Structural and Functional Analysis of Cell Wall-anchored Polypeptide Adhesin BspA in Streptococcus agalactiae

An opportunistic pathogen under stress: how Group B Streptococcus responds to cytotoxic reactive species and conditions of metal ion imbalance to survive

Group B Streptococcus (GBS; also known as Streptococcus agalactiae) is an opportunistic bacterial pathogen that causes sepsis, meningitis, pneumonia, and skin and soft tissue infections in neonates and healthy or immunocompromised adults. GBS is well-adapted to survive in humans due to a plethora of virulence mechanisms that afford responses to support bacterial survival in dynamic host environments. These mechanisms and responses include counteraction of cell death from exposure to excess metal ions that can cause mismetallation and cytotoxicity, and strategies to combat molecules such as reactive oxygen and nitrogen species that are generated as part of innate host defence. Cytotoxicity from reactive molecules can stem from damage to proteins, DNA, and membrane lipids, potentially leading to bacterial cell death inside phagocytic cells or within extracellular spaces within the host. Deciphering the ways in which GBS responds to the stress of cytotoxic reactive molecules within the host will benefit the development of novel therapeutic and preventative strategies to manage the burden of GBS disease. This review summarizes knowledge of GBS carriage in humans and the mechanisms used by the bacteria to circumvent killing by these important elements of host immune defence: oxidative stress, nitrosative stress, and stress from metal ion intoxication/mismetallation.

Structural foundation for the role of enterococcal PrgB in conjugation, biofilm formation, and virulence

Type 4 Secretion Systems are a main driver for the spread of antibiotic resistance genes and virulence factors in bacteria. In Gram-positives, these secretion systems often rely on surface adhesins to enhance cellular aggregation and mating-pair formation. One of the best studied adhesins is PrgB from the conjugative plasmid pCF10 of Enterococcus faecalis, which has been shown to play major roles in conjugation, biofilm formation, and importantly also in bacterial virulence. Since prgB orthologs exist on a large number of conjugative plasmids in various different species, this makes PrgB a model protein for this widespread virulence factor. After characterizing the polymer adhesin domain of PrgB previously, we here report the structure for almost the entire remainder of PrgB, which reveals that PrgB contains four immunoglobulin (Ig)-like domains. Based on this new insight, we re-evaluate previously studied variants and present new in vivo data where specific domains or conserved residues have been removed. For the first time, we can show a decoupling of cellular aggregation from biofilm formation and conjugation in prgB mutant phenotypes. Based on the presented data, we propose a new functional model to explain how PrgB mediates its different functions. We hypothesize that the Ig-like domains act as a rigid stalk that presents the polymer adhesin domain at the right distance from the cell wall.

The commensal protist Tritrichomonas musculus exhibits a dynamic life cycle that induces extensive remodeling of the gut microbiota

Commensal protists and gut bacterial communities exhibit complex relationships, mediated at least in part through host immunity. To improve our understanding of this tripartite interplay, we investigated community and functional dynamics between the murine protist Tritrichomonas musculus ( T. mu ) and intestinal bacteria in healthy and B cell-deficient mice. We identified dramatic, protist-driven remodeling of resident microbiome growth and activities, in parallel with T. mu functional changes, accelerated in the absence of B cells. Metatranscriptomic data revealed nutrient-based competition between bacteria and the protist. Single cell transcriptomics identified distinct T. mu life stages, providing new evidence for trichomonad sexual replication and the formation of pseudocysts. Unique cell states were validated in situ through microscopy and flow cytometry. Our results reveal complex microbial dynamics during the establishment of a commensal protist in the gut, and provide valuable datasets to drive future mechanistic studies.

Interspecies Interactions within the Host: the Social Network of Group B Streptococcus

Group B Streptococcus (GBS) is a pervasive neonatal pathogen accounting for a combined half a million deaths and stillbirths annually. The most common source of fetal or neonatal GBS exposure is the maternal microbiota. GBS asymptomatically colonizes the gastrointestinal and vaginal mucosa of 1 in 5 individuals globally, although its precise role in these niches is not well understood. To prevent vertical transmission, broad-spectrum antibiotics are administered to GBS-positive mothers during labor in many countries. Although antibiotics have significantly reduced GBS early-onset neonatal disease, there are several unintended consequences, including an altered neonatal microbiota and increased risk for other microbial infections. Additionally, the incidence of late-onset GBS neonatal disease remains unaffected and has sparked an emerging hypothesis that GBS-microbe interactions in developing neonatal gut microbiota may be directly involved in this disease process. This review summarizes our current understanding of GBS interactions with other resident microbes at the mucosal surface from multiple angles, including clinical association studies, agriculture and aquaculture observations, and experimental animal model systems. We also include a comprehensive review of in vitro findings of GBS interactions with other bacterial and fungal microbes, both commensal and pathogenic, along with newly established animal models of GBS vaginal colonization and in utero or neonatal infection. Finally, we provide a perspective on emerging areas of research and current strategies to design microbe-targeting prebiotic or probiotic therapeutic intervention strategies to prevent GBS disease in vulnerable populations.

The Group B Streptococcal Adhesin BspC Interacts with Host Cytokeratin 19 To Promote Colonization of the Female Reproductive Tract

Streptococcus agalactiae, otherwise known as Group B Streptococcus (GBS), is an opportunistic pathogen that vaginally colonizes approximately one third of healthy women. During pregnancy, this can lead to in utero infection, resulting in premature rupture of membranes, chorioamnionitis, and stillbirths. Furthermore, GBS causes serious infection in newborns, including sepsis, pneumonia, and meningitis. Previous studies have indicated that GBS antigen (Ag) I/II family proteins promote interaction with vaginal epithelial cells; thus, we hypothesized that the Ag I/II Group B streptococcal surface protein C (BspC) contributes to GBS colonization of the female reproductive tract (FRT). Here, we show that a ΔbspC mutant has decreased bacterial adherence to vaginal, ecto-, and endocervical cells, as well as decreased auto-aggregation and biofilm-like formation on cell monolayers. Using a murine model of vaginal colonization, we observed that the ΔbspC mutant strain exhibited a significant fitness defect compared to wild-type (WT) GBS and was less able to ascend to the cervix and uterus in vivo, resulting in reduced neutrophil chemokine signaling. Furthermore, we determined that BspC interacts directly with the host intermediate filament protein cytokeratin 19 (K19). Surface localization of K19 was increased during GBS infection, and interaction was mediated by the BspC variable (V) domain. Finally, mice treated with a drug that targets the BspC V-domain exhibited reduced bacterial loads in the vaginal lumen and reproductive tissues. These results demonstrate the importance of BspC in promoting GBS colonization of the FRT and that it may be targeted therapeutically to reduce GBS vaginal persistence and ascending infection. IMPORTANCE Group B Streptococcus (GBS) asymptomatically colonizes the female reproductive tract (FRT) of up to one third of women, but GBS carriage can lead to adverse pregnancy outcomes, including premature rupture of membranes, preterm labor, and chorioamnionitis. GBS colonization during pregnancy is also the largest predisposing factor for neonatal GBS disease, including pneumonia, sepsis, and meningitis. The molecular interactions between bacterial surface proteins and the host cell receptors that promote GBS colonization are vastly understudied, and a better understanding would facilitate development of novel therapeutics to prevent GBS colonization and disease. Here, we characterize the role of the GBS surface protein BspC in colonization of the FRT. We show for the first time that GBS infection induces cytokeratin 19 (K19) surface localization on vaginal epithelial cells; GBS then uses the BspC V-domain to interact with K19 to promote colonization and ascending infection. Furthermore, this interaction can be targeted therapeutically to reduce GBS carriage.

Interactions between Candida albicans and the resident microbiota

Candida albicans is a prevalent, opportunistic human fungal pathogen. It usually dwells in the human body as a commensal, however, once in its pathogenic state, it causes diseases ranging from debilitating superficial to life-threatening systemic infections. The switch from harmless colonizer to virulent pathogen is, in most cases, due to perturbation of the fungus-host-microbiota interplay. In this review, we focused on the interactions between C. albicans and the host microbiota in the mouth, gut, blood, and vagina. We also highlighted important future research directions. We expect that the evaluation of these interplays will help better our understanding of the etiology of fungal infections and shed new light on the therapeutic approaches.

Proteomic Characterization of Virulence Factors and Related Proteins in Enterococcus Strains from Dairy and Fermented Food Products

Enterococcus species are Gram-positive bacteria that are normal gastrointestinal tract inhabitants that play a beneficial role in the dairy and meat industry. However, Enterococcus species are also the causative agents of health care-associated infections that can be found in dairy and fermented food products. Enterococcal infections are led by strains of Enterococcus faecalis and Enterococcus faecium, which are often resistant to antibiotics and biofilm formation. Enterococci virulence factors attach to host cells and are also involved in immune evasion. LC-MS/MS-based methods offer several advantages compared with other approaches because one can directly identify microbial peptides without the necessity of inferring conclusions based on other approaches such as genomics tools. The present study describes the use of liquid chromatography−electrospray ionization tandem mass spectrometry (LC−ESI−MS/MS) to perform a global shotgun proteomics characterization for opportunistic pathogenic Enterococcus from different dairy and fermented food products. This method allowed the identification of a total of 1403 nonredundant peptides, representing 1327 proteins. Furthermore, 310 of those peptides corresponded to proteins playing a direct role as virulence factors for Enterococcus pathogenicity. Virulence factors, antibiotic sensitivity, and proper identification of the enterococcal strain are required to propose an effective therapy. Data are available via ProteomeXchange with identifier PXD036435. Label-free quantification (LFQ) demonstrated that the majority of the high-abundance proteins corresponded to E. faecalis species. Therefore, the global proteomic repository obtained here can be the basis for further research into pathogenic Enterococcus species, thus facilitating the development of novel therapeutics.

Targeting the BspC-vimentin interaction to develop anti-virulence therapies during Group B streptococcal meningitis

Bacterial infections are a major cause of morbidity and mortality worldwide and the rise of antibiotic resistance necessitates development of alternative treatments. Pathogen adhesins that bind to host cells initiate disease pathogenesis and represent potential therapeutic targets. We have shown previously that the BspC adhesin in Group B Streptococcus (GBS), the leading cause of bacterial neonatal meningitis, interacts with host vimentin to promote attachment to brain endothelium and disease development. Here we determined that the BspC variable (V-) domain contains the vimentin binding site and promotes GBS adherence to brain endothelium. Site directed mutagenesis identified a binding pocket necessary for GBS host cell interaction and development of meningitis. Using a virtual structure-based drug screen we identified compounds that targeted the V-domain binding pocket, which blocked GBS adherence and entry into the brain in vivo. These data indicate the utility of targeting the pathogen-host interface to develop anti-virulence therapeutics.

Lactococcus lactis, an Attractive Cell Factory for the Expression of Functional Membrane Proteins

Membrane proteins play key roles in most crucial cellular processes, ranging from cell-to-cell communication to signaling processes. Despite recent improvements, the expression of functionally folded membrane proteins in sufficient amounts for functional and structural characterization remains a challenge. Indeed, it is still difficult to predict whether a protein can be overproduced in a functional state in some expression system(s), though studies of high-throughput screens have been published in recent years. Prokaryotic expression systems present several advantages over eukaryotic ones. Among them, Lactococcus lactis (L. lactis) has emerged in the last two decades as a good alternative expression system to E. coli. The purpose of this chapter is to describe L. lactis and its tightly inducible system, NICE, for the effective expression of membrane proteins from both prokaryotic and eukaryotic origins.

Bacterial and Host Determinants of Group B Streptococcal Vaginal Colonization and Ascending Infection in Pregnancy

Group B streptococcus (GBS) is a gram-positive bacteria that asymptomatically colonizes the vaginal tract. However, during pregnancy maternal GBS colonization greatly predisposes the mother and baby to a wide range of adverse outcomes, including preterm birth (PTB), stillbirth, and neonatal infection. Although many mechanisms involved in GBS pathogenesis are partially elucidated, there is currently no approved GBS vaccine. The development of a safe and effective vaccine that can be administered during or prior to pregnancy remains a principal objective in the field, because current antibiotic-based therapeutic strategies do not eliminate all cases of invasive GBS infections. Herein, we review our understanding of GBS disease pathogenesis at the maternal-fetal interface with a focus on the bacterial virulence factors and host defenses that modulate the outcome of infection. We follow GBS along its path from an asymptomatic colonizer of the vagina to an invasive pathogen at the maternal-fetal interface, noting factors critical for vaginal colonization, ascending infection, and vertical transmission to the fetus. Finally, at each stage of infection we emphasize important host-pathogen interactions, which, if targeted therapeutically, may help to reduce the global burden of GBS.

Molecular Characteristics of IS1216 Carrying Multidrug Resistance Gene Cluster in Serotype III/Sequence Type 19 Group B Streptococcus

Streptococcus agalactiae is the leading cause of meningitis in newborns and a significant cause of invasive diseases in pregnant women and adults with underlying diseases. Antibiotic resistance against erythromycin and clindamycin in group B streptococcus (GBS) isolates has been increasing worldwide. GBS expresses the Srr1 and Srr2 proteins, which have important roles in bacterial infection. They have been investigated as novel vaccine candidates against GBS infection, with promising results. But a recent study detected non-srr1/2-expressing clinical isolates belonging to serotype III. Thus, we aimed to analyze the genotypes of non-srr1/2 GBS clinical isolates collected between 2013 and 2016 in South Korea. Forty-one (13.4%) of the 305 serotype III isolates were identified as non-srr1/2 strains, including sequence type 19 (ST19) (n = 16) and ST27 (n = 18) strains. The results of the comparative genomic analysis of the ST19/serotype III/non-srr1/2 strains further revealed four unique gene clusters. Site 4 in the srr1 gene locus was replaced by an lsa(E)-lnu(B)-aadK-aac-aph-aadE-carrying multidrug-resistant gene cluster flanked by two IS1216 transposases with 99% homology to the enterococcal plasmid pKUB3007-1. Despite the Srr1 and Srr2 deficiencies, which resulted in reduced fibrinogen binding, the adherence of non-srr1/2 strains to endothelial and epithelial cells was comparable to that of Srr1- or Srr2-expressing strains. Moreover, their virulence in mouse models of meningitis was not significantly affected. Furthermore, additional adhesin-encoding genes, including a gene encoding a BspA-like protein, which may contribute to colonization by non-srr1/2 strains, were identified via whole-genome analysis. Thus, our study provides important findings that can aid in the development of vaccines and antibiotics against GBS. IMPORTANCE Most previously isolated group B streptococcus (GBS) strains express either the Srr1 or Srr2 glycoprotein, which plays an important role in bacterial colonization and invasion. These glycoproteins are potential protein vaccine candidates. In this study, we first report GBS clinical isolates in which the srr1/2 gene was deleted or replaced with foreign genes. Despite Srr1/2 deficiency, in vitro adherence to mammalian cells and in vivo virulence in murine models were not affected, suggesting that the isolates might have another adherence mechanism that enhanced their virulence aside from Srr1/2-fibrinogen-mediated adherence. In addition, several non-srr1/2 isolates replaced the srr1/2 gene with the lnu(B) and lsa(E) antibiotic resistance genes flanked by IS1216, effectively causing multidrug resistance. Collectively, we believe that our study identifies the underlying genes responsible for the pathogenesis of new GBS serotype III. Furthermore, our study emphasizes the need for alternative antibiotics for patients who are allergic to β-lactams and for those who are pregnant.

Genomic and phenotypic diversity of Enterococcus faecalis isolated from endophthalmitis

Enterococcus faecalis are hospital-associated opportunistic pathogens and also causative agents of post-operative endophthalmitis. Patients with enterococcal endophthalmitis often have poor visual outcomes, despite appropriate antibiotic therapy. Here we investigated the genomic and phenotypic characteristics of E. faecalis isolates collected from 13 patients treated at the University of Pittsburgh Medical Center Eye Center over 19 years. Comparative genomic analysis indicated that patients were infected with E. faecalis belonging to diverse multi-locus sequence types (STs) and resembled E. faecalis sampled from clinical, commensal, and environmental sources. We identified known E. faecalis virulence factors and antibiotic resistance genes in each genome, including genes conferring resistance to aminoglycosides, erythromycin, and tetracyclines. We assessed all isolates for their cytolysin production, biofilm formation, and antibiotic susceptibility, and observed phenotypic differences between isolates. Fluoroquinolone and cephalosporin susceptibilities were particularly variable between isolates, as were biofilm formation and cytolysin production. In addition, we found evidence of E. faecalis adaptation during recurrent endophthalmitis by identifying genetic variants that arose in sequential isolates sampled over eight months from the same patient. We identified a mutation in the DNA mismatch repair gene mutS that was associated with an increased rate of spontaneous mutation in the final isolate from the patient. Overall this study documents the genomic and phenotypic variability among E. faecalis causing endophthalmitis, as well as possible adaptive mechanisms underlying bacterial persistence during recurrent ocular infection.

Bacterial protein domains with a novel Ig-like fold target human CEACAM receptors

Streptococcus agalactiae, also known as group B Streptococcus (GBS), is the major cause of neonatal sepsis in humans. A critical step to infection is adhesion of bacteria to epithelial surfaces. GBS adhesins have been identified to bind extracellular matrix components and cellular receptors. However, several putative adhesins have no host binding partner characterised. We report here that surface-expressed β protein of GBS binds to human CEACAM1 and CEACAM5 receptors. A crystal structure of the complex showed that an IgSF domain in β represents a novel Ig-fold subtype called IgI3, in which unique features allow binding to CEACAM1. Bioinformatic assessment revealed that this newly identified IgI3 fold is not exclusively present in GBS but is predicted to be present in adhesins from other clinically important human pathogens. In agreement with this prediction, we found that CEACAM1 binds to an IgI3 domain found in an adhesin from a different streptococcal species. Overall, our results indicate that the IgI3 fold could provide a broadly applied mechanism for bacteria to target CEACAMs.

A current review of pathogenicity determinants of Streptococcus sp

The genus Streptococcus comprises important pathogens, many of them are part of the human or animal microbiota. Advances in molecular genetics, taxonomic approaches and phylogenomic studies have led to the establishment of at least 100 species that have a severe impact on human health and are responsible for substantial economic losses to agriculture. The infectivity of the pathogens is linked to cell-surface components and/or secreted virulence factors. Bacteria have evolved sophisticated and multifaceted adaptation strategies to the host environment, including biofilm formation, survival within professional phagocytes, escape the host immune response, amongst others. This review focuses on virulence mechanism and zoonotic potential of Streptococcus species from pyogenic (S. agalactiae, S. pyogenes) and mitis groups (S. pneumoniae).

Potential role of the gut microbiota in neuromyelitis optica spectrum disorder: Implication for intervention

The gut microbiota plays an important role in the occurrence and development of neuroimmunological diseases. Neuromyelitis optica spectrum disorder (NMOSD) is an autoimmune disease of the central nervous system that is characterized by the peripheral production of the disease-specific serum autoantibody aquaporin-4 (AQP4)-IgG. Recently, accumulating evidence has provided insights into the associations of gut microbiota dysbiosis and intestinal mucosal barrier destruction with NMOSD, but the underlying pathogenesis remains unclear. Thus, a microbiota intervention might be a potential therapeutic strategy for NMOSD by regulating the gut microbiota, repairing the intestinal mucosal barrier, and modulating intestinal immunity and peripheral immunity.

The Multifaceted Nature of Streptococcal Antigen I/II Proteins in Colonization and Disease Pathogenesis

Streptococci are Gram-positive bacteria that belong to the natural microbiota of humans and animals. Certain streptococcal species are known as opportunistic pathogens with the potential to cause severe invasive disease. Antigen I/II (AgI/II) family proteins are sortase anchored cell surface adhesins that are nearly ubiquitous across streptococci and contribute to many streptococcal diseases, including dental caries, respiratory tract infections, and meningitis. They appear to be multifunctional adhesins with affinities to various host substrata, acting to mediate attachment to host surfaces and stimulate immune responses from the colonized host. Here we will review the literature including recent work that has demonstrated the multifaceted nature of AgI/II family proteins, focusing on their overlapping and distinct functions and their important contribution to streptococcal colonization and disease.

Cytotoxicity of Streptococcus agalactiae secretory protein on tilapia cultured cells

Streptococcus agalactiae secrete virulence factors believed to be able of killing host tissues, especially under elevated water temperature. A direct effect of S. agalactiae secretory products on tilapia cells was tested on the tilapia kidney (TK-1) cell culture. The bacteria were cultured under four different temperature levels: 22, 29, 32 and 37°C; the cell-free portion was processed through SDS-PAGE; and distinct bands were identified by LC-MS/MS. At least, three virulence factors were identified, Bsp, PcsB and CAMP factor, with increasing levels as the cultured temperature rose. Expressions of bsp, pcsB and cfb were also up-regulated with the rising of the temperature in S. agalactiae culture. The supernatant from the bacteria cultured under specified temperatures was added into TK-1 cell-cultured wells. Morphological damage and mortality of the cultured cells, as determined by MTT method, were increased progressively from the supernatant treatment according to the rise of temperature in S. agalactiae culture. This study suggests that the production of the three virulence factors of S. agalactiae reported herein is temperature-dependent, and it is likely that CAMP factor directly kills the TK-1 cells since the other two types of protein are involved in S. agalactiae cell division and the bacterial adherence to host tissues.

Characterization of novel antigenic vaccine candidates for nile tilapia (Oreochromis niloticus) against Streptococcus agalactiae infection

Streptococcus agalactiae is one of the important pathogens responsible for high mortality and economic losses of the tilapia industry worldwide. Based on ten serovars of S. agalactiae infection, subunit vaccine with conserved antigens is promising strategy corresponding stimulated long-term immunity and provides protection for animals against different serotypes of S. agalactiae. In the present study, eight proteins (AP, AL, LivK, ESAT6, essA, essB, essC and esaA) were selected from the S. agalactiae serotype Ia genome as immunogenic antigens with bioinformation and immune experiment assays. These recombinant proteins were successfully obtained through expression in Escherichia coli and the immunogenicity was assessed in tilapia challenge model. The results showed that the recombinant proteins caused high-level-specific antibodies production and high lysozyme activities, suggesting that the recombinant proteins induced specific humoral immune response and innate immune response of tilapia. The signficant increase were observed in the cytokines levels of TNF-α, IL-1β, IFN-γ, cc1, cc2 and immune-related genes levels of CD8α and MHC factors in the spleen and head kidney tissues, suggesting that the recombinant proteins induced immune response of tilapia through cytokines signal pathway and activated high cytotoxic T-lymphocyte (CTL) activity of tilapia. Furthermore, vaccinated tilapia conferred high levels of protection against challenge with a lethal dose of highly virulent serovar Ⅰa (highest RPS was 91.60% in AL and essC protein groups). Our results indicated that the eight recombinant proteins induced high level of immune responses and offered protection against S. agalactiae infection, could be potential subunit vaccine candidates.

The streptococcal multidomain fibrillar adhesin CshA has an elongated polymeric architecture

The cell surfaces of many bacteria carry filamentous polypeptides termed adhesins that enable binding to both biotic and abiotic surfaces. Surface adherence is facilitated by the exquisite selectivity of the adhesins for their cognate ligands or receptors and is a key step in niche or host colonization and pathogenicity. Streptococcus gordonii is a primary colonizer of the human oral cavity and an opportunistic pathogen, as well as a leading cause of infective endocarditis in humans. The fibrillar adhesin CshA is an important determinant of S. gordonii adherence, forming peritrichous fibrils on its surface that bind host cells and other microorganisms. CshA possesses a distinctive multidomain architecture comprising an N-terminal target-binding region fused to 17 repeat domains (RDs) that are each ∼100 amino acids long. Here, using structural and biophysical methods, we demonstrate that the intact CshA repeat region (CshA_RD1-17, domains 1-17) forms an extended polymeric monomer in solution. We recombinantly produced a subset of CshA RDs and found that they differ in stability and unfolding behavior. The NMR structure of CshA_RD13 revealed a hitherto unreported all β-fold, flanked by disordered interdomain linkers. These findings, in tandem with complementary hydrodynamic studies of CshA_RD1-17, indicate that this polypeptide possesses a highly unusual dynamic transitory structure characterized by alternating regions of order and disorder. This architecture provides flexibility for the adhesive tip of the CshA fibril to maintain bacterial attachment that withstands shear forces within the human host. It may also help mitigate deleterious folding events between neighboring RDs that share significant structural identity without compromising mechanical stability.

The Fungal Pathogen Candida albicans Promotes Bladder Colonization of Group B Streptococcus

Group B Streptococcus (GBS) is a common cause of bacterial urinary tract infections (UTI) in susceptible populations, including pregnant women and the elderly. However, the factors that govern GBS persistence and disease severity in this niche are not fully understood. Here, we report that the presence of the fungus Candida albicans, a common urogenital colonizer, can promote GBS UTI. Co-inoculation of GBS with C. albicans increased bacterial adherence to bladder epithelium and promoted GBS colonization in vivo in a C. albicans adhesin-dependent manner. This study demonstrates that fungal colonization of the urogenital tract may be an important determinant of bacterial pathogenesis during UTI.

The Group B Streptococcal surface antigen I/II protein, BspC, interacts with host vimentin to promote adherence to brain endothelium and inflammation during the pathogenesis of meningitis

Streptococcus agalactiae (Group B Streptococcus, GBS) normally colonizes healthy adults but can cause invasive disease, such as meningitis, in the newborn. To gain access to the central nervous system, GBS must interact with and penetrate brain or meningeal blood vessels; however, the exact mechanisms are still being elucidated. Here, we investigate the contribution of BspC, an antigen I/II family adhesin, to the pathogenesis of GBS meningitis. Disruption of the bspC gene reduced GBS adherence to human cerebral microvascular endothelial cells (hCMEC), while heterologous expression of BspC in non-adherent Lactococcus lactis conferred bacterial attachment. In a murine model of hematogenous meningitis, mice infected with ΔbspC mutants exhibited lower mortality as well as decreased brain bacterial counts and inflammatory infiltrate compared to mice infected with WT GBS strains. Further, BspC was both necessary and sufficient to induce neutrophil chemokine expression. We determined that BspC interacts with the host cytoskeleton component vimentin and confirmed this interaction using a bacterial two-hybrid assay, microscale thermophoresis, immunofluorescent staining, and imaging flow cytometry. Vimentin null mice were protected from WT GBS infection and also exhibited less inflammatory cytokine production in brain tissue. These results suggest that BspC and the vimentin interaction is critical for the pathogenesis of GBS meningitis.

Group B Streptococcus (GBS) typically colonizes healthy adults but can cause severe disease in immune-compromised individuals, including newborns. Despite wide-spread intrapartum antibiotic prophylaxis given to pregnant women, GBS remains a leading cause of neonatal meningitis. To cause meningitis, GBS must interact with and penetrate the blood-brain barrier (BBB), which separates bacteria and immune cells in the blood from the brain. In order to develop targeted therapies to treat GBS meningitis, it is important to understand the mechanisms of BBB crossing. Here, we describe the role of the GBS surface factor, BspC, in promoting meningitis and discover the host ligand for BspC, vimentin, which is an intermediate filament protein that is constitutively expressed by endothelial cells. We determined that BspC interacts with the C-terminal domain of cell-surface vimentin to promote bacterial attachment to brain endothelial cells and that purified BspC protein can induce immune signaling pathways. In a mouse model of hematogenous meningitis, we observed that a GBS mutant lacking BspC was less virulent compared to WT GBS and resulted in less inflammatory disease. We also observed that mice lacking vimentin were protected from GBS infection. These results reveal the importance of the BspC-vimentin interaction in the progression of GBS meningitis disease.

The Streptococcus agalactiae complement interfering protein combines multiple complement-inhibitory mechanisms by interacting with both C4 and C3 ligands

Group B Streptococcus (GBS) colonizes the human lower intestinal and genital tracts and constitutes a major threat to neonates from pregnant carrier mothers and to adults with underlying morbidity. The pathogen expresses cell-surface virulence factors that enable cell adhesion and penetration and that counteract innate and adaptive immune responses. Among these, the complement interfering protein (CIP) was recently described for its capacity to interact with the human C4b ligand and to interfere with the classical- and lectin-complement pathways. In the present study, we provide evidence that CIP can also interact with C3, C3b, and C3d. Immunoassay-based competition experiments showed that binding of CIP to C3d interferes with the interaction between C3d and the complement receptor 2/cluster of differentiation 21 (CR2/CD21) receptor on B cells. By B-cell intracellular signaling assays, CIP was confirmed to down-regulate CR2/CD21-dependent B-cell activation. The CIP domain involved in C3d binding was mapped via hydrogen deuterium exchange–mass spectrometry. The data obtained reveal a new role for this GBS polypeptide at the interface between the innate and adaptive immune responses, adding a new member to the growing list of virulence factors secreted by gram-positive pathogens that incorporate multiple immunomodulatory functions.—Giussani, S., Pietrocola, G., Donnarumma, D., Norais, N., Speziale, P., Fabbrini, M., Margarit, I. The Streptococcus agalactiae complement interfering protein combines multiple complement-inhibitory mechanisms by interacting with both C4 and C3 ligands.

Enterococcus adhesin PrgB facilitates type IV secretion by condensation of extracellular DNA

Conjugative type IV secretion systems (T4SSs) are multi-protein complexes in Gram-negative and Gram-positive (G+) bacteria, responsible for spreading antibiotic resistances and virulence factors among different species. Compared to Gram-negative bacteria, which establish close contacts for conjugative transfer via sex pili, G+ T4SSs are suggested to employ surface adhesins instead. One example is pCF10, an enterococcal conjugative sex-pheromone responsive plasmid with a narrow host range, thus disseminating genetic information only among closely related species. This MicroCommentary is dedicated to the crystal structure of the pCF10-encoded adhesion domain of PrgB presented by Schmitt et al. The authors show in their work that this adhesion domain is responsible for biofilm formation, tight binding and condensation of extracellular DNA (eDNA) and conjugative transfer of pCF10. A sophisticated two-step mechanism for highly efficient conjugative transfer is postulated, including the formation of PrgB-mediated long-range intercellular contacts by binding and establishment of shorter-range contacts via condensation of eDNA. PrgB binding to lipoteichoic acid on the recipient cell surface stabilizes junctions between the mating partners. The major findings by Schmitt et al. will be brought into a broader context and potential medical applications targeting eDNA as essential component in biofilm formation and conjugation will be discussed.

Enteric bacteria boost defences against oxidative stress in Entamoeba histolytica

Oxidative stress is one of the strongest toxic factors in nature: it can harm or even kill cells. Cellular means of subverting the toxicity of oxidative stress are important for the success of infectious diseases. Many types of bacterium inhabit the intestine, where they can encounter pathogens. During oxidative stress, we analyzed the interplay between an intestinal parasite (the pathogenic amoeba Entamoeba histolytica - the agent of amoebiasis) and enteric bacteria (microbiome residents, pathogens and probiotics). We found that live enteric bacteria protected E. histolytica against oxidative stress. By high-throughput RNA sequencing, two amoebic regulatory modes were observed with enteric bacteria but not with probiotics. The first controls essential elements of homeostasis, and the second the levels of factors required for amoeba survival. Characteristic genes of both modes have been acquired by the amoebic genome through lateral transfer from the bacterial kingdom (e.g. glycolytic enzymes and leucine-rich proteins). Members of the leucine-rich are homologous to proteins from anti-bacterial innate immune such as Toll-like receptors. The factors identified here suggest that despite its old age in evolutionary terms, the protozoan E. histolytica displays key characteristics of higher eukaryotes' innate immune systems indicating that components of innate immunity existed in the common ancestor of plants and animals.

Streptococcus agalactiae Non-Pilus, Cell Wall-Anchored Proteins: Involvement in Colonization and Pathogenesis and Potential as Vaccine Candidates

Group B Streptococcus (GBS) remains an important etiological agent of several infectious diseases including neonatal septicemia, pneumonia, meningitis, and orthopedic device infections. This pathogenicity is due to a variety of virulence factors expressed by Streptococcus agalactiae. Single virulence factors are not sufficient to provoke a streptococcal infection, which is instead promoted by the coordinated activity of several pathogenicity factors. Such determinants, mostly cell wall-associated and secreted proteins, include adhesins that mediate binding of the pathogen to host extracellular matrix/plasma ligands and cell surfaces, proteins that cooperate in the invasion of and survival within host cells and factors that neutralize phagocytosis and/or modulate the immune response. The genome-based approaches and bioinformatics tools and the extensive use of biophysical and biochemical methods and animal model studies have provided a great wealth of information on the molecular structure and function of these virulence factors. In fact, a number of new GBS surface-exposed or secreted proteins have been identified (GBS immunogenic bacterial adhesion protein, leucine-rich repeat of GBS, serine-rich repeat proteins), the three-dimensional structures of known streptococcal proteins (αC protein, C5a peptidase) have been solved and an understanding of the pathogenetic role of "old" and new determinants has been better defined in recent years. Herein, we provide an update of our current understanding of the major surface cell wall-anchored proteins from GBS, with emphasis on their biochemical and structural properties and the pathogenetic roles they may have in the onset and progression of host infection. We also focus on the antigenic profile of these compounds and discuss them as targets for therapeutic intervention.

Surface-exposed loops and an acidic patch in the Scl1 protein of group A Streptococcus enable Scl1 binding to wound-associated fibronectin

Keratinized epidermis constitutes a powerful barrier of the mucosa and skin, effectively preventing bacterial invasion, unless it is wounded and no longer protective. Wound healing involves deposition of distinct extracellular matrix (ECM) proteins enriched in cellular fibronectin (cFn) isoforms containing extra domain A (EDA). The streptococcal collagen-like protein 1 (Scl1) is a surface adhesin of group A Streptococcus (GAS), which contains an N-terminal variable (V) domain and a C-terminally located collagen-like domain. During wound infection, Scl1 selectively binds EDA/cFn isoforms and laminin, as well as low-density lipoprotein (LDL), through its V domain. The trimeric V domain has a six-helical bundle fold composed of three pairs of anti-parallel α-helices interconnected by hypervariable loops, but the roles of these structures in EDA/cFn binding are unclear. Here, using recombinant Scl (rScl) constructs to investigate structure-function determinants of the Scl1-EDA/cFn interaction, we found that full-length rScl1, containing both the globular V and the collagen domains, is necessary for EDA/cFn binding. We established that the surface-exposed loops, interconnecting conserved α-helices, guide recognition and binding of Scl1-V to EDA and binding to laminin and LDL. Moreover, electrostatic surface potential models of the Scl1-V domains pointed to a conserved, negatively charged pocket, surrounded by positively charged and neutral regions, as a determining factor for the binding. In light of these findings, we propose an updated model of EDA/cFn recognition by the Scl1 adhesin from GAS, representing a significant step in understanding the Scl1-ECM interactions within the wound microenvironment that underlie GAS pathogenesis.

Coassociation between Group B Streptococcus and Candida albicans Promotes Interactions with Vaginal Epithelium

Group B Streptococcus (GBS) is a leading cause of neonatal sepsis, pneumonia, and meningitis worldwide. In the majority of cases, GBS is transmitted vertically from mother to neonate, making maternal vaginal colonization a key risk factor for neonatal disease. The fungus Candida albicans is an opportunistic pathogen of the female genitourinary tract and the causative agent of vaginal thrush. Carriage of C. albicans has been shown to be an independent risk factor for vaginal colonization by GBS. However, the nature of interactions between these two microbes is poorly understood. This study provides evidence of a reciprocal, synergistic interplay between GBS and C. albicans that may serve to promote their cocolonization of the vaginal mucosa. GBS strains NEM316 (serotype III) and 515 (serotype Ia) are shown to physically interact with C. albicans, with the bacteria exhibiting tropism for candidal hyphal filaments. This interaction enhances association levels of both microbes with the vaginal epithelial cell line VK2/E6E7. The ability of GBS to coassociate with C. albicans is dependent upon expression of the hypha-specific adhesin Als3. In turn, expression of GBS antigen I/II family adhesins (Bsp polypeptides) facilitates this coassociation and confers upon surrogate Lactococcus lactis the capacity to exhibit enhanced interactions with C. albicans on vaginal epithelium. As genitourinary tract colonization is an essential first step in the pathogenesis of GBS and C. albicans, the coassociation mechanism reported here may have important implications for the risk of disease involving both of these pathogens.

Surface proteins involved in the adhesion of Streptococcus salivarius to human intestinal epithelial cells

The adhesion properties of 14 Streptococcus salivarius strains to mucus (HT29-MTX) and non-mucus secreting (Caco-2/TC7) human intestinal epithelial cells were investigated. Ability to adhere to these two eukaryotic cell lines greatly differs between strains. The presence of mucus played a major factor in adhesion, likely due to high adhesiveness to mucins present in the native human mucus layer covering the whole cell surface. Only one S. salivarius strain (F6-1), isolated from the feces of a healthy baby, was found to strongly adhere to HT-29 MTX cells at a level comparable to that of Lactobacillus rhamnosus GG, a probiotic strain considered to be highly adherent. By sequencing the genome of F6-1, we were able to identify 36 genes encoding putative surface proteins. Deletion mutants were constructed for six of them and their adhesion abilities on HT-29 MTX cells were checked. Our study confirmed that four of these genes encode adhesins involved in the adhesion of S. salivarius to host cells. Such adhesins were also identified in other S. salivarius strains.

Group B Streptococcal Maternal Colonization and Neonatal Disease: Molecular Mechanisms and Preventative Approaches

Group B Streptococcus (GBS) colonizes the gastrointestinal and vaginal epithelium of a significant percentage of healthy women, with potential for ascending intrauterine infection or transmission during parturition, creating a risk of serious disease in the vulnerable newborn. This review highlights new insights on the bacterial virulence determinants, host immune responses, and microbiome interactions that underpin GBS vaginal colonization, the proximal step in newborn infectious disease pathogenesis. From the pathogen perspective, the function GBS adhesins and biofilms, β-hemolysin/cytolysin toxin, immune resistance factors, sialic acid mimicry, and two-component transcriptional regulatory systems are reviewed. From the host standpoint, pathogen recognition, cytokine responses, and the vaginal mucosal and placental immunity to the pathogen are detailed. Finally, the rationale, efficacy, and potential unintended consequences of current universal recommended intrapartum antibiotic prophylaxis are considered, with updates on new developments toward a GBS vaccine or alternative approaches to reducing vaginal colonization.

Molecular Mechanisms of Drug Action: X-ray Crystallography at the Basis of Structure-based and Ligand-based Drug Design

Biological systems are recognized for their complexity and diversity and yet we sometimes manage to cure disease via the administration of small chemical drug molecules. At first, active ingredients were found accidentally and at that time there did not seem a need to understand the molecular mechanism of drug functioning. However, the urge to develop new drugs, the discovery of multipurpose characteristics of some drugs, and the necessity to remove unwanted secondary drug effects, incited the pharmaceutical sector to rationalize drug design. This did not deliver success in the years directly following its conception, but it drove the evolution of biochemical and biophysical techniques to enable the characterization of molecular mechanisms of drug action. Functional and structural data generated by biochemists and structural biologists became a valuable input for computational biologists, chemists and bioinformaticians who could extrapolate in silico, based on variations in the structural aspects of the drug molecules and their target. This opened up new avenues with much improved predictive power because of a clearer perception of the role and impact of structural elements in the intrinsic affinity and specificity of the drug for its target. In this chapter, we review how crystal structures can initiate structure-based drug design in general.

The Streptococcus gordonii Adhesin CshA Protein Binds Host Fibronectin via a Catch-Clamp Mechanism

Adherence of bacteria to biotic or abiotic surfaces is a prerequisite for host colonization and represents an important step in microbial pathogenicity. This attachment is facilitated by bacterial adhesins at the cell surface. Because of their size and often elaborate multidomain architectures, these polypeptides represent challenging targets for detailed structural and functional characterization. The multifunctional fibrillar adhesin CshA, which mediates binding to both host molecules and other microorganisms, is an important determinant of colonization by Streptococcus gordonii, an oral commensal and opportunistic pathogen of animals and humans. CshA binds the high-molecular-weight glycoprotein fibronectin (Fn) via an N-terminal non-repetitive region, and this protein-protein interaction has been proposed to promote S. gordonii colonization at multiple sites within the host. However, the molecular details of how these two proteins interact have yet to be established. Here we present a structural description of the Fn binding N-terminal region of CshA, derived from a combination of X-ray crystallography, small angle X-ray scattering, and complementary biophysical methods. In vitro binding studies support a previously unreported two-state "catch-clamp" mechanism of Fn binding by CshA, in which the disordered N-terminal domain of CshA acts to "catch" Fn, via formation of a rapidly assembled but also readily dissociable pre-complex, enabling its neighboring ligand binding domain to tightly clamp the two polypeptides together. This study presents a new paradigm for target binding by a bacterial adhesin, the identification of which will inform future efforts toward the development of anti-adhesive agents that target S. gordonii and related streptococci.