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

Characterization of a Myeloid Differentiation Factor 2-Derived Peptide that Facilitates THP-1 Macrophage-Mediated Phagocytosis of Gram-Negative Bacteria

Myeloid differentiation factor 2 (MD2), the TLR4 coreceptor, has been shown to possess opsonic activity and has been implicated in phagocytosis and intracellular killing of Gram-negative bacteria. However, any MD2 protein segment involved in phagocytosis of Gram-negative bacteria is not yet known. A short synthetic MD2 segment, MD54 (amino acid regions 54 to 69), was shown to interact with a Gram-negative bacterial outer membrane component, LPS, earlier. Furthermore, the MD54 peptide induced aggregation of LPS and facilitated its internalization in THP-1 cells. Currently, it has been investigated if MD2-derived MD54 possesses any opsonic property and role in phagocytosis of Gram-negative bacteria. Remarkably, we observed that MD54 facilitated agglutination of Gram-negative bacteria, Escherichia coli (ATCC 25922) and Pseudomonas aeruginosa (ATCC BAA-427), but not of Gram-positive bacteria, Bacillus subtilis (ATCC 6633) and Staphylococcus aureus (ATCC 25923). The MD54-opsonized Gram-negative bacteria internalized within PMA-treated THP-1 cells and were killed over a longer incubation period. However, both internalization and intracellular killing of the MD54-opsonized Gram-negative bacteria within THP-1 phagocytes were appreciably inhibited in the presence of a phagocytosis inhibitor, cytochalasin D. Furthermore, MD54 facilitated the clearance of Gram-negative bacteria E. coli (ATCC 25922) and P. aeruginosa (ATCC BAA-427) from the infected BALB/c mice whereas an MD54 analog, MMD54, was inactive. Overall, for the first time, the results revealed that a short MD2-derived peptide can specifically agglutinate Gram-negative bacteria, act as an opsonin for these bacteria, and facilitate their phagocytosis by THP-1 phagocytes. The results suggest that the MD54 segment could have a crucial role in MD2-mediated host-pathogen interaction involving the Gram-negative bacteria.

Synthetic and natural antimicrobials as a control against food borne pathogens: A review

Food borne pathogens are one of the most common yet concerning cause of illnesses around the globe. These microbes invade the body via food items, through numerous mediums of contamination and it is impossible to completely eradicate these organisms from food. Extensive research has been made regarding their treatment. Unfortunately, the only available treatment currently is by antibiotics. Recent exponential increase in antibiotic resistance and the side effect of synthetic compounds have established a need for alternate therapies that could be utilized either on their own or along with antibiotics to provide protection against food-borne diseases. The aim of this review is to provide information regarding some common food borne diseases, their current and possible natural treatment. It will include details regarding some common foodborne pathogens, the disease they cause, prevalence, manifestations and treatment of the respective disease. Some natural modes of potential treatment will be summarized, which including phytochemicals, derived from plants either as crude extracts or as purified form and Bacteriocins as microbial based treatment, obtained from various types of bacteria. The paper will describe their mechanism of action, classification, susceptible organisms, some antimicrobial compounds and producing organisms, application in food systems and as potential treatment. Along with that, synthetic treatment i.e., antibiotics will be discussed including the first-line treatment of some common food borne infections, prevalence and mechanism of resistance against antibiotics in the pathogens.

Lpp of Escherichia coli K1 inhibits host ROS production to counteract neutrophil-mediated elimination

Escherichia coli (E. coli) is the most common Gram-negative bacterial organism causing neonatal meningitis. The pathogenesis of E. coli meningitis, especially how E. coli escape the host immune defenses, remains to be clarified. Here we show that deletion of bacterial Lpp encoding lipoprotein significantly reduces the pathogenicity of E. coli K1 to induce high-degree of bacteremia necessary for meningitis. The Lpp-deleted E. coli K1 is found to be susceptible to the intracellular bactericidal activity of neutrophils, without affecting the release of neutrophil extracellular traps. The production of reactive oxygen species (ROS), representing the primary antimicrobial mechanism in neutrophils, is significantly increased in response to Lpp-deleted E. coli. We find this enhanced ROS response is associated with the membrane translocation of NADPH oxidase p47^phox and p67^phox in neutrophils. Then we constructed p47^phox knockout mice and we found the incidence of bacteremia and meningitis in neonatal mice induced by Lpp-deleted E. coli is significantly recovered by p47^phox knockout. Proteomic profile analysis show that Lpp deficiency induces upregulation of flagellar protein FliC in E. coli. We further demonstrate that FliC is required for the ROS induction in neutrophils by Lpp-deleted E. coli. Taken together, these data uncover the novel role of Lpp in facilitating intracellular survival of E. coli K1 within neutrophils. It can be inferred that Lpp of E. coli K1 is able to suppress FliC expression to restrain the activation of NADPH oxidase in neutrophils resulting in diminished bactericidal activity, thus protecting E. coli K1 from the elimination by neutrophils.

Hcp Proteins of the Type VI Secretion System Promote Avian Pathogenic E. coli DE205B (O2:K1) to Induce Meningitis in Rats

Avian pathogenic Escherichia coli (APEC) is an important extra-intestinal pathogenic E. coli (ExPEC), which often causes systemic infection in poultry and causes great economic loss to the breeding industry. In addition, as a major source of human ExPEC infection, the potential zoonotic risk of APEC has been an ongoing concern. Previous studies have pointed out that APEC is a potential zoonotic pathogen, which has high homology with human pathogenic E. coli such as uro-pathogenic E. coli (UPEC) and neonatal meningitis E. coli (NMEC), shares multiple virulence factors and can cause mammalian diseases. Previous studies have reported that O18 and O78 could cause different degrees of meningitis in neonatal rats, and different serotypes had different degrees of zoonotic risk. Here, we compared APEC DE205B (O2:K1) with NMEC RS218 (O18:K1:H7) by phylogenetic analysis and virulence gene identification to analyze the potential risk of DE205B in zoonotic diseases. We found that DE205B possessed a variety of virulence factors associated with meningitis and, through phylogenetic analysis, had high homology with RS218. DE205B could colonize the cerebrospinal fluid (CSF) of rats, and cause meningitis and nerve damage. Symptoms and pathological changes in the brain were similar to RS218. In addition, we found that DE205B had a complete T6SS, of which Hcp protein was its important structural protein. Hcp1 induced cytoskeleton rearrangement in human brain microvascular endothelial cells (HBMECs), and Hcp2 was mainly involved in the invasion of DE205B in vitro. In the meningitis model of rats, deletion of hcp2 gene reduced survival in the blood and the brain invasiveness of DE205B. Compared with WT group, Δhcp2 group induced lower inflammation and neutrophils infiltration in brain tissue, alleviating the process of meningitis. Together, these results suggested that APEC DE205B had close genetic similarities to NMEC RS218, and a similar mechanism in causing meningitis and being a risk for zoonosis. This APEC serotype provided a basis for zoonotic research.

Salmonella Typhimurium outer membrane protein A (OmpA) renders protection from nitrosative stress of macrophages by maintaining the stability of bacterial outer membrane

Bacterial porins are highly conserved outer membrane proteins used in the selective transport of charged molecules across the membrane. In addition to their significant contributions to the pathogenesis of Gram-negative bacteria, their role(s) in salmonellosis remains elusive. In this study, we investigated the role of outer membrane protein A (OmpA), one of the major outer membrane porins of Salmonella, in the pathogenesis of Salmonella Typhimurium (STM). Our study revealed that OmpA plays an important role in the intracellular virulence of Salmonella. An ompA deficient strain of Salmonella (STM ΔompA) showed compromised proliferation in macrophages. We found that the SPI-2 encoded virulence factors such as sifA and ssaV are downregulated in STM ΔompA. The poor colocalization of STM ΔompA with LAMP-1 showed that disruption of SCV facilitated its release into the cytosol of macrophages, where it was assaulted by reactive nitrogen intermediates (RNI). The enhanced recruitment of nitrotyrosine on the cytosolic population of STM ΔompAΔsifA and ΔompAΔssaV compared to STM ΔsifA and ΔssaV showed an additional role of OmpA in protecting the bacteria from host nitrosative stress. Further, we showed that the generation of greater redox burst could be responsible for enhanced sensitivity of STM ΔompA to the nitrosative stress. The expression of several other outer membrane porins such as ompC, ompD, and ompF was upregulated in STM ΔompA. We found that in the absence of ompA, the enhanced expression of ompF increased the outer membrane porosity of Salmonella and made it susceptible to in vitro and in vivo nitrosative stress. Our study illustrates a novel mechanism for the strategic utilization of OmpA by Salmonella to protect itself from the nitrosative stress of macrophages.

Salmonella Typhimurium majorly uses SPI-1 and SPI-2 encoded T3SS and virulence factors for thriving in the host macrophages. But the role of non-SPI genes in Salmonella pathogenesis remains unknown. This article illustrates a novel mechanism of how a non-SPI virulent protein, OmpA, helps Salmonella Typhimurium to survive in murine macrophages. Our data revealed that Salmonella lacking OmpA (STM ΔompA) is deficient in producing SPI-2 effector proteins and has a severe defect in maintaining the stability of its outer membrane. It is released into the cytosol of macrophages during infection after disrupting the SCV membrane. STM ΔompA was severely challenged with reactive nitrogen intermediates in the cytosol, which reduced their proliferation in macrophages. We further showed that the deletion of OmpA increased the expression of other larger porins (ompC, ompD, and ompF) on the surface of Salmonella. It was observed that the enhanced expression of OmpF in STM ΔompA increased the outer membrane permeability and made the bacteria more susceptible to in vitro and in vivo nitrosative stress. Altogether our study proposes new insights into the role of Salmonella OmpA as an essential virulence factor.

Cross-Kingdom Infection of Macrophages Reveals Pathogen- and Immune-Specific Global Reprogramming and Adaptation

The relationship between the human microbiota and infectious disease outcome is a rapidly expanding area of study. Understanding how the host responds to changes in its symbiotic relationship with microbes provides new insight into how disruption can promote disease.

Collaborative Action of Microglia and Astrocytes Mediates Neutrophil Recruitment to the CNS to Defend against Escherichia coli K1 Infection

Escherichia coli K1 is a leading cause of neonatal bacterial meningitis. Recruitment of neutrophils to the central nervous system (CNS) via local immune response plays a critical role in defense against E. coli K1 infection; however, the mechanism underlying this recruitment remains unclear. In this study, we report that microglia and astrocytes are activated in response to stimulation by E. coli K1 and/or E. coli K1-derived outer membrane vesicles (OMVs) and work collaboratively to drive neutrophil recruitment to the CNS. Microglial activation results in the release of the pro-inflammatory cytokine TNF-α, which activates astrocytes, resulting in the production of CXCL1, a chemokine critical for recruiting neutrophils. Mice lacking either microglia or TNF-α exhibit impaired production of CXCL1, impaired neutrophil recruitment, and an increased CNS bacterial burden. C-X-C chemokine receptor 2 (CXCR2)-expressing neutrophils primarily respond to CXCL1 released by astrocytes. This study provides further insights into how immune responses drive neutrophil recruitment to the brain to combat E. coli K1 infection. In addition, we show that direct recognition of E. coli K1 by microglia is prevented by the K1 capsule. This study also reveals that OMVs are sufficient to induce microglial activation.

Roles of the Tol/Pal System in Bacterial Pathogenesis and Its Application to Antibacterial Therapy

The Tol/Pal system (also written as “The Tol-Pal system”) is a set of protein complexes produced by most Gram-negative bacteria. It comprises the inner membrane-associated and the outer membrane-anchored subunits composed of the TolA, TolQ, and TolR proteins and the TolB and Pal proteins, respectively. Although the Tol/Pal system was first defined as bacterial proteins involved in colicin uptake of Escherichia coli, its global roles have been characterized in several studies as mentioned in this article. Pathogenesis of many Gram-negative pathogens is sustained by the Tol/Pal system. It is also essential for cell growth and fitness in some pathogens. Therefore, the Tol/Pal system is proposed as a potential target for antimicrobial chemotherapy. Although the tol/pal mutants are low in virulence, they still have the ability to stimulate the immune system. The Pal protein is highly immunogenic and induces both adaptive and innate immune responses. Therefore, the tol/pal mutant strains and Pal proteins also have potential vaccine properties. For these reasons, the Tol/Pal system represents a promising research target in the development of antibacterial therapeutic strategies for refractory infections caused by multi-drug-resistant (MDR), Gram-negative pathogens. In this paper, we summarize studies on the Tol/Pal system associated with bacterial pathogenesis and vaccine development.

Nucleotide receptors mediate protection against neonatal sepsis and meningitis caused by alpha-hemolysin expressing Escherichia coli K1

Neonatal meningitis-associated Escherichia coli (NMEC) is among the leading causes of bacterial meningitis and sepsis in newborn infants. Several virulence factors have been identified as common among NMEC, and have been shown to play an important role in the development of bacteremia and/or meningitis. However, there is significant variability in virulence factor expression between NMEC isolates, and relatively little research has been done to assess the impact of variable virulence factor expression on immune cell activation and the outcome of infection. Here, we investigated the role of NMEC strain-dependent P2X receptor (P2XR) signaling on the outcome of infection in neonatal mice. We found that alpha-hemolysin (HlyA)-expressing NMEC (HlyA⁺ ) induced robust P2XR-dependent macrophage cell death in vitro, while HlyA^- NMEC did not. P2XR-dependent cell death was inflammasome independent, suggesting an uncoupling of P2XR and inflammasome activation in the context of NMEC infection. In vivo inhibition of P2XRs was associated with increased mortality in neonatal mice infected with HlyA⁺ NMEC, but had no effect on the survival of neonatal mice infected with HlyA^- NMEC. Furthermore, we found that P2XR-dependent protection against HlyA⁺ NMEC in vivo required macrophages, but not neutrophils or NLRP3. Taken together, these data suggest that HlyA⁺ NMEC activates P2XRs which in turn confers macrophage-dependent protection against infection in neonates. In addition, our findings indicate that strain-dependent virulence factor expression should be taken into account when studying the immune response to NMEC.

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

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

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

Roles of OmpA in Type III Secretion System-Mediated Virulence of Enterohemorrhagic Escherichia coli

Outer membrane proteins are commonly produced by gram-negative bacteria, and they have diverse functions. A subgroup of proteins, which includes OmpA, OmpW and OmpX, is often involved in bacterial pathogenesis. Here we show that OmpA, rather than OmpW or OmpX, contributes to the virulence of enterohemorrhagic Escherichia coli (EHEC) through its type III secretion system (T3SS). Deletion of ompA decreased secretion of the T3SS proteins EspA and EspB; however, the expression level of the LEE genes that encode a set of T3SS proteins did not decrease. The ompA mutant had less abilities to form A/E lesions in host epithelial cells and lyse human red blood cells than the parent strain. Moreover, the virulence of an ompA mutant of Citrobacter rodentium (traditionally used to estimate T3SS-associated virulence in mice) was attenuated. Mice infected with the ompA mutant survived longer than those infected with the parent strain. Furthermore, mice infected with ompA developed symptoms of diarrhea more slowly than mice infected with the parent strain. Altogether, these results suggest that OmpA sustains the activity of the T3SS and is required for optimal virulence in EHEC. This work expands the roles of outer membrane proteins in bacterial pathogenesis.

A previously uncharacterized two-component signaling system in uropathogenic Escherichia coli coordinates protection against host-derived oxidative stress with activation of hemolysin-mediated host cell pyroptosis

Uropathogenic Escherichia coli (UPEC) deploy an array of virulence factors to successfully establish urinary tract infections. Hemolysin is a pore-forming toxin, and its expression correlates with the severity of UPEC infection. Two-component signaling systems (TCSs) are a major mechanism by which bacteria sense environmental cues and respond by initiating adaptive responses. Here, we began this study by characterizing a novel TCS (C3564/C3565, herein renamed orhK/orhR for oxidative resistance and hemolysis kinase/regulator) that is encoded on a UPEC pathogenicity island, using bioinformatic and biochemical approaches. A prevalence analysis indicates that orhK/orhR is highly associated with the UPEC pathotype, and it rarely occurs in other E. coli pathotypes tested. We then demonstrated that OrhK/OrhR directly activates the expression of a putative methionine sulfoxide reductase system (C3566/C3567) and hemolysin (HlyA) in response to host-derived hydrogen peroxide (H₂O₂) exposure. OrhK/OrhR increases UPEC resistance to H₂O₂in vitro and survival in macrophages in cell culture via C3566/C3567. Additionally, OrhK/OrhR mediates hemolysin-induced renal epithelial cell and macrophage death via a pyroptosis pathway. Reducing intracellular H₂O₂ production by a chemical inhibitor impaired OrhK/OrhR-mediated activation of c3566-c3567 and hlyA. We also uncovered that UPEC links the two key virulence traits by cotranscribing the c3566-c3567 and hlyCABD operons. Taken together, our data suggest a paradigm in which a signal transduction system coordinates both bacterial pathogen defensive and offensive traits in the presence of host-derived signals; and this exquisite mechanism likely contributes to hemolysin-induced severe pathological outcomes.

Uropathogenic Escherichia coli (UPEC) is the primary cause of urinary tract infections, and approximately half of UPEC isolates produce a pore-forming toxin, hemolysin. Clinically, hemolysin carriage is associated with severe pathology and symptoms during UPEC infections. However, overexpression of hemolysin can be detrimental to UPEC colonization. Therefore, fine-tuning of hemolysin expression in response to in vivo-relevant signals is critical for optimal UPEC fitness in the urinary tract. In this study, we describe a virulence strategy employed by UPEC, i.e., the bacteria use a two-component signaling (TCS) system to coordinate oxidative stress resistance and hemolysin-mediated pyroptosis of host cells in response to host-derived oxidative signals. The TCS achieves this coordination by cotranscribing genes encoding the oxidative stress resistance and the hemolysin. As a result, UPEC is able to link defense to offense, and this exquisite virulence mechanism likely contributes to UPEC fitness in vivo and hemolysin-induced severe pathological outcomes.

Designer protein pseudo-capsids targeting intracellular bacteria

The emergence of multidrug-resistant bacteria stimulates the search for antimicrobial materials capable of addressing challenges conventional antibiotics fail to address. The ability to target intracellular bacteria remains one of the most fundamental tasks for contemporary antimicrobial treatments. Here we report engineered protein pseudo-capsids targeting bacteria internalised in macrophages. Using a combination of live-cell imaging and single-cell electron microscopy analysis we show that these materials effectively disrupt the bacteria without affecting the host cells. The study offers a disruptive antimicrobial strategy demonstrating potential for developing principally more challenging mechanisms for bacteria to overcome.

Combatting intracellular pathogens using bacteriophage delivery

Intracellular pathogens reside in specialised compartments within the host cells restricting the access of antibiotics. Insufficient intracellular delivery of antibiotics along with several other resistance mechanisms weaken the efficacy of current therapies. An alternative to antibiotic therapy could be bacteriophage (phage) therapy. Although phage therapy has been in practice for a century against various bacterial infections, the efficacy of phages against intracellular bacteria is still being explored. In this review, we will discuss the advancement and challenges in phage therapy, particularly against intracellular bacterial pathogens. Finally, we will highlight the uptake mechanisms and approaches to overcome the challenges to phage therapy against intracellular bacteria.

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

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

Preconceptual Priming Overrides Susceptibility to Escherichia coli Systemic Infection during Pregnancy

Maternal sepsis is a leading cause of morbidity and mortality during pregnancy. Escherichia coli is a primary cause of bacteremia in women and occurs more frequently during pregnancy. Several key outstanding questions remain regarding how to identify women at highest infection risk and how to boost immunity against E. coli infection during pregnancy. Here, we show that pregnancy-induced susceptibility to E. coli systemic infection extends to rodents as a model of human infection. Mice infected during pregnancy contain >100-fold-more recoverable bacteria in target tissues than nonpregnant controls. Infection leads to near complete fetal wastage that parallels placental plus congenital fetal invasion. Susceptibility in maternal tissues positively correlates with the number of concepti, suggesting important contributions by expanded placental-fetal target tissue. Remarkably, these pregnancy-induced susceptibility phenotypes are also efficiently overturned in mice with resolved sublethal infection prior to pregnancy. Preconceptual infection primes the accumulation of E. coli-specific IgG and IgM antibodies, and adoptive transfer of serum containing these antibodies to naive recipient mice protects against fetal wastage. Together, these results suggest that the lack of E. coli immunity may help discriminate individuals at risk during pregnancy, and that overriding susceptibility to E. coli prenatal infection by preconceptual priming is a potential strategy for boosting immunity in this physiological window of vulnerability.IMPORTANCE Pregnancy makes women especially vulnerable to infection. The most common cause of bloodstream infection during pregnancy is by a bacterium called Escherichia coli This bacterium is a very common cause of bloodstream infection, not just during pregnancy but in all individuals, from newborn babies to the elderly, probably because it is always present in our intestine and can intermittently invade through this mucosal barrier. We first show that pregnancy in animals also makes them more susceptible to E. coli bloodstream infection. This is important because many of the dominant factors likely to control differences in human infection susceptibility can be property controlled for only in animals. Despite this vulnerability induced by pregnancy, we also show that animals with resolved E. coli infection are protected against reinfection during pregnancy, including having resistance to most infection-induced pregnancy complications. Protection against reinfection is mediated by antibodies that can be measured in the blood. This information may help to explain why most women do not develop E. coli infection during pregnancy, enabling new approaches for identifying those at especially high risk of infection and strategies for preventing infection during pregnancy.

Diversification of OmpA and OmpF of Yersinia ruckeri is independent of the underlying species phylogeny and evidence of virulence-related selection

Yersinia ruckeri is the causative agent of enteric redmouth disease (ERM) which causes economically significant losses in farmed salmonids, especially Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss, Walbaum). However, very little is known about the genetic relationships of disease-causing isolates in these two host species or about factors responsible for disease. Phylogenetic analyses of 16 representative isolates based on the nucleotide sequences of 19 housekeeping genes suggests that pathogenic Atlantic salmon and rainbow trout isolates represent distinct host-specific lineages. However, the apparent phylogenies of certain isolates has been influenced by horizontal gene transfer and recombinational exchange. Splits decomposition analysis demonstrated a net-like phylogeny based on the housekeeping genes, characteristic of recombination. Comparative analysis of the distribution of individual housekeeping gene alleles across the isolates demonstrated evidence of genomic mosaicism and recombinational exchange involving certain Atlantic salmon and rainbow trout isolates. Comparative nucleotide sequence analysis of the key outer membrane protein genes ompA and ompF revealed that the corresponding gene trees were both non-congruent with respect to the housekeeping gene phylogenies providing evidence that horizontal gene transfer has influenced the evolution of both these surface protein-encoding genes. Analysis of inferred amino acid sequence variation in OmpA identified a single variant, OmpA.1, that was present in serotype O1 and O8 isolates representing typical pathogenic strains in rainbow trout and Atlantic salmon, respectively. In particular, the sequence of surface-exposed loop 3 differed by seven amino acids to that of other Y. ruckeri isolates. These findings suggest that positive selection has likely influenced the presence of OmpA.1 in these isolates and that loop 3 may play an important role in virulence. Amino acid sequence variation of OmpF was greater than that of OmpA and was similarly restricted mainly to the surface-exposed loops. Two OmpF variants, OmpF.1 and OmpF.2, were associated with pathogenic rainbow trout and Atlantic salmon isolates, respectively. These OmpF proteins had very similar amino acid sequences suggesting that positive evolutionary pressure has also favoured the selection of these variants in pathogenic strains infecting both species.

Propionate Induces Virulent Properties of Crohn's Disease-Associated Escherichia coli

Crohn's disease (CD) is a severe chronic immune-mediated granulomatous inflammatory disease of the gastrointestinal tract. The mechanisms of CD pathogenesis remain obscure. Metagenomic analysis of samples from CD patients revealed that several of them have the elevated level of Escherichia coli with adhesive-invasive phenotype (AIEC). Previously, we isolated an E. coli strain CD isolate ZvL2 from a patient with CD, which features AIEC phenotype. Here, we demonstrate that prolonged growth on propionate containing medium stimulates virulent properties of CD isolate ZvL2, while prolonged growth on glucose reduces these properties to levels indistinguishable from laboratory strain K-12 MG1655. Propionate presence also boosts the ability of CD isolate ZvL2 to penetrate and colonize macrophages. The effect of propionate is reversible, re-passaging of CD isolate on M9 medium supplemented with glucose leads to the loss of its virulent properties. Proteome analysis of CD isolate ZvL2 growth in medium supplemented with propionate or glucose revealed that propionate induces expression porins OmpA and OmpW, transcription factors PhoP and OmpR, and universal stress protein UspE, which were previously found to be important for macrophage colonization by enteropathogenic bacteria.

Lipid rafts and pathogens: the art of deception and exploitation

Lipid rafts, solid regions of the plasma membrane enriched in cholesterol and glycosphingolipids, are essential parts of a cell. Functionally, lipid rafts present a platform that facilitates interaction of cells with the outside world. However, the unique properties of lipid rafts required to fulfill this function at the same time make them susceptible to exploitation by pathogens. Many steps of pathogen interaction with host cells, and sometimes all steps within the entire lifecycle of various pathogens, rely on host lipid rafts. Such steps as binding of pathogens to the host cells, invasion of intracellular parasites into the cell, the intracellular dwelling of parasites, microbial assembly and exit from the host cell, and microbe transfer from one cell to another all involve lipid rafts. Interaction also includes modification of lipid rafts in host cells, inflicted by pathogens from both inside and outside the cell, through contact or remotely, to advance pathogen replication, to utilize cellular resources, and/or to mitigate immune response. Here, we provide a systematic overview of how and why pathogens interact with and exploit host lipid rafts, as well as the consequences of this interaction for the host, locally and systemically, and for the microbe. We also raise the possibility of modulation of lipid rafts as a therapeutic approach against a variety of infectious agents.

Chryseobacterium indologenes and Chryseobacterium gleum interact and multiply intracellularly in Acanthamoeba castellanii

Chryseobacterium indologenes and Chryseobacterium gleum are Gram negative environmental bacteria that have been frequently reported to implicate in fatal nosocomial infections, such as bacteraemia and ventilator-associated pneumonia in immunocompromised individuals in the past decades. The interaction between Chryseobacterium spp. and Acanthamoeba castellanii, a free-living amoeba ubiquitous in the environment, has not been explored previously. In this study, C. indologenes and C. gleum were co-cultured with A. castellanii trophozoites and their interactions were evaluated. Our results showed that when co-cultured with A. castellanii, bacterial numbers of C. indologenes and C. gleum increased significantly (p < 0.05), indicating growth-supporting role of A. castellanii. Specifically, our findings showed that C. indologenes and C. gleum were able to associate, invade and/or taken up by A. castellani trophozoites, and multiply intracellularly at similar rates (p > 0.05). Interestingly, the two Chryseobacterium spp. associated, invaded and/or taken up by A. castellanii at significantly higher rates than Escherichia coli K1, a neuropathogenic bacterial strain known to interact and replicate intracellularly in A. castellanii (p < 0.05). However, the ability of both Chryseobacterium spp. to multiply in A. castellanii was significantly weaker than E. coli K1 (p < 0.001). This is the first time that Chryseobacterium spp. and A. castellanii were shown to interact with each other. The ability to survive intracellularly in A. castellanii may confer protection to C. indologenes and C. gleum and assist in the survival and transmission of Chryseobacterium spp. to susceptible hosts within a hospital setting. Future studies will determine the ability of C. indologenes and C. gleum survival in A. castellanii cysts and the possible molecular mechanisms involved in such interactions.

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

Objective

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

Results

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

Strategies used by bacterial pathogens to cross the blood-brain barrier

The skull, spine, meninges, and cellular barriers at the blood-brain and the blood-cerebrospinal fluid interfaces well protect the brain and meningeal spaces against microbial invasion. However, once in the bloodstream, a range of pathogenic bacteria is able to reach the brain and cause meningitis. Despite advances in antibacterial therapy, bacterial meningitis remains one of the most important infectious diseases worldwide. The most common causative bacteria in children and adults are Streptococcus pneumoniae and Neisseria meningitidis associated with high morbidity and mortality, while among neonates, most cases of bacterial meningitis are due to group B Streptococcus and Escherichia coli. Here we summarise our current knowledge on the strategies used by these bacterial pathogens to survive in the bloodstream, to colonise the brain vasculature and to cross the blood-brain barrier.

Antigenic epitopes on the outer membrane protein A of Escherichia coli identified with single-chain variable fragment (scFv) antibodies

Bacterial meningitis is a severe disease that is fatal to one-third of patients. The major cause of meningitis in neonates is Escherichia coli (E. coli) K1. This bacterium synthesizes an outer membrane protein A (OmpA) that is responsible for the adhesion to (and invasion of) endothelial cells. Thus, the OmpA protein represents a potential target for developing diagnostic and therapeutic agents for meningitis. In this study, we expressed recombinant OmpA proteins with various molecular weights in E. coli. The sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) was performed to check the molecular size of OmpA's full length (FL) and truncated proteins. OmpA-FL protein was purified for immunizing chickens to produce immunoglobulin yolk (IgY) antibodies. We applied phage display technology to construct antibody libraries (OmpA-FL scFv-S 1.1 × 10⁷ and OmpA-FL scFv-L 5.01 × 10⁶) to select specific anti-OmpA-FL scFv antibodies; these were characterized by their binding ability to recombinant or endogenous OmpA using ELISA, immunofluorescent staining, and confirmed with immunoblotting. We found 12 monoclonal antibodies that react to OmpA fragments; seven scFvs recognize fragments spanning amino acid (aa) residues 1-346, aa 1-287, aa 1-167, and aa 60-192, while five scFvs recognize fragments spanning aa 1-346 and aa 1-287 only. Two fragments (aa 246-346 and aa 287-346) were not recognized with any of the 12 scFvs. Together, the data suggest three antigenic epitopes (60 aa-160 aa, 161 aa-167 aa, 193 aa-245 aa) recognized by monoclonal antibodies. These scFv antibodies show strong reactivity against OmpA proteins. We believe that antibodies show promising diagnostic agents for E. coli K1 meningitis.

Pathogenomics of Virulence Traits of Plesiomonas shigelloides That Were Deemed Inconclusive by Traditional Experimental Approaches

One of the major challenges of modern medicine includes the failure of conventional protocols to characterize the pathogenicity of emerging pathogens. This is particularly apparent in the case of Plesiomonas shigelloides. Although a number of infections have been linked to this microorganism, experimental evidence of its virulence factors (VFs), obtained by traditional approaches, is somewhat inconclusive. Hence, it remains unclear whether P. shigelloides is a true or opportunistic one. In the current study, four publicly available whole-genome sequences of P. shigelloides (GN7, NCTC10360, 302-73, and LS1) were profiled using bioinformatics platforms to determine the putative candidate VFs to characterize the bacterial pathogenicity. Overall, 134 unique open reading frames (ORFs) were identified that were homologous or orthologous to virulence genes identified in other pathogens. Of these, 52.24% (70/134) were jointly shared by the strains. The numbers of strain-specific virulence traits were 4 in LS1; 7 in NCTC10360; 10 in 302-73; and 15 in GN7. The pathogenicity islands (PAIs) common to all the strains accounted for 24.07% ORFs. The numbers of PAIs exclusive to each strain were 8 in 302-73; 11 in NCTC10360; 14 in GN7; and 18 in LS1. A PAI encoding Vibrio cholerae ToxR-activated gene d protein was specific to 302-73, GN7, and NCTC10360 strains. Out of 33 antibiotic multi-resistance genes identified, 16 (48.48%) genes were intrinsic to all strains. Further, 17 (22.08%) of 77 antibiotic resistance islands were found in all the strains. Out of 23 identified distinct insertion sequences, 13 were only harbored by strain LS1. The number of intact prophages identified in the strains was 1 in GN7; 2 in 302-73; and 2 in NCTC10360. Further, 1 CRISPR element was identified in LS1; 2 in NCTC10360; and 8 in 302-73. Fifteen (78.95%) of 19 secretion systems and secretion effector variants were identified in all the strains. In conclusion, certain P. shigelloides strains might possess VFs associated with gastroenteritis and extraintestinal infections. However, the role of host factors in the onset of infections should not be undermined.

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

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

Enterotoxigenic Escherichia coli is phagocytosed by macrophages underlying villus-like intestinal epithelial cells: modeling ex vivo innate immune defenses of the human gut

There is a paucity of information on diarrheagenic enterotoxigenic Escherichia coli (ETEC)'s interaction with innate immune cells, in part due to the lack of reliable models that recapitulate infection in human gut. In a recent publication, we described the development of an ex vivo enteroid-macrophage co-culture model using human primary cells. We reported that macrophages residing underneath the epithelial monolayer acquired "resident macrophage" phenotype characterized by lower production of inflammatory cytokines and strong phagocytic activity. These macrophages extended projections across the epithelium, which captured ETEC applied to the apical side of the epithelium and reduced luminal bacterial load. Additional evidence presented in this addendum confirms these findings and further demonstrates that macrophage adaptation occurs regardless of the stage of differentiation of epithelial cells, and that ETEC uptake arises rapidly after infection. The enteroid-macrophage co-culture represents a novel and relevant tool to study host-cell interactions and pathogenesis of enteric infections in humans.

Escherichia coli type III secretion system 2 regulator EtrA promotes virulence of avian pathogenic Escherichia coli

The Escherichia coli type III secretion system 2 (ETT2) is found in most E. coli strains, including pathogenic and commensal strains. Although many ETT2 gene clusters carry multiple genetic mutations or deletions, ETT2 is known to be involved in bacterial virulence. In enterohaemorrhagic E. coli (EHEC), ETT2 affects adhesion through the regulator EtrA, which regulates transcription and secretion of the type III secretion system (T3SS) encoded by the locus of enterocyte effacement (LEE). To date, no studies have been conducted on the role of EtrA in the virulence of avian pathogenic E. coli (APEC), which harbours only ETT2. Thus, we constructed etrA mutant and complemented strains of APEC and evaluated their phenotypes and pathogenicities. We found that the etrA gene deletion significantly reduced bacterial survival in macrophages, and proliferation and virulence in ducks. In addition, the etrA gene deletion reduced expression of the APEC fimbriae genes. Upregulation of genes encoding the pro-inflammatory cytokines interleukin (IL)-1β and IL-8 was also observed in HD-11 macrophages infected with the etrA gene mutant strain compared to the wild-type strain. Furthermore, the altered capacities of the mutant strain were restored by genetic complementation. Our observations demonstrate that the ETT2 regulator EtrA contributes to the virulence of APEC.

Virulence characteristics of Acinetobacter baumannii clinical isolates vary with the expression levels of omps

PURPOSE

We investigated the expression levels of virulence factors (ompA, omp33-36 and carO) in five clinical isolates and in a standard ATCC 19606 strain of Acinetobacter baumannii to determine their effect on the virulence characteristics of the isolates.

METHODOLOGY

The mRNA levels of omps and proinflammatory cytokines were analyzed by quantitative real-time PCR. For adherence assay, after human lung epithelial cells (A549) were co-cultured with A. baumannii at 37 °C for 2 h, the cell-adherent bacteria was counted. Pearson correlation analysis was used to compare the omps mRNA levels, the proinflammatory cytokines and the number of adherent bacteria.

RESULTS

The mRNA levels of ompA in the clinical isolates were higher and similar compared with those in ATCC 19606, whereas the mRNA levels of omp33-36 in the clinical isolates were lower and similar compared with those in ATCC 19606. The mRNA levels of carO in the clinical isolates were significantly higher than those in ATCC 19606. The number of cell-adherent clinical isolates was higher than that of cell-adherent ATCC 19606. Furthermore, the number of cell-adherent clinical isolates was positively and significantly correlated with ompA mRNA level. The mRNA levels of TNF-α, IL-6 and IL-8 in A549 cells co-cultured with the clinical isolates were lower than those in A549 cells co-cultured with ATCC 19606. Moreover, the mRNA levels of TNF-α, IL-6 and IL-8 were negatively and significantly correlated with those of carO in the isolates.

CONCLUSION

These results provide insights into the renewed virulence characteristics of A. baumannii clinical isolates that depend on cell adherence capacity and the expression level of omp mRNAs.

The Burkholderia cenocepacia peptidoglycan-associated lipoprotein is involved in epithelial cell attachment and elicitation of inflammation

The Burkholderia cepacia complex (Bcc) is a group of Gram-negative opportunistic pathogens causing infections in people with cystic fibrosis (CF). Bcc is highly antibiotic resistant, making conventional antibiotic treatment problematic. The identification of novel targets for anti-virulence therapies should improve therapeutic options for infected CF patients. We previously identified that the peptidoglycan-associated lipoprotein (Pal) was immunogenic in Bcc infected CF patients; however, its role in Bcc pathogenesis is unknown. The virulence of a pal deletion mutant (Δpal) in Galleria mellonella was 88-fold reduced (p < .001) compared to wild type. The lipopolysaccharide profiles of wild type and Δpal were identical, indicating no involvement of Pal in O-antigen transport. However, Δpal was more susceptible to polymyxin B. Structural elucidation by X-ray crystallography and calorimetry demonstrated that Pal binds peptidoglycan fragments. Δpal showed a 1.5-fold reduced stimulation of IL-8 in CF epithelial cells relative to wild type (p < .001), demonstrating that Pal is a significant driver of inflammation. The Δpal mutant had reduced binding to CFBE41o^- cells, but adhesion of Pal-expressing recombinant E. coli to CFBE41o^- cells was enhanced compared to wild-type E. coli (p < .0001), confirming that Pal plays a direct role in host cell attachment. Overall, Bcc Pal mediates host cell attachment and stimulation of cytokine secretion, contributing to Bcc pathogenesis.

Pertussis Toxin Exploits Host Cell Signaling Pathways Induced by Meningitis-Causing E. coli K1-RS218 and Enhances Adherence of Monocytic THP-1 Cells to Human Cerebral Endothelial Cells

Pertussis toxin (PTx), the major virulence factor of the whooping cough-causing bacterial pathogen Bordetella pertussis, permeabilizes the blood–brain barrier (BBB) in vitro and in vivo. Breaking barriers might promote translocation of meningitis-causing bacteria across the BBB, thereby facilitating infection. PTx activates several host cell signaling pathways exploited by the neonatal meningitis-causing Escherichia coli K1-RS218 for invasion and translocation across the BBB. Here, we investigated whether PTx and E. coli K1-RS218 exert similar effects on MAPK p38, NF-κB activation and transcription of downstream targets in human cerebral endothelial TY10 cells using qRT-PCR, Western blotting, and ELISA in combination with specific inhibitors. PTx and E. coli K1-RS218 activate MAPK p38, but only E. coli K1-RS218 activates the NF-κB pathway. mRNA and protein levels of p38 and NF-κB downstream targets including IL-6, IL-8, CxCL-1, CxCL-2 and ICAM-1 were increased. The p38 specific inhibitor SB203590 blocked PTx-enhanced activity, whereas E. coli K1-RS218’s effects were inhibited by the NF-κB inhibitor Bay 11-7082. Further, we found that PTx enhances the adherence of human monocytic THP-1 cells to human cerebral endothelial TY10 cells, thereby contributing to enhanced translocation. These modulations of host cell signaling pathways by PTx and meningitis-causing E. coli support their contributions to pathogen and monocytic THP-1 cells translocation across the BBB.

The emerging problems of Klebsiella pneumoniae infections: carbapenem resistance and biofilm formation

Klebsiella pneumoniae is an opportunistic pathogen that commonly causes nosocomial infections in the urinary tract, respiratory tract, lung, wound sites and blood in individuals with debilitating diseases. Klebsiella pneumoniae is still a cause of severe pneumonia in alcoholics in Africa and Asia, and the predominant primary pathogen of primary liver abscess in Taiwan and Southeast Asia, particularly in Asian and Hispanic patients, and individuals with diabetes mellitus. In the United States and Europe, K. pneumoniae infections are most frequently associated with nosocomial infections. The emergence of antibiotic-resistant strains of K. pneumoniae worldwide has become a cause of concern where extended-spectrum β-lactamases (ESBLs) and carbapenemase-producing strains have been isolated with increasing frequency. The pathogen's ability to form biofilms on inserted devices such as urinary catheter has been proposed as one of the important mechanisms in nosocomially acquired and persistent infections, adding to the increased resistance to currently used antibiotics. In this review, infections caused by K. pneumoniae, antibiotic resistance and formation of biofilm will be discussed.

Novel antimicrobial peptides with promising activity against multidrug resistant Salmonella enterica serovar Choleraesuis and its stress response mechanism

AIMS

To evaluate the antibacterial efficacy of novel antimicrobial peptides (AMPs) against multidrug-resistant (MDR) Salmonella enterica serovar Choleraesuis (Salm. Choleraesuis) and to delineate the AMP-responsive mechanisms of wild-type (WT) and MDR strains.

METHODS AND RESULTS

Proteomic approaches were performed based on two-dimensional gel electrophoresis and liquid chromatography-electrospray ionization-quadrupole- time-of-flight tandem mass spectrometry to analyse the protein profiles of these two strains upon exposure to AMP GW-Q6. Quantitative real-time PCR was conducted to determine the mRNA expression level of the target genes. Furthermore, lipopolysaccharide (LPS) competition analysis was used to verify whether LPS may serve as the potential binding target when AMP approach and adhere to the bacterial surface.

CONCLUSIONS

The minimal inhibitory concentration assay revealed that our AMPs were even more effective against the MDR strains (4-32 μg ml(-1) ), compared with those for the WT (8-64 μg ml(-1) ). LPS dose-dependently suppressed the GW-Q6 antimicrobial activity. Clusters of orthologous groups analysis showed that the majority of the AMP-responsive proteins were involved in cell envelope biogenesis and outer membrane, translation and chaperones.

SIGNIFICANCE AND IMPACT OF THE STUDY

These results indicated that the novel AMP GW-Q6 serves as a potential candidate for antimicrobial drug development against MDR strains. These findings will also be helpful for expanding our knowledge on the molecular mechanisms of AMP-microbe interaction and the pathogenicity of salmonellosis caused by MDR strains of Salm. Choleraesuis.

The effects of cytotoxic necrotizing factor 1 expression in the uptake of Escherichia coli K1 by macrophages and the onset of meningitis in newborn mice

Macrophages are a permissive niche for E. coli K1 multiplication for which the interaction of the bacterial outer membrane protein A and its cognate receptor CD64 are critical. Using in vitro immunofluorescence and live microscopy with ex vivo macrophage cultures from RFP-Lifeact mice, we show that cytotoxic necrotizing factor 1 (CNF1) secreted by E. coli K1 sequesters cellular actin toward microspike formation, thereby limiting actin availability for OmpA-mediated bacterial invasion. Surprisingly, the observed effects of CNF1 occur despite the absence of 67-kDa laminin receptor in macrophages. Concomitantly, the CNF1 deletion mutant of E. coli K1 (Δcnf1) invades macrophages and the brains of newborn mice in greater numbers compared to wild-type. However, the Δcnf1 strain induces less severe pathology in the brain. These results suggest a novel role for CNF1 in limiting E. coli K1 entry into macrophages while exacerbating disease severity in the brains of newborn mice.

Host-pathogen interactions in bacterial meningitis

Bacterial meningitis is a devastating disease occurring worldwide with up to half of the survivors left with permanent neurological sequelae. Due to intrinsic properties of the meningeal pathogens and the host responses they induce, infection can cause relatively specific lesions and clinical syndromes that result from interference with the function of the affected nervous system tissue. Pathogenesis is based on complex host–pathogen interactions, some of which are specific for certain bacteria, whereas others are shared among different pathogens. In this review, we summarize the recent progress made in understanding the molecular and cellular events involved in these interactions. We focus on selected major pathogens, Streptococcus pneumonia, S. agalactiae (Group B Streptococcus), Neisseria meningitidis, and Escherichia coli K1, and also include a neglected zoonotic pathogen, Streptococcus suis. These neuroinvasive pathogens represent common themes of host–pathogen interactions, such as colonization and invasion of mucosal barriers, survival in the blood stream, entry into the central nervous system by translocation of the blood–brain and blood–cerebrospinal fluid barrier, and induction of meningeal inflammation, affecting pia mater, the arachnoid and subarachnoid spaces.

Homology analysis and cross-immunogenicity of OmpA from pathogenic Yersinia enterocolitica, Yersinia pseudotuberculosis and Yersinia pestis

The outer membrane protein A (OmpA) is one of the intra-species conserved proteins with immunogenicity widely found in the family of Enterobacteriaceae. Here we first confirmed OmpA is conserved in the three pathogenic Yersinia: Yersinia pestis, Yersinia pseudotuberculosis and pathogenic Yersinia enterocolitica, with high homology at the nucleotide level and at the amino acid sequence level. The identity of ompA sequences for 262 Y. pestis strains, 134 Y. pseudotuberculosis strains and 219 pathogenic Y. enterocolitica strains are 100%, 98.8% and 97.7% similar. The main pattern of OmpA of pathogenic Yersinia are 86.2% and 88.8% identical at the nucleotide and amino acid sequence levels, respectively. Immunological analysis showed the immunogenicity of each OmpA and cross-immunogenicity of OmpA for pathogenic Yersinia where OmpA may be a vaccine candidate for Y. pestis and other pathogenic Yersinia.

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

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

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSION

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

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

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

High HMGB1 level is associated with poor outcome of septicemic melioidosis

OBJECTIVES

A high level of HMGB1 (high-mobility group box 1) - a late onset inflammatory mediator - is a marker of lethal sepsis in several infectious diseases. The level of HMGB1 in the plasma of Burkholderia pseudomallei-infected patients was investigated together with the severity of the disease. The neutralization of HMGB1 to improve survival was also tested in a mouse model.

METHODS

HMGB1 levels in the plasma of 77 septic patients, 40 with B. pseudomallei infection and 37 with other bacterial infections, were determined by ELISA. Neutralizing antibody against purified recombinant HMGB1 was prepared in rabbits (rab-a-HMGB1) and its potential as an adjunct therapy was evaluated in B. pseudomallei-infected Balb/c mice treated with suboptimal doses of ceftazidime.

RESULTS

The plasma from B. pseudomallei-infected patients showed significantly higher HMGB1 levels than the plasma from other septic patients (median 11.1 ng/ml vs. 7.1 ng/ml). The HMGB1 level was significantly higher in patients with melioidosis who died than in those who survived (median 14.8 ng/ml vs. 9.2 ng/ml). Moreover, the HMGB1 level was significantly correlated with the clinical severity score (SOFA score). In the mouse study, although the rab-a-HMGB1 by itself could not improve the survival outcome of B. pseudomallei-infected mice, it could nevertheless enhance the effectiveness of suboptimal doses of ceftazidime in the treatment of these animals.

CONCLUSION

The level of HMGB1 in septic melioidosis patients can be used as a marker of late severe sepsis. Neutralizing antibody to HMGB1 may be used as an adjunct therapy to improve the outcome of melioidosis.

Pathogenesis of adherent-invasive Escherichia coli

The etiology of Crohn's disease (CD) is complex and involves both host susceptibility factors (i.e., the presence of particular genetic alleles) and environmental factors, including bacteria. In this regard, adherent-invasive Escherichia coli (AIEC), have recently emerged as an exciting potential etiological agent of CD. AIEC are distinguished from commensal strains of E. coli through their ability to adhere to and invade epithelial cells and replicate in macrophages. Recent molecular analyses have identified genes required for both invasion of epithelial cells and replication in the macrophage. However, these genetic studies, in combination with recent genome sequencing projects, have revealed that the pathogenesis of this group of bacteria cannot be explained by the presence of AIEC-specific genes. In this article, we review the role of AIEC as a pathobiont in the pathology of CD. We also describe the emerging link between AIEC and autophagy, and we propose a model for AIEC pathogenesis.

Mechanisms of phagocytosis and host clearance of Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic bacterial pathogen responsible for a high incidence of acute and chronic pulmonary infection. These infections are particularly prevalent in patients with chronic obstructive pulmonary disease and cystic fibrosis: much of the morbidity and pathophysiology associated with these diseases is due to a hypersusceptibility to bacterial infection. Innate immunity, primarily through inflammatory cytokine production, cellular recruitment, and phagocytic clearance by neutrophils and macrophages, is the key to endogenous control of P. aeruginosa infection. In this review, we highlight recent advances toward understanding the innate immune response to P. aeruginosa, with a focus on the role of phagocytes in control of P. aeruginosa infection. Specifically, we summarize the cellular and molecular mechanisms of phagocytic recognition and uptake of P. aeruginosa, and how current animal models of P. aeruginosa infection reflect clinical observations in the context of phagocytic clearance of the bacteria. Several notable phenotypic changes to the bacteria are consistently observed during chronic pulmonary infections, including changes to mucoidy and flagellar motility, that likely enable or reflect their ability to persist. These traits are likewise examined in the context of how the bacteria avoid phagocytic clearance, inflammation, and sterilizing immunity.

Structure-Function Analysis of DipA, a Francisella tularensis Virulence Factor Required for Intracellular Replication

Francisella tularensis is a highly infectious bacterium whose virulence relies on its ability to rapidly reach the macrophage cytosol and extensively replicate in this compartment. We previously identified a novel Francisella virulence factor, DipA (FTT0369c), which is required for intramacrophage proliferation and survival, and virulence in mice. DipA is a 353 amino acid protein with a Sec-dependent signal peptide, four Sel1-like repeats (SLR), and a C-terminal coiled-coil (CC) domain. Here, we determined through biochemical and localization studies that DipA is a membrane-associated protein exposed on the surface of the prototypical F. tularensis subsp. tularensis strain SchuS4 during macrophage infection. Deletion and substitution mutagenesis showed that the CC domain, but not the SLR motifs, of DipA is required for surface exposure on SchuS4. Complementation of the dipA mutant with either DipA CC or SLR domain mutants did not restore intracellular growth of Francisella, indicating that proper localization and the SLR domains are required for DipA function. Co-immunoprecipitation studies revealed interactions with the Francisella outer membrane protein FopA, suggesting that DipA is part of a membrane-associated complex. Altogether, our findings indicate that DipA is positioned at the host–pathogen interface to influence the intracellular fate of this pathogen.

The KdpD/KdpE two-component system: integrating K⁺ homeostasis and virulence

The two-component system (TCS) KdpD/KdpE, extensively studied for its regulatory role in potassium (K⁺) transport, has more recently been identified as an adaptive regulator involved in the virulence and intracellular survival of pathogenic bacteria, including Staphylococcus aureus, entero-haemorrhagic Escherichia coli, Salmonella typhimurium, Yersinia pestis, Francisella species, Photorhabdus asymbiotica, and mycobacteria. Key homeostasis requirements monitored by KdpD/KdpE and other TCSs such as PhoP/PhoQ are critical to survival in the stressful conditions encountered by pathogens during host interactions. It follows these TCSs may therefore acquire adaptive roles in response to selective pressures associated with adopting a pathogenic lifestyle. Given the central role of K⁺ in virulence, we propose that KdpD/KdpE, as a regulator of a high-affinity K⁺ pump, has evolved virulence-related regulatory functions. In support of this hypothesis, we review the role of KdpD/KdpE in bacterial infection and summarize evidence that (i) KdpD/KdpE production is correlated with enhanced virulence and survival, (ii) KdpE regulates a range of virulence loci through direct promoter binding, and (iii) KdpD/KdpE regulation responds to virulence-related conditions including phagocytosis, exposure to microbicides, quorum sensing signals, and host hormones. Furthermore, antimicrobial stress, osmotic stress, and oxidative stress are associated with KdpD/KdpE activity, and the system's accessory components (which allow TCS fine-tuning or crosstalk) provide links to stress response pathways. KdpD/KdpE therefore appears to be an important adaptive TCS employed during host infection, promoting bacterial virulence and survival through mechanisms both related to and distinct from its conserved role in K⁺ regulation.

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

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

Identification of a novel Francisella tularensis factor required for intramacrophage survival and subversion of innate immune response

Francisella tularensis, the causative agent of tularemia, is one of the deadliest agents of biological warfare and bioterrorism. Extremely high virulence of this bacterium is associated with its ability to dampen or subvert host innate immune response. The objectives of this study were to identify factors and understand the mechanisms of host innate immune evasion by F. tularensis. We identified and explored the pathogenic role of a mutant interrupted at gene locus FTL_0325, which encodes an OmpA-like protein. Our results establish a pathogenic role of FTL_0325 and its ortholog FTT0831c in the virulent F. tularensis SchuS4 strain in intramacrophage survival and suppression of proinflammatory cytokine responses. This study provides mechanistic evidence that the suppressive effects on innate immune responses are due specifically to these proteins and that FTL_0325 and FTT0831c mediate immune subversion by interfering with NF-κB signaling. Furthermore, FTT0831c inhibits NF-κB activity primarily by preventing the nuclear translocation of p65 subunit. Collectively, this study reports a novel F. tularensis factor that is required for innate immune subversion caused by this deadly bacterium.

Loss of outer membrane protein C in Escherichia coli contributes to both antibiotic resistance and escaping antibody-dependent bactericidal activity

Outer membrane proteins (OMPs) serve as the permeability channels for nutrients, toxins, and antibiotics. In Escherichia coli, OmpA has been shown to be involved in bacterial virulence, and OmpC is related to multidrug resistance. However, it is unclear whether OmpC also has a role in the virulence of E. coli. The aims of this study were to characterize the role of OmpC in antimicrobial resistance and bacterial virulence in E. coli. The ompC deletion mutant showed significantly decreased susceptibility to carbapenems and cefepime. To investigate the survival of E. coli exposed to the innate immune system, a human blood bactericidal assay showed that the ompC mutant increased survival in blood and serum but not in complement-inactivated serum. These effects were also demonstrated in the natural selection of OmpC mutants. Also, C1q interacted with E. coli through a complex of antibodies bound to OmpC as a major target. Bacterial survival was increased in the wild-type strain in a dose-dependent manner by adding free recombinant OmpC protein or anti-C1q antibody to human serum. These results demonstrated that the interaction of OmpC-specific antibody and C1q was the key step in initiating the antibody-dependent classical pathway for the clearance of OmpC-expressing E. coli. Anti-OmpC antibody was detected in human sera, indicating that OmpC is an immunogen. These data indicate that the loss of OmpC in E. coli is resistant to not only antibiotics, but also the serum bactericidal effect, which is mediated from the C1q and anti-OmpC antibody-dependent classical pathway.

Outer membrane protein A and OprF: versatile roles in Gram-negative bacterial infections

Outer membrane protein A (OmpA) is an abundant protein of Escherichia coli and other enterobacteria and has a multitude of functions. Although the structural features and porin function of OmpA have been well studied, its role in the pathogenesis of various bacterial infections has emerged only during the last decade. The four extracellular loops of OmpA interact with a variety of host tissues for adhesion to and invasion of the cell and for evasion of host-defense mechanisms when inside the cell. This review describes how various regions present in the extracellular loops of OmpA contribute to the pathogenesis of neonatal meningitis induced by E. coli K1 and to many other functions. In addition, the function of OmpA-like proteins, such as OprF of Pseudomonas aeruginosa, is discussed.

Interaction of Escherichia coli K1 and K5 with Acanthamoeba castellanii trophozoites and cysts

The existence of symbiotic relationships between Acanthamoeba and a variety of bacteria is well-documented. However, the ability of Acanthamoeba interacting with host bacterial pathogens has gained particular attention. Here, to understand the interactions of Escherichia coli K1 and E. coli K5 strains with Acanthamoeba castellanii trophozoites and cysts, association assay, invasion assay, survival assay, and the measurement of bacterial numbers from cysts were performed, and nonpathogenic E. coli K12 was also applied. The association ratio of E. coli K1 with A. castellanii was 4.3 cfu per amoeba for 1 hr but E. coli K5 with A. castellanii was 1 cfu per amoeba for 1 hr. By invasion and survival assays, E. coli K5 was recovered less than E. coli K1 but still alive inside A. castellanii. E. coli K1 and K5 survived and multiplied intracellularly in A. castellanii. The survival assay was performed under a favourable condition for 22 hr and 43 hr with the encystment of A. castellanii. Under the favourable condition for the transformation of trophozoites into cysts, E. coli K5 multiplied significantly. Moreover, the pathogenic potential of E. coli K1 from A. castellanii cysts exhibited no changes as compared with E. coli K1 from A. castellanii trophozoites. E. coli K5 was multiplied in A. castellanii trophozoites and survived in A. castellanii cysts. Therefore, this study suggests that E. coli K5 can use A. castellanii as a reservoir host or a vector for the bacterial transmission.