Entry and intracellular replication of Escherichia coli K1 in macrophages require expression of outer membrane protein A

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.

Characterization of Escherichia coli isolated from gut biopsies of newly diagnosed patients with inflammatory bowel disease

BACKGROUND

Mucosal-associated Escherichia coli may play a role in the pathogenesis of inflammatory bowel diseases (IBDs). In this study we assessed mucosal-associated E. coli in adults at the time of first diagnosis.

MATERIALS AND METHODS

E. coli were isolated from 59 right colon biopsies of 34 newly diagnosed adult IBD patients (Crohn's disease [CD] = 23, ulcerative colitis [UC] = 11) and 25 healthy controls (HC). Strains were serotyped, phylotyped into A, B1, B2, or D, and tested for their ability to survive in macrophages. The presence of various virulence factors was also assessed. The fimH subunit of type 1 fimbriae was sequenced and phylogenetically analyzed.

RESULTS

A total of 65 E. coli were isolated from CD (29 isolates from 23 patients), UC (11 isolates from 11 patients), and HC (25 isolates from 25 subjects). All E. coli were positive for fimH, crl, and cgsA and negative for vt1, vt2, hlyA, cnf, and eae. Significant positive associations were between CD and in between CD and afae (P = 0.002), and between UC and ompA (P = 0.02), afae (P = 0.03), and USP (P = 0.04). The B2+D phylotype was significantly associated with inflammation (P = 0.04) as it was with serine protease autotransporters (SPATE), malX, ompA, and kpsMTII (P < 0.05). Macrophage survival was the highest in UC-isolated E. coli (P = 0.04). FimH amino acid substitutions N91S, S99N, and A223V were associated with IBD (P < 0.05).

CONCLUSIONS

Adherent invasive E. coli are present at first diagnosis, suggesting that they may have a role in the early stages of disease onset.

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.

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.

Evidence of cellular immune response to outer membrane protein of Salmonella typhimurium in patients with enthesitis-related arthritis subtype of juvenile idiopathic arthritis

OBJECTIVE

enthesitis-related arthritis subtype of juvenile idiopathic arthritis (JIA-ERA) clinically resembles reactive arthritis (ReA). In patients with ReA the immune response is targeted at the outer membrane protein (OMP) of Salmonella typhimurium. We studied the immune response in JIA-ERA to S. typhimurium OMP.

METHODS

synovial fluid mononuclear cells (SFMC) and peripheral blood mononuclear cells (PBMC) were isolated from blood and SF of patients with JIA-ERA. Lymphocyte transformation test was done with S. typhimurium OMP and crude bacterial lysates of Yersinia enterocolitica, Shigella flexneri, and S. typhimurium. IgG and IgA ELISA were performed in serum and SF using S. typhimurium OMP as antigen and compared with sera from healthy controls.

RESULTS

in PBMC samples (n = 25) an antigen-specific proliferative response was seen in 13 patients and a cross-reactive response in 6. Among these 19 patients, 12 showed response to OMP. In SFMC (n = 15) antigen-specific responses were seen in 3 patients and cross-reactive responses in 9. Among these 12 patients, 11 showed response to OMP. The IgG and IgA anti-OMP antibody concentrations in serum and SF were similar in patients and controls.

CONCLUSION

in JIA-ERA, OMP is the major antigenic target recognized by both SFMC and PBMC. Response to OMP is independent of specific bacterial response, suggesting that OMP is the immunodominant antigen. In these patients, absence of significant humoral response suggests response to OMP is mainly T cell mediated.

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.

Procession to pediatric bacteremia and sepsis: covert operations and failures in diplomacy

Despite advances in diagnosis and treatment, bacterial sepsis remains a major cause of pediatric morbidity and mortality, particularly among neonates, the critically ill, and the growing immunocompromised patient population. Sepsis is the end point of a complex and dynamic series of events in which both host and microbial factors drive high morbidity and potentially lethal physiologic alterations. In this article we provide a succinct overview of the events that lead to pediatric bloodstream infections (BSIs) and sepsis, with a focus on the molecular mechanisms used by bacteria to subvert host barriers and local immunity to gain access to and persist within the systemic circulation. In the events preceding and during BSI and sepsis, Gram-positive and Gram-negative pathogens use a battery of factors for translocation, inhibition of immunity, molecular mimicry, intracellular survival, and nutrient scavenging. Gaps in understanding the molecular pathogenesis of bacterial BSIs and sepsis are highlighted as opportunities to identify and develop new therapeutics.

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 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.

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.

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.

Blockade of virus infection by human CD4+ T cells via a cytokine relay network

CD4(+) T cells directly participate in bacterial clearance through secretion of proinflammatory cytokines. Although viral clearance relies heavily on CD8(+) T cell functions, we sought to determine whether human CD4(+) T cells could also directly influence viral clearance through cytokine secretion. We found that IFN-gamma and TNF-alpha, secreted by IL-12-polarized Th1 cells, displayed potent antiviral effects against a variety of viruses. IFN-gamma and TNF-alpha acted directly to inhibit hepatitis C virus replication in an in vitro replicon system, and neutralization of both cytokines was required to block the antiviral activity that was secreted by Th1 cells. IFN-gamma and TNF-alpha also exerted antiviral effects against vesicular stomatitis virus infection, but in this case, functional type I IFN receptor activity was required. Thus, in cases of vesicular stomatitis virus infection, the combination of IFN-gamma and TNF-alpha secreted by human Th1 cells acted indirectly through the IFN-alpha/beta receptor. These results highlight the importance of CD4(+) T cells in directly regulating antiviral responses through proinflammatory cytokines acting in both a direct and indirect manner.

Survival and replication of Escherichia coli O157:H7 inside the mice peritoneal macrophages

The replication of Escherichia coli O157:H7 on the resident peritoneal macrophages of four mice strains (BALB/c, CD1, C57BL, and Swiss) has been investigated. Macrophagial bactericidal killing activity was estimated via studying their ability to internalize (gentamicin-protected) E. coli during 2, 4, 24, and 48 h assays. Host genetic background has been found to show no significant effect on the ability of resident peritoneal macrophages to kill E. coli O157:H7.

Effects of complement regulators bound to Escherichia coli K1 and Group B Streptococcus on the interaction with host cells

Escherichia coli K1 and Group B Streptococcus (GBS) are the most common bacteria that cause meningitis during the neonatal period. Complement, the first line of defence in the host, acts on these bacteria to opsonize with various components of complement for subsequent presentation to phagocytes. To counteract these opsonization effects, E. coli and GBS bind to the complement regulators C4 binding protein and Factor H, respectively. Nonetheless, the deposition of complement components on these two bacteria from neonatal serum and their effect on the host cell interaction is unclear. Here we demonstrated that the deposition of complement proteins from adult serum prevented the invasion of E. coli into human brain microvascular endothelial cells, whereas the invasion of GBS was enhanced. In contrast, treatment with cord serum had no effect on the invasion of both these bacteria. We also examined the effect of the deposited complement proteins on phagocytosis using THP-1 cells and THP-1 cells differentiated into macrophages. Escherichia coli treated with adult serum neither attached nor entered these cells, whereas GBS was phagocytosed and survived efficiently. We further demonstrate that the inhibitory effect of complement proteins is the result of the bound complement inhibitors C4b-binding protein, in the case of E. coli, and Factor H, in the case of GBS. Taken together, these results suggest that E. coli and GBS utilize contrasting mechanisms of complement-mediated interactions with their target cells for successful establishment of disease.

A molecular Swiss army knife: OmpA structure, function and expression

The OmpA outer membrane protein of Escherichia coli and other enterobacteria is a multifaceted protein. This protein is expressed to very high levels and ompA is tightly regulated at the posttranscriptional level. It can function as an adhesin and invasin, participate in biofilm formation, act as both an immune target and evasin, and serves as a receptor for several bacteriophages. Many of these properties are due to four short protein loops that emanate from the protein to the outside of the cell. Herein it is described how the structure of this protein relates to its many functions.

Identification of a new OmpA-like protein in Neisseria gonorrhoeae involved in the binding to human epithelial cells and in vivo colonization

Outer membrane protein As (OmpAs) are highly conserved proteins within the Enterobacteriaceae family. OmpA contributes to the maintenance of structural membrane integrity and invasion into mammalian cells. In Escherichia coli K1 OmpA also contributes to serum resistance and is involved in the virulence of the bacterium. Here we describe the identification of an OmpA-like protein in Neisseria gonorrhoeae (Ng-OmpA). We show that the gonococcal OmpA-like protein, similarly to E. coli OmpA, plays a significant role in the adhesion and invasion into human cervical carcinoma and endometrial cells and is required for entry into macrophages and intracellular survival. Furthermore, the isogenic knockout ompA mutant demonstrates reduced recovery in a mouse model of infection when compared with the wild-type strain, suggesting that Ng-OmpA plays an important role in the in vivo colonization. All together, these data suggest that the newly identified surface exposed protein Ng-OmpA represents a novel virulence factor of gonococcus.

Going for baroque at the Escherichia coli K1 cell surface

Phase variation is usually thought of as the stochastic switching between alternatively expressed ('on') and unexpressed ('off') phenotypic states. However, coupling synthesis of a monotonous homopolysaccharide to a mechanism of random but incomplete chemical modification produces almost infinite structural variation. Potentially limitless variability implies that evolution can produce highly ornate or extravagant flourishes reminiscent of the baroque style. Here, we describe an analysis of capsular polysialic acid form variation in Escherichia coli K1, demonstrating that the large number of variant structures is controlled by a single contingency locus. The mechanism for generating maximum structural diversity from maximal genetic parsimony is conferred by a simple translational switch carried on a K1-specific prophage.

The capsule plays an important role in Escherichia coli K1 interactions with Acanthamoeba

Escherichia coli K1 is shown to bind to, associate with, invade and survive inside Acanthamoeba, but the precise mechanisms associated with these events are unclear. We have previously shown that outer membrane protein A and lipopolysaccharide are critical bacterial determinants involved in E. coli K1 interactions with Acanthamoeba. Using an isogenic K1 capsule-deletion mutant (lacking the neuDB genes cluster that is necessary for the production of cytoplasmic precursors to the exopolysaccharide capsule), we observed that the capsule modulates and enhances E. coli K1 association and survival inside Acanthamoeba. The capsule-deletion mutant exhibited significantly reduced association compared with the wild type strain, E44. Similarly, the K1 capsule-deletion mutant exhibited limited ability for invasion/uptake by and survival inside Acanthamoeba. Next, we determined whether E. coli K1 survive inside Acanthamoeba during the encystment process and that viable bacteria can be isolated from the mature cysts. Using encystment assays, our findings revealed that E. coli K1, but not its capsule-deletion mutant, exhibit survival inside Acanthamoeba cysts. We believe this is the first demonstration that the K1 capsule plays an important role in E. coli K1 interactions with Acanthamoeba.

Escherichia coli interactions with Acanthamoeba: a symbiosis with environmental and clinical implications

The ability of Acanthamoeba to feed on Gram-negative bacteria, as well as to harbour potential pathogens, such as Legionella pneumophila, Coxiella burnetii, Pseudomonas aeruginosa, Vibrio cholerae, Helicobacter pylori, Listeria monocytogenes and Mycobacterium avium, suggest that both amoebae and bacteria are involved in complex interactions, which may play important roles in the environment and in human health. In this study, Acanthamoeba castellanii (a keratitis isolate belonging to the T4 genotype) was used and its interactions with Escherichia coli (strain K1, a cerebrospinal fluid isolate from a meningitis patient, O18 : K1 : H7, and a K-12 laboratory strain, HB101) were studied. The invasive K1 isolate exhibited a significantly higher association with A. castellanii than the non-invasive K-12 isolate. Similarly, K1 showed significantly increased invasion and/or uptake by A. castellanii in gentamicin protection assays than the non-invasive K-12. Using several mutants derived from K1, it was observed that outer-membrane protein A (OmpA) and LPS were crucial bacterial determinants responsible for E. coli K1 interactions with A. castellanii. Once inside the cell, E. coli K1 remained viable and multiplied within A. castellanii, while E. coli K-12 was killed. Again, OmpA and LPS were crucial for E. coli K1 intracellular survival in A. castellanii. In conclusion, these findings suggest that E. coli K1 interactions with A. castellanii are carefully regulated by the virulence of E. coli.

Logarithmic phase Escherichia coli K1 efficiently avoids serum killing by promoting C4bp-mediated C3b and C4b degradation

Meningitis caused by Escherichia coli K1 is a serious illness in neonates with neurological sequelae in up to 50% of survivors. A high degree of bacteremia is required for E. coli K1 to cross the blood-brain barrier, which suggests that the bacterium must evade the host defence mechanisms and survive in the bloodstream. We previously showed that outer membrane protein A (OmpA) of E. coli binds C4b-binding protein (C4bp), an inhibitor of complement activation via the classical pathway. Nevertheless, the exact mechanism by which E. coli K1 survives in serum remains elusive. Here, we demonstrate that log phase (LP) OmpA+ E. coli K1 avoids serum bactericidal activity more effectively than postexponential phase bacteria. OmpA- E. coli cannot survive in serum grown to either phase. The increased serum resistance of LP OmpA+ E. coli is the result of increased binding of C4bp, with a concomitant decrease in the deposition of C3b and the downstream complement proteins responsible for the formation of the membrane attack complex. C4bp bound to E. coli K1 acts as a cofactor to factor I in the cleavage of both C3b and C4b, which shuts down the ensuing complement cascade. Accordingly, a peptide corresponding to the complement control protein domain 3 of C4bp sequence, was able to compete with C4bp binding to OmpA and cause increased deposition of C3b. Thus, binding of C4bp appears to be responsible for survival of E. coli K1 in human serum.

Escherichia coliK1 inhibits proinflammatory cytokine induction in monocytes by preventing NF-κB activation

Phagocytes are well-known effectors of the innate immune system to produce proinflammatory cytokines and chemokines such as tumor necrosis factor alpha (TNF-alpha), interleukin (IL)-1beta, and IL-8 during infections. Here, we show that infection of monocytes with wild-type Escherichia coli K1, which causes meningitis in neonates, suppresses the production of cytokines and chemokines (TNF-alpha, regulated on activation, normal T expressed and secreted, macrophage-inflammatory protein-1beta, IL-1beta, and IL-8). In contrast, infection of monocytes with a mutant E. coli, which lacks outer membrane protein A (OmpA- E. coli) resulted in robust production of cytokines and chemokines. Wild-type E. coli K1 (OmpA+ E. coli) prevented the phosphorylation and its degradation of inhibitor of kappaB, thereby blocking the translocation of nuclear factor (NF)-kappaB to the nucleus. OmpA+ E. coli-infected cells, subsequently subjected to lipopolysaccharide challenge, were crippled severely in their ability to activate NF-kappaB to induce cytokine/chemokine production. Selective inhibitors of the extracellular signal-regulated kinase (ERK) 1/2 pathway and p38 mitogen-activated protein kinase (MAPK), but not Jun N-terminal kinase, significantly reduced the activation of NF-kappaB and the production of cytokines and chemokines induced by OmpA- E. coli, indicating a role for these kinases in the NF-kappaB/cytokine pathway. It is interesting that the phosphorylation of ERK 1/2 and p38 MAPK was notably reduced in monocytes infected with OmpA+ E. coli when compared with monocytes infected with OmpA- E. coli, suggesting that the modulation of upstream events common for NF-kappaB and MAPKs by the bacterium is possible. The ability of OmpA+ E. coli K1 to inhibit the macrophage response temporarily may enable bacterial survival and growth within the host for the onset of meningitis by E. coli K1.

Serum amyloid A protein binds to outer membrane protein A of gram-negative bacteria

Serum amyloid A (SAA) is the major acute phase protein in man and most mammals. We observed SAA binding to a surprisingly large number of Gram-negative bacteria, including Escherichia coli, Salmonella typhimurium, Shigella flexneri, Klebsiella pneumoniae, Vibrio cholerae, and Pseudomonas aeruginosa. The binding was found to be high affinity and rapid. Importantly, this binding was not inhibited by high density lipoprotein with which SAA is normally complexed in serum. Binding was also observed when bacteria were offered serum containing SAA. Ligand blots following SDS-PAGE or two-dimensional gels revealed two major ligands of 29 and 35 kDa that bound SAA when probing with radiolabeled SAA or SAA and monoclonal anti-SAA. Following fractionation the ligand was found in the outer membrane fraction of E. coli and was identified by matrix-assisted laser desorption ionization time-of-flight mass spectrometry to be outer membrane protein A (OmpA). OmpA-deficient E. coli did not bind SAA, and following purification of OmpA the protein retained binding activity. The ligands on other bacteria were likely to be homologues of OmpA because wild type, but not OprF-deficient, P. aeruginosa bound SAA.

Inhibition of apoptosis by Escherichia coli K1 is accompanied by increased expression of BclXL and blockade of mitochondrial cytochrome c release in macrophages

Escherichia coli K1 survival in the blood is a critical step for the onset of meningitis in neonates. Therefore, the circulating bacteria are impelled to avoid host defense mechanisms by finding a niche to survive and multiply. Our recent studies have shown that E. coli K1 enters and survives in both monocytes and macrophages in the newborn rat model of meningitis as well as in macrophage cell lines. Here we demonstrate that E. coli K1 not only extends the survival of human and murine infected macrophage cell lines but also renders them resistant to apoptosis induced by staurosporine. Macrophages infected with wild-type E. coli expressing outer membrane protein A (OmpA), but not with OmpA- E. coli, are resistant to DNA fragmentation and phosphatidylserine exposure induced by staurosporine. Infection with OmpA+ E. coli induces the expression of Bcl(XL), an antiapoptotic protein, both at the mRNA level as assessed by gene array analysis and at the protein level as evaluated by immunoblotting. OmpA- E. coli infection of macrophages induced the release of cytochrome c from mitochondria into the cytosol and the activation of caspases 3, 6, and 9, events that were significantly blocked in OmpA+ E. coli-infected macrophages. In addition, OmpA+ E. coli-infected cells were resistant to a decrease in the transmembrane potential of mitochondria induced by staurosporine as measured by the MitoCapture fluorescence technique. Complementation of OmpA- E. coli with a plasmid containing the ompA gene restored the ability of OmpA- E. coli to inhibit the apoptosis of infected macrophages, further demonstrating that E. coli OmpA expression is critical for inducing macrophage survival and thereby finding a safe haven for its growth.