Deciphering the roles of outer membrane protein A extracellular loops in the pathogenesis of Escherichia coli K1 meningitis

A high-throughput sequencing approach identifies immunotherapeutic targets for bacterial meningitis in neonates

BACKGROUND

Worldwide, Escherichia coli is the leading cause of neonatal Gram-negative bacterial meningitis, but full understanding of the pathogenesis of this disease is not yet achieved. Moreover, to date, no vaccine is available against bacterial neonatal meningitis.

METHODS

Here, we used Transposon Sequencing of saturated banks of mutants (TnSeq) to evaluate E. coli K1 genetic fitness in murine neonatal meningitis. We identified E. coli K1 genes encoding for factors important for systemic dissemination and brain infection, and focused on products with a likely outer-membrane or extra-cellular localization, as these are potential vaccine candidates. We used in vitro and in vivo models to study the efficacy of active and passive immunization.

RESULTS

We selected for further study the conserved surface polysaccharide Poly-β-(1-6)-N-Acetyl Glucosamine (PNAG), as a strong candidate for vaccine development. We found that PNAG was a virulence factor in our animal model. We showed that both passive and active immunization successfully prevented and/or treated meningitis caused by E. coli K1 in neonatal mice. We found an excellent opsonophagocytic killing activity of the antibodies to PNAG and in vitro these antibodies were also able to decrease binding, invasion and crossing of E. coli K1 through two blood brain barrier cell lines. Finally, to reinforce the potential of PNAG as a vaccine candidate in bacterial neonatal meningitis, we demonstrated that Group B Streptococcus, the main cause of neonatal meningitis in developed countries, also produced PNAG and that antibodies to PNAG could protect in vitro and in vivo against this major neonatal pathogen.

INTERPRETATION

Altogether, these results indicate the utility of a high-throughput DNA sequencing method to identify potential immunotherapy targets for a pathogen, including in this study a potential broad-spectrum target for prevention of neonatal bacterial infections.

FUNDINGS

ANR Seq-N-Vaq, Charles Hood Foundation, Hearst Foundation, and Groupe Pasteur Mutualité.

Human Monoclonal Antibodies to Escherichia coli Outer Membrane Protein A Porin Domain Cause Aggregation but Do Not Alter In Vivo Bacterial Burdens in a Murine Sepsis Model

Escherichia coli is one of the most frequent human pathogens, increasingly exhibits antimicrobial resistance, and has complex interactions with the host immune system. E. coli exposure or infection can result in the generation of antibodies specific for outer membrane protein A (OmpA), a multifunctional porin. We identified four OmpA-specific naturally occurring antibodies from healthy human donor B cells and assessed their interactions with E. coli and OmpA. These antibodies are highly specific for OmpA, exhibiting no cross-reactivity to a strain lacking ompA and retaining binding to both laboratory and clinical isolates of E. coli in enzyme-linked immunosorbent assay (ELISA) and immunofluorescence assays. One monoclonal antibody (Mab), designated ECOL-11, is specific for the extracellular N-terminal porin domain of OmpA and induces growth phase-specific bacterial aggregation. This aggregation is not induced by the fragment antigen binding (Fab) form of the MAb, suggesting the importance of bivalency for this aggregating activity. ECOL-11 decreases adhesion and phagocytosis of E. coli by RAW 264.7 macrophage-like cells, possibly by inhibiting the adhesion functions of OmpA. Despite this in vitro phenotype, organ E. coli burdens were not altered by antibody prophylaxis in a murine model of lethal E. coli septic shock. Our findings support the importance of OmpA at the host-pathogen interface and begin to explore the implications and utility of E. coli-specific antibodies in human hosts.

The Potentials of Melatonin in the Prevention and Treatment of Bacterial Meningitis Disease

Bacterial meningitis (BM) is an acute infectious central nervous system (CNS) disease worldwide, occurring with 50% of the survivors left with a long-term serious sequela. Acute bacterial meningitis is more prevalent in resource-poor than resource-rich areas. The pathogenesis of BM involves complex mechanisms that are related to bacterial survival and multiplication in the bloodstream, increased permeability of blood-brain barrier (BBB), oxidative stress, and excessive inflammatory response in CNS. Considering drug-resistant bacteria increases the difficulty of meningitis treatment and the vaccine also has been limited to several serotypes, and the morbidity rate of BM still is very high. With recent development in neurology, there is promising progress for drug supplements of effectively preventing and treating BM. Several in vivo and in vitro studies have elaborated on understanding the significant mechanism of melatonin on BM. Melatonin is mainly secreted in the pineal gland and can cross the BBB. Melatonin and its metabolite have been reported as effective antioxidants and anti-inflammation, which are potentially useful as prevention and treatment therapy of BM. In bacterial meningitis, melatonin can play multiple protection effects in BM through various mechanisms, including immune response, antibacterial ability, the protection of BBB integrity, free radical scavenging, anti-inflammation, signaling pathways, and gut microbiome. This manuscript summarizes the major neuroprotective mechanisms of melatonin and explores the potential prevention and treatment approaches aimed at reducing morbidity and alleviating nerve injury of BM.

AOA-2 Derivatives as Outer Membrane Protein A Inhibitors for Treatment of Gram-Negative Bacilli Infections

Previously, we identified that a cyclic hexapeptide AOA-2 inhibited the interaction of Gram-negative bacilli (GNB) like Acinetobacter baumannii, Pseudomonas aeruginosa, and Escherichia coli to host cells thereby preventing the development of infection in vitro and in a murine sepsis peritoneal model. In this work, we aimed to evaluate in vitro a library of AOA-2 derivatives in order to improve the effect of AOA-2 against GNB infections. Ten AOA-2 derivatives were synthetized for the in vitro assays. Their toxicities to human lung epithelial cells (A549 cells) for 24 h were evaluated by determining the A549 cells viability using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. The effect of these peptide derivatives and AOA-2 at 250, 125, 62.5, and 31.25 μg/mL on the attachment of A. baumannii ATCC 17978, P. aeruginosa PAO1 and E. coli ATCC 25922 strains to A549 cells was characterized by adherence and viability assays. None of the 10 derivatives showed toxicity to A549 cells. RW01 and RW06 have reduced more the adherence of ATCC 17978, PAO1 and ATCC 2599 strains to A549 cells when compared with the original compound AOA-2. Moreover, both peptides have increased slightly the viability of infected A549 cells by PAO1 and ATCC 25922 than those observed with AOA-2. Finally, RW01 and RW06 have potentiated the activity of colistin against ATCC 17978 strain in the same level with AOA-2. The optimization program of AOA-2 has generated two derivatives (RW01 and RW06) with best effect against interaction of GNB with host cells, specifically against P. aeruginosa and E. coli.

Outer membrane protein A (OmpA) of extraintestinal pathogenic Escherichia coli

Objective

Extraintestinal Pathogenic E. coli (ExPEC), are responsible for host diseases such as Neonatal Meningitis Escherichia coli (NMEC), the second-leading cause of neonatal bacterial meningitis, Avian Pathogenic E. coli (APEC), a cause of extraintestinal disease in poultry, and Uropathogenic E. coli (UPEC), the most common cause of urinary tract infections. Virulence factors associated with NMEC include outer membrane protein A (OmpA) and type I fimbriae (FimH), which also occur in APEC and UPEC. OmpA contributes to NMEC’s ability to cross the blood–brain barrier, persist in the bloodstream and has been identified as a potential vaccine target for ExPEC, however the protein has amino acid variants, which may influence virulence of strains or alter vaccine efficacy. Although OmpA is present in virtually all E. coli, differences in its amino acid residues have yet to be surveyed in ExPEC.

Results

Here the ompA gene (n = 399) from ExPEC collections were sequenced and translated in silico. Twenty-five different OmpA polymorphism patterns were identified. Seven polymorphism patterns were significantly associated with an ExPEC subpathotype, but chromosomal history most likely accounts for most differences found. The differences in OmpA protein sequences suggest that OmpA may influence variation in virulence and host specificity within ExPEC subpathotypes.

Blp1 protein shows virulence-associated features and elicits protective immunity to Acinetobacter baumannii infection

BACKGROUND

Multidrug resistant Acinetobacter baumannii is one of the major infection agents causing nosocomial pneumonia. Therefore, new therapeutic approaches against this bacterium are needed. Surface-exposed proteins from bacterial pathogens are implicated in a variety of virulence-related traits and are considered as promising candidates for vaccine development.

RESULTS

We show in this study that a large Blp1 protein from opportunistic pathogen A. baumannii is encoded in all examined clinical strains of globally spread international clonal lineages I (IC I) and II (IC II). The two blp1 gene variants exhibit lineage-specific distribution profile. By characterization of blp1 deletion mutants and their complementation with blp1 alleles we show that blp1 gene is required for A. baumannii biofilm formation and adhesion to epithelial cells in IC I strain but not in the IC II strain. Nevertheless both alleles are functional in restoring the deficient phenotypes of IC I strain. Moreover, the blp1 gene is required for the establishing of A. baumannii virulence phenotype in nematode and murine infection models. Additionally, we demonstrate that C-terminal 711 amino acid fragment of Blp1 elicits an efficient protection to lethal A. baumannii infection in a murine model using active and passive immunization approaches. Antiserum obtained against Blp1-specific antigen provides opsonophagocytic killing of A. baumannii in vitro.

CONCLUSIONS

Lineage-specific variants of surface-exposed components of bacterial pathogens complicate the development of new therapeutic approaches. Though we demonstrated different impact of Blp1 variants on adherence of IC I and IC II strains, Blp1-specific antiserum neutralized A. baumannii strains of both clonal lineages. Together with the observed increased survival rate in vaccinated mice these results indicate that A. baumannii Blp1 protein could be considered as a new vaccine candidate.

Amino Acid Substitutions and Differential Gene Expression of Outer Membrane Proteins in Adherent-Invasive Escherichia coli

Variations in the sequence and/or the expression of outer membrane proteins (OMPs) may modulate bacterial virulence. OmpA and OmpC have been involved in the interaction of adherent-invasive Escherichia coli (AIEC) strain LF82 with intestinal epithelial cells (IECs). Scarce data exist about OMPs sequence variants in a collection of AIEC strains, and no study of OMPs expression during infection exists. We aimed to determine whether particular mutations or differential expression of OMPs are associated with AIEC virulence. The ompA, ompC, and ompF genes in 14 AIEC and 30 non-AIEC strains were sequenced by Sanger method, and the protein expression profile was analyzed by urea-SDS-PAGE. Gene expression was determined during in vitro bacterial infection of intestine-407 cells by RT-qPCR. The distribution of amino acid substitutions in OmpA-A200V, OmpC-S89N, V220I, and W231D associated with pathotype and specific changes (OmpA-A200V, OmpC-V220I, D232A, OmpF-E51V, and M60K) correlated with adhesion and/or invasion indices but no particular variants were found specific of AIEC. OMPs protein levels did not differ according to pathotype when growing in Mueller-Hinton broth. Interestingly, higher OMPs gene expression levels were reported in non-AIEC growing in association with cells compared with those non-AIEC strains growing in the supernatants of infected cultures (p < 0.028), whereas in AIEC strains ompA expression was the only increased when growing in association with cells (p = 0.032), but they did not significantly alter ompC and ompF expression under this condition (p > 0.146). Despite no particular OMPs sequence variants have been found as a common and distinctive trait in AIEC, some mutations could facilitate a better interaction with the host. Moreover, the different behavior between pathotypes regarding OMPs gene expression at different stages of infection could be related with the virulence of the strains.

Past and Current Perspectives in Modeling Bacteria and Blood-Brain Barrier Interactions

The central nervous system (CNS) barriers are highly specialized cellular barriers that promote brain homeostasis while restricting pathogen and toxin entry. The primary cellular constituent regulating pathogen entry in most of these brain barriers is the brain endothelial cell (BEC) that exhibits properties that allow for tight regulation of CNS entry. Bacterial meningoencephalitis is a serious infection of the CNS and occurs when bacteria can cross specialized brain barriers and cause inflammation. Models have been developed to understand the bacterial - BEC interaction that lead to pathogen crossing into the CNS, however, these have been met with challenges due to these highly specialized BEC phenotypes. This perspective provides a brief overview and outlook of the in vivo and in vitro models currently being used to study bacterial brain penetration, and opinion on improved models for the future.

Connection between gut microbiome and brain development in preterm infants

Dysbiosis of the gut microbiome in preterm infants predisposes the neonate to various major morbidities including neonatal necrotizing enterocolitis and sepsis in the neonatal intensive care unit, and adverse neurological outcomes later in life. There are parallel early developmental windows for the gut microbiota and the nervous system during prenatal to postnatal of life. Therefore, preterm infants represent a unique population in which optimization of initial colonization and microbiota development can affect brain development and enhance neurological outcomes. In this review, we will first discuss the factors affecting the assembly of neonatal gut microbiota and the contribution of dysbiosis in preterm infants to neuroinflammation and neurodevelopmental disorders. We then will discuss the emerging pathways connecting the gut microbiome and brain development. Further we will discuss the significance of current models for alteration of the gut microbiome (including humanized gnotobiotic models and exposure to antibiotics) to brain development and functions. Understanding the role of early optimization of the microbiome in brain development is of paramount importance for developing microbiome-targeted therapies and protecting infants from prematurity-related neurodevelopmental diseases.

Stable isotope labelling by amino acids in cell culture (SILAC) applied to quantitative proteomics of Edwardsiella tarda ATCC 15947 under prolonged cold stress

Edwardsiella tarda poses a threat to human health and has resulted in enormous economic losses in aquaculture. Low temperatures are usually applied to contain the growth of this microorganism. In this study, stable isotope labelling by amino acids in cell culture (SILAC) was used to conduct comparative proteomic quantitation of E. tarda ATCC 15947 under cold stress for two weeks. We identified 1391 proteins, of which 898 were quantifiable. Of these, 72 proteins were upregulated and 164 were downregulated in response to cold stress. Even though E. tarda ATCC 15947 is not a psychrophile, several key proteins related to DNA synthesis and transcription were significantly upregulated. Additionally, proteins related to haemolytic activities and gluconeogenesis were upregulated, even though E. tarda ATCC 15497 is considered non-virulent in aquaculture. This study therefore delineated the specific proteomic response of this E. tarda ATCC 15947 to prolonged cold stress.

Ring-enhancing lesions in neonatal meningitis: an analysis of neuroradiology pitfalls through exemplificative cases and a review of the literature

Very often the clinical course of neonatal meningitis (NM) is characterized by sudden worsening, at times associated with cerebrovascular complications and strokes or the formation of cerebral abscesses. The immediate recognition of these pathological patterns is pivotal in providing clinicians with useful information to differentiate between those different pathological entities, which may both present as ring-enhancing lesions on neuroradiology investigations. Understanding their natural history and diagnostic features is of paramount importance to timely adopt the most appropriate medical and surgical management.

Estrogen receptor-β of microglia underlies sexual differentiation of neuronal protection via ginsenosides in mice brain

AIMS

Streptococcus pneumoniae infection in acute bacterial meningitis can lead to widespread brain damage and mortality. Inflammatory responses by immune cells in the brain are thought to determine the degree of brain injury. Yet, the mechanisms underlying host responses to pneumococcal meningitis are largely unknown. To explore host responses as a potential therapeutic target for preventing brain injury after pneumococcal meningitis.

METHODS

We evaluated signaling mechanisms that minimize neuronal damage caused by pneumococcal infection; specifically, we assessed pathways related to neuronal survival after enhancing estrogen receptor-β (ER-β) expression using a natural therapeutic substance known as ginsenoside Rb1 and Rg3 enhanced ginseng.

RESULTS

Tissue damage caused by bacterial infection was reduced in Rb1/Rg3-treated mice as a result of microglial activation and the inhibition of apoptosis. Furthermore, Rb1 upregulated the expression of brain-derived neurotrophic factor (BDNF) as well as anti-apoptotic factors including Bcl-2 and Bcl-xL. Using BV2 microglial cells in vitro, Rb1 treatment inhibited microglial apoptosis in a manner associated with JAK2/STAT5 phosphorylation.

CONCLUSION

After S. pneumoniae infection in mice, particularly in female mice, Rb1-containing ginseng increased bacterial clearance and survival. These findings inform our understanding of the host immune response to pneumococcal meningitis.

Effect of Neuroinflammation on Synaptic Organization and Function in the Developing Brain: Implications for Neurodevelopmental and Neurodegenerative Disorders

The brain is a plastic organ where both the intrinsic CNS milieu and extrinsic cues play important roles in shaping and wiring neural connections. The perinatal period constitutes a critical time in central nervous system development with extensive refinement of neural connections, which are highly sensitive to fetal and neonatal compromise, such as inflammatory challenges. Emerging evidence suggests that inflammatory cells in the brain such as microglia and astrocytes are pivotal in regulating synaptic structure and function. In this article, we will review the role of glia cells in synaptic physiology and pathophysiology, including microglia-mediated elimination of synapses. We propose that activation of the immune system dynamically affects synaptic organization and function in the developing brain. We will discuss the role of neuroinflammation in altered synaptic plasticity following perinatal inflammatory challenges and potential implications for neurodevelopmental and neurodegenerative disorders.

Evaluation of three recombinant outer membrane proteins, OmpA1, Tdr, and TbpA, as potential vaccine antigens against virulent Aeromonas hydrophila infection in channel catfish (Ictalurus punctatus)

A virulent clonal population of Aeromonas hydrophila (VAh) is recognized as the etiological agent in outbreaks of motile aeromonas septicemia (MAS) in catfish aquaculture in the southeastern United States since 2009. Genomic subtraction revealed three outer membrane proteins present in VAh strain ML09-119 but not in low virulence reference A. hydrophila strains: major outer membrane protein OmpA1, TonB-dependent receptor (Tdr), and transferrin-binding protein A (TbpA). Here, the genes encoding ompA1, tdr, and tbpA were cloned from A. hydrophila ML09-119 and expressed in Escherichia coli. The purified recombinant OmpA1, Tdr, and TbpA proteins had estimated molecular weights of 37.26, 78.55, and 41.67 kDa, respectively. Catfish fingerlings vaccinated with OmpA1, Tdr, and TbpA emulsified with non-mineral oil adjuvant were protected against subsequent VAh strain ML09-119 infection with 98.59%, 95.59%, and 47.89% relative percent survival (RPS), respectively. Furthermore, the mean liver, spleen, and anterior kidney bacterial concentrations were significantly lower in catfish vaccinated with the OmpA1 and Tdr than the sham-vaccinated control group. ELISA demonstrated that catfish immunized with OmpA1, Tdr, and TbpA produce significant antibody response by 21 days post-immunization. Therefore, OmpA1 and Tdr proteins could be used as potential candidates for vaccine development against virulent A. hydrophila infection. However, TbpA protein failed to provide strong protection.

An Overview of Two-Component Signal Transduction Systems Implicated in Extra-Intestinal Pathogenic E. coli Infections

Extra-intestinal pathogenic E. coli (ExPEC) infections are common in mammals and birds. The predominant ExPEC types are avian pathogenic E. coli (APEC), neonatal meningitis causing E. coli/meningitis associated E. coli (NMEC/MAEC), and uropathogenic E. coli (UPEC). Many reviews have described current knowledge on ExPEC infection strategies and virulence factors, especially for UPEC. However, surprisingly little has been reported on the regulatory modules that have been identified as critical in ExPEC pathogenesis. Two-component systems (TCSs) comprise the predominant method by which bacteria respond to changing environments and play significant roles in modulating bacterial fitness in diverse niches. Recent studies have highlighted the potential of manipulating signal transduction systems as a means to chemically re-wire bacterial pathogens, thereby reducing selective pressure and avoiding the emergence of antibiotic resistance. This review begins by providing a brief introduction to characterized infection strategies and common virulence factors among APEC, NMEC, and UPEC and continues with a comprehensive overview of two-component signal transduction networks that have been shown to influence ExPEC pathogenesis.

Immunogenicity of amino acids 1-150 of Streptococcus GapC displayed on the surface of Escherichia coli

Streptococcus is one of the main pathogens that cause bovine mastitis. They includes into S.agalactiae, S.dysgalactiae, and S.uberis. The GapC protein is a virulence factor that is expressed on the surface of Streptococcus species. GapC is highly antigenic and immunization with GapC confers cross-protection against all three species. Our previous data showed that amino acids 1-150 of GapC (GapC_1-150) of S. dysgalactiae conferred similar immunoprotection compared to full-length GapC. Thus, the present study aimed to construct a recombinant Escherichia coli XL1-Blue strain that displayed GapC_1-150 on its surface, and to investigate the immunogenicity of the surface-localized GapC_1-150. To do so, the ompA gene of the E. coli XL1-Blue strain was replaced with the lpp'-ompA-gapC1_1-150 or lpp'-ompA genes by λ Red recombination, the former of which fused GapC_1-150 to an Lpp lipoprotein signal peptide and amino acids 1-159 of OmpA; the recombinant strains were named XL1-Blue/LOG76 and XL1-Blue/LO11, respectively. GapC_1-150 was confirmed to localize to the surface of the XL1-Blue/LOG76 strain by an indirect enzyme-linked immunosorbent assay (ELISA), a fluorescence-activated cell sorter analysis, and laser-scanning confocal microscopy. Then, ICR mice were immunized intramuscularly with the XL1-Blue/LOG76 or XL1-Blue/LO11 strains, or recombinant GapC_1-150. The sera of the immunized mice were collected and the anti-GapC_1-150 antibody levels were detected by ELISA. Lymphocytes secreting interleukin (IL)-4 and interferon-γ were detected by an enzyme-linked ImmunoSpot assay, as was the level of IL-17A level in the supernatant of cultured splenic lymphocytes. The mice immunized with the XL1-Blue/LOG76 strain or GapC_1-150 exhibited better cellular and humoral immunity. Lastly, the immunized mice were challenged with S. uberis, S. dysgalactiae, and S. agalactiae strains, and mice that were immunized with the XL1-Blue/LOG76 strain were better protected than those that were immunized with the XL1-Blue/LO11 strain. These results indicate that it is feasible to display GapC_1-150 on the E. coli surface as a vaccine against Streptococcus species.

Immunization with r-Lactococcus lactis expressing outer membrane protein A of Shigella dysenteriae type-1: evaluation of oral and intranasal route of administration

AIMS

To evaluate the comparative immunogenic potential of food grade Lactococcus lactis expressing outer membrane protein A (OmpA) of Shigella dysenteriae type-1 (SD-1) when administered either orally or intranasally.

METHODS AND RESULTS

OmpA of SD-1 was cloned and expressed first in Escherichia coli and then in L. lactis. Presence of recombinant gene was confirmed by restriction enzyme digestion and immunoblot analysis. Using immobilized metal affinity chromatography, OmpA was purified from recombinant E. coliBL21 (DE3) and subcutaneously administered in BALB/c mice. Detection of OmpA-specific IgG antibodies by enzyme-linked immunosorbent assay (ELISA) confirmed the immunogenicity of OmpA. In order to establish r-L. lactis as a mucosal delivery vehicle, it was administered orally and nasally in BALB/c mice. Serum IgG and faecal IgA were assessed through ELISA to compare the relative potential of immunization routes and immunogenic potential of r-L. lactis. Immunization via the oral route proved superior to intranasal exposure.

CONCLUSION

Recombinant L. lactis expressing OmpA of SD-1 was found to be immunogenic. Oral administration of r-L. lactis elicited higher systemic and mucosal immune response when compared with the nasal route.

SIGNIFICANCE AND IMPACT OF THE STUDY

Using food grade recombinant L. lactis has implications in the development of a prophylactic against multidrug-resistant Shigella, which can be used as a prospective vaccine candidate. Evaluating mucosal routes of immunization demonstrated that the oral route of administration elicited better immune response against OmpA of Shigella.

Physiology and pathogenicity of cpdB deleted mutant of avian pathogenic Escherichia coli

Avian colibacillosis is one of the most common infectious diseases caused partially or entirely by avian pathogenic Escherichia coli (APEC) in birds. In addition to spontaneous infection, APEC can also cause secondary infections that result in greater severity of illness and greater losses to the poultry industry. In order to assess the role of 2', 3'-cyclic phosphodiesterase (cpdB) in APEC on disease physiology and pathogenicity, an avian pathogenic Escherichia coli-34 (APEC-34) cpdB mutant was obtained using the Red system. The cpdB mutant grew at a slower rate than the natural strain APEC-34. Scanning electron microscopy (SEM) indicated that the bacteria of the cpdB mutant were significantly longer than the bacteria observed in the natural strain (P<0.01), and that the width of the cpdB mutant was significantly smaller than its natural counterpart (P<0.01). In order to evaluate the role of cpdB in APEC in the colonization of internal organs (lung, liver and spleen) in poultry, seven-day-old SPF chicks were infected with 10⁹CFU/chick of the cpdB mutant or the natural strain. No colonizations of cpdB mutants were observed in the internal organs 10days following the infection, though numerous natural strains were observed at 20days following infection. Additionally, the relative expression of division protein ftsZ, outer membrane protein A ompA, ferric uptake regulator fur and tryptophanase tnaA genes in the mutant strain were all significantly lower than in the natural strain (P<0.05 or P<0.01). These results suggested that cpdB is involved in the long-term colonization of APEC in the internal organs of the test subjects. The deletion of the cpdB gene also significantly affected the APEC growth and morphology.

Community-acquired bacterial meningitis

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

Role of OmpA2 surface regions of Porphyromonas gingivalis in host-pathogen interactions with oral epithelial cells

Outer membrane protein A (OmpA) is a key outer membrane protein found in Gram‐negative bacteria that contributes to several crucial processes in bacterial virulence. In Porphyromonas gingivalis, OmpA is predicted as a heterotrimer of OmpA1 and OmpA2 subunits encoded by adjacent genes. Here we describe the role of OmpA and its individual subunits in the interaction of P. gingivalis with oral cells. Using knockout mutagenesis, we show that OmpA2 plays a significant role in biofilm formation and interaction with human epithelial cells. We used protein structure prediction software to identify extracellular loops of OmpA2, and determined these are involved in interactions with epithelial cells as evidenced by inhibition of adherence and invasion of P. gingivalis by synthetic extracellular loop peptides and the ability of the peptides to mediate interaction of latex beads with human cells. In particular, we observe that OmpA2‐loop 4 plays an important role in the interaction with host cells. These data demonstrate for the first time the important role of P. gingivalis OmpA2 extracellular loops in interaction with epithelial cells, which may help design novel peptide‐based antimicrobial therapies for periodontal disease.

Colicins U and Y inhibit growth of Escherichia coli strains via recognition of conserved OmpA extracellular loop 1

Interactions of colicins U and Y with the OmpA (Outer membrane protein A) receptor molecule were studied using site-directed mutagenesis and colicin binding assay. A systematic mutagenesis of the colicin-susceptible OmpA sequence from Escherichia coli (OmpA_EC) to the colicin-resistant OmpA sequence from Serratia marcescens (OmpA_SM) was performed in regions corresponding to extracellular OmpA loops 1-4. Susceptibility to colicins U and Y was significantly affected by the OmpA mutation in loop 1. As with functional analysis, a decrease in binding capacity of His-tagged colicin U was found for recombinant OmpA with a mutated segment in loop 1 compared to control OmpA_EC. To verify the importance of the identified amino acid residues in OmpA loop 1, we introduced loop 1 from OmpA_EC into OmpA_SM, which resulted in the substantial increase of susceptibility to colicins U and Y. In addition, colicins U and Y were tested against a panel of 118 bacteriocin non-producing strains of four Escherichia species, including E. coli (39 strains), E. fergusonii (10 strains), E. hermannii (42 strains), and E. vulneris (27 strains). A majority (82%) of E. coli strains was susceptible to colicins U and Y. Interestingly, colicins U and Y also inhibited all of the 30 tested multidrug-resistant E. coli O25b-ST131 isolates. These findings, together with the fact that OmpA loop 1 is important for bacterial virulence and is evolutionary conserved, offer the potential of using colicins U and Y as specific anti-OmpA loop 1 directed antibacterial proteins.

In Silico Structure and Sequence Analysis of Bacterial Porins and Specific Diffusion Channels for Hydrophilic Molecules: Conservation, Multimericity and Multifunctionality

Diffusion channels are involved in the selective uptake of nutrients and form the largest outer membrane protein (OMP) family in Gram-negative bacteria. Differences in pore size and amino acid composition contribute to the specificity. Structure-based multiple sequence alignments shed light on the structure-function relations for all eight subclasses. Entropy-variability analysis results are correlated to known structural and functional aspects, such as structural integrity, multimericity, specificity and biological niche adaptation. The high mutation rate in their surface-exposed loops is likely an important mechanism for host immune system evasion. Multiple sequence alignments for each subclass revealed conserved residue positions that are involved in substrate recognition and specificity. An analysis of monomeric protein channels revealed particular sequence patterns of amino acids that were observed in other classes at multimeric interfaces. This adds to the emerging evidence that all members of the family exist in a multimeric state. Our findings are important for understanding the role of members of this family in a wide range of bacterial processes, including bacterial food uptake, survival and adaptation mechanisms.

Genome-Wide Analysis of Oceanimonas sp. GK1 Isolated from Gavkhouni Wetland (Iran) Demonstrates Presence of Genes for Virulence and Pathogenicity

OBJECTIVE

The bacterium Oceanimonas sp. (O. sp.) GK1 is a member of the Aeromonadaceae family and its genome represents several virulence genes involved in fish and human pathogenicity. In this original research study we aimed to identify and characterize the putative virulence factors and pathogenicity of this halotolerant marine bacterium using genome wide analysis.

MATERIALS AND METHODS

The genome data of O. sp. GK1 was obtained from NCBI. Comparative genomic study was done using MetaCyc database.

RESULTS

Whole genome data analysis of the O. sp. GK1 revealed that the bacterium possesses some important virulence genes (e.g. ZOT, RTX toxin, thermostable hemolysin, lateral flagella and type IV pili) which have been implicated in adhesion and biofilm formation and infection in some other pathogenic bacteria.

CONCLUSION

This is the first report of the putative pathogenicity of O. sp.GK1. The genome wide analysis of the bacterium demonstrates the presence of virulence genes causing infectious diseases in many warmand cold-blooded animals.

Serotype O18 avian pathogenic and neonatal meningitis Escherichia coli strains employ similar pathogenic strategies for the onset of meningitis

Neonatal meningitis Escherichia coli K1 (NMEC) are thought to be transmitted from mothers to newborns during delivery or by nosocomial infections. However, the source of E. coli K1 causing these infections is not clear. Avian pathogenic E. coli (APEC) have the potential to cause infection in humans while human E. coli have potential to cause colibacillosis in poultry, suggesting that these strains may lack host specificity. APEC strains are capable of causing meningitis in newborn rats; however, it is unclear whether these bacteria use similar mechanisms to that of NMEC to establish disease. Using four representative APEC and NMEC strains that belong to serotype O18, we demonstrate that these strains survive in human serum similar to that of the prototypic NMEC strain E44, a derivative of RS218. These bacteria also bind and enter both macrophages and human cerebral microvascular endothelial cells (HCMEC/D3) with similar frequency as that of E44. The amino acid sequences of the outer membrane protein A (OmpA), an important virulence factor in the pathogenesis of meningitis, are identical within these representative APEC and NMEC strains. Further, these strains also require FcγRI-α chain (CD64) and Ecgp96 as receptors for OmpA in macrophages and HCMEC/D3, respectively, to bind and enter these cells. APEC and NMEC strains induce meningitis in newborn mice with varying degree of pathology in the brains as assessed by neutrophil recruitment and neuronal apoptosis. Together, these results suggest that serotype O18 APEC strains utilize similar pathogenic mechanisms as those of NMEC strains in causing meningitis.

The Borrelia afzelii outer membrane protein BAPKO_0422 binds human factor-H and is predicted to form a membrane-spanning β-barrel

The deep evolutionary history of the Spirochetes places their branch point early in the evolution of the diderms, before the divergence of the present day Proteobacteria. As a spirochete, the morphology of the Borrelia cell envelope shares characteristics of both Gram-positive and Gram-negative bacteria. A thin layer of peptidoglycan, tightly associated with the cytoplasmic membrane, is surrounded by a more labile outer membrane (OM). This OM is rich in lipoproteins but with few known integral membrane proteins. The outer membrane protein A (OmpA) domain is an eight-stranded membrane-spanning β-barrel, highly conserved among the Proteobacteria but so far unknown in the Spirochetes. In the present work, we describe the identification of four novel OmpA-like β-barrels from Borrelia afzelii, the most common cause of erythema migrans (EM) rash in Europe. Structural characterization of one these proteins (BAPKO_0422) by SAXS and CD indicate a compact globular structure rich in β-strand consistent with a monomeric β-barrel. Ab initio molecular envelopes calculated from the scattering profile are consistent with homology models and demonstrate that BAPKO_0422 adopts a peanut shape with dimensions 25×45 Å (1 Å=0.1 nm). Deviations from the standard C-terminal signature sequence are apparent; in particular the C-terminal phenylalanine residue commonly found in Proteobacterial OM proteins is replaced by isoleucine/leucine or asparagine. BAPKO_0422 is demonstrated to bind human factor H (fH) and therefore may contribute to immune evasion by inhibition of the complement response. Encoded by chromosomal genes, these proteins are highly conserved between Borrelia subspecies and may be of diagnostic or therapeutic value.

Recombinant OmpA protein fragments mediate interleukin-17 regulation to prevent Escherichia coli meningitis

BACKGROUND

Neonates are at a higher risk for bacterial meningitis than children of other age groups. Although the mortality rates have decreased over the past few decades, neonatal meningitis is still a severe disease with high morbidity. For bacterial meningitis, antibiotic therapy is the primary choice for management. However, neurologic complications often cannot be averted; ∼40% of survivors exhibit neurological sequelae. Escherichia coli infection is the common cause of neonatal meningitis. Previously, we have demonstrated that the recombinant loop 1-3, loop 2-3, and loop 2-4 fragments of OmpA protein can protect mice from death after intracerebral E. coli infection. In this study, the protective effects of the recombinant OmpA protein fragments in E. coli intracerebral infections were investigated.

METHODS

The effects of E. coli intracerebral infection on cytokine and chemokine expression were determined. We also used various recombinant fragments of the OmpA protein to investigate the effects of these recombinant OmpA protein fragments on cytokine and chemokine expression.

RESULTS

In this study, we demonstrated that the expression of interleukin-17 and other cytokines, chemokines, inducible nitric oxide synthase, and cyclooxygenase-2 are involved in the inflammatory processes of intracerebral E. coli infection. We also demonstrated that specific recombinant OmpA protein fragments (L1-3, L2-3, L2-4, and L3) can regulate cytokine, chemokine, nitric oxide synthase, and cyclooxygenase-2 expression and, subsequently, protect mice from death caused by intracerebral infection of E. coli.

CONCLUSION

This finding indicates the potential for developing a new therapeutic approach to improve the prognosis of bacterial meningitis.

Key residues of S. flexneri OmpA mediate infection by bacteriophage Sf6

Many viruses, including bacteriophage, have the inherent ability to utilize several types of proteinaceous receptors as an attachment mechanism to infect cells, yet the molecular mechanisms that drive receptor binding have not been elucidated. Using bacteriophage Sf6 and its host, Shigella flexneri, we investigated how Sf6 utilizes outer membrane protein A (OmpA) for infection. Specifically, we identified that surface loops of OmpA mediate Shigella infection. We further characterized which residues in the surface loops are responsible for Sf6 binding and productive infection using a combination of in vivo and in vitro approaches including site-directed mutagenesis, phage plaque assays, circular dichroism spectroscopy, and in vitro genome ejection assays. Our data indicate that Sf6 can productively interact with other bacterial OmpAs as long as they share homology in loops 2 and 4, suggesting that these loops may determine host specificity. Our data provide a model in which Sf6 interacts with OmpA using the surface of the protein and new insights into viral attachment through binding to membrane protein receptors.

Recombinant outer membrane protein A fragments protect against Escherichia coli meningitis

BACKGROUND

Although the mortality rates have decreased over the past few decades, neonatal meningitis is still a severe disease with high morbidity. Moreover, approximately 40% of survivors exhibit neurological sequelae. Escherichia coli is the major Gram-negative bacterial pathogen in neonatal meningitis. The N-terminal β-barrel domain of the outer membrane protein A (OmpA) of E. coli is essential for effective protein conformation and function and contains four surface-exposed hydrophilic loops. In this study, we expressed different fragments of the four ring structures of the N-terminal domain, and investigated whether these recombinant OmpA fragments can protect mice from death after E. coli infection.

METHODS

We expressed the recombinant proteins of the following OmpA fragments by using molecular cloning of Loop 1-2, Loop 1-3, Loop 1-4, Loop 2-3, Loop 2-4, and Loop 3-4. Animal experiments were subsequently performed to investigate the effects of these recombinant OmpA fragments on the survival of C57BL/6 mice after intracerebral E. coli RS218 administration.

RESULTS

This study demonstrated that the recombinant Loop 1-3, Loop 2-3, and Loop 2-4 fragments of OmpA can protect mice from intracerebral E. coli infection.

CONCLUSION

In bacterial meningitis, although antibiotic therapy is the first choice for management, neurological complications can seldom be averted. Based on the results of the present study, we intend to establish an effective therapeutic application for E. coli meningitis.

The interaction of N-glycans in Fcγ receptor I α-chain with Escherichia coli K1 outer membrane protein A for entry into macrophages: experimental and computational analysis

Neonatal meningitis, caused by Escherichia coli K1, is a serious central nervous system disease. We have established that macrophages serve as permissive niches for E. coli K1 to multiply in the host and for attaining a threshold level of bacterial load, which is a prerequisite for the onset of the disease. Here, we demonstrate experimentally that three N-glycans in FcγRIa interact with OmpA of E. coli K1 for binding to and entering the macrophages. Adoptive transfer of FcγRIa(-/-) bone marrow-derived macrophages transfected with FcγRIa into FcγRIa(-/-) newborn mice renders them susceptible to E. coli K1-induced meningitis. In contrast, mice that received bone marrow-derived macrophages transfected with FcγRIa in which N-glycosylation sites 1, 4, and 5 are mutated to alanines exhibit resistance to E. coli K1 infection. Our molecular dynamics and simulation studies predict that N-glycan 5 exhibits strong binding at the barrel site of OmpA formed by loops 3 and 4, whereas N-glycans 1 and 4 interact with loops 1, 3, and 4 of OmpA at tip regions. Molecular modeling data also suggest no role for the IgG binding site in the invasion process. In agreement, experimental mutations in IgG binding site had no effect on the E. coli K1 entry into macrophages in vitro or on the onset of meningitis in newborn mice. Together, this integration of experimental and computational studies reveals how the N-glycans in FcγRIa interact with the OmpA of E. coli K1 for inducing the disease pathogenesis.

Identification of minimum carbohydrate moiety in N-glycosylation sites of brain endothelial cell glycoprotein 96 for interaction with Escherichia coli K1 outer membrane protein A

Bacterial meningitis is a serious central nervous system infection and Escherichia coli K1 (E. coli K1) is one of the leading etiological agents that cause meningitis in neonates. Outer membrane protein A (OmpA) of E. coli K1 is a major virulence factor in the pathogenesis of meningitis, and interacts with human brain microvascular endothelial cells (HBMEC) to cross the blood-brain barrier. Using site-directed mutagenesis, we demonstrate that two N-glycosylation sites (NG1 and NG2) in the extracellular domain of OmpA receptor, Ecgp96 are critical for bacterial binding to HBMEC. E. coli K1 invasion assays using CHO-Lec1 cells that express truncated N-glycans, and sequential digestion of HBMEC surface N-glycans using specific glycosidases showed that GlcNAc1-4GlcNAc epitopes are sufficient for OmpA interaction with HBMEC. Lack of NG1 and NG2 sites in Ecgp96 inhibits E. coli K1 OmpA induced F-actin polymerization, phosphorylation of protein kinase C-α, and disruption of transendothelial electrical resistance required for efficient invasion of E. coli K1 in HBMEC. Furthermore, the microvessels of cortex and hippocampus of the brain sections of E. coli K1 infected mice showed increased expression of glycosylated Ecgp96. Therefore, the interface of OmpA and GlcNAc1-4GlcNAc epitope interaction would be a target for preventative strategies against E. coli K1 meningitis.

Bacterial self-defence: how Escherichia coli evades serum killing

The ability to survive the bactericidal action of serum is advantageous to extraintestinal pathogenic Escherichia coli that gain access to the bloodstream. Evasion of the innate defences present in serum, including complement and antimicrobial peptides, involves multiple factors. Serum resistance mechanisms utilized by E. coli include the production of protective extracellular polysaccharide capsules and expression of factors that inhibit or interfere with the complement cascade. Recent studies have also highlighted the importance of structural integrity of the cell envelope in serum survival. These survival strategies are outlined in this review with particular attention to novel findings and recent insights into well-established resistance mechanisms.

Attenuation of biopterin synthesis prevents Escherichia coli K1 invasion of brain endothelial cells and the development of meningitis in newborn mice

Elevated levels of pterins and nitric oxide (NO) are observed in patients with septic shock and bacterial meningitis. We demonstrate that Escherichia coli K1 infection of human brain microvascular endothelial cells (HBMECs) induces the expression of guanosine triphosphate cyclohydrolase (GCH1), the rate-limiting enzyme in pterin synthesis, thereby elevating levels of biopterin. DAHP (2,4-diamino hydroxyl pyrimidine), a specific inhibitor of GCH1, prevented biopterin and NO production and invasion of E. coli K1 in HBMECs. GCH1 interaction with Ecgp96, the receptor for outer membrane protein A of E. coli K1, also increases on infection, and suppression of Ecgp96 expression prevents GCH1 activation and biopterin synthesis. Pretreatment of newborn mice with DAHP prevented the production of biopterin and the development of meningitis. These results suggest a novel role for biopterin synthesis in the pathogenesis of E. coli K1 meningitis.

Regulation of Toll-like receptor 2 interaction with Ecgp96 controls Escherichia coli K1 invasion of brain endothelial cells

The interaction of outer membrane protein A (OmpA) with its receptor, Ecgp96 (a homologue of Hsp90β), is critical for the pathogenesis of Escherichia coli K1 meningitis. Since Hsp90 chaperones Toll-like receptors (TLRs), we examined the role of TLRs in E. coli K1 infection. Herein, we show that newborn TLR2(-/-) mice are resistant to E. coli K1 meningitis, while TLR4(-/-) mice succumb to infection sooner. In vitro, OmpA+ E. coli infection selectively upregulates Ecgp96 and TLR2 in human brain microvascular endothelial cells (HBMEC), whereas OmpA- E. coli upregulates TLR4 in these cells. Furthermore, infection with OmpA+ E. coli causes Ecgp96 and TLR2 translocate to the plasma membrane of HBMEC as a complex. Immunoprecipitation studies of the plasma membrane fractions from infected HBMEC reveal that the C termini of Ecgp96 and TLR2 are critical for OmpA+ E. coli invasion. Knockdown of TLR2 using siRNA results in inefficient membrane translocation of Ecgp96 and significantly reduces invasion. In addition, the interaction of Ecgp96 andTLR2 induces a bipartite signal, one from Ecgp96 through PKC-α while the other from TLR2 through MyD88, ERK1/2 and NF-κB. This bipartite signal ultimately culminates in the efficient production of NO, which in turn promotes E. coli K1 invasion of HBMEC.

Defense at the border: the blood-brain barrier versus bacterial foreigners

Bacterial meningitis is among the top ten causes of infectious disease-related deaths worldwide, with up to half of the survivors left with permanent neurological sequelae. The blood-brain barrier (BBB), composed mainly of specialized brain microvascular endothelial cells, maintains biochemical homeostasis in the CNS by regulating the passage of nutrients, molecules and cells from the blood to the brain. Despite its highly restrictive nature, certain bacterial pathogens are able to gain entry into the CNS resulting in serious disease. In recent years, important advances have been made in understanding the molecular and cellular events that are involved in the development of bacterial meningitis. In this review, we summarize the progress made in elucidating the molecular mechanisms of bacterial BBB-crossing, highlighting common themes of host-pathogen interaction, and the potential role of the BBB in innate defense during infection.

Insights into the structure and assembly of Escherichia coli outer membrane protein A

Outer membrane protein A (OmpA) of Escherichia coli is a paradigm for the biogenesis of outer membrane proteins; however, the structure and assembly of OmpA have remained controversial. A review of studies to date supports the hypothesis that native OmpA is a single-domain large pore, while a two-domain narrow-pore structure is a folding intermediate or minor conformer. The in vitro refolding of OmpA to the large-pore conformation requires isolation of the protein from outer membranes with retention of an intact disulfide bond followed by sufficient incubation in lipids at temperatures of ≥ 26 °C to overcome the high energy of activation for refolding. The in vivo maturation of the protein involves covalent modification of serines in the eighth β-barrel of the N-terminal domain by oligo-(R)-3-hydroxybutyrates as the protein is escorted across the cytoplasm by SecB for post-translational secretion across the secretory translocase in the inner membrane. After cleavage of the signal sequence, protein chaperones, such as Skp, DegP and SurA, guide OmpA across the periplasm to the β-barrel assembly machinery (BAM) complex in the outer membrane. During this passage, a disulfide bond is formed between C290 and C302 by DsbA, and the hydrophobicity of segments of the C-terminal domain, which are destined for incorporation as β-barrels in the outer membrane bilayer, is increased by covalent attachment of oligo-(R)-3-hydroxybutyrates. With the aid of the BAM complex, OmpA is then assembled into the outer membrane as a single-domain large pore.

gp96 expression in neutrophils is critical for the onset of Escherichia coli K1 (RS218) meningitis

Despite the fundamental function of neutrophils (PMNs) in innate immunity, their role in Escherichia coli K1 (EC-K1) induced meningitis is unexplored. Here we show that PMN-depleted mice are resistant to EC-K1 (RS218) meningitis. EC-K1 survives and multiplies in PMNs for which outer membrane protein A (OmpA) expression is essential. EC-K1infection of PMNs increases the cell surface expression of gp96, which acts as a receptor for bacterial entry. Suppression of gp96 expression in newborn mice prevents the onset of EC-K1 meningitis. Infection of PMNs with EC-K1 suppresses oxidative burst by down regulating rac1, rac2 and gp91^phox transcription both in vitro and in vivo. The interaction of loop 2 of OmpA with gp96 is essential for EC-K1-mediated inhibition of oxidative burst. These results reveal that EC-K1 exploits surface expressed gp96 in PMNs to prevent oxidative burst for the onset of neonatal meningitis.