Ferredoxin is involved in secretion of cytotoxic necrotizing factor 1 across the cytoplasmic membrane in Escherichia coli K1

The Cytotoxic Necrotizing Factors (CNFs)-A Family of Rho GTPase-Activating Bacterial Exotoxins

The cytotoxic necrotizing factors (CNFs) are a family of Rho GTPase-activating single-chain exotoxins that are produced by several Gram-negative pathogenic bacteria. Due to the pleiotropic activities of the targeted Rho GTPases, the CNFs trigger multiple signaling pathways and host cell processes with diverse functional consequences. They influence cytokinesis, tissue integrity, cell barriers, and cell death, as well as the induction of inflammatory and immune cell responses. This has an enormous influence on host-pathogen interactions and the severity of the infection. The present review provides a comprehensive insight into our current knowledge of the modular structure, cell entry mechanisms, and the mode of action of this class of toxins, and describes their influence on the cell, tissue/organ, and systems levels. In addition to their toxic functions, possibilities for their use as drug delivery tool and for therapeutic applications against important illnesses, including nervous system diseases and cancer, have also been identified and are discussed.

Oral and intestinal bacterial exotoxins: Potential linked to carcinogenesis

Growing evidence suggests that imbalances in resident microbes (dysbiosis) can promote chronic inflammation, immune-subversion, and production of carcinogenic metabolites, thus leading to neoplasia. Yet, evidence to support a direct link of individual bacteria species to human sporadic cancer is still limited. This chapter focuses on several emerging bacterial toxins that have recently been characterized for their potential oncogenic properties toward human orodigestive cancer and the presence of which in human tissue samples has been documented. These include cytolethal distending toxins produced by various members of gamma and epsilon Proteobacteria, Dentilisin from mammalian oral Treponema, Pasteurella multocida toxin, two Fusobacterial toxins, FadA and Fap2, Bacteroides fragilis toxin, colibactin, cytotoxic necrotizing factors and α-hemolysin from Escherichia coli, and Salmonella enterica AvrA. It was clear that these bacterial toxins have biological activities to induce several hallmarks of cancer. Some toxins directly interact with DNA or chromosomes leading to their breakdowns, causing mutations and genome instability, and others modulate cell proliferation, replication and death and facilitate immune evasion and tumor invasion, prying specific oncogene and tumor suppressor pathways, such as p53 and β-catenin/Wnt. In addition, most bacterial toxins control tumor-promoting inflammation in complex and diverse mechanisms. Despite growing laboratory evidence to support oncogenic potential of selected bacterial toxins, we need more direct evidence from human studies and mechanistic data from physiologically relevant experimental animal models, which can reflect chronic infection in vivo, as well as take bacterial-bacterial interactions among microbiome into consideration.

Virulence factor-dependent basolateral invasion of choroid plexus epithelial cells by pathogenic Escherichia coli in vitro

Escherichia coli is the most common Gram-negative causative agent of neonatal meningitis and E. coli meningitis is associated with high morbidity and mortality. Previous research has been carried out with regard to the blood-brain barrier and thereby unveiled an assortment of virulence factors involved in E. coli meningitis. Little, however, is known about the role of the blood-cerebrospinal fluid (CSF) barrier (BCSFB), in spite of several studies suggesting that the choroid plexus (CP) is a possible entry point for E. coli into the CSF spaces. Here, we used a human CP papilloma (HIBCPP) cell line that was previously established as valid model for the study of the BCSFB. We show that E. coli invades HIBCPP cells in a polar fashion preferentially from the physiologically relevant basolateral side. Moreover, we demonstrate that deletion of outer membrane protein A, ibeA or neuDB genes results in decreased cell infection, while absence of fimH enhances invasion, although causing reduced adhesion to the apical side of HIBCPP cells. Our findings suggest that the BCSFB might constitute an entry point for E. coli into the central nervous system, and HIBCPP cells are a valuable tool for investigating E. coli entry of the BCSFB.

Human Meningitis-Associated Escherichia coli

Escherichia coli is the most common Gram-negative bacillary organism causing meningitis, and E. coli meningitis continues to be an important cause of mortality and morbidity throughout the world. Our incomplete knowledge of its pathogenesis contributes to such mortality and morbidity. Recent reports of E. coli strains producing CTX-M-type or TEM-type extended-spectrum β-lactamases create a challenge. Studies using in vitro and in vivo models of the blood-brain barrier have shown that E. coli meningitis follows a high degree of bacteremia and invasion of the blood-brain barrier. E. coli invasion of the blood-brain barrier, the essential step in the development of E. coli meningitis, requires specific microbial and host factors as well as microbe- and host-specific signaling molecules. Blockade of such microbial and host factors contributing to E. coli invasion of the blood-brain barrier is shown to be efficient in preventing E. coli penetration into the brain. The basis for requiring a high degree of bacteremia for E. coli penetration of the blood-brain barrier, however, remains unclear. Continued investigation on the microbial and host factors contributing to a high degree of bacteremia and E. coli invasion of the blood-brain barrier is likely to identify new targets for prevention and therapy of E. coli meningitis.

Outer-membrane vesicles from Gram-negative bacteria: biogenesis and functions

Outer-membrane vesicles (OMVs) are spherical buds of the outer membrane filled with periplasmic content and are commonly produced by Gram-negative bacteria. The production of OMVs allows bacteria to interact with their environment, and OMVs have been found to mediate diverse functions, including promoting pathogenesis, enabling bacterial survival during stress conditions and regulating microbial interactions within bacterial communities. Additionally, because of this functional versatility, researchers have begun to explore OMVs as a platform for bioengineering applications. In this Review, we discuss recent advances in the study of OMVs, focusing on new insights into the mechanisms of biogenesis and the functions of these vesicles.

Cytotoxic necrotizing factor 1 contributes to Escherichia coli meningitis

E. coli is the most common Gram-negative bacteria causing neonatal meningitis, and E. coli meningitis continues to be an important cause of mortality and morbidity throughout the world. Recent reports of E. coli meningitis caused by antimicrobial resistant strains are a particular concern. These findings indicate that a novel strategy is needed to identify new targets for prevention and therapy of E. coli meningitis. Cytotoxic necrotizing factor 1 (CNF1) is a bacterial virulence factor associated principally with E. coli strains causing urinary tract infection and meningitis. We have shown that CNF1 contributes to E. coli invasion of the blood-brain barrier and penetration into the brain, the essential step in the development of E. coli meningitis, and identified the host receptor for CNF1, 37-kDa laminin receptor precursor (37LRP). CNF1, however, is a cytoplasmic protein and its contribution to E. coli invasion of the blood-brain barrier requires its secretion from the bacterial cytoplasm. No signal peptide is found in the CNF1 sequence. CNF1 secretion is, therefore, a strategy utilized by meningitis-causing E. coli to invade the blood-brain barrier. Elucidation of the mechanisms involved in CNF1 secretion, as shown in this report with the involvement of Fdx and YgfZ provides the novel information on potential targets for prevention and therapy of E. coli meningitis by virtue of targeting the secretion of CNF1.

Analysis of the regulated transcriptome of Neisseria meningitidis in human blood using a tiling array

Neisseria meningitidis is the major cause of septicemia and meningococcal meningitis. During the course of infection, the bacterium must adapt to different host environments as a crucial factor for survival and dissemination; in particular, one of the crucial factors in N. meningitidis pathogenesis is the ability to grow and survive in human blood. We recently showed that N. meningitidis alters the expression of 30% of the open reading frames (ORFs) of the genome during incubation in human whole blood and suggested the presence of fine regulation at the gene expression level in order to control this step of pathogenesis. In this work, we used a customized tiling oligonucleotide microarray to define the changes in the whole transcriptional profile of N. meningitidis in a time course experiment of ex vivo bacteremia by incubating bacteria in human whole blood and then recovering RNA at different time points. The application of a newly developed bioinformatic tool to the tiling array data set allowed the identification of new transcripts--small intergenic RNAs, cis-encoded antisense RNAs, mRNAs with extended 5' and 3' untranslated regions (UTRs), and operons--differentially expressed in human blood. Here, we report a panel of expressed small RNAs, some of which can potentially regulate genes involved in bacterial metabolism, and we show, for the first time in N. meningitidis, extensive antisense transcription activity. This analysis suggests the presence of a circuit of regulatory RNA elements used by N. meningitidis to adapt to proliferate in human blood that is worthy of further investigation.

mRNA context dependent regulation of cytotoxic necrotizing factor 1 translation by GidA, a tRNA modification enzyme in Escherichia coli

Cytotoxic necrotizing factor 1 (CNF1), the paradigm of Rho GTPase activating bacterial toxins has been shown to promote E. coli invasion of human brain microvascular endothelial cells (HBMEC), which constitute the blood-brain barrier, but its synthesis and secretion is unclear. In this study, we performed mini Tn5 mutagenesis screen to identify genetic requirements for CNF1 production and secretion. Transposon mutagenesis screen of meningitis-causing E. coli K1 strain RS218 revealed that CNF1 production was markedly decreased in a transposon mutant (NBC-28G9) where transposon insertion occurred in the 5' end of gidA gene. In contrast, total deletion of gidA gene has less drastic effect on the production of CNF1. The N-terminus truncated GidA exhibited dominant negative effect on the production of CNF1. The inhibition of CNF1 production by N-terminus truncated GidA was shown to occur at the translational level. This was supported by our demonstrations that cnf1 mRNA transcription levels did not differ between strains RS218 and NBC-28G9; and the production of recombinant CNF1 under the control of artificial promoter was also repressed by truncated GidA. Progressive deletion of DNA regions in cnf1 gene identified two putative regions that were responsible for translational inhibition mediated by truncated GidA.

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

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

YgfZ contributes to secretion of cytotoxic necrotizing factor 1 into outer-membrane vesicles in Escherichia coli

Cytotoxic necrotizing factor 1 (CNF1), a Rho GTPase-activating bacterial toxin, has been shown to contribute to invasion by meningitis-causing Escherichia coli K1 of human brain microvascular endothelial cells (HBMEC), which constitute the blood-brain barrier. However, CNF1 is a cytosolic protein and it remains unclear how its secretion occurs, contributing to E. coli invasion of HBMEC. To investigate the genetic requirement for CNF1 secretion in E. coli K1 strain RS218, we performed mini-Tn5 in vitro mutagenesis and constructed a transposon mutant library of strain NBC, in which β-lactamase was fused to the C-terminus of CNF1 in the chromosome of strain RS218. We identified a transposon mutant (NBC-1E6) that exhibited reduced β-lactamase activity in its culture supernatant and had the transposon inserted into the ygfZ gene. When ygfZ was deleted from the genome of strain RS218 (ΔygfZ), the translocation of CNF1 into HBMEC was impaired. Subcellular localization analysis of CNF1 demonstrated that YgfZ, a periplasmic protein, contributes to secretion of CNF1 into outer-membrane vesicles (OMVs). The ΔygfZ mutant was significantly defective in invasion of HBMEC compared to the parent E. coli K1 strain. The defects of the ΔygfZ mutant in CNF1 secretion into OMVs and translocation into HBMEC as well as invasion of HBMEC were abrogated by complementation with ygfZ. Taken together, our findings demonstrate that YgfZ contributes to CNF1 secretion into OMVs in meningitis-causing E. coli K1.

Cytotoxic Necrotizing Factors (CNFs)-A Growing Toxin Family

The Escherichia coli Cytotoxic Necrotizing Factors, CNF1, CNF2, CNF3 and CNFY from Yersinia pseudotuberculosis belong to a family of deamidating toxins. CNFs deamidate glutamine 63/61 in the switch II region of Rho GTPases that is essential for GTP hydrolysing activity. Deamidation leads to constitutive activation of Rho GTPases. However, cellular mechanisms like proteasomal degradation of the activated Rho proteins restrict the action of the GTPases. This review describes the differences between the toxin family members concerning expression, cellular entry and substrate specificity.

A bacteria-specific 2[4Fe-4S] ferredoxin is essential in Pseudomonas aeruginosa

Background

Ferredoxins are small iron-sulfur proteins belonging to all domains of life. A sub-group binds two [4Fe-4S] clusters with unequal and extremely low values of the reduction potentials. These unusual properties are associated with two specific fragments of sequence. The functional importance of the very low potential ferredoxins is unknown.

Results

A bioinformatic screening of the sequence features defining very low potential 2[4Fe-4S] ferredoxins has revealed the almost exclusive presence of the corresponding fdx gene in the Proteobacteria phylum, without occurrence in Archaea and Eukaryota. The transcript was found to be monocistronic in Pseudomonas aeruginosa, and not part of an operon in most bacteria. Only fdx genes of bacteria which anaerobically degrade aromatic compounds belong to operons. As this pathway is not present in all bacteria having very low potential 2[4Fe-4S] ferredoxins, these proteins cannot exclusively be reductants of benzoyl CoA reductases. Expression of the ferredoxin gene did not change in response to varying growth conditions, including upon macrophage infection or aerobic growth with 4-hydroxy benzoate as carbon source. However, it increased along the growth curve in Pseudomonas aeruginosa and in Escherichia coli. The sequence immediately 5' upstream of the coding sequence contributed to the promotor activity. Deleting the fdx gene in Pseudomonas aeruginosa abolished growth, unless a plasmid copy of the gene was provided to the deleted strain.

Conclusions

The gene of the very low potential 2[4Fe-4S] ferredoxin displays characteristics of a housekeeping gene, and it belongs to the minority of genes that are essential in Pseudomonas aeruginosa. These data identify a new potential antimicrobial target in this and other pathogenic Proteobacteria.