Group B streptococcal capsular sialic acids interact with siglecs (immunoglobulin-like lectins) on human leukocytes

Protease SfpB plays an important role in cell membrane stability and immune system evasion in Streptococcus agalactiae

Bacteria possess the ability to develop diverse and ingenious strategies to outwit the host immune system, and proteases are one of the many weapons employed by bacteria. This study sought to identify S. agalactiae additional serine protease and determine its role in virulence. The S. agalactiae THN0901 genome features one S8 family serine peptidase B (SfpB), acting as a secreted and externally exposed entity. A S8 family serine peptidase mutant strain (ΔsfpB) and complement strain (CΔsfpB) were generated through homologous recombination. Compared to the wild-type strain THN0901, the absorption of EtBr dyes was significantly reduced (P < 0.01) in ΔsfpB, implying an altered cell membrane permeability. In addition, the ΔsfpB strain had a significantly lower survival rate in macrophages (P < 0.01) and a 61.85 % lower adhesion ability to the EPC cells (P < 0.01) compared to THN0901. In the in vivo colonization experiment using tilapia as a model, 210 fish were selected and injected with different bacterial strains at a concentration of 3 × 106 CFU/tail. At 6, 12, 24, 48, 72 and 96 h post-injection, three fish were randomly selected from each group and their brain, liver, spleen, and kidney tissues were isolated. Subsequently, it was demonstrated that the ΔsfpB strain exhibited a markedly diminished capacity for colonization in tilapia. Additionally, the cumulative mortality of ΔsfpB in fish after intraperitoneal injection was reduced by 19.92-23.85 %. In conclusion, the findings in this study have demonstrated that the SfpB plays a significant role in S. agalactiae cell membrane stability and immune evasion. The immune evasion is fundamental for the development and transmission of invasive diseases, the serine protease SfpB may be a promising candidate for the development of antimicrobial agents to reduce the transmission of S. agalactiae.

Re-framing the importance of Group B Streptococcus as a gut-resident pathobiont

Streptococcus agalactiae (Group B Streptococcus, GBS) is a Gram-positive bacterial species that causes disease in humans across the lifespan. While antibiotics are used to mitigate GBS infections, it is evident that antibiotics disrupt human microbiomes (which can predispose people to other diseases later in life), and antibiotic resistance in GBS is on the rise. Taken together, these unintended negative impacts of antibiotics highlight the need for precision approaches for minimizing GBS disease. One possible approach involves selectively depleting GBS in its commensal niches before it can cause disease at other body sites or be transmitted to at-risk individuals. One understudied commensal niche of GBS is the adult gastrointestinal (GI) tract, which may predispose colonization at other body sites in individuals at risk for GBS disease. However, a better understanding of the host-, microbiome-, and GBS-determined variables that dictate GBS GI carriage is needed before precise GI decolonization approaches can be developed. In this review, we synthesize current knowledge of the diverse body sites occupied by GBS as a pathogen and as a commensal. We summarize key molecular factors GBS utilizes to colonize different host-associated niches to inform future efforts to study GBS in the GI tract. We also discuss other GI commensals that are pathogenic in other body sites to emphasize the broader utility of precise de-colonization approaches for mitigating infections by GBS and other bacterial pathogens. Finally, we highlight how GBS treatments could be improved with a more holistic understanding of GBS enabled by continued GI-focused study.

The role of sialidases in the pathogenesis of bacterial vaginosis and their use as a promising pharmacological target in bacterial vaginosis

Bacterial vaginosis (BV) is an infection of the genital tract characterized by disturbance of the normally Lactobacilli-dominated vaginal flora due to the overgrowth of Gardnerella and other anaerobic bacteria. Gardnerella vaginalis, an anaerobic pathogen and the major pathogen of BV, produces sialidases that cleave terminal sialic acid residues off of human glycans. By desialylation, sialidases not only alter the function of sialic acid-containing glycoconjugates but also play a vital role in the attachment, colonization and spread of many other vaginal pathogens. With known pathogenic effects, excellent performance of sialidase-based diagnostic tests, and promising therapeutic potentials of sialidase inhibitors, sialidases could be used as a biomarker of BV. This review explores the sources of sialidases and their role in vaginal dysbiosis, in aims to better understand their participation in the pathogenesis of BV and their value in the diagnosis and treatment of BV.

The glycopatterns of Pseudomonas aeruginosa as a potential biomarker for its carbapenem resistance

IMPORTANCE

Bacterial surface glycans are an attractive therapeutic target in response to antibiotics; however, current knowledge of the corresponding mechanisms is rather limited. Antimicrobial susceptibility testing, genome sequencing, and MALDI-TOF MS, commonly used in recent years to analyze bacterial resistance, are unable to rapidly and efficiently establish associations between glycans and resistance. The discovery of new antimicrobial strategies still requires the introduction of promising analytical methods. In this study, we applied lectin microarray technology and a machine-learning model to screen for important glycan structures associated with carbapenem-resistant P. aeruginosa. This work highlights that specific glycopatterns can be important biomarkers associated with bacterial antibiotic resistance, which promises to provide a rapid entry point for exploring new resistance mechanisms in pathogens.

Combinatorial multimer staining and spectral flow cytometry facilitate quantification and characterization of polysaccharide-specific B cell immunity

Bacterial capsular polysaccharides are important vaccine immunogens. However, the study of polysaccharide-specific immune responses has been hindered by technical restrictions. Here, we developed and validated a high-throughput method to analyse antigen-specific B cells using combinatorial staining with fluorescently-labelled capsular polysaccharide multimers. Concurrent staining of 25 cellular markers further enables the in-depth characterization of polysaccharide-specific cells. We used this assay to simultaneously analyse 14 Streptococcus pneumoniae or 5 Streptococcus agalactiae serotype-specific B cell populations. The phenotype of polysaccharide-specific B cells was associated with serotype specificity, vaccination history and donor population. For example, we observed a link between non-class switched (IgM+) memory B cells and vaccine-inefficient S. pneumoniae serotypes 1 and 3. Moreover, B cells had increased activation in donors from South Africa, which has high-incidence of S. agalactiae invasive disease, compared to Dutch donors. This assay allows for the characterization of heterogeneity in B cell immunity that may underlie immunization efficacy.

Synthesis and Binding Mode Predictions of Novel Siglec-7 Ligands

Siglec-7 regulates immune cell activity and is a promising target for immunomodulation. Here, we report the discovery of novel sialic acid derivatives binding to Siglec-7. Synthesis and affinity measurements are complemented by high-quality models of sialoside-Siglec-7 complexes based on molecular dynamics (MD) simulations on the microsecond time scale. We provide details for the predicted binding modes for the new ligands, e.g., that an extension of the carbon backbone leads to a different molecular interaction pattern with the receptor and the nearby water structure than found for known Siglec-7 ligands. Further on, we uncover some shortcomings of the GLYCAM06 and GAFF2 force fields when used for the simulation of sialoside-based glycomimetics. Our results open new opportunities for the rational design of Siglec-7 inhibitors. In addition, we provide strategies on how to use and visualize MD simulations to describe and investigate sialoside-Siglec complexes in general.

Siglecs family used by pathogens for immune escape may engaged in immune tolerance in pregnancy

The Siglecs family is a group of type I sialic acid-binding immunoglobulin-like receptors that regulate cellular signaling by recognizing sialic acid epitopes. Siglecs are predominantly expressed on the surface of leukocytes, where they play a crucial role in regulating immune activity. Pathogens can exploit inhibitory Siglecs by utilizing their sialic acid components to promote invasion or suppress immune functions, facilitating immune evasion. The establishing of an immune-balanced maternal-fetal interface microenvironment is essential for a successful pregnancy. Dysfunctional immune cells may lead to adverse pregnancy outcomes. Siglecs are important for inducing a phenotypic switch in leukocytes at the maternal-fetal interface toward a less toxic and more tolerant phenotype. Recent discoveries regarding Siglecs in the reproductive system have drawn further attention to their potential roles in reproduction. In this review, we primarily discuss the latest advances in understanding the impact of Siglecs as immune regulators on infections and pregnancy.

Porcine Macrophage Markers and Populations: An Update

Besides its importance as a livestock species, pig is increasingly being used as an animal model for biomedical research. Macrophages play critical roles in immunity to pathogens, tissue development, homeostasis and tissue repair. These cells are also primary targets for replication of viruses such as African swine fever virus, classical swine fever virus, and porcine respiratory and reproductive syndrome virus, which can cause huge economic losses to the pig industry. In this article, we review the current status of knowledge on porcine macrophages, starting by reviewing the markers available for their phenotypical characterization and following with the characteristics of the main macrophage populations described in different organs, as well as the effect of polarization conditions on their phenotype and function. We will also review available cell lines suitable for studies on the biology of porcine macrophages and their interaction with pathogens.

Evolutionary and functional history of the Escherichia coli K1 capsule

Escherichia coli is a leading cause of invasive bacterial infections in humans. Capsule polysaccharide has an important role in bacterial pathogenesis, and the K1 capsule has been firmly established as one of the most potent capsule types in E. coli through its association with severe infections. However, little is known about its distribution, evolution and functions across the E. coli phylogeny, which is fundamental to elucidating its role in the expansion of successful lineages. Using systematic surveys of invasive E. coli isolates, we show that the K1-cps locus is present in a quarter of bloodstream infection isolates and has emerged in at least four different extraintestinal pathogenic E. coli (ExPEC) phylogroups independently in the last 500 years. Phenotypic assessment demonstrates that K1 capsule synthesis enhances E. coli survival in human serum independent of genetic background, and that therapeutic targeting of the K1 capsule re-sensitizes E. coli from distinct genetic backgrounds to human serum. Our study highlights that assessing the evolutionary and functional properties of bacterial virulence factors at population levels is important to better monitor and predict the emergence of virulent clones, and to also inform therapies and preventive medicine to effectively control bacterial infections whilst significantly lowering antibiotic usage.

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.

Discovery, classification, evolution and diversity of Siglecs

Immunoglobulin (Ig) superfamily proteins play diverse roles in vertebrates, including regulation of cellular responses by sensing endogenous or exogenous ligands. Siglecs are a family of glycan-recognizing proteins belonging to the Ig superfamily (i.e., I-type lectins). Siglecs are expressed on various leukocyte types and are involved in diverse aspects of immunity, including the regulation of inflammatory responses, leukocyte proliferation, host-microbe interaction, and cancer immunity. Sialoadhesin/Siglec-1, CD22/Siglec-2, and myelin-associated glycoprotein/Siglec-4 were among the first to be characterized as members of the Siglec family, and along with Siglec-15, they are relatively well-conserved among tetrapods. Conversely, CD33/Siglec-3-related Siglecs (CD33rSiglecs, so named as they show high sequence similarity with CD33/Siglec-3) are encoded in a gene cluster with many interspecies variations and even intraspecies variations within some lineages such as humans. The rapid evolution of CD33rSiglecs expressed on leukocytes involved in innate immunity likely reflects the selective pressure by pathogens that interact and possibly exploit these Siglecs. Human Siglecs have several additional unique and/or polymorphic properties as compared with closely related great apes, changes possibly related to the loss of the sialic acid Neu5Gc, another distinctly human event in sialobiology. Multiple changes in human CD33rSiglecs compared to great apes include many examples of human-specific expression in non-immune cells, coinciding with human-specific diseases involving such cell types. Some Siglec gene polymorphisms have dual consequences-beneficial in a situation but detrimental in another. The association of human Siglec gene polymorphisms with several infectious and non-infectious diseases likely reflects the ongoing competition between the host and microbial pathogens.

Relationship between Biofilm Production and High Somatic Cell Count in Streptococcus agalactiae Isolated from Milk of Cows with Subclinical Mastitis

Streptococcus agalactiae (S. agalactiae) is one of the main agents that causes mastitis in dairy cows, mainly inducing the subclinical form, which is characterized by a high somatic cell count (SCC). The aim of this study was to correlate the increase in SCC caused by S. agalactiae in cows with subclinical mastitis to the presence of genes related to adhesion and invasion in bovine mammary epithelial cells (BMEC) and biofilm formation. Considering the 145 isolates tested, 57.2% presented the capsular type Ia and 42.8% presented type III. We identified the virulence genes among the isolates and determined nine genetic profiles. The most common profile was identified in 69 isolates (47.5%): Ia, fbsA⁺, fbsB^-, pI1^-, pI2a^-, pI2b⁺, and hylb⁺. All isolates produced biofilm, with 58.6% classified as strong producers, 29% as moderate producers and 12.4% as weak producers. No statistical correlation was found between the presence of virulence genes and increased SCC or biofilm production. However, biological evidence was observed between increased SCC and biofilm production. One isolate from each profile was randomly subjected to adhesion and invasion assays, and all of them adhered to BEMC, but none were able to invade. Our results showed that different genetic profiles do not provide advantages for bacteria to invade BMEC in vitro. In addition, biofilm production appears to be related to high SCC.

Inflammasome activation by Gram-positive bacteria: Mechanisms of activation and regulation

The inflammasomes are intracellular multimeric protein complexes consisting of an innate immune sensor, the adapter protein ASC and the inflammatory caspases-1 and/or -11 and are important for the host defense against pathogens. Activaton of the receptor leads to formation of the inflammasomes and subsequent processing and activation of caspase-1 that cleaves the proinflammatory cytokines IL-1β and IL-18. Active caspase-1, and in some instances caspase-11, cleaves gasdermin D that translocates to the cell membrane where it forms pores resulting in the cell death program called pyroptosis. Inflammasomes can detect a range of microbial ligands through direct interaction or indirectly through diverse cellular processes including changes in ion fluxes, production of reactive oxygen species and disruption of various host cell functions. In this review, we will focus on the NLRP3, NLRP6, NLRC4 and AIM2 inflammasomes and how they are activated and regulated during infections with Gram-positive bacteria, including Staphylococcus spp., Streptococcus spp. and Listeria monocytogenes.

Group B Streptococcus: Virulence Factors and Pathogenic Mechanism

Group B Streptococcus (GBS) or Streptococcus agalactiae is a major cause of neonatal mortality. When colonizing the lower genital tract of pregnant women, GBS may cause premature birth and stillbirth. If transmitted to the newborn, it may result in life-threatening illnesses, including sepsis, meningitis, and pneumonia. Moreover, through continuous evolution, GBS can use its original structure and unique factors to greatly improve its survival rate in the human body. This review discusses the key virulence factors that facilitate GBS invasion and colonization and their action mechanisms. A comprehensive understanding of the role of virulence factors in GBS infection is crucial to develop better treatment options and screen potential candidate molecules for the development of the vaccine.

Biological functions of sialic acid as a component of bacterial endotoxin

Lipopolysaccharide (endotoxin, LPS) is an important Gram-negative bacteria antigen. LPS of some bacteria contains sialic acid (Neu5Ac) as a component of O-antigen (O-Ag), in this review we present an overview of bacteria in which the presence of Neu5Ac has been confirmed in their outer envelope and the possible ways that bacteria can acquire Neu5Ac. We explain the role of Neu5Ac in bacterial pathogenesis, and also involvement of Neu5Ac in bacterial evading the host innate immunity response and molecular mimicry phenomenon. We also highlight the role of sialic acid in the mechanism of bacterial resistance to action of serum complement. Despite a number of studies on involvement of Neu5Ac in bacterial pathogenesis many aspects of this phenomenon are still not understood.

CRISPR Contributes to Adhesion, Invasion, and Biofilm Formation in Streptococcus agalactiae by Repressing Capsular Polysaccharide Production

The clustered regularly interspaced palindromic repeat (CRISPR)-associated (Cas) system functions classically as a prokaryotic defense system against invading mobile genetic elements, such as phages, plasmids, and viruses. Our previous study revealed that CRISPR deletion caused increased transcription of capsular polysaccharide (CPS) synthesis-related genes and severely attenuated virulence in the hypervirulent piscine Streptococcus agalactiae strain GD201008-001. Here, we found that CRISPR deficiency resulted in reduced adhesion, invasion, and biofilm formation abilities in this strain by upregulating the production of CPS. However, enhanced CPS production was not responsible for the attenuated phenotype of the ΔCRISPR mutant. RNA degradation assays indicated that inhibited transcription of the cps operon by CRISPR RNA (crRNA) was not due to the base pairing of the crRNA with the cps mRNA but to the repression of the promoter activity of cpsA, which is a putative transcriptional regulator of the capsule locus. IMPORTANCE Beyond protection from invading nucleic acids, CRISPR-Cas systems have been shown to have an important role in regulating bacterial endogenous genes. In this study, we demonstrate that crRNA inhibits the transcription of the cps operon by repressing the activity of promoter PcpsA, leading to increases in the abilities of adhesion, invasion, and biofilm formation in S. agalactiae. This study highlights the regulatory role of crRNA in bacterial physiology and provides a new explanation for the mechanism of crRNA-mediated endogenous gene regulation in S. agalactiae.

The Immunological Synapse: An Emerging Target for Immune Evasion by Bacterial Pathogens

Similar to other pathogens, bacteria have developed during their evolution a variety of mechanisms to overcome both innate and acquired immunity, accounting for their ability to cause disease or chronic infections. The mechanisms exploited for this critical function act by targeting conserved structures or pathways that regulate the host immune response. A strategic potential target is the immunological synapse (IS), a highly specialized structure that forms at the interface between antigen presenting cells (APC) and T lymphocytes and is required for the establishment of an effective T cell response to the infectious agent and for the development of long-lasting T cell memory. While a variety of bacterial pathogens are known to impair or subvert cellular processes essential for antigen processing and presentation, on which IS assembly depends, it is only recently that the possibility that IS may be a direct target of bacterial virulence factors has been considered. Emerging evidence strongly supports this notion, highlighting IS targeting as a powerful, novel means of immune evasion by bacterial pathogens. In this review we will present a brief overview of the mechanisms used by bacteria to affect IS assembly by targeting APCs. We will then summarize what has emerged from the current handful of studies that have addressed the direct impact of bacterial virulence factors on IS assembly in T cells and, based on the strategic cellular processes targeted by these factors in other cell types, highlight potential IS-related vulnerabilities that could be exploited by these pathogens to evade T cell mediated immunity.

Evolution of Human-Specific Alleles Protecting Cognitive Function of Grandmothers

The myelomonocytic receptor CD33 (Siglec-3) inhibits innate immune reactivity by extracellular V-set domain recognition of sialic acid (Sia)-containing "self-associated molecular patterns" (SAMPs). We earlier showed that V-set domain-deficient CD33-variant allele, protective against late-onset Alzheimer's Disease (LOAD), is derived and specific to the hominin lineage. We now report multiple hominin-specific CD33 V-set domain mutations. Due to hominin-specific, fixed loss-of-function mutation in the CMAH gene, humans lack N-glycolylneuraminic acid (Neu5Gc), the preferred Sia-ligand of ancestral CD33. Mutational analysis and molecular dynamics (MD)-simulations indicate that fixed change in amino acid 21 of hominin V-set domain and conformational changes related to His45 corrected for Neu5Gc-loss by switching to N-acetylneuraminic acid (Neu5Ac)-recognition. We show that human-specific pathogens Neisseria gonorrhoeae and Group B Streptococcus selectively bind human CD33 (huCD33) as part of immune-evasive molecular mimicry of host SAMPs and that this binding is significantly impacted by amino acid 21 modification. In addition to LOAD-protective CD33 alleles, humans harbor derived, population-universal, cognition-protective variants at several other loci. Interestingly, 11 of 13 SNPs in these human genes (including CD33) are not shared by genomes of archaic hominins: Neanderthals and Denisovans. We present a plausible evolutionary scenario to compile, correlate, and comprehend existing knowledge about huCD33-evolution and suggest that grandmothering emerged in humans.

Sialic acid diversity in the human gut: Molecular impacts and tools for future discovery

Sialic acids are a family of structurally related sugars that are prevalent in mucosal surfaces, including the human intestine. In the gut, sialic acids have diverse biological roles at the interface of the host epithelium and the microbiota. N-acetylneuraminic acid (Neu5Ac), the best studied sialic acid, is a nutrient source for bacteria and, when displayed on the cell surface, a binding site for host immune factors, viruses, and bacterial toxins. Neu5Ac is extensively modified by host and microbial enzymes, and the impacts of Neu5Ac derivatives on host-microbe interactions, and generally on human and microbial biology, remain underexplored. In this mini-review, we highlight recent reports describing how host and microbial proteins differentiate Neu5Ac and its derivatives, draw attention to gaps in knowledge related to sialic acid biology, and suggest cutting-edge methodologies that may expand our appreciation and understanding of Neu5Ac in health and disease.

Group B Streptococcal Neonatal Meningitis

Neonatal bacterial meningitis is a devastating disease, associated with high mortality and neurological disability, in both developed and developing countries. Streptococcus agalactiae, commonly referred to as group B Streptococcus (GBS), remains the most common bacterial cause of meningitis among infants younger than 90 days. Maternal colonization with GBS in the gastrointestinal and/or genitourinary tracts is the primary risk factor for neonatal invasive disease. Despite prophylactic intrapartum antibiotic administration to colonized women and improved neonatal intensive care, the incidence and morbidity associated with GBS meningitis have not declined since the 1970s. Among meningitis survivors, a significant number suffer from complex neurological or neuropsychiatric sequelae, implying that the pathophysiology and pathogenic mechanisms leading to brain injury and devastating outcomes are not yet fully understood. It is imperative to develop new therapeutic and neuroprotective approaches aiming at protecting the developing brain. In this review, we provide updated clinical information regarding the understanding of neonatal GBS meningitis, including epidemiology, diagnosis, management, and human evidence of the disease's underlying mechanisms. Finally, we explore the experimental models used to study GBS meningitis and discuss their clinical and physiologic relevance to the complexities of human disease.

Bacterial and Host Determinants of Group B Streptococcal Infection of the Neonate and Infant

Group B streptococci (GBS) are Gram-positive β-hemolytic bacteria that can cause serious and life-threatening infections in neonates manifesting as sepsis, pneumonia, meningitis, osteomyelitis, and/or septic arthritis. Invasive GBS infections in neonates in the first week of life are referred to as early-onset disease (EOD) and thought to be acquired by the fetus through exposure to GBS in utero or to vaginal fluids during birth. Late-onset disease (LOD) refers to invasive GBS infections between 7 and 89 days of life. LOD transmission routes are incompletely understood, but may include breast milk, household contacts, nosocomial, or community sources. Invasive GBS infections and particularly meningitis may result in significant neurodevelopmental injury and long-term disability that persists into childhood and adulthood. Globally, EOD and LOD occur in more than 300,000 neonates and infants annually, resulting in 90,000 infant deaths and leaving more than 10,000 infants with a lifelong disability. In this review, we discuss the clinical impact of invasive GBS neonatal infections and then summarize virulence and host factors that allow the bacteria to exploit the developing neonatal immune system and target organs. Specifically, we consider the mechanisms known to enable GBS invasion into the neonatal lung, blood vessels and brain. Understanding mechanisms of GBS invasion and pathogenesis relevant to infections in the neonate and infant may inform the development of therapeutics to prevent or mitigate injury, as well as improve risk stratification.

Inhibitory pattern recognition receptors

Pathogen- and damage-associated molecular patterns are sensed by the immune system's pattern recognition receptors (PRRs) upon contact with a microbe or damaged tissue. In situations such as contact with commensals or during physiological cell death, the immune system should not respond to these patterns. Hence, immune responses need to be context dependent, but it is not clear how context for molecular pattern recognition is provided. We discuss inhibitory receptors as potential counterparts to activating pattern recognition receptors. We propose a group of inhibitory pattern recognition receptors (iPRRs) that recognize endogenous and microbial patterns associated with danger, homeostasis, or both. We propose that recognition of molecular patterns by iPRRs provides context, helps mediate tolerance to microbes, and helps balance responses to danger signals.

O-Acetylation of Capsular Polysialic Acid Enables Escherichia coli K1 Escaping from Siglec-Mediated Innate Immunity and Lysosomal Degradation of E. coli-Containing Vacuoles in Macrophage-Like Cells

Escherichia coli K1 causes bacteremia and meningitis in human neonates. The K1 capsule, an α2,8-linked polysialic acid (PSA) homopolymer, is its essential virulence factor. PSA is usually partially modified by O-acetyl groups. It is known that O-acetylation alters the antigenicity of PSA, but its impact on the interactions between E. coli K1 and host cells is unclear. In this study, a phase variant was obtained by passage of E. coli K1 parent strain, which expressed a capsule with 44% O-acetylation whereas the capsule of the parent strain has only 3%. The variant strain showed significantly reduced adherence and invasion to macrophage-like cells in comparison to the parent strain. Furthermore, we found that O-acetylation of PSA enhanced the modulation of trafficking of E. coli-containing vacuoles (ECV), enabling them to avoid fusing with lysosomes in these cells. Intriguingly, by using quartz crystal microbalance, we demonstrated that the PSA purified from the parent strain interacted with human sialic acid-binding immunoglobulin-like lectins (Siglecs), including Siglec-5, Siglec-7, Siglec-11, and Siglec-14. However, O-acetylated PSA from the variant interacted much less and also suppressed the production of Siglec-mediated proinflammatory cytokines. The adherence of the parent strain to human macrophage-like cells was significantly blocked by monoclonal antibodies against Siglec-11 and Siglec-14. Furthermore, the variant strain caused increased bacteremia and higher lethality in neonatal mice compared to the parent strain. These data elucidate that O-acetylation of K1 capsule enables E. coli to escape from Siglec-mediated innate immunity and lysosomal degradation; therefore, it is a strategy used by E. coli K1 to regulate its virulence.

IMPORTANCEEscherichia coli K1 is a leading cause of neonatal meningitis. The mortality and morbidity of this disease remain significantly high despite antibiotic therapy. One major limitation on advances in prevention and therapy for meningitis is an incomplete understanding of its pathogenesis. E. coli K1 is surrounded by PSA, which is observed to have high-frequency variation of O-acetyl modification. Here, we present an in-depth study of the function of O-acetylation in PSA at each stage of host-pathogen interaction. We found that a high level of O-acetylation significantly interfered with Siglec-mediated bacterial adherence to macrophage-like cells, and blunted the proinflammatory response. Furthermore, the O-acetylation of PSA modulated the trafficking of ECVs to prevent them from fusing with lysosomes, enabling them to escape degradation by lysozymes within these cells. Elucidating how subtle modification of the capsule enhances bacterial defenses against host innate immunity will enable the future development of effective drugs or vaccines against infection by E. coli K1.

Siglecs Modulate Activities of Immune Cells Through Positive and Negative Regulation of ROS Generation

Reactive oxygen species (ROS) are a group of oxygen-containing highly-reactive molecules produced from oxidative metabolic processes or in response to intracellular signals like cytokines and external stimuli like pathogen attack. They regulate a range of physiological processes and are involved in innate immune responses against infectious agents. Deregulation of ROS contributes to a plethora of disease conditions. Sialic acids are carbohydrates, present on cell surfaces or soluble proteins. Sialic acid-binding immunoglobulin-like lectins (Siglecs) recognize and bind to sialic acids. These are widely expressed on various types of immune cells. Siglecs modulate immune activation and can promote or inhibit ROS generation under different contexts. Siglecs promote ROS-dependent cell death in neutrophils and eosinophils while limiting oxidative stress associated with chronic obstructive pulmonary disease (COPD), sickle cell disease (SCD), coronavirus disease-2019 (COVID-19), etc. This review distinguishes itself in summarizing the current understanding of the role of Siglecs in moderating ROS production and their distinct effect on different immune cells; that ultimately determine the cellular response and the disease outcome. This is an important field of investigation having scope for both expansion and medical importance.

Analysis of Susceptibility to the Antimicrobial and Anti-Biofilm Activity of Human Milk Lactoferrin in Clinical Strains of Streptococcus agalactiae With Diverse Capsular and Sequence Types

Group B Streptococcus (GBS) is one of the leading infection-related causes of adverse maternal and neonatal outcomes. This includes chorioamnionitis, which leads to preterm ruptures of membranes and can ultimately result in preterm or stillbirth. Infection can also lead to maternal and neonatal sepsis that may contribute to mortality. Currently, treatment for GBS infection include a bolus of intrapartum antibiotic prophylaxis to mothers testing positive for GBS colonization during late pregnancy. Lactoferrin is an antimicrobial peptide expressed in human breast milk, mucosal epithelia, and secondary granules of neutrophils. We previously demonstrated that lactoferrin possesses antimicrobial and antibiofilm properties against several strains of GBS. This is largely due to the ability of lactoferrin to bind and sequester iron. We expanded upon that study by assessing the effects of purified human breast milk lactoferrin against a panel of phenotypically and genetically diverse isolates of GBS. Of the 25 GBS isolates screened, lactoferrin reduced bacterial growth in 14 and biofilm formation in 21 strains. Stratifying the data, we observed that colonizing strains were more susceptible to the growth inhibition activity of lactoferrin than invasive isolates at lactoferrin concentrations between 250-750 µg/mL. Treatment with 750 µg/mL of lactoferrin resulted in differences in bacterial growth and biofilm formation between discrete sequence types. Differences in bacterial growth were also observed between capsular serotypes 1a and III. Maternally isolated strains were more susceptible to lactoferrin with respect to bacterial growth, but not biofilm formation, compared to neonatal sepsis isolates. Finally, high biofilm forming GBS strains were more impacted by lactoferrin across all isolates tested. Taken together, this study demonstrates that lactoferrin possesses antimicrobial and antibiofilm properties against a wide range of GBS isolates, with maternally isolated colonizing strains being the most susceptible.

Sweet impersonators: Molecular mimicry of host glycans by bacteria

All bacteria display surface-exposed glycans that can play an important role in their interaction with the host and in select cases mimic the glycans found on host cells, an event called molecular or glycan mimicry. In this review, we highlight the key bacteria that display human glycan mimicry and provide an overview of the involved glycan structures. We also discuss the general trends and outstanding questions associated with human glycan mimicry by bacteria. Finally, we provide an overview of several techniques that have emerged from the discipline of chemical glycobiology, which can aid in the study of the composition, variability, interaction and functional role of these mimicking glycans.

Group B Streptococcus cpsE Is Required for Serotype V Capsule Production and Aids in Biofilm Formation and Ascending Infection of the Reproductive Tract during Pregnancy

Group B Streptococcus (GBS) is an encapsulated Gram-positive pathogen that causes ascending infections of the reproductive tract during pregnancy. The capsule of this organism is a critical virulence factor that has been implicated in a variety of cellular processes to promote pathogenesis. Primarily comprised of carbohydrates, the GBS capsule and its synthesis is driven by the capsule polysaccharide synthesis (cps) operon. The cpsE gene within this operon encodes a putative glycosyltransferase that is responsible for the transfer of a Glc-1-P from UDP-Glc to an undecaprenyl lipid molecule. We hypothesized that the cpsE gene product is important for GBS virulence and ascending infection during pregnancy. Our work demonstrates that a GBS cpsE mutant secretes fewer carbohydrates, has a reduced capsule, and forms less biofilm than the wild-type parental strain. We show that, compared to the parental strain, the ΔcpsE deletion mutant is more readily taken up by human placental macrophages and has a significantly attenuated ability to invade and proliferate in the mouse reproductive tract. Taken together, these results demonstrate that the cpsE gene product is an important virulence factor that aids in GBS colonization and invasion of the gravid reproductive tract.

Identification of an N-acetylneuraminic acid-presenting bacteria isolated from a human microbiome

N-Acetylneuraminic acid is the most abundant sialic acid (SA) in humans and is expressed as the terminal sugar on intestinal mucus glycans. Several pathogenic bacteria harvest and display host SA on their own surfaces to evade Siglec-mediated host immunity. While previous studies have identified bacterial enzymes associated with SA catabolism, no reported methods permit the selective labeling, tracking, and quantitation of SA-presenting microbes within complex multi-microbial systems. We combined metabolic labeling, click chemistry, 16S rRNA gene, and whole-genome sequencing to track and identify SA-presenting microbes from a cultured human fecal microbiome. We isolated a new strain of Escherichia coli that incorporates SA onto its own surface and encodes for the nanT, neuA, and neuS genes necessary for harvesting and presenting SA. Our method is applicable to the identification of SA-presenting bacteria from human, animal, and environmental microbiomes, as well as providing an entry point for the investigation of surface-expressed SA-associated structures.

Structural and Biosynthetic Diversity of Nonulosonic Acids (NulOs) That Decorate Surface Structures in Bacteria

Nonulosonic acids (NulOs) are a diverse family of 9-carbon α-keto acid sugars that are involved in a wide range of functions across all branches of life. The family of NulOs includes the sialic acids as well as the prokaryote-specific NulOs. Select bacteria biosynthesize the sialic acid N-acetylneuraminic acid (Neu5Ac), and the ability to produce this sugar and its subsequent incorporation into cell-surface structures is implicated in a variety of bacteria-host interactions. Furthermore, scavenging of sialic acid from the environment for energy has been characterized across a diverse group of bacteria, mainly human commensals and pathogens. In addition to sialic acid, bacteria have the ability to biosynthesize prokaryote-specific NulOs, of which there are several known isomers characterized. These prokaryotic NulOs are similar in structure to Neu5Ac but little is known regarding their role in bacterial physiology. Here, we discuss the diversity in structure, the biosynthesis pathways, and the functions of bacteria-specific NulOs. These carbohydrates are phylogenetically widespread among bacteria, with numerous structurally unique modifications recognized. Despite the diversity in structure, the NulOs are involved in similar functions such as motility, biofilm formation, host colonization, and immune evasion.

Development of BODIPY labelled sialic acids as sialyltransferase substrates for direct detection of terminal galactose on N- and O-linked glycans

Glycans on proteins and cell surfaces are useful biomarkers for determining functional interactions with glycan binding proteins, potential disease states, or indeed level of differentiation. The ability to rapidly and sensitively detect or tag specific glycans on proteins provides a diagnostic tool with wide application in chemical glycobiology. The monosaccharide N-acetylneuraminic acid (sialic acid) is a key player in these interactions and the manipulation and control of sialylation levels has been an important research focus, particularly in the development of therapeutic proteins. Using sialyltransferases to tag specific glycans provides a rapid means of determining what types of glycans are present. We have synthesized two variants of sialic acid carrying the fluorophore BODIPY (4,4 -Difluoro-4-boro-3a,4a-diaza-s-indacene) and examined its use with several different sialyltransferases on a variety of protein substrates and cell surface glycans. Our data show that there are significant differences between various enzymes ability to transfer the labelled sialic acids, and that the type of N-glycan and target protein strongly influences this activity.

A Siglec-1-like lectin from Nile tilapia (Oreochromis niloticus) possesses functions of agglutination and mediation of macrophage phagocytic activity

Siglec-1, one of the sialic acid-binding immunoglobulin-type lectins, is closely related to the recognition of host-pathogen and cell-cell interactions in the adaptive and innate immune systems. In this communication, a Siglec-1-like gene (OnSiglec-1-like) from Nile tilapia (Oreochromis niloticus) was analyzed. Relative expression revealed that the OnSiglec-1-like was expressed in all tested tissues, and the highest expression was found in the anterior kidney. Upon Streptococcus agalactiae (S. agalactiae) infection, the expression of OnSiglec-1-like was up-regulated in anterior kidney and spleen significantly in vivo. Additionally, the same phenomenon was observed in anterior kidney leukocytes upon LPS and S. agalactiae challenges as well in vitro. Western-blotting and ELISA analyses revealed that recombinant OnSiglec-1-like protein possessed high binding activity to LTA, LPS and S. agalactiae. Further, the recombinant OnSiglec-1-like was able to agglutinate S. agalactiae. Moreover, with the digestion of specific sialidase, the phagocytic ability of macrophages to S. agalactiae was greatly enhanced. Taken together, these results indicated that the Siglec-1-like possesses conserved functions of agglutination and promotion of macrophage phagocytic activity in Nile tilapia.

Like mother, like microbe: human milk oligosaccharide mediated microbiome symbiosis

Starting shortly after parturition, and continuing throughout our lifetime, the gut microbiota coevolves with our metabolic and neurological programming. This symbiosis is regulated by a complex interplay between the host and environmental factors, including diet and lifestyle. Not surprisingly, the development of this microbial community is of critical importance to health and wellness. In this targeted review, we examine the gut microbiome from birth to 2 years of age to characterize the role human milk oligosaccharides play in early formation of microbial flora.

Siglecs at the Host-Pathogen Interface

Siglecs are sialic acid (Sia) recognizing immunoglobulin-like receptors expressed on the surface of all the major leukocyte lineages in mammals. Siglecs recognize ubiquitous Sia epitopes on various glycoconjugates in the cell glycocalyx and transduce signals to regulate immunological and inflammatory activities of these cells. The subset known as CD33-related Siglecs is principally inhibitory receptors that suppress leukocyte activation, and recent research has shown that a number of bacterial pathogens use Sia mimicry to engage these Siglecs as an immune evasion strategy. Conversely, Siglec-1 is a macrophage phagocytic receptor that engages GBS and other sialylated bacteria to promote effective phagocytosis and antigen presentation for the adaptive immune response, whereas certain viruses and parasites use Siglec-1 to gain entry to immune cells as a proximal step in the infectious process. Siglecs are positioned in crosstalk with other host innate immune sensing pathways to modulate the immune response to infection in complex ways. This chapter summarizes the current understanding of Siglecs at the host-pathogen interface, a field of study expanding in breadth and medical importance, and which provides potential targets for immune-based anti-infective strategies.

Siglec-14 Enhances NLRP3-Inflammasome Activation in Macrophages

Pathogenic microorganisms are sensed by the inflammasome, resulting in the release of the pro-immune and proinflammatory cytokine interleukin-1β (IL-1β). In humans, the paired <underline>s</underline>ialic acid-binding Ig-like lectin receptors Siglec-5 (inhibitory) and Siglec-14 (activating) have been shown to have reciprocal roles in regulating macrophage immune responses, but their interaction with IL-1β signaling and the inflammasome has not been characterized. Here we show that in response to known inflammasome activators (ATP, nigericin) or the sialic acid-expressing human bacterial pathogen group B Streptococcus (GBS), the presence of Siglec-14 enhances, whereas Siglec-5 reduces, inflammasome activation and macrophage IL-1β release. Human THP-1 macrophages stably transfected with Siglec-14 exhibited increased caspase-1 activation, IL-1β release and pyroptosis after GBS infection, in a manner blocked by a specific inhibitor of nucleotide-binding domain leucine-rich repeat protein 3 (NLRP3), a protein involved in inflammasome assembly. Another leading pathogen, Streptococcus pneumoniae, lacks sialic acid but rather prominently expresses a sialidase, which cleaves sialic acid from macrophages, eliminating cis- interactions with the lectin receptor, thus attenuating Siglec-14 induced IL-1β secretion. Vimentin, a cytoskeletal protein released during macrophage inflammatory activation is known to induce the inflammasome. We found that vimentin has increased interaction with Siglec-14 compared to Siglec-5, and this interaction heightened IL-1β production by Siglec-14-expressing cells. Siglec-14 is absent from some humans because of a SIGLEC5/14 fusion polymorphism, and we found increased IL-1β expression in primary macrophages from SIGLEC14+/+ individuals compared to those with the SIGLEC14-/+ and SIGLEC14-/- genotypes. Collectively, our results identify a new immunoregulatory role of Siglec-14 as a positive regulator of NLRP3 inflammasome activation.

Morphological characterization of the adherence and invasion of Streptococcus agalactiae to the intestinal mucosa of tilapia Oreochromis sp.: An in vitro model

Streptococcosis in tilapia Oreochromis sp. is possibly the most important bacterial disease for fish production worldwide. In Colombia, streptococcosis is caused by Streptococcus agalactiae (GBS), but in other countries, Streptococcus iniae is also involved. Prevention of streptococcosis is required and must be addressed for economic, social, international trade and public health reasons. This research used an in vitro culture of tilapia intestine to detail the intestinal mucosal response once the pathogen contacts the epithelium. We show that S. agalactiae sheds off its capsule to adhere to the epithelium. The bacterium adheres as a single individuum, in groups or in chains and is able to divide on the apical border of enterocytes. GBS adheres at and invades exclusively through the apical portion of the intestinal folds, using the transepithelial route. Once within the cytoplasm of enterocytes, the bacteria continue to divide. On the basolateral side of the epithelium, the microorganisms leave the cells to reach the propria and travel through the microcirculation. No evidence of an immuno-inflammatory reaction or goblet cell response in the epithelium or the lamina propria was seen during the process of adherence and invasion of the pathogen.

Glycointeractions in bacterial pathogenesis

Many important interactions between bacterial pathogens and their hosts are highly specific binding events that involve host or pathogen carbohydrate structures (glycans). Glycan interactions can mediate adhesion, invasion and immune evasion and can act as receptors for toxins. Several bacterial pathogens can also enzymatically alter host glycans to reveal binding targets, degrade the host cell glycans or alter the function of host glycoproteins. In recent years, high-throughput screening technologies, such as lectin, glycan and mucin microarrays, have transformed the field by identifying new bacterial-host glycointeractions, which are crucial for colonization, persistence and disease. In this Review, we discuss interactions involving both host and bacterial glycans that have a role in bacterial pathogenesis. We also highlight recent technological advances that have illuminated the glycoscience of microbial pathogenesis.

Diminished Capsule Exacerbates Virulence, Blood-Brain Barrier Penetration, Intracellular Persistence, and Antibiotic Evasion of Hyperhemolytic Group B Streptococci

Group B streptococci (GBS) are encapsulated, β-hemolytic bacteria that are a common cause of infections in human newborns and certain adults. Two factors important for GBS virulence are the sialic acid capsular polysaccharide that promotes immune evasion and the hemolytic pigment that induces host cell cytotoxcity. These virulence factors are often oppositely regulated by the CovR/CovS two-component system. Clinical GBS strains exhibiting hyperhemolysis and low capsule due to pathoadaptive covR/S mutations have been isolated from patients. Given the importance of capsule to GBS virulence, we predicted that a decrease or loss of capsule would attenuate the virulence of covR/S mutants. Surprisingly, hyperhemolytic GBS with low or no capsule exhibit increased virulence, intracellular persistence, and blood-brain barrier penetration, which was independent of a Trojan horse mechanism of barrier penetration. Additionally, intracellular persistence enabled both hemolytic and hyperhemolytic GBS to evade antibiotics routinely used to treat these infections. The finding that diminished capsule expression promotes GBS virulence, intracellular persistence, and antibiotic evasion has important implications for sustained antibiotic therapy and efficacy of capsule-based vaccines.

Dual actions of group B Streptococcus capsular sialic acid provide resistance to platelet-mediated antimicrobial killing

Circulating platelets have important functions in thrombosis and in modulating immune and inflammatory responses. However, the role of platelets in innate immunity to bacterial infection is largely unexplored. While human platelets rapidly kill Staphylococcus aureus, we found the neonatal pathogen group B Streptococcus (GBS) to be remarkably resistant to platelet killing. GBS possesses a capsule polysaccharide (CPS) with terminal α2,3-linked sialic acid (Sia) residues that mimic a common epitope present on the human cell surface glycocalyx. A GBS mutant deficient in CPS Sia was more efficiently killed by human platelets, thrombin-activated platelet releasate, and synthetic platelet-associated antimicrobial peptides. GBS Sia is known to bind inhibitory Sia-recognizing Ig superfamily lectins (Siglecs) to block neutrophil and macrophage activation. We show that human platelets also express high levels of inhibitory Siglec-9 on their surface, and that GBS can engage this receptor in a Sia-dependent manner to suppress platelet activation. In a mouse i.v. infection model, antibody-mediated platelet depletion increased susceptibility to platelet-sensitive S. aureus but did not alter susceptibility to platelet-resistant GBS. Elimination of murine inhibitory Siglec-E partially reversed platelet suppression in response to GBS infection. We conclude that GBS Sia has dual roles in counteracting platelet antimicrobial immunity: conferring intrinsic resistance to platelet-derived antimicrobial components and inhibiting platelet activation through engagement of inhibitory Siglecs. We report a bacterial virulence factor for evasion of platelet-mediated innate immunity.

Evolution of the exclusively human pathogen Neisseria gonorrhoeae: Human-specific engagement of immunoregulatory Siglecs

Neisseria gonorrhoeae causes the sexually transmitted disease gonorrhea exclusively in humans and uses multiple strategies to infect, including acquisition of host sialic acids that cap and mask lipooligosaccharide termini, while restricting complement activation. We hypothesized that gonococci selectively target human anti-inflammatory sialic acid-recognizing Siglec receptors on innate immune cells to blunt host responses and that pro-inflammatory Siglecs and SIGLEC pseudogene polymorphisms represent host evolutionary adaptations to counteract this interaction. N. gonorrhoeae can indeed engage multiple human but not chimpanzee CD33rSiglecs expressed on innate immune cells and in the genitourinary tract--including Siglec-11 (inhibitory) and Siglec-16 (activating), which we detected for the first time on human cervical epithelium. Surprisingly, in addition to LOS sialic acid, we found that gonococcal porin (PorB) mediated binding to multiple Siglecs. PorB also bound preferentially to human Siglecs and not chimpanzee orthologs, modulating host immune reactions in a human-specific manner. Lastly, we studied the distribution of null SIGLEC polymorphisms in a Namibian cohort with a high prevalence of gonorrhea and found that uninfected women preferentially harbor functional SIGLEC16 alleles encoding an activating immune receptor. These results contribute to the understanding of the human specificity of N. gonorrhoeae and how it evolved to evade the human immune defense.

The Double Life of Group B Streptococcus: Asymptomatic Colonizer and Potent Pathogen

Group B streptococcus (GBS) is a β-hemolytic gram-positive bacterium that colonizes the lower genital tract of approximately 18% of women globally as an asymptomatic member of the gastrointestinal and/or vaginal flora. If established in other host niches, however, GBS is highly pathogenic. During pregnancy, ascending GBS infection from the vagina to the intrauterine space is associated with preterm birth, stillbirth, and fetal injury. In addition, vertical transmission of GBS during or after birth results in life-threatening neonatal infections, including pneumonia, sepsis, and meningitis. Although the mechanisms by which GBS traffics from the lower genital tract to vulnerable host niches are not well understood, recent advances have revealed that many of the same bacterial factors that promote asymptomatic vaginal carriage also facilitate dissemination and virulence. Furthermore, highly pathogenic GBS strains have acquired unique factors that enhance survival in invasive niches. Several host factors also exist that either subdue GBS upon vaginal colonization or alternatively permit invasive infection. This review summarizes the GBS and host factors involved in GBS's state as both an asymptomatic colonizer and an invasive pathogen. Gaining a better understanding of these mechanisms is key to overcoming the challenges associated with vaccine development and identification of novel strategies to mitigate GBS virulence.

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.

Structural and functional characterization of a modified legionaminic acid involved in glycosylation of a bacterial lipopolysaccharide

Nonulosonic acids (NulOs) are a diverse family of α-keto acid carbohydrates present across all branches of life. Bacteria biosynthesize NulOs among which are several related prokaryotic-specific isomers and one of which, N-acetylneuraminic acid (sialic acid), is common among all vertebrates. Bacteria display various NulO carbohydrates on lipopolysaccharide (LPS), and the identities of these molecules tune host-pathogen recognition mechanisms. The opportunistic bacterial pathogen Vibrio vulnificus possesses the genes for NulO biosynthesis; however, the structures and functions of the V. vulnificus NulO glycan are unknown. Using genetic and chemical approaches, we show here that the major NulO produced by a clinical V. vulnificus strain CMCP6 is 5-N-acetyl-7-N-acetyl-d-alanyl-legionaminic acid (Leg5Ac7AcAla). The CMCP6 strain could catabolize modified legionaminic acid, whereas V. vulnificus strain YJ016 produced but did not catabolize a NulO without the N-acetyl-d-alanyl modification. In silico analysis suggested that Leg5Ac7AcAla biosynthesis follows a noncanonical pathway but appears to be present in several bacterial species. Leg5Ac7AcAla contributed to bacterial outer-membrane integrity, as mutant strains unable to produce or incorporate Leg5Ac7AcAla into the LPS have increased membrane permeability, sensitivity to bile salts and antimicrobial peptides, and defects in biofilm formation. Using the crustacean model, Artemia franciscana, we demonstrate that Leg5Ac7AcAla-deficient bacteria have decreased virulence potential compared with WT. Our data indicate that different V. vulnificus strains produce multiple NulOs and that the modified legionaminic acid Leg5Ac7AcAla plays a critical role in the physiology, survivability, and pathogenicity of V. vulnificus CMCP6.

Possible Influences of Endogenous and Exogenous Ligands on the Evolution of Human Siglecs

Sialic acids, a group of acidic sugars abundantly expressed in the tissues of deuterostome animals but rarely found in microbes, serve as a "signature of self" for these animals. Cognate sensors for sialic acids include Siglecs, a family of transmembrane lectins of vertebrate immune systems that recognize glycans containing sialic acids. A type of sialic acid called N-glycolylneuraminic acid (Neu5Gc) is abundant in many mammalian lineages including great apes, the closest extant relatives of modern human, but was lost in the lineage leading to modern human via the pseudogenization of the CMAH gene encoding the enzyme that converts N-acetylneuraminic acid (Neu5Ac) to Neu5Gc. Loss of Neu5Gc appears to have influenced the evolution of human Siglecs, such as the adjustment of sialic acid binding preferences and the inactivation of at least one Siglec. In addition, various mechanistic studies using model systems and genetic association studies have revealed that some human Siglecs interact with pathogens and influence the outcome of infections, and these pathogens in turn likely influence the evolution of these Siglecs. By understanding the evolutionary forces affecting Siglecs, we shall achieve a better appreciation of Siglec functions, and by understanding Siglec functions, we can obtain deeper insight into the evolutionary processes driving Siglec evolution.

Siglec-7 engagement by GBS β-protein suppresses pyroptotic cell death of natural killer cells

Natural killer (NK) cells are innate immune lymphocytes that recognize and destroy abnormal host cells, such as tumor cells or those infected by viral pathogens. To safely accomplish these functions, NK cells display activating receptors that detect stress molecules or viral ligands displayed at the cell surface, balanced by inhibitory receptors that bind to self-molecules. To date, such activating and inhibitory receptors on NK cells are not known to recognize bacterial determinants. Moreover, NK cell responses to direct interactions with extracellular bacteria are poorly explored. In this study, we observed the human neonatal pathogen group B Streptococcus (GBS) can directly engage human NK cells. The interaction was mediated through the B6N segment of streptococcal β-protein, binding to the inhibitory receptor Siglec-7 via its amino-terminal V-set domain. Unlike classical Siglec binding, the interaction is also independent of its sialic acid recognition property. In contrast to WT GBS, mutants lacking β-protein induced efficient pyroptosis of NK cells through the NLRP3 inflammasome, with production and secretion of the proinflammatory cytokine IL-1β and dissemination of the cytotoxic molecule granzyme B. We postulate that GBS evolved β-protein engagement of inhibitory human Siglec-7 to suppress the pyroptotic response of NK cells and thereby block recruitment of a broader innate immune response, i.e., by "silencing the sentinel."

Stable Expression of Modified Green Fluorescent Protein in Group B Streptococci To Enable Visualization in Experimental Systems

Group B streptococcus (GBS) is a Gram-positive bacterium associated with various diseases in humans and animals. Many studies have examined GBS physiology, virulence, and microbe-host interactions using diverse imaging approaches, including fluorescence microscopy. Strategies to label and visualize GBS using fluorescence biomarkers have been limited to antibody-based methods or nonspecific stains that bind DNA or protein; an effective plasmid-based system to label GBS with a fluorescence biomarker would represent a useful visualization tool. In this study, we developed and validated a green fluorescent protein (GFP)-variant-expressing plasmid, pGU2664, which can be applied as a marker to visualize GBS in experimental studies. The synthetic constitutively active CP25 promoter drives strong and stable expression of the GFPmut3 biomarker in GBS strains carrying pGU2664. GBS maintains GFPmut3 activity at different phases of growth. The application of fluorescence polarization enables easy discrimination of GBS GFPmut3 activity from the autofluorescence of culture media commonly used to grow GBS. Differential interference contrast microscopy, in combination with epifluorescence microscopy to detect GFPmut3 in GBS, enabled visualization of bacterial attachment to live human epithelial cells in real time. Plasmid pGU2664 was also used to visualize phenotypic differences in the adherence of wild-type GBS and an isogenic gene-deficient mutant strain lacking CovR (the control of virulence regulator) in adhesion assays. The system for GFPmut3 expression in GBS described in this study provides a new tool for the visualization of this organism in diverse research applications. We discuss the advantages and consider the limitations of this fluorescent biomarker system developed for GBS.IMPORTANCE Group B streptococcus (GBS) is a bacterium associated with various diseases in humans and animals. This study describes the development of a strategy to label and visualize GBS using a fluorescence biomarker, termed GFPmut3. We show that this biomarker can be successfully applied to track the growth of bacteria in liquid medium, and it enables the detailed visualization of GBS in the context of live human cells in real-time microscopic analysis. The system for GFPmut3 expression in GBS described in this study provides a new tool for the visualization of this organism in diverse research applications.

The first report of siglec-3/CD33 gene in a teleost (rock bream, Oplegnathus fasciatus): An analysis of its spatial expression during stimulation to red seabream iridovirus (RSIV) and two bacterial pathogens

Siglec-3/CD33 is a myeloid-specific inhibitory receptor that is expressed on cells of the immune system, where it is believed to play a regulatory role, modulating the inflammatory and immune responses. We characterized CD33 (RbCD33) in rock bream which is a transmembrane protein with two IG-like domains and a cytoplasmic tail. It has a deduced amino acid sequence of 390 residues and has tyrosine-based signaling motifs in the cytoplasmic tail. The RbCD33 mRNA was highly expressed in peripheral blood leukocytes and was also detected in the muscle, spleen, skin, head kidney, gills, trunk kidney, heart, stomach, brain, intestine and liver by quantitative real-time PCR. A temporal variation in expression of RbCD33 was observed in different tissues after stimulating with E. tarda, S. iniae and red seabream iridovirus (RSIV). In the head kidney tissue, E. tarda and S. iniae induced RbCD33, while a down regulation was observed with RSIV. In addition, in spleen tissue, S. iniae caused a very high induction of RbCD33 in comparison with an E. tarda and RSIV challenge. In the liver and gill tissues, all three pathogens induced a high expression of RbCD33. The expression pattern in various tissues and its high induction after pathogen stimulation suggests that RbCD33 plays an important role in initiating the immune response via the inhibition of signal transduction of the myeloid lineage cells.

l-rhamnose-binding lectins (RBLs) in Nile tilapia, Oreochromis niloticus: Characterization and expression profiling in mucosal tissues

Rhamnose-binding lectins (RBLs) are crucial elements associated with innate immune responses to infections and have been characterized from a variety of teleost fishes. Given the importance of RBL in teleost fishes, we sought to study the diversity and expression profiles of RBLs in an important cultured fish, Nile tilapia (Oreochromis niloticus) following experimental infection with Streptococcus agalactiae, a major cause of streptococcosis in farmed tilapia. In this study, four predicted RBL genes were identified from Nile tilapia and were designated as OnRBL3a, OnRBL3b, OnRBL3c, and OnRBL3d. These OnRBLs were composed of two tandem-repeated type five carbohydrate recognition domains (CRDs), classified as type IIIc, and all clustered together phylogenetically. OnRBL-CRDs shared conserved topology of eight cysteine residues, characteristic peptide motifs of -YGR- and -DPC- (or -FGR- and -DTC-), and similar exon/intron organization. OnRBLs had the highest expression in immune-related tissues, gills, intestine or liver. However, the changes of OnRBL expression in the gills and intestine at 2 h, 4 h and 24 h post S. agalactiae challenge were modest, suggesting that tilapia may not mediate the entry or confront the infection of S. agalactiae through induction of RBL genes. The observed expression pattern may be related to the RBL type and CRD composition, S. agalactiae pathogenesis, the accessibility of ligands on the bacterial surface, and/or the species of fish. OnRBLs characterized in this study were the first RBL members identified in Nile tilapia and their characterization will expand our knowledge of RBLs in immunity.

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.

Enterococcus faecalis Promotes Innate Immune Suppression and Polymicrobial Catheter-Associated Urinary Tract Infection

Enterococcus faecalis, a member of the human gastrointestinal microbiota, is an opportunistic pathogen associated with hospital-acquired wound, bloodstream, and urinary tract infections. E. faecalis can subvert or evade immune-mediated clearance, although the mechanisms are poorly understood. In this study, we examined E. faecalis-mediated subversion of macrophage activation. We observed that E. faecalis actively prevents NF-κB signaling in mouse RAW264.7 macrophages in the presence of Toll-like receptor agonists and during polymicrobial infection with Escherichia coliE. faecalis and E. coli coinfection in a mouse model of catheter-associated urinary tract infection (CAUTI) resulted in a suppressed macrophage transcriptional response in the bladder compared to that with E. coli infection alone. Finally, we demonstrated that coinoculation of E. faecalis with a commensal strain of E. coli into catheterized bladders significantly augmented E. coli CAUTI. Taken together, these results support the hypothesis that E. faecalis suppression of NF-κB-driven responses in macrophages promotes polymicrobial CAUTI pathogenesis, especially during coinfection with less virulent or commensal E. coli strains.