Outer membrane protein A of Escherichia coli contributes to invasion of brain microvascular endothelial cells

Hcp family proteins secreted via the type VI secretion system coordinately regulate Escherichia coli K1 interaction with human brain microvascular endothelial cells

Type VI secretion systems (T6SSs) are involved in the pathogenicity of several gram-negative bacteria. Based on sequence analysis, we found that a cluster of Escherichia coli virulence factors (EVF) encoding a putative T6SS exists in the genome of the meningitis-causing E. coli K1 strain RS218. The T6SS-associated deletion mutants exhibited significant defects in binding to and invasion of human brain microvascular endothelial cells (HBMEC) compared with the parent strain. Hcp family proteins (the hallmark of T6SS), including Hcp1 and Hcp2, were localized in the bacterial outer membrane, but the involvements of Hcp1 and Hcp2 have been shown to differ in E. coli-HBMEC interaction. The deletion mutant of hcp2 showed defects in the bacterial binding to and invasion of HBMEC, while Hcp1 was secreted in a T6SS-dependent manner and induced actin cytoskeleton rearrangement, apoptosis, and the release of interleukin-6 (IL-6) and IL-8 in HBMEC. These findings demonstrate that the T6SS is functional in E. coli K1, and two Hcp family proteins participate in different steps of E. coli interaction with HBMEC in a coordinate manner, e.g., binding to and invasion of HBMEC, the cytokine and chemokine release followed by cytoskeleton rearrangement, and apoptosis in HBMEC. This is the first demonstration of the role of T6SS in meningitis-causing E. coli K1, and T6SS-associated Hcp family proteins are likely to contribute to the pathogenesis of E. coli meningitis.

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.

Outer membrane protein A of bovine and ovine isolates of Mannheimia haemolytica is surface exposed and contains host species-specific epitopes

Mannheimia haemolytica is the etiological agent of pneumonic pasteurellosis of cattle and sheep; two different OmpA subclasses, OmpA1 and OmpA2, are associated with bovine and ovine isolates, respectively. These proteins differ at the distal ends of four external loops, are involved in adherence, and are likely to play important roles in host adaptation. M. haemolytica is surrounded by a polysaccharide capsule, and the degree of OmpA surface exposure is unknown. To investigate surface exposure and immune specificity of OmpA among bovine and ovine M. haemolytica isolates, recombinant proteins representing the transmembrane domain of OmpA from a bovine serotype A1 isolate (rOmpA1) and an ovine serotype A2 isolate (rOmpA2) were overexpressed, purified, and used to generate anti-rOmpA1 and anti-rOmpA2 antibodies, respectively. Immunogold electron microscopy and immunofluorescence techniques demonstrated that OmpA1 and OmpA2 are surface exposed, and are not masked by the polysaccharide capsule, in a selection of M. haemolytica isolates of various serotypes and grown under different growth conditions. To explore epitope specificity, anti-rOmpA1 and anti-rOmpA2 antibodies were cross-absorbed with the heterologous isolate to remove cross-reacting antibodies. These cross-absorbed antibodies were highly specific and recognized only the OmpA protein of the homologous isolate in Western blot assays. A wider examination of the binding specificities of these antibodies for M. haemolytica isolates representing different OmpA subclasses revealed that cross-absorbed anti-rOmpA1 antibodies recognized OmpA1-type proteins but not OmpA2-type proteins; conversely, cross-absorbed anti-rOmpA2 antibodies recognized OmpA2-type proteins but not OmpA1-type proteins. Our results demonstrate that OmpA1 and OmpA2 are surface exposed and could potentially bind to different receptors in cattle and sheep.

Identification of immunogenic proteins within distinct molecular mass fractions of Flavobacterium psychrophilum

Flavobacterium psychrophilum is the aetiological agent of bacterial coldwater disease (CWD), and this pathogen has large economic impacts on salmonid aquaculture worldwide. Previously, it was demonstrated that high levels of protection against F. psychrophilum challenge were conferred to rainbow trout, Oncorhynchus mykiss (Walbaum), by immunization with distinct molecular mass fractions of the bacterium, and specific antibodies were correlated with protection. In this study, an immunoproteomic analysis of F. psychrophilum was performed using two-dimensional polyacrylamide gel electrophoresis and Western blotting with serum from fish immunized with high- and mid-molecular mass fractions of the bacterium. Mass spectrometry was used to determine the protein identity, and 15 immunogenic proteins were positively identified following Mascot searches of the F. psychrophilum genome. Based on known function and immunogenicity of homologous proteins in other bacterial pathogens, antibodies specific for several of the identified proteins may be important for protective immunity from CWD. These include outer membrane protein OmpA (P60), trigger factor, ClpB, elongation factor G, gliding motility protein GldN and a conserved hypothetical protein. This work increases the understanding of the protective humoral immune response of rainbow trout against these distinct molecular mass fractions of F. psychrophilum and provides new potential targets for recombinant protein vaccine development.

The outer membrane protein A (OmpA) of Yersinia pestis promotes intracellular survival and virulence in mice

The plague bacterium Yersinia pestis has a number of well-described strategies to protect itself from both host cells and soluble factors. In an effort to identify additional anti-host factors, we employed a transposon site hybridization (TraSH)-based approach to screen 10(5)Y. pestis mutants in an in vitro infection system. In addition to loci encoding various components of the well-characterized type III secretion system (T3SS), our screen unambiguously identified ompA as a pro-survival gene. We go on to show that an engineered Y. pestis ΔompA strain, as well as a ΔompA strain of the closely related pathogen Yersinia pseudotuberculosis, have fully functioning T3SSs but are specifically defective in surviving within macrophages. Additionally, the Y. pestis ΔompA strain was out competed by the wild-type strain in a mouse co-infection assay. Unlike in other bacterial pathogens in which OmpA can promote adherence, invasion, or serum resistance, the OmpA of Y. pestis is restricted to enhancing intracellular survival. Our data show that OmpA of the pathogenic Yersinia is a virulence factor on par with the T3SS.

Global discovery of small RNAs in Yersinia pseudotuberculosis identifies Yersinia-specific small, noncoding RNAs required for virulence

A major class of bacterial small, noncoding RNAs (sRNAs) acts by base-pairing with mRNAs to alter the translation from and/or stability of the transcript. Our laboratory has shown that Hfq, the chaperone that mediates the interaction of many sRNAs with their targets, is required for the virulence of the enteropathogen Yersinia pseudotuberculosis. This finding suggests that sRNAs play a critical role in the regulation of virulence in this pathogen, but these sRNAs are not known. Using a deep sequencing approach, we identified the global set of sRNAs expressed in vitro by Y. pseudotuberculosis. Sequencing of RNA libraries from bacteria grown at 26 °C and 37 °C resulted in the identification of 150 unannotated sRNAs. The majority of these sRNAs are Yersinia specific, without orthologs in either Escherichia coli or Salmonella typhimurium. Six sRNAs are Y. pseudotuberculosis specific and are absent from the genome of the closely related species Yersinia pestis. We found that the expression of many sRNAs conserved between Y. pseudotuberculosis and Y. pestis differs in both timing and dependence on Hfq, suggesting evolutionary changes in posttranscriptional regulation between these species. Deletion of multiple sRNAs in Y. pseudotuberculosis leads to attenuation of the pathogen in a mouse model of yersiniosis, as does the inactivation in Y. pestis of a conserved, Yersinia-specific sRNA in a mouse model of pneumonic plague. Finally, we determined the regulon controlled by one of these sRNAs, revealing potential virulence determinants in Y. pseudotuberculosis that are regulated in a posttranscriptional manner.

Investigation on mechanisms of glycopeptide nanoparticles for drug delivery across the blood-brain barrier

AIM

Nanoneuroscience, based on the use polymeric nanoparticles (NPs), represents an emerging field of research for achieving an effective therapy for neurodegenerative diseases. In particular, poly-lactide-co-glycolide (PLGA) glyco-heptapetide-conjugated NPs (g7-NPs) were shown to be able to cross the blood-brain barrier (BBB). However, the in vivo mechanisms of the BBB crossing of this kind of NP has not been investigated until now. This article aimed to develop a deep understanding of the mechanism of BBB crossing of the modified NPs.

MATERIALS & METHODS

Loperamide and rhodamine-123 (model drugs unable to cross the BBB) were loaded into NPs, composed of a mixture of PLGA, differently modified with g7 or with a random sequence of the same aminoamids (random-g7). To study brain targeting of these model drugs, loaded NPs were administered via the tail vein in rats in order to perform both pharmacological studies and biodistribution analysis along with fluorescent, confocal and electron microscopy analysis, in order to achieve the NP BBB crossing mechanism. Computational analysis on the conformation of the g7- and random-g7-NPs of the NP surface was also developed.

RESULTS

Only loperamide delivered to the brain with g7-NPs created a high central analgesia, corresponding to the 14% of the injected dose, and data were confirmed by biodistribution studies. Electron photomicrographs showed the ability of g7-NPs in crossing the BBB as evidenced by several endocytotic vesicles and macropinocytotic processes. The computational analysis on g7 and random-g7 showed a different conformation (linear vs globular), thus suggesting a different interaction with the BBB.

CONCLUSION

Taken together, this evidence suggested that g7-NP BBB crossing is enabled by multiple pathways, mainly membrane-membrane interaction and macropinocytosis-like mechanisms. The results of the computational analysis showed the Biousian structure of the g7 peptide, in contrast to random-g7 peptide (globular conformation), suggesting that this difference is pivotal in explaining the BBB crossing and allowing us to hypothesize regarding the mechanism of BBB crossing by g7-NPs.

Identification of the immunogenic outer membrane protein A antigen of Haemophilus parasuis by a proteomics approach and passive immunization with monoclonal antibodies in mice

Monoclonal antibodies (MAbs) against Haemophilus parasuis were generated by fusing spleen cells from BALB/c mice immunized with whole bacterial cells with SP2/0 murine myeloma cells. Desirable hybridomas were screened by enzyme-linked immunosorbent assay (ELISA). Neutralizing MAb 1D8 was selected in protection assays. ELISA results demonstrated that 1D8 can react with all 15 serotypes of H. parasuis and field isolate H. parasuis HLJ-018. Passive immunization studies showed that mice inoculated intraperitoneally with 1D8 had significantly reduced prevalence of H. parasuis colonization in the blood, lung, spleen, and liver and had prolonged survival time compared to that of the control group. Furthermore, the passive transfer experiment indicated that MAb 1D8 can protect mice from both homologous and heterologous challenges with H. parasuis. Using two-dimensional gel electrophoresis (2-DE), the immunoreactive protein target for MAb 1D8 was identified. The data presented confirm the protective role of MAb 1D8 and identify OmpA as the target of the protective monoclonal antibody. The data suggest that OmpA is a promising candidate for a subunit vaccine against H. parasuis.

Aggregatibacter actinomycetemcomitans Omp29 is associated with bacterial entry to gingival epithelial cells by F-actin rearrangement

The onset and progressive pathogenesis of periodontal disease is thought to be initiated by the entry of Aggregatibacter actinomycetemcomitans (Aa) into periodontal tissue, especially gingival epithelium. Nonetheless, the mechanism underlying such bacterial entry remains to be clarified. Therefore, this study aimed to investigate the possible role of Aa outer membrane protein 29 kD (Omp29), a homologue of E. coli OmpA, in promoting bacterial entry into gingival epithelial cells. To accomplish this, Omp29 expression vector was incorporated in an OmpA-deficient mutant of E. coli. Omp29⁺/OmpA⁻E. coli demonstrated 22-fold higher entry into human gingival epithelial line cells (OBA9) than Omp29⁻/OmpA⁻E. coli. While the entry of Aa and Omp29⁺/OmpA⁻E. coli into OBA9 cells were inhibited by anti-Omp29 antibody, their adherence to OBA9 cells was not inhibited. Stimulation of OBA9 cells with purified Omp29 increased the phosphorylation of focal adhesion kinase (FAK), a pivotal cell-signaling molecule that can up-regulate actin rearrangement. Furthermore, Omp29 increased the formation of F-actin in OBA9 cells. The internalization of Omp29-coated beads and the entry of Aa into OBA9 were partially inhibited by treatment with PI3-kinase inhibitor (Wortmannin) and Rho GTPases inhibitor (EDIN), both known to convey FAK-signaling to actin-rearrangement. These results suggest that Omp29 is associated with the entry of Aa into gingival epithelial cells by up-regulating F-actin rearrangement via the FAK signaling pathway.

Outer membrane vesicles as a candidate vaccine against edwardsiellosis

Infection with Edwardsiella tarda, a gram-negative bacterium, causes high morbidity and mortality in both marine and freshwater fish. Outer membrane vesicles (OMVs) released from gram-negative bacteria are known to play important roles in bacterial pathogenesis and host immune responses, but no such roles for E. tarda OMVs have yet been described. In the present study, we investigated the proteomic composition of OMVs and the immunostimulatory effect of OMVs in a natural host, as well as the efficacy of OMVs when used as a vaccine against E. tarda infection. A total of 74 proteins, from diverse subcellular fractions, were identified in OMVs. These included a variety of important virulence factors, such as hemolysin, OmpA, porin, GAPDH, EseB, EseC, EseD, EvpC, EvpP, lipoprotein, flagellin, and fimbrial protein. When OMVs were administrated to olive flounder, significant induction of mRNAs encoding IL-1β, IL-6, TNFα, and IFNγ was observed, compared with the levels seen in fish injected with formalin-killed E. tarda. In a vaccine trial, olive flounder given OMVs were more effectively protected (p<0.0001) than were control fish. Investigation of OMVs may be useful not only for understanding the pathogenesis of E. tarda but also in development of an effective vaccine against edwardsiellosis.

OmpA is a virulence factor of Riemerella anatipestifer

Riemerella anatipestifer infection is probably the most economically important disease of farm ducks worldwide. The pathogen R. anatipestifer causes septicemia anserum exsudativa in ducks, but little is known about the molecular basis of its pathogenesis and the virulence factors involved. In this study, by deleting ompA gene from R. anatipestifer serotype 2 strain Th4, we constructed a mutant strain Th4ΔompA to investigate whether R. anatipestifer OmpA is an important virulence factor. Results showed that although the growth curve, bacterial and colony morphology of Th4ΔompA in tryptic soybean broth (TSB) or on TSB agar were similar to its parent strain Th4, the adhesion and invasion capacities of mutant strain to Vero cells were decreased significantly. Furthermore, the median lethal dose (LD(50)) of both strains was determined to measure the virulence with 10-day-old Cherry Valley ducklings. The results showed that LD(50) of Th4ΔompA mutant was >10(10) colony forming units (CFU), it was attenuated significantly in comparison with that of Th4 which LD(50) was 4.41 × 10(8) CFU. Additional analysis indicated that blood bacterial loading of ducklings infected with the Th4ΔompA mutant were much lower than those of Th4-infected ducklings. The results demonstrate that OmpA is a virulence factor of R. anatipestifer, and that it may act as an adhesin.

Klebsiella pneumoniae outer membrane protein A is required to prevent the activation of airway epithelial cells

Outer membrane protein A (OmpA) is a class of proteins highly conserved among the Enterobacteriaceae family and throughout evolution. Klebsiella pneumoniae is a capsulated gram-negative pathogen. It is an important cause of community-acquired and nosocomial pneumonia. Evidence indicates that K. pneumoniae infections are characterized by a lack of an early inflammatory response. Data from our laboratory indicate that K. pneumoniae CPS helps to suppress the host inflammatory response. However, it is unknown whether K. pneumoniae employs additional factors to modulate host inflammatory responses. Here, we report that K. pneumoniae OmpA is important for immune evasion in vitro and in vivo. Infection of A549 and normal human bronchial cells with 52OmpA2, an ompA mutant, increased the levels of IL-8. 52145-Δwca(K2)ompA, which does not express CPS and ompA, induced the highest levels of IL-8. Both mutants could be complemented. In vivo, 52OmpA2 induced higher levels of tnfα, kc, and il6 than the wild type. ompA mutants activated NF-κB, and the phosphorylation of p38, p44/42, and JNK MAPKs and IL-8 induction was via NF-κB-dependent and p38- and p44/42-dependent pathways. 52OmpA2 engaged TLR2 and -4 to activate NF-κB, whereas 52145-Δwca(K2)ompA activated not only TLR2 and TLR4 but also NOD1. Finally, we demonstrate that the ompA mutant is attenuated in the pneumonia mouse model. The results of this study indicate that K. pneumoniae OmpA contributes to attenuate airway cell responses. This may facilitate pathogen survival in the hostile environment of the lung.

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

Outer membrane protein A (OmpA) has been implicated as an important virulence factor in several gram-negative bacterial infections such as Escherichia coli K1, a leading cause of neonatal meningitis associated with significant mortality and morbidity. In this study, we generated E. coli K1 mutants that express OmpA in which three or four amino acids from various extracellular loops were changed to alanines, and we examined their ability to survive in several immune cells. We observed that loop regions 1 and 2 play an important role in the survival of E. coli K1 inside neutrophils and dendritic cells, and loop regions 1 and 3 are needed for survival in macrophages. Concomitantly, E. coli K1 mutants expressing loop 1 and 2 mutations were unable to cause meningitis in a newborn mouse model. Of note, mutations in loop 4 of OmpA enhance the severity of the pathogenesis by allowing the pathogen to survive better in circulation and to produce high bacteremia levels. These results demonstrate, for the first time, the roles played by different regions of extracellular loops of OmpA of E. coli K1 in the pathogenesis of meningitis and may help in designing effective preventive strategies against this deadly disease.

Extracellular loops of the Eschericia coli outer membrane protein A contribute to the pathogenesis of meningitis

Neonatal meningitis by Eschericia coli RS218 occurs due to bacteremia and its transmigration across the blood-brain barrier. Although the outer membrane protein A (OmpA), a molecule with extracellular loops has been shown to contribute to the above phenomenon, we do not know the exact the role of these individual loops. Using bacterial strains whose individual loops have been removed, we demonstrated that whereas Loops1 and 2 contribute to 70%-80% bacterial survival in serum, bacterial entry into human brain microvascular endothelial cells (HBMEC) is governed by Loops1, 2, and 3. Cellular invasion was shown to require activation of host cytosolic phospholipase A2 (cPLA2α) by Loops1 and 2 but not 3. This suggests 2 distinct pathways for bacterial entry into host cells. Loop 4 played no role in either serum survival, cellular entry, or cPLA2α signaling. These findings demonstrate for the first time the different contributions of extracellular loops of OmpA to the pathogenesis of E. coli meningitis.

Fcγ receptor I alpha chain (CD64) expression in macrophages is critical for the onset of meningitis by Escherichia coli K1

Neonatal meningitis due to Escherichia coli K1 is a serious illness with unchanged morbidity and mortality rates for the last few decades. The lack of a comprehensive understanding of the mechanisms involved in the development of meningitis contributes to this poor outcome. Here, we demonstrate that depletion of macrophages in newborn mice renders the animals resistant to E. coli K1 induced meningitis. The entry of E. coli K1 into macrophages requires the interaction of outer membrane protein A (OmpA) of E. coli K1 with the alpha chain of Fcγ receptor I (FcγRIa, CD64) for which IgG opsonization is not necessary. Overexpression of full-length but not C-terminal truncated FcγRIa in COS-1 cells permits E. coli K1 to enter the cells. Moreover, OmpA binding to FcγRIa prevents the recruitment of the γ-chain and induces a different pattern of tyrosine phosphorylation of macrophage proteins compared to IgG2a induced phosphorylation. Of note, FcγRIa^−/− mice are resistant to E. coli infection due to accelerated clearance of bacteria from circulation, which in turn was the result of increased expression of CR3 on macrophages. Reintroduction of human FcγRIa in mouse FcγRIa^−/− macrophages in vitro increased bacterial survival by suppressing the expression of CR3. Adoptive transfer of wild type macrophages into FcγRIa^−/− mice restored susceptibility to E. coli infection. Together, these results show that the interaction of FcγRI alpha chain with OmpA plays a key role in the development of neonatal meningitis by E. coli K1.

Escherichia coli K1 is the most common cause of meningitis in premature infants; the mortality rate of this disease ranges from 5% to 30%. A better understanding of the pathogenesis of E. coli K1 meningitis is needed to develop new preventative strategies. We have shown that outer membrane protein A (OmpA) of E. coli K1, independent of antibody opsonization, is critical for bacterial entrance and survival within macrophages. Using a newborn mouse model, we found that depletion of macrophages renders the animals resistant to E. coli K1 induced meningitis. OmpA binds to α-chain of Fcγ-receptor I (FcγRIa) in macrophages, but does not induce expected gamma chain association and signaling. FcγRIa knockout mice are resistant to E. coli K1 infection because their macrophages express more CR3 and are thus able to kill bacteria with greater efficiency, preventing the development of high-grade bacteremia, a pre-requisite for the onset of meningitis. These novel observations demonstrate that inhibiting OmpA binding to FcγRIa is a promising therapeutic target for treatment or prevention of neonatal meningitis.

Effects of ibeA deletion on virulence and biofilm formation of avian pathogenic Escherichia coli

The ibeA gene is located on a genomic island, GimA, which is involved in the pathogenesis of neonatal meningitis Escherichia coli (NMEC) and avian pathogenic E. coli (APEC). The prevalence of ibeA in the APEC collection in China was investigated, and 20 of 467 strains (4.3%) were positive. In addition, analysis of the association of the E. coli reference (ECOR) groups with positive strains revealed that ibeA was linked to group B2. The ibeA gene in DE205B was analyzed and compared to those of APEC and NMEC, which indicated that the specificity of ibeA was not consistent along pathotypes. The invasion of chicken embryo fibroblast DF-1 cells by APEC DE205B and RS218 was observed, which suggested that DF-1 cells could be a model to study the mechanism of APEC invasion. The inactivation of ibeA in APEC DE205B led to the reduced capacity to invade DF-1 cells, defective virulence in vivo, and decreased biofilm formation compared to the wild-type strain. In addition, strain AAEC189 expressing ibeA exhibited enhanced invasion capacity and biofilm formation. The results of the quantitative real-time reverse transcription-PCR (qRT-PCR) analysis and animal system infection experiments indicated that the loss of ibeA decreased the colonization and proliferation capacities of APEC in the brain during system infection.

Experimental validation of the predicted binding site of Escherichia coli K1 outer membrane protein A to human brain microvascular endothelial cells: identification of critical mutations that prevent E. coli meningitis

Escherichia coli K1, the most common cause of meningitis in neonates, has been shown to interact with GlcNAc1-4GlcNAc epitopes of Ecgp96 on human brain microvascular endothelial cells (HBMECs) via OmpA (outer membrane protein A). However, the precise domains of extracellular loops of OmpA interacting with the chitobiose epitopes have not been elucidated. We report the loop-barrel model of these OmpA interactions with the carbohydrate moieties of Ecgp96 predicted from molecular modeling. To test this model experimentally, we generated E. coli K1 strains expressing OmpA with mutations of residues predicted to be critical for interaction with the HBMEC and tested E. coli invasion efficiency. For these same mutations, we predicted the interaction free energies (including explicit calculation of the entropy) from molecular dynamics (MD), finding excellent correlation (R(2) = 90%) with experimental invasion efficiency. Particularly important is that mutating specific residues in loops 1, 2, and 4 to alanines resulted in significant inhibition of E. coli K1 invasion in HBMECs, which is consistent with the complete lack of binding found in the MD simulations for these two cases. These studies suggest that inhibition of the interactions of these residues of Loop 1, 2, and 4 with Ecgp96 could provide a therapeutic strategy to prevent neonatal meningitis due to E. coli K1.

Cell-free synthesis and folding of transmembrane OmpA reveals higher order structures and premature truncations

We use a cell-free transcription-translation system to monitor the effect of different lipids on the synthesis and folding of the transmembrane domain of the outer membrane protein OmpA from E. coli under physiological conditions. Folding is consistent with previous observations made in vitro at high pH. Synthesis and folding yields are optimal in phosphocholine lipids, particularly in short chain lipids and small vesicles, while lipid rafts do not promote folding compared to the folding in the absence of lipids. Truncated species are observed during translation in the presence of the periplasmic chaperone Skp, which likely binds to the newly synthesized polypeptide chain during cell-free translation and thus prematurely terminate polypeptide chain synthesis. In contrast, folded and unfolded dimers of OmpA correlate negatively with folding yields. This suggests that dimer formation competes with folding and insertion of monomeric OmpA, though folded dimers slowly appear to convert to folded monomers.

Escherichia coli K1 promotes the ligation of CD47 with thrombospondin-1 to prevent the maturation of dendritic cells in the pathogenesis of neonatal meningitis

Dendritic cells (DCs) are professional APCs providing a critical link between adaptive and innate immune responses. Our previous studies have shown that Escherichia coli K1 internalization of myeloid DCs suppressed the maturation of the cells for which outer membrane protein A (OmpA) expression is essential. In this study, we demonstrate that infection of DCs with OmpA(+) E. coli significantly upregulates the expression of CD47, an integrin-associated protein, and its natural ligand thrombospondin 1 (TSP-1). Pretreatment of DCs with anti-CD47 blocking Ab or knocking down the expression of CD47 or TSP-1, but not signal regulatory protein alpha by small interfering RNA, abrogated the suppressive effect of E. coli K1. Ligation of CD47 with a mAb prevented the maturation and cytokine production by DCs upon stimulation with LPS similar to the inhibitory effect induced by OmpA(+) E. coli. In agreement with the in vitro studies, suppression of CD47 or TSP-1 expression in newborn mice by a novel in vivo small interfering RNA technique protected the animals against E. coli K1 meningitis. Reconstitution of CD47 knockdown mice with CD47(+) DCs renders the animals susceptible to meningitis by E. coli K1, substantiating the role of CD47 expression in DCs for the occurrence of meningitis. Our results demonstrate a role for CD47 for the first time in bacterial pathogenesis and may be a novel target for designing preventive approaches for E. coli K1 meningitis.

Mutation of the gene encoding a major outer-membrane protein in Xanthomonas campestris pv. campestris causes pleiotropic effects, including loss of pathogenicity

Xanthomonas campestris pv. campestris (Xcc) is the phytopathogen that causes black rot in crucifers. The xanthan polysaccharide and extracellular enzymes produced by this organism are virulence factors, the expression of which is upregulated by Clp (CRP-like protein) and DSF (diffusible signal factor), which is synthesized by RpfF. It is also known that biofilm formation/dispersal, regulated by the effect of controlled synthesis of DSF on cell-cell signalling, is required for virulence. Furthermore, a deficiency in DSF causes cell aggregation with concomitant production of a gum-like substance that can be dispersed by addition of DSF or digested by exogenous endo-beta-1,4-mannanase expressed by Xcc. In this study, Western blotting of proteins from a mopB mutant (XcMopB) showed Xcc MopB to be the major outer-membrane protein (OMP); Xcc MopB shared over 97 % identity with homologues from other members of Xanthomonas. Similarly to the rpfF mutant, XcMopB formed aggregates with simultaneous production of a gummy substance, but these aggregates could not be dispersed by DSF or endo-beta-1,4-mannanase, indicating that different mechanisms were involved in aggregation. In addition, XcMopB showed surface deformation, altered OMP composition, impaired xanthan production, increased sensitivity to stressful conditions including SDS, elevated temperature and changes in pH, reduced adhesion and motility and defects in pathogenesis. The finding that the major OMP is required for pathogenicity is unprecedented in phytopathogenic bacteria.

The GimA locus of extraintestinal pathogenic E. coli: does reductive evolution correlate with habitat and pathotype?

IbeA (invasion of brain endothelium), which is located on a genomic island termed GimA, is involved in the pathogenesis of several extraintestinal pathogenic E. coli (ExPEC) pathotypes, including newborn meningitic E. coli (NMEC) and avian pathogenic E. coli (APEC). To unravel the phylogeny of GimA and to investigate its island character, the putative insertion locus of GimA was determined via Long Range PCR and DNA-DNA hybridization in 410 E. coli isolates, including APEC, NMEC, uropathogenic (UPEC), septicemia-associated E. coli (SEPEC), and human and animal fecal isolates as well as in 72 strains of the E. coli reference (ECOR) collection. In addition to a complete GimA (∼20.3 kb) and a locus lacking GimA we found a third pattern containing a 342 bp remnant of GimA in this strain collection. The presence of GimA was almost exclusively detected in strains belonging to phylogenetic group B2. In addition, the complete GimA was significantly more frequent in APEC and NMEC strains while the GimA remnant showed a higher association with UPEC strains. A detailed analysis of the ibeA sequences revealed the phylogeny of this gene to be consistent with that obtained by Multi Locus Sequence Typing of the strains. Although common criteria for genomic islands are partially fulfilled, GimA rather seems to be an ancestral part of phylogenetic group B2, and it would therefore be more appropriate to term this genomic region GimA locus instead of genomic island. The existence of two other patterns reflects a genomic rearrangement in a reductive evolution-like manner.

Virulence and immunomodulatory roles of bacterial outer membrane vesicles

Outer membrane (OM) vesicles are ubiquitously produced by Gram-negative bacteria during all stages of bacterial growth. OM vesicles are naturally secreted by both pathogenic and nonpathogenic bacteria. Strong experimental evidence exists to categorize OM vesicle production as a type of Gram-negative bacterial virulence factor. A growing body of data demonstrates an association of active virulence factors and toxins with vesicles, suggesting that they play a role in pathogenesis. One of the most popular and best-studied pathogenic functions for membrane vesicles is to serve as natural vehicles for the intercellular transport of virulence factors and other materials directly into host cells. The production of OM vesicles has been identified as an independent bacterial stress response pathway that is activated when bacteria encounter environmental stress, such as what might be experienced during the colonization of host tissues. Their detection in infected human tissues reinforces this theory. Various other virulence factors are also associated with OM vesicles, including adhesins and degradative enzymes. As a result, OM vesicles are heavily laden with pathogen-associated molecular patterns (PAMPs), virulence factors, and other OM components that can impact the course of infection by having toxigenic effects or by the activation of the innate immune response. However, infected hosts can also benefit from OM vesicle production by stimulating their ability to mount an effective defense. Vesicles display antigens and can elicit potent inflammatory and immune responses. In sum, OM vesicles are likely to play a significant role in the virulence of Gram-negative bacterial pathogens.

IL-10 administration reduces PGE-2 levels and promotes CR3-mediated clearance of Escherichia coli K1 by phagocytes in meningitis

Ineffectiveness of antibiotics in treating neonatal Escherichia coli K1 meningitis and the emergence of antibiotic-resistant strains evidently warrants new prevention strategies. We observed that administration of interleukin (IL)-10 during high-grade bacteremia clears antibiotic-sensitive and -resistant E. coli from blood of infected mice. Micro-CT studies of brains from infected animals displayed gross morphological changes similar to those observed in infected human neonates. In mice, IL-10, but not antibiotic or anti-TNF antibody treatment prevented brain damage caused by E. coli. IL-10 administration elevated CR3 expression in neutrophils and macrophages of infected mice, whereas infected and untreated mice displayed increased expression of FcγRI and TLR2. Neutrophils or macrophages pretreated with IL-10 ex vivo exhibited a significantly greater microbicidal activity against E. coli compared with cells isolated from wild-type or IL-10^−/− mice. The protective effect of IL-10 was abrogated when CR3 was knocked-down in vivo by siRNA. The increased expression of CR3 in phagocytes was caused by inhibition of prostaglandin E-2 (PGE-2) levels, which were significantly increased in neutrophils and macrophages upon E. coli infection. These findings describe a novel modality of IL-10–mediated E. coli clearance by diverting the entry of bacteria via CR3 and preventing PGE-2 formation in neonatal meningitis.

Genome sequence of Cronobacter sakazakii BAA-894 and comparative genomic hybridization analysis with other Cronobacter species

BACKGROUND

The genus Cronobacter (formerly called Enterobacter sakazakii) is composed of five species; C. sakazakii, C. malonaticus, C. turicensis, C. muytjensii, and C. dublinensis. The genus includes opportunistic human pathogens, and the first three species have been associated with neonatal infections. The most severe diseases are caused in neonates and include fatal necrotizing enterocolitis and meningitis. The genetic basis of the diversity within the genus is unknown, and few virulence traits have been identified.

METHODOLOGY/PRINCIPAL FINDINGS

We report here the first sequence of a member of this genus, C. sakazakii strain BAA-894. The genome of Cronobacter sakazakii strain BAA-894 comprises a 4.4 Mb chromosome (57% GC content) and two plasmids; 31 kb (51% GC) and 131 kb (56% GC). The genome was used to construct a 387,000 probe oligonucleotide tiling DNA microarray covering the whole genome. Comparative genomic hybridization (CGH) was undertaken on five other C. sakazakii strains, and representatives of the four other Cronobacter species. Among 4,382 annotated genes inspected in this study, about 55% of genes were common to all C. sakazakii strains and 43% were common to all Cronobacter strains, with 10-17% absence of genes.

CONCLUSIONS/SIGNIFICANCE

CGH highlighted 15 clusters of genes in C. sakazakii BAA-894 that were divergent or absent in more than half of the tested strains; six of these are of probable prophage origin. Putative virulence factors were identified in these prophage and in other variable regions. A number of genes unique to Cronobacter species associated with neonatal infections (C. sakazakii, C. malonaticus and C. turicensis) were identified. These included a copper and silver resistance system known to be linked to invasion of the blood-brain barrier by neonatal meningitic strains of Escherichia coli. In addition, genes encoding for multidrug efflux pumps and adhesins were identified that were unique to C. sakazakii strains from outbreaks in neonatal intensive care units.

CTX-M beta-lactamase production and virulence of Escherichia coli K1

We report a patient with neonatal meningitis caused by a CTX-M-1-producing Escherichia coli K1 strain. The influence of CTX-M production on virulence was investigated in cell culture and a newborn mouse model of meningitis. CTX-M production had no influence on virulence but was a major factor in clinical outcome.

Inhibition of inducible nitric oxide controls pathogen load and brain damage by enhancing phagocytosis of Escherichia coli K1 in neonatal meningitis

Escherichia coli K1 is a leading cause of neonatal meningitis in humans. In this study, we sought to determine the pathophysiologic relevance of inducible nitric oxide (iNOS) in experimental E. coli K1 meningitis. By using a newborn mouse model of meningitis, we demonstrate that E. coli infection triggered the expression of iNOS in the brains of mice. Additionally, iNOS-/- mice were resistant to E. coli K1 infection, displaying normal brain histology, no bacteremia, no disruption of the blood-brain barrier, and reduced inflammatory response. Treatment with an iNOS specific inhibitor, aminoguanidine (AG), of wild-type animals before infection prevented the development of bacteremia and the occurrence of meningitis. The infected animals treated with AG after the development of bacteremia also completely cleared the pathogen from circulation and prevented brain damage. Histopathological and micro-CT analysis of brains revealed significant damage in E. coli K1-infected mice, which was completely abrogated by AG administration. Peritoneal macrophages and polymorphonuclear leukocytes isolated from iNOS-/- mice or pretreated with AG demonstrated enhanced uptake and killing of the bacteria compared with macrophages and polymorphonuclear leukocytes from wild-type mice in which E. coli K1 survive and multiply. Thus, NO produced by iNOS may be beneficial for E. coli to survive inside the macrophages, and prevention of iNOS could be a therapeutic strategy to treat neonatal E. coli meningitis.

OmpA of uropathogenic Escherichia coli promotes postinvasion pathogenesis of cystitis

Type 1 pilus directs bladder epithelial binding and invasion by uropathogenic Escherichia coli (UPEC) in the initial stage of cystitis, but the bacterial determinants of postinvasion events in the pathogenesis of cystitis are largely undetermined. We show here that the UPEC outer membrane protein A (OmpA), a monomeric, major, integral protein component of the bacterial outer membrane, functions as a critical determinant of intracellular virulence for UPEC, promoting persistent infection within bladder epithelium. Using a murine urinary tract infection (UTI) model, we demonstrate that whereas deletion of the UPEC ompA gene did not disrupt initial epithelial binding and invasion by UPEC, it did preclude completion of the intracellular bacterial community (IBC) pathway, accompanied by diminishing bacterial loads in the bladder. This defect in epithelial persistence of the ompA mutant was enhanced in competitive infections with wild-type UPEC. Microscopic examinations revealed that the ompA mutant formed significantly fewer IBCs, and those that were initiated were unable to progress past the early stages of maturation. These defects could be corrected by complementation of ompA. In addition, expression of ompA during wild-type UTI was sharply increased at time points correlated with IBC development and the arrival of host immune effector cells. Our findings establish OmpA as a key UPEC virulence factor that functions after epithelial invasion to facilitate IBC maturation and chronic bacterial persistence.

Nitric oxide/cGMP signalling induces Escherichia coli K1 receptor expression and modulates the permeability in human brain endothelial cell monolayers during invasion

Escherichia coli K1 invasion of human brain microvascular endothelial cells (HBMEC) mediated by outer membrane protein A (OmpA) results in the leakage of HBMEC monolayers. Despite the influence of nitric oxide (NO) in endothelial cell tight junction integrity, its role in E. coli-induced HBMEC monolayer permeability is poorly defined. Here, we demonstrate that E. coli invasion of HBMEC stimulates NO production by increasing the inducible nitric oxide synthase (iNOS) expression. Exposure to NO-producing agents enhanced the invasion of OmpA(+)E. coli and thereby increased the permeability of HBMEC. OmpA(+)E. coli-induced NO production lead to increased generation of cGMP and triggered the expression of OmpA receptor, Ec-gp96 in HBMEC. Pre-treatment of HBMEC with iNOS inhibitors or by introducing siRNA to iNOS, but not to eNOS or cGMP inhibitors abrogated the E. coli-induced expression of Ec-gp96. Overexpression of the C-terminal truncated Ec-gp96 in HBMEC prevented NO production and its downstream effector, cGMP generation and consequently, the invasion of OmpA(+)E. coli. NO/cGMP production also activates PKC-alpha, which is previously shown to be involved in HBMEC monolayer leakage. These results indicate that NO/cGMP signalling pathway plays a novel role in OmpA(+)E. coli invasion of HBMEC by enhancing the surface expression of Ec-gp96.

The type III secretion system is involved in the invasion and intracellular survival of Escherichia coli K1 in human brain microvascular endothelial cells

Type III secretion systems (T3SSs) have been documented in many Gram-negative bacteria, including enterohemorrhagic Escherichia coli. We have previously shown the existence of a putative T3SS in meningitis-causing E. coli K1 strains, referred to as E. coli type III secretion 2 (ETT2). The sequence of ETT2 in meningitis-causing E. coli K1 strain EC10 (O7:K1) revealed that ETT2 comprises the epr, epa and eiv genes, but bears mutations, deletions and insertions. We constructed the EC10 mutants deleted of ETT2 or eivA gene, and their contributions to bacterial pathogenesis were evaluated in human brain microvascular endothelial cells (HBMECs). The deletion mutant of ETT2 exhibited defects in invasion and intracellular survival compared with the parental E. coli K1 strain EC10. The mutant deleted of eivA within ETT2 was also significantly defective in invasion and intracellular survival in HBMECs, and the defects of the eiv mutant were restored to the levels of the parent strain EC10 by transcomplementation. These findings suggest that ETT2 plays a role in the pathogenesis of E. coli K1 infection, including meningitis.

The group B streptococcal serine-rich repeat 1 glycoprotein mediates penetration of the blood-brain barrier

BACKGROUND

Group B Streptococcus (GBS) is the leading cause of bacterial meningitis in newborn infants. Because GBS is able to invade, survive, and cross the blood-brain barrier, we sought to identify surface-expressed virulence factors that contribute to blood-brain barrier penetration and the pathogenesis of meningitis.

METHODS

Targeted deletion and insertional mutants were generated in different GBS clinical isolates. Wild-type and mutant bacteria were analyzed for their capacity to adhere to and invade human brain microvascular endothelial cells (hBMECs) and to penetrate the blood-brain barrier using our model of hematogenous meningitis.

RESULTS

Analysis of a GBS (serotype V) clinical isolate revealed the presence of a surface-anchored serine-rich protein, previously designated serine-rich repeat 1 (Srr-1). GBS Srr-1 is a glycosylated protein with high molecular weight. Deletion of srr1 in NCTC 10/84 resulted in a significant decrease in adherence to and invasion of hBMECs. Additional mutants in other GBS serotypes commonly associated with meningitis showed a similar decrease in hBMEC invasion, compared with parental strains. Finally, in mice, wild-type GBS penetrated the blood-brain barrier and established meningitis more frequently than did the Deltasrr1 mutant strain.

CONCLUSIONS

Our data suggest that GBS Srr glycoproteins play an important role in crossing the blood-brain barrier and in the development of streptococcal meningitis.

Identification of Major Antigenic Proteins of Edwardsiella Tarda Recognized by Japanese Flounder Antibody

Edwardsiella tarda is a fish pathogen that causes systemic infections in fresh water and marine fish. Determining the antigenic proteins is important for the development of an immunodiagnostic tests and a vaccine for effective infection control in fish. In the current study, antigens were detected by immunoblotting and affinity column chromatography using a Japanese flounder ( Paralichthys olivaceus) antibody produced by experimental infection with E. tarda. GroEL, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), outer membrane protein A, filament protein, 30S ribosomal protein S6, 50S ribosomal protein L9, cold shock protein, and carbon storage protein were identified as antigens of E. tarda through biochemical analyses of the molecular weights, isoelectric points, and N-terminal amino-acid sequences. These proteins can be easily detected in flounder infected with E. tarda and are potential diagnostic markers.

OmpA is the critical component for Escherichia coli invasion-induced astrocyte activation

Escherichia coli is the major Gram-negative bacterial pathogen in neonatal meningitis. Outer membrane protein A (OmpA) is a conserved major protein in the E. coli outer membrane and is involved in several host-cell interactions. To characterize the role of OmpA in the invasion of astrocytes by E. coli, we investigated OmpA-positive and OmpA-negative E. coli strains. Outer membrane protein A E44, E105, and E109 strains adhered to and invaded C6 glioma cells 10- to 15-fold more efficiently than OmpA-negative strains. Actin rearrangement, protein tyrosine kinase, and phosphoinositide 3-kinase activation were required for OmpA-mediated invasion by E. coli. In vitro infection of C6 cells and intracerebral injection into mice of the E44 strain induced expression of the astrocyte differentiation marker glial fibrillary acidic protein and the inflammatory mediators cyclooxygenase 2 and nitric oxide synthase 2. After intracerebral infection with E44, all C57BL/6 mice died within 36hours, whereas 80% of mice injected with E44 premixed with recombinant OmpA protein survived. Astrocyte activation and neutrophil infiltration were reduced in brain tissue sections in the mice given OmpA. Taken together, these data suggest that OmpA-mediated invasion plays an important role in the early stage of E.coli-induced brain damage, and that it may have therapeutic use in E. coli meningitis.

Characteristics of Bacteroides fragilis lacking the major outer membrane protein, OmpA

OmpA1 is the major outer membrane protein of the Gram-negative anaerobic pathogen Bacteroides fragilis. We identified three additional conserved ompA homologues (ompA2-ompA4) and three less homologous ompA-like genes (ompAs 5, 6 and 7) in B. fragilis. We constructed an ompA1 disruption mutant in B. fragilis 638R (WAL6 OmegaompA1) using insertion-mediated mutagenesis. WAL6 OmegaompA1 formed much smaller colonies and had smaller, rounder forms on Gram stain analysis than the parental strain or other unrelated disruption mutants. SDS-PAGE and Western blot analysis (with anti-OmpA1 IgY) of the OMP patterns of WAL6 OmegaompA1 grown in both high- and low-salt media did not reveal any other OmpA proteins even under osmotic stress. An ompA1 deletant (WAL186DeltaompA1) was constructed using a two-step double-crossover technique, and an ompA 'reinsertant', WAL360+ompA1, was constructed by reinserting the ompA gene into WAL186DeltaompA1. WAL186DeltaompA1 was significantly more sensitive to exposure to SDS, high salt and oxygen than the parental (WAL108) or reinsertant (WAL360+ompA1) strain. No significant change was seen in MICs of a variety of antimicrobials for either WAL6 OmegaompA1 or WAL186DeltaompA1 compared to WAL108. RT-PCR revealed that all of the ompA genes are transcribed in the parental strain and in the disruption mutant, but, as expected, ompA1 is not transcribed in WAL186DeltaompA1. Unexpectedly, ompA4 is also not transcribed in WAL186DeltaompA1. A predicted structure indicated that among the four OmpA homologues, the barrel portion is more conserved than the loops, except for specific conserved patches on loop 1 and loop 3. The presence of multiple copies of such similar genes in one organism would suggest a critical role for this protein in B. fragilis.

Identification of IbeR as a stationary-phase regulator in meningitic Escherichia coli K1 that carries a loss-of-function mutation in rpoS

IbeR is a regulator present in meningitic Escherichia coli strain E44 that carries a loss-of-function mutation in the stationary-phase (SP) regulatory gene rpoS. In order to determine whether IbeR is an SP regulator in E44, two-dimensional gel electrophoresis and LC-MS were used to compare the proteomes of a noninvasive ibeR deletion mutant BR2 and its parent strain E44 in the SP. Four up-regulated (TufB, GapA, OmpA, AhpC) and three down-regulated (LpdA, TnaA, OpmC) proteins in BR2 were identified when compared to E44. All these proteins contribute to energy metabolism or stress resistance, which is related to SP regulation. One of the down-regulated proteins, tryptophanase (TnaA), which is regulated by RpoS in other E. coli strains, is associated with SP regulation via production of a signal molecule indole. Our studies demonstrated that TnaA was required for E44 invasion, and that indole was able to restore the noninvasive phenotype of the tnaA mutant. The production of indole was significantly reduced in BR2, indicating that ibeR is required for the indole production via tnaA. Survival studies under different stress conditions indicated that IbeR contributed to bacteria stress resistance in the SP. Taken together, IbeR is a novel regulator contributing to the SP regulation.

Brain damage in newborn rat model of meningitis by Enterobacter sakazakii: a role for outer membrane protein A

Enterobacter sakazakii (ES) is an emerging pathogen that causes sepsis, meningitis, and necrotizing enterocolitis in neonates. Very limited information is available regarding the pathogenesis of these diseases and the specific virulence factors of ES. Here, we demonstrate, for the first time using a newborn rat model, that outer membrane protein A (OmpA) expression is important for the onset of meningitis by ES. Orally administered OmpA(+) ES traverses the intestinal barrier, multiplies in blood, and subsequently penetrates the blood-brain barrier. OmpA(+) ES were present in high numbers in the brains of infected animals along with associated neutrophil infiltration, hemorrhage, and gliosis. In contrast, OmpA(-) ES could not bind to the intestinal epithelial cells in vitro and in vivo efficiently. The bound OmpA(+) ES also caused apoptosis of enterocytes in the intestinal segments of infected animals; OmpA(-) ES did not. Furthermore, OmpA(-) ES are very susceptible to blood and serum killing, whereas OmpA(+) ES are resistant. Of note, 100% mortality rates were observed in OmpA(+) ES-infected newborn rats, whereas OmpA(-) ES-infected rats survived without any pathological manifestations. The inability of OmpA(-) ES to cause disease was restored by complementation with the ompA gene. These results suggest that OmpA expression in ES is necessary for the colonization of the gastrointestinal tract and for subsequent survival in blood to cause meningitis.

Acinetobacter baumannii invades epithelial cells and outer membrane protein A mediates interactions with epithelial cells

Background

Acinetobacter baumannii is a nosocomial pathogen of increasing importance, but the pathogenic mechanism of this microorganism has not been fully explored. This study investigated the potential of A. baumannii to invade epithelial cells and determined the role of A. baumannii outer membrane protein A (AbOmpA) in interactions with epithelial cells.

Results

A. baumannii invaded epithelial cells by a zipper-like mechanism, which is associated with microfilament- and microtubule-dependent uptake mechanisms. Internalized bacteria were located in the membrane-bound vacuoles. Pretreatment of recombinant AbOmpA significantly inhibited the adherence to and invasion of A. baumannii in epithelial cells. Cell invasion of isogenic AbOmpA^- mutant significantly decreased as compared with wild-type bacteria. In a murine pneumonia model, wild-type bacteria exhibited a severe lung pathology and induced a high bacterial burden in blood, whereas AbOmpA^- mutant was rarely detected in blood.

Conclusion

A. baumannii adheres to and invades epithelial cells. AbOmpA plays a major role in the interactions with epithelial cells. These findings contribute to the understanding of A. baumannii pathogenesis in the early stage of bacterial infection.

Klebsiella pneumoniae OmpA confers resistance to antimicrobial peptides

A Klebsiella pneumoniae ompA mutant was more susceptible to antimicrobial peptides (APs) than the wild type. Susceptibility did not result from surface changes other than the absence of OmpA. Our data suggest that OmpA is implicated in the activation of yet-unknown systems dedicated to ameliorating AP cytotoxicity.

Identification of Mannheimia haemolytica adhesins involved in binding to bovine bronchial epithelial cells

Mannheimia haemolytica, a commensal organism of the upper respiratory tract in cattle, is the principal bacterial pathogen associated with the bovine respiratory disease complex. Adherence to the respiratory mucosa is a crucial event in its pathogenesis. However, the bacterial components that contribute to this process are not fully characterized. In this study, we demonstrated that M. haemolytica adhered to bovine bronchial epithelial cells (BBEC) in vitro and that adherence was inhibited by anti-M. haemolytica antibody. Western blot analysis of M. haemolytica proteins that bind to BBEC showed a dominant protein band with an apparent molecular mass of approximately 30 kDa. Peptide sequences for the 30-kDa BBEC-binding proteins, as determined by liquid chromatography-tandem mass spectrometry, matched two M. haemolytica surface proteins: heat-modifiable outer membrane protein A (OmpA) and lipoprotein 1 (Lpp1). Western blotting showed that the 30-kDa protein band is recognized by both anti-M. haemolytica OmpA and anti-Lpp1 antibodies. Furthermore, incubation with anti-OmpA and anti-Lpp1 antibodies significantly inhibited M. haemolytica binding to BBEC monolayers. In summary, these results suggest that OmpA and Lpp1 contribute to adherence of M. haemolytica to bovine respiratory epithelial cells.

Proteomics in gram-negative bacterial outer membrane vesicles

Gram-negative bacteria constitutively secrete outer membrane vesicles (OMVs) into the extracellular milieu. Recent research in this area has revealed that OMVs may act as intercellular communicasomes in polyspecies communities by enhancing bacterial survival and pathogenesis in hosts. However, the mechanisms of vesicle formation and the pathophysiological roles of OMVs have not been clearly defined. While it is obvious that mass spectrometry-based proteomics offers great opportunities for improving our knowledge of bacterial OMVs, limited proteomic data are available for OMVs. The present review aims to give an overview of the previous biochemical, biological, and proteomic studies in the emerging field of bacterial OMVs, and to give future directions for high-throughput and comparative proteomic studies of OMVs that originate from diverse Gram-negative bacteria under various environmental conditions. This article will hopefully stimulate further efforts to construct a comprehensive proteome database of bacterial OMVs that will help us not only to elucidate the biogenesis and functions of OMVs but also to develop diagnostic tools, vaccines, and antibiotics effective against pathogenic bacteria.

Escherichia coli interaction with human brain microvascular endothelial cells induces signal transducer and activator of transcription 3 association with the C-terminal domain of Ec-gp96, the outer membrane protein A receptor for invasion

Our inability to develop new therapeutic strategies to prevent meningitis due to Escherichia coli K1 is attributed to our incomplete understanding of the pathophysiology of the disease. Previously, we demonstrated that outer membrane protein A of E. coli interacts with a gp96 homologue, Ec-gp96, on human brain microvascular endothelial cells (HBMEC) for invasion. However, signalling events mediated by Ec-gp96 that allow internalization of E. coli are incompletely understood. Here, we demonstrate that signal transducer and activator of transcription 3 (Stat3) activation and its interaction with Ec-gp96 were critical for E. coli invasion. The activated Stat3 was colocalized with Ec-gp96 at the actin condensation sites, and overexpressing a dominant negative (DN) form of Stat3 in HBMEC significantly abrogated the invasion. Furthermore, overexpression of Ec-gp96Delta200, the C-terminal 214-amino-acid truncated Ec-gp96, prevented the invasion of E. coli in HBMEC. In contrast, lack of ATP binding by gp96 did not affect the invasion. Overexpression of DN forms of either phosphatidyl inositol-3 kinase (PI3-kinase) subunit p85 or protein kinase C-alpha (PKC-alpha) had no effect on the activation of Stat3 and its association with Ec-gp96, whereas overexpression of DN-Stat3 abolished the activation of both PI3-kinase and PKC-alpha. Together, our findings identified a novel interaction of Stat3 with Ec-gp96, upstream of PI3-kinase and PKC-alpha activation that is required for the invasion of E. coli into HBMEC.

Endocytosis at the blood–brain barrier: From basic understanding to drug delivery strategies

The blood-brain barrier (BBB) protects the central nervous system (CNS) from potentially harmful xenobiotics and endogenous molecules. Anatomically, it comprises the brain microvasculature whose functionality is nevertheless influenced by associated astrocyte, pericyte and neuronal cells. The highly restrictive paracellular pathway within brain microvasculature restricts significant CNS penetration to only those drugs whose physicochemical properties afford ready penetration into hydrophobic cell membranes or are capable of exploiting endogenous active transport processes such as solute carriers or endocytosis pathways. Endocytosis at the BBB is an essential pathway by which the brain obtains its nutrients and affords communication with the periphery. The development of strategies to exploit these endocytic pathways for the purposes of drug delivery to the CNS is still an immature field although some impressive results have been documented with the targeting of particular receptors. This current article initially provides an overview of general endocytosis processes and pathways showing evidence of their functional existence within the BBB. Subsequent sections provide, in an entity-specific manner, comprehensive reviews on BBB transport investigations of endocytosis involving: transferrin and the targeting of the transferrin receptor; hormones; cytokines; cell penetrating peptides; microorganisms and toxins, and nanoparticles aimed at more effectively delivering drugs to the CNS.

An insect symbiosis is influenced by bacterium-specific polymorphisms in outer-membrane protein A

Beneficial bacterial symbioses are ubiquitous in nature. However, the functional and molecular basis of host tolerance to resident symbiotic microbes, in contrast to resistance to closely related bacteria that are recognized as foreign, remain largely unknown. We used the tsetse fly (Glossina morsitans), which depends on symbiotic flora for fecundity and has limited exposure to foreign microbes, to investigate the tolerance phenomenon exhibited during symbiosis. We examined the potential role of bacterium-specific polymorphisms present in the major bacterial surface protein, outer-membrane protein A (OmpA), on host infection outcomes. Tsetse were successfully superinfected with their mutualistic facultative symbiont, Sodalis glossinidius, whereas infections with Escherichia coli K12 were lethal. In contrast, tsetse were resistant to an E. coli OmpA mutant strain, whereas recombinant Sodalis expressing E. coli OmpA became pathogenic. Profiling of tsetse immunity-related gene expression incriminated peptidoglycan recognition protein (pgrp)-lb as a determinant of the infection outcomes we observed. RNAi-induced knockdown of tsetse pgrp-lb significantly reduced host mortality after infection with otherwise lethal E. coli K12. Our results show that polymorphisms in the exposed loop domains of OmpA represent a microbial adaptation that mediates host tolerance of endogenous symbiotic bacteria.

Outer membrane protein A expression in Enterobacter sakazakii is required to induce microtubule condensation in human brain microvascular endothelial cells for invasion

Enterobacter sakazakii (ES) causes neonatal meningitis and necrotizing enterocolitis with case-fatality rates among infected infants ranging from 40 to 80%. Very little is known about the mechanisms by which these organisms cause disease. Here, we demonstrate that ES invades human brain microvascular endothelial cells (HBMEC) with higher frequency when compared with epithelial cells and endothelial cells from different origins. The entry of ES into HBMEC requires the expression of outer membrane protein A (OmpA), as the OmpA-deletion mutant was sevenfold less invasive than the wild type ES and the bacterium does not multiply inside HBMEC. Anti-OmpA antibodies generated against the OmpA of Escherichia coli K1, which also recognize the OmpA of ES, did not prevent the invasion of ES in HBMEC. ES invasion depends on microtubule condensation in HBMEC and is independent of actin filament reorganization. Both PI3-kinase and PKC-alpha were activated during ES entry into HBMEC between 15 min and 30 min of infection. Concomitantly, overexpression of dominant negative forms of PI3-kinase and PKC-alpha significantly inhibited the invasion of ES into HBMEC. In summary, ES invasion of HBMEC is dependent on the expression of OmpA similar to that of E. coli K1; however, the epitopes involved in the interaction with HBMEC appears to be different.

Cytokine release and expression induced by OmpA proteins from the Gram-negative anaerobes, Porphyromonas asaccharolytica and Bacteroides fragilis

OmpA proteins from Gram-negative anaerobes Porphyromonas asaccharolytica and Bacteroides fragilis induced release and expression of IL-1alpha, tumor necrosis factor (TNF)-alpha, IFN-gamma, IL-6, and IL-10 from murine splenocytes in vitro in a dose-dependent fashion. The release of the cytokines induced by B. fragilis Bf-OmpA was at much lower levels compared with P. asaccharolytica Omp-PA; Bf-OmpA did not induce release of IL-10. Omp-PA and Bf-OmpA were able to upregulate mRNA expression of the tested cytokines. The results obtained with refolded Bf-OmpA were similar to those with native Bf-OmpA. The data presented in this research demonstrate for the first time that Omps from anaerobic bacteria can induce the release of cytokines, suggesting that Omp-PA and Bf-OmpA may play important roles in the pathogenic processes of these bacteria.