic response induced by the beta-lactam antibiotic.

Group B Streptococcus engages an inhibitory Siglec through sialic acid mimicry to blunt innate immune and inflammatory responses in vivo

Group B Streptococcus (GBS) is a common agent of bacterial sepsis and meningitis in newborns. The GBS surface capsule contains sialic acids (Sia) that engage Sia-binding immunoglobulin-like lectins (Siglecs) on leukocytes. Here we use mice lacking Siglec-E, an inhibitory Siglec of myelomonocytic cells, to study the significance of GBS Siglec engagement during in vivo infection. We found GBS bound to Siglec-E in a Sia-specific fashion to blunt NF-κB and MAPK activation. As a consequence, Siglec-E-deficient macrophages had enhanced pro-inflammatory cytokine secretion, phagocytosis and bactericidal activity against the pathogen. Following pulmonary or low-dose intravenous GBS challenge, Siglec-E KO mice produced more pro-inflammatory cytokines and exhibited reduced GBS invasion of the central nervous system. In contrast, upon high dose lethal challenges, cytokine storm in Siglec-E KO mice was associated with accelerated mortality. We conclude that GBS Sia mimicry influences host innate immune and inflammatory responses in vivo through engagement of an inhibitory Siglec, with the ultimate outcome of the host response varying depending upon the site, stage and magnitude of infection.

The bacterium Group B Streptococcus (GBS) causes serious infections such as meningitis in human newborn babies. The surface of GBS is coated with a capsule made of sugar molecules. Prominent among these is sialic acid (Sia), a human-like sugar that interacts with protein receptors called Siglecs on the surface of our white blood cells. In a test tube, GBS Sia binding to human Siglecs can suppress white blood cell activation, reducing their bacterial killing abilities; however, the significance of this during actual infection was unknown. To answer this question, we studied mice for which a key white blood cell Siglec has been genetically deleted. When infected with GBS, white blood cells from the mutant mice are not shut off by the pathogen's Sia-containing sugar capsule. The white blood cells from the Siglec-deficient mice are better at killing GBS and are able to clear infection more quickly than a normal mouse. However, if the mice are given an overwhelming dose of GBS bacteria, exaggerated white blood activation can trigger shock and more rapid death. These studies show how “molecular mimicry” of sugar molecules in the host can influence a bacterial pathogen's interaction with the immune system and the outcome of infection.

Concepts and mechanisms: crossing host barriers

The human body is bordered by the skin and mucosa, which are the cellular barriers that define the frontier between the internal milieu and the external nonsterile environment. Additional cellular barriers, such as the placental and the blood-brain barriers, define protected niches within the host. In addition to their physiological roles, these host barriers provide both physical and immune defense against microbial infection. Yet, many pathogens have evolved elaborated mechanisms to target this line of defense, resulting in a microbial invasion of cells constitutive of host barriers, disruption of barrier integrity, and systemic dissemination and invasion of deeper tissues. Here we review representative examples of microbial interactions with human barriers, including the intestinal, placental, and blood-brain barriers, and discuss how these microbes adhere to, invade, breach, or compromise these barriers.

Group B Streptococcus CovR regulation modulates host immune signalling pathways to promote vaginal colonization

Streptococcus agalactiae (Group B Streptococcus, GBS) is a frequent commensal organism of the vaginal tract of healthy women. However, GBS can transition to a pathogen in susceptible hosts, but host and microbial factors that contribute to this conversion are not well understood. GBS CovR/S (CsrR/S) is a two component regulatory system that regulates key virulence elements including adherence and toxin production. We performed global transcription profiling of human vaginal epithelial cells exposed to WT, CovR deficient, and toxin deficient strains, and observed that insufficient regulation by CovR and subsequent increased toxin production results in a drastic increase in host inflammatory responses, particularly in cytokine signalling pathways promoted by IL-8 and CXCL2. Additionally, we observed that CovR regulation impacts epithelial cell attachment and intracellular invasion. In our mouse model of GBS vaginal colonization, we further demonstrated that CovR regulation promotes vaginal persistence, as infection with a CovR deficient strainresulted in a heightened host immune response as measured by cytokine production and neutrophil activation. Using CXCr2 KO mice, we determined that this immune alteration occurs, at least in part, via signalling through the CXCL2 receptor. Taken together, we conclude that CovR is an important regulator of GBS vaginal colonization and loss of this regulatory function may contribute to the inflammatory havoc seen during the course of infection.

A hemolytic pigment of Group B Streptococcus allows bacterial penetration of human placenta

Microbial infection of the amniotic fluid is a significant cause of fetal injury, preterm birth, and newborn infections. Group B Streptococcus (GBS) is an important human bacterial pathogen associated with preterm birth, fetal injury, and neonatal mortality. Although GBS has been isolated from amniotic fluid of women in preterm labor, mechanisms of in utero infection remain unknown. Previous studies indicated that GBS are unable to invade human amniotic epithelial cells (hAECs), which represent the last barrier to the amniotic cavity and fetus. We show that GBS invades hAECs and strains lacking the hemolysin repressor CovR/S accelerate amniotic barrier failure and penetrate chorioamniotic membranes in a hemolysin-dependent manner. Clinical GBS isolates obtained from women in preterm labor are hyperhemolytic and some are associated with covR/S mutations. We demonstrate for the first time that hemolytic and cytolytic activity of GBS is due to the ornithine rhamnolipid pigment and not due to a pore-forming protein toxin. Our studies emphasize the importance of the hemolytic GBS pigment in ascending infection and fetal injury.

Group B Streptococcus hijacks the host plasminogen system to promote brain endothelial cell invasion

Group B Streptococcus (GBS) is the leading cause of meningitis in neonates. We have previously shown that plasminogen, once recruited to the GBS cell surface and converted into plasmin by host-derived activators, leads to an enhancement of bacterial virulence. Here, we investigated whether plasmin(ogen) bound at the GBS surface contributes to blood-brain barrier penetration and invasion of the central nervous system. For that purpose, GBS strain NEM316 preincubated with or without plasminogen plus tissue type plasminogen activator was analyzed for the capacity to adhere to, invade and transmigrate the human brain microvascular endothelial cell (hBMEC) monolayer, and to penetrate the central nervous system using a neonatal mouse model. At earlier times of infection, plasmin(ogen)-treated GBS exhibited a significant increase in adherence to and invasion of hBMECs. Later, injury of hBMECs were observed with plasmin(ogen)-treated GBS that displayed a plasmin-like activity. The same results were obtained when hBMECs were incubated with whole human plasma and infected with untreated GBS. To confirm that the observed effects were due to the recruitment and activation of plasminogen on GBS surface, the bacteria were first incubated with epsilon-aminocaproic acid (εACA), an inhibitor of plasminogen binding, and thereafter with plasmin(ogen). A significant decrease in the hBMECs injury that was correlated with a decrease of the GBS surface proteolytic activity was observed. Furthermore, plasmin(ogen)-treated GBS infected more efficiently the brain of neonatal mice than the untreated bacteria, indicating that plasmin(ogen) bound to GBS surface may facilitate the traversal of the blood-brain barrier. A higher survival rate was observed in offspring born from εACA-treated mothers, compared to untreated mice, and no brain infection was detected in these neonates. Our findings suggest that capture of the host plasmin(ogen) by the GBS surface promotes the crossing of the blood-brain barrier and contributes to the establishment of meningitis.

Adult zebrafish model of bacterial meningitis in Streptococcus agalactiae infection

Streptococcus agalactiae (Group B Streptococcus, GBS) is the major cause of severe bacterial disease and meningitis in newborns. The zebrafish (Danio rerio) has recently emerged as a valuable and powerful vertebrate model for the study of human streptococcal infections. In the present study we demonstrate that adult zebrafish are susceptible to GBS infection through the intraperitoneal and intramuscular routes of infection. Following intraperitoneal challenge with GBS, zebrafish developed a fulminant infection 24-48 h post-injection, with signs of pathogenesis including severe inflammation at the injection site and meningoencephalitis. Quantification of blood and brain bacterial load confirmed that GBS is capable of replicating in the zebrafish bloodstream and penetrating the blood-brain barrier, resulting in the induction of host inflammatory immune responses in the brain. Additionally, we show that GBS mutants previously described as avirulent in the mice model, have an impaired ability to cause meningitis in this new in vivo model. Taken together, our data demonstrates that adult zebrafish may be used as a bacterial meningitis model as a means for deciphering the pathogenesis and development of invasive GBS disease.

Peptide sequences mediating tropism to intact blood-brain barrier: an in vivo biodistribution study using phage display

Peptide motifs that demonstrate tropism for the blood brain barrier (BBB) are of real translational value in developing innovative delivery strategies for biological brain targeted therapies. In vivo peptide-phage display affords peptide selection against the full complement of biological markers within the correct cellular macro- and micro-environments. Here a stringent in vivo biopanning protocol was employed in the rat aimed at identifying cyclic 7-mer peptide motifs that mediate tropism to brain microvasculature. Five rounds of biopanning identified 349 unique peptide motifs in the brain tissue gray matter compartment (microvasculature and parenchyma). While in general no consensus was evident linking peptide physico-chemical properties and brain tropism, peptides bearing c-SxTSSTx-c or c-xxxSSTx-c motifs were found to be present in high abundance. Based on amino acid frequency distribution of the 349 unique peptides sequences a theoretical 'idealized' peptide pattern, c-PP(S/P)SSST-c, could be derived. For the most abundant experimental peptide sequence found in brain tissue, c-SYTSSTM-c, an in vivo pharmacokinetic and whole body tissue biodistribution study was performed. Based upon tissue exposure data (i.e. tissue AUC((0-infinity))) the sequence c-SYTSSTM-c efficiently retargeted phage virions to the brain providing an approximate 5-fold greater (P<0.05) accumulation in brain over control phage; in all other organs no significant (P>0.05) difference in tissue tropism between c-SYTSSTM-c and control phages were evident. This peptide and more generally the peptide motifs, -SxTSSTx- or -xxxSSTx-, warrant further investigation as agents mediating sequence-dependent tropism to brain microvasculature potentially able to deliver biologic cargo to the CNS.

Effective plasmid DNA and small interfering RNA delivery to diseased human brain microvascular endothelial cells

Expression of exogenous DNA or small interfering RNA (siRNA) in vitro is significantly affected by the particular delivery system utilized. In this study, we evaluated the transfection efficiency of plasmid DNA and siRNA into human brain microvascular endothelial cells (HBMEC) and meningioma cells, which constitute the blood-cerebrospinal fluid barrier, a target of meningitis-causing pathogens. Chemical transfection methods and various lipofection reagents including Lipofectamin™, FuGene™, or jetPRIME®, as well as physical transfection methods and electroporation techniques were applied. To monitor the transfection efficiencies, HBMEC and meningioma cells were transfected with the reporter plasmid pTagGFP2-actin vector, and efficiency of transfection was estimated by fluorescence microscopy and flow cytometry. We established protocols based on electroporation using Cell Line Nucleofector® Kit V with the Amaxa® Nucleofector® II system from Lonza and the Neon® Transfection system from Invitrogen resulting in up to 41 and 82% green fluorescent protein-positive HBMEC, respectively. Optimal transfection solutions, pulse programs and length were evaluated. We furthermore demonstrated that lipofection is an efficient method to transfect meningioma cells with a transfection efficiency of about 81%. Finally, we applied the successful electroporation protocols to deliver synthetic siRNA to HBMEC and analyzed the role of the actin-binding protein cortactin in Neisseria meningitidis pathogenesis.

Binding of glycoprotein Srr1 of Streptococcus agalactiae to fibrinogen promotes attachment to brain endothelium and the development of meningitis

The serine-rich repeat glycoprotein Srr1 of Streptococcus agalactiae (GBS) is thought to be an important adhesin for the pathogenesis of meningitis. Although expression of Srr1 is associated with increased binding to human brain microvascular endothelial cells (hBMEC), the molecular basis for this interaction is not well defined. We now demonstrate that Srr1 contributes to GBS attachment to hBMEC via the direct interaction of its binding region (BR) with human fibrinogen. When assessed by Far Western blotting, Srr1 was the only protein in GBS extracts that bound fibrinogen. Studies using recombinant Srr1-BR and purified fibrinogen in vitro confirmed a direct protein-protein interaction. Srr1-BR binding was localized to amino acids 283–410 of the fibrinogen Aα chain. Structural predictions indicated that the conformation of Srr1-BR is likely to resemble that of SdrG and other related staphylococcal proteins that bind to fibrinogen through a “dock, lock, and latch” mechanism (DLL). Deletion of the predicted latch domain of Srr1-BR abolished the interaction of the BR with fibrinogen. In addition, a mutant GBS strain lacking the latch domain exhibited reduced binding to hBMEC, and was significantly attenuated in an in vivo model of meningitis. These results indicate that Srr1 can bind fibrinogen directly likely through a DLL mechanism, which has not been described for other streptococcal adhesins. This interaction was important for the pathogenesis of GBS central nervous system invasion and subsequent disease progression.

Streptococcus agalactiae (Group B streptococcus, GBS) is a leading cause of meningitis in newborns and infants. This life-threatening infection of the brain and surrounding tissues continues to result in a high incidence of morbidity and mortality, despite antibiotic therapy. A key factor in disease production is the ability of this organism to invade the central nervous system, via the bloodstream. We now report that a GBS surface protein called Srr1 binds fibrinogen, a major protein in human blood. This interaction enhances the attachment of GBS to brain vascular endothelial cells, and contributes to the development of meningitis. A mutation in Srr1 that specifically disrupted binding to fibrinogen significantly reduced GBS attachment to brain endothelium, and markedly reduced virulence in an in vivo model of GBS disease. These studies have identified a new mechanism by which Srr1 contributes to GBS invasion of the central nervous system and may provide a basis for novel therapies targeting Srr1 binding.