Expression of cnf1 by Escherichia coli J96 involves a large upstream DNA region including the hlyCABD operon, and is regulated by the RfaH protein

Multidrug resistance in pathogenic Escherichia coli isolates from urinary tract infections in dogs, Spain

Escherichia coli (E. coli) is a pathogen frequently isolated in cases of urinary tract infections (UTIs) in both humans and dogs and evidence exists that dogs are reservoirs for human infections. In addition, E. coli is associated to increasing antimicrobial resistance rates. This study focuses on the analysis of antimicrobial resistance and the presence of selected virulence genes in E. coli isolates from a Spanish dog population suffering from UTI. This collection of isolates showed an extremely high level of phenotypic resistance to 1st-3rd generation cephalosporins, followed by penicillins, fluoroquinolones and amphenicols. Apart from that, 13.46% of them were considered extended-spectrum beta-lactamase producers. An alarmingly high percentage (71.15%) of multidrug resistant isolates were also detected. There was a good correlation between the antimicrobial resistance genes found and the phenotypic resistance expressed. Most of the isolates were classified as extraintestinal pathogenic E. coli, and two others harbored virulence factors related to diarrheagenic pathotypes. A significant relationship between low antibiotic resistance and high virulence factor carriage was found, but the mechanisms behind it are still poorly understood. The detection of high antimicrobial resistance rates to first-choice treatments highlights the need of constant antimicrobial resistance surveillance, as well as continuous revision of therapeutic guidelines for canine UTI to adapt them to changes in antimicrobial resistance patterns.

Increased Levels of (p)ppGpp Correlate with Virulence and Biofilm Formation, but Not with Growth, in Strains of Uropathogenic Escherichia coli

Urinary tract infections are one of the most frequent bacterial diseases worldwide. UPECs are the most prominent group of bacterial strains among pathogens responsible for prompting such infections. As a group, these extra-intestinal infection-causing bacteria have developed specific features that allow them to sustain and develop in their inhabited niche of the urinary tract. In this study, we examined 118 UPEC isolates to determine their genetic background and antibiotic resistance. Moreover, we investigated correlations of these characteristics with the ability to form biofilm and to induce a general stress response. We showed that this strain collection expressed unique UPEC attributes, with the highest representation of FimH, SitA, Aer, and Sfa factors (100%, 92.5%, 75%, and 70%, respectively). According to CRA (Congo red agar) analysis, the strains particularly predisposed to biofilm formation represented 32.5% of the isolates. Those biofilm forming strains presented a significant ability to accumulate multi-resistance traits. Most notably, these strains presented a puzzling metabolic phenotype-they showed elevated basal levels of (p)ppGpp in the planktonic phase and simultaneously exhibited a shorter generation time when compared to non-biofilm-forming strains. Moreover, our virulence analysis showed these phenotypes to be crucial for the development of severe infections in the Galleria mellonella model.

The cnf1 gene is associated with an expanding Escherichia coli ST131 H30Rx/C2 subclade and confers a competitive advantage for gut colonization

Epidemiological projections point to acquisition of ever-expanding multidrug resistance (MDR) by Escherichia coli, a commensal of the digestive tract and a source of urinary tract pathogens. Bioinformatics analyses of a large collection of E. coli genomes from EnteroBase, enriched in clinical isolates of worldwide origins, suggest the Cytotoxic Necrotizing Factor 1 (CNF1)-toxin encoding gene, cnf1, is preferentially distributed in four common sequence types (ST) encompassing the pandemic E. coli MDR lineage ST131. This lineage is responsible for a majority of extraintestinal infections that escape first-line antibiotic treatment, with known enhanced capacities to colonize the gastrointestinal tract. Statistical projections based on this dataset point to a global expansion of cnf1-positive multidrug-resistant ST131 strains from subclade H30Rx/C2, accounting for a rising prevalence of cnf1-positive strains in ST131. Despite the absence of phylogeographical signals, cnf1-positive isolates segregated into clusters in the ST131-H30Rx/C2 phylogeny, sharing a similar profile of virulence factors and the same cnf1 allele. The suggested dominant expansion of cnf1-positive strains in ST131-H30Rx/C2 led us to uncover the competitive advantage conferred by cnf1 for gut colonization to the clinical strain EC131GY ST131-H30Rx/C2 versus cnf1-deleted isogenic strain. Complementation experiments showed that colon tissue invasion was compromised in the absence of deamidase activity on Rho GTPases by CNF1. Hence, gut colonization factor function of cnf1 was confirmed for another clinical strain ST131-H30Rx/C2. In addition, functional analysis of the cnf1-positive clinical strain EC131GY ST131-H30Rx/C2 and a cnf1-deleted isogenic strain showed no detectable impact of the CNF1 gene on bacterial fitness and inflammation during the acute phase of bladder monoinfection. Together these data argue for an absence of role of CNF1 in virulence during UTI, while enhancing gut colonization capacities of ST131-H30Rx/C2 and suggested expansion of cnf1-positive MDR isolates in subclade ST131-H30Rx/C2.

YbdO Promotes the Pathogenicity of Escherichia coli K1 by Regulating Capsule Synthesis

Escherichia coli K1 is the most popular neonatal meningitis-causing Gram-negative bacterium. As a key virulence determinant, the K1 capsule enhances the survival of E. coli K1 in human brain microvascular endothelial cells (HBMECs) upon crossing the blood–brain barrier; however, the regulatory mechanisms of capsule synthesis during E. coli K1 invasion of HBMECs remain unclear. Here, we identified YbdO as a transcriptional regulator that promotes E. coli K1 invasion of HBMECs by directly activating K1 capsule gene expression to increase K1 capsule synthesis. We found that ybdO deletion significantly reduced HBMEC invasion by E. coli K1 and meningitis occurrence in mice. Additionally, electrophoretic mobility shift assay and chromatin immunoprecipitation–quantitative polymerase chain reaction analysis indicated that YbdO directly activates kpsMT and neuDBACES expression, which encode products involved in K1 capsule transport and synthesis by directly binding to the kpsM promoter. Furthermore, ybdO transcription was directly repressed by histone-like nucleoid structuring protein (H-NS), and we observed that acidic pH similar to that of early and late endosomes relieves this transcriptional repression. These findings demonstrated the regulatory mechanism of YbdO on K1 capsule synthesis, providing further insights into the evolution of E. coli K1 pathogenesis and host–pathogen interaction.

Whole Genome Sequencing and Biological Characteristics of Two Strains of Porcine Escherichia coli Isolated from Saba Pigs

Escherichia coli (E. coli) is an important pathogen that causes diarrhea and death in piglets. In this work, whole genome sequencing of two E. coli strains (ZB-1, ZWW-1) isolated from Saba pigs. And focus on the relationship between drug resistance, pathogenic phenotype and genotype of the two strains. This study analyzed the drug susceptibility of the two strains. The LD50 values, tissue bacterial load and intestinal pathological changes in mice infected with the two strains. The differences in gene functions such as drug resistance, virulence, and unique genes between the two strains, as well as the genetic evolutionary relationship of housekeeping genes were analyzed. The results showed that the two strains had the same resistance phenotype to most drugs. The LD50 value, tissue load, and pathological changes in mice infected with strain ZB-1 revealed that this strain was more virulent and pathogenic than strain ZWW-1. In addition, the housekeeping genes contained in the two strains are in the same large branch as E. coli of different species, and the genetic evolution is stable. All of them carry EPEC-type strain-specific virulence genes escV and ent, indicating that they are all new members of EPEC-type strains. This study laid the foundation for understanding the genetic background and biological characteristics of E. coli from Saba pigs.

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.

Escherichia coli Isolated from Diabetic Foot Osteomyelitis: Clonal Diversity, Resistance Profile, Virulence Potential, and Genome Adaptation

This study assessed the clonal diversity, the resistance profile and the virulence potential of Escherichia coli strains isolated from diabetic foot infection (DFI) and diabetic foot osteomyelitis (DFOM). A retrospective single-centre study was conducted on patients diagnosed with E. coli isolated from deep DFI and DFOM at Clinique du Pied Diabétique Gard-Occitanie (France) over a two-year period. Phylogenetic backgrounds, virulence factors (VFs) and antibiotic resistance profiles were determined. Whole-genome analysis of E. coli strains isolated from same patients at different periods were performed. From the two-years study period, 35 E. coli strains isolated from 33 patients were analysed; 73% were isolated from DFOM. The majority of the strains belonged to the virulent B2 and D phylogenetic groups (82%). These isolates exhibited a significant higher average of VFs number than strains belonging to other groups (p < 0.001). papG2 gene was significantly more detected in strains belonging to B2 phylogroup isolated from DFI compared to DFOM (p = 0.003). The most prevalent antibiotic resistance pattern was observed for ampicillin (82%), cotrimoxazole (45%), and ciprofloxacin (33%). The genome analysis of strains isolated at two periods in DFOM showed a decrease of the genome size, and this decrease was more important for the strain isolated at nine months (vs. four months). A shared mutation on the putative acyl-CoA dehydrogenase-encoding gene aidB was observed on both strains. E. coli isolates from DFOM were highly genetically diverse with different pathogenicity traits. Their adaptation in the bone structure could require genome reduction and some important modifications in the balance virulence/resistance of the bacteria.

Adherent/invasive Escherichia coli (AIEC) isolates from asymptomatic people: new E. coli ST131 O25:H4/H30-Rx virotypes

BACKGROUND

The widespread Escherichia coli clone ST131 implicated in multidrug-resistant infections has been recently reported, the majority belonging to O25:H4 serotype and classified into five main virotypes in accordance with the virulence genes carried.

METHODS

Pathogenicity Islands I and II (PAI-I and PAI-II) were determined using conventional PCR protocols from a set of four E. coli CTXR ST131 O25:H4/H30-Rx strains collected from healthy donors' stool. The virulence genes patterns were also analyzed and compared them with the virotypes reported previously; then adherence, invasion, macrophage survival and biofilm formation assays were evaluated and AIEC pathotype genetic determinants were investigated.

FINDINGS

Non-reported virulence patterns were found in our isolates, two of them carried satA, papA, papGII genes and the two-remaining isolates carried cnfI, iroN, satA, papA, papGII genes, and none of them belonged to classical ST131 virotypes, suggesting an endemic distribution of virulence genes and two new virotypes. The presence of PAI-I and PAI-II of Uropathogenic E. coli was determined in three of the four strains, furthermore adherence and invasion assays demonstrated higher degrees of attachment/invasion compared with the control strains. We also amplified intI1, insA and insB genes in all four samples.

INTERPRETATION

The results indicate that these strains own non-reported virotypes suggesting endemic distribution of virulence genes, our four strains also belong to an AIEC pathotype, being this the first report of AIEC in México and the association of AIEC with healthy donors.

Cytotoxic Escherichia coli strains encoding colibactin and cytotoxic necrotizing factor (CNF) colonize laboratory macaques

Background

Many Escherichia coli strains are considered to be a component of the normal flora found in the human and animal intestinal tracts. While most E. coli strains are commensal, some strains encode virulence factors that enable the bacteria to cause intestinal and extra-intestinal clinically-relevant infections. Colibactin, encoded by a genomic island (pks island), and cytotoxic necrotizing factor (CNF), encoded by the cnf gene, are genotoxic and can modulate cellular differentiation, apoptosis and proliferation. Some commensal and pathogenic pks+ and cnf+ E. coli strains have been associated with inflammation and cancer in humans and animals.

Results

In the present study, E. coli strains encoding colibactin and CNF were identified in macaque samples. We performed bacterial cultures utilizing rectal swabs and extra-intestinal samples from clinically normal macaques. A total of 239 E. coli strains were isolated from 266 macaques. The strains were identified biochemically and selected isolates were serotyped as O88:H4, O25:H4, O7:H7, OM:H14, and OM:H16. Specific PCR for pks and cnf1 gene amplification, and phylogenetic group identification were performed on all E. coli strains. Among the 239 isolates, 41 (17.2%) were pks+/cnf1−, 19 (7.9%) were pks−/cnf1+, and 31 (13.0%) were pks+/cnf1+. One hundred forty-eight (61.9%) E. coli isolates were negative for both genes (pks−/cnf1−). In total, 72 (30.1%) were positive for pks genes, and 50 (20.9%) were positive for cnf1. No cnf2+ isolates were detected. Both pks+ and cnf1+ E. coli strains belonged mainly to phylogenetic group B2, including B2₁. Colibactin and CNF cytotoxic activities were observed using a HeLa cell cytotoxicity assay in representative isolates. Whole genome sequencing of 10 representative E. coli strains confirmed the presence of virulence factors and antibiotic resistance genes in rhesus macaque E. coli isolates.

Conclusions

Our findings indicate that colibactin- and CNF-encoding E. coli colonize laboratory macaques and can potentially cause clinical and subclinical diseases that impact macaque models.

Electronic supplementary material

The online version of this article (10.1186/s13099-017-0220-y) contains supplementary material, which is available to authorized users.

Coordinated delivery and function of bacterial MARTX toxin effectors

Bacteria often coordinate virulence factors to fine-tune the host response during infection. These coordinated events can include toxins counteracting or amplifying effects of another toxin or though regulating the stability of virulence factors to remove their function once it is no longer needed. Multifunctional autoprocessing repeats-in toxin (MARTX) toxins are effector delivery toxins that form a pore into the plasma membrane of a eukaryotic cell to deliver multiple effector proteins into the cytosol of the target cell. The function of these proteins includes manipulating actin cytoskeletal dynamics, regulating signal transduction pathways and inhibiting host secretory pathways. Investigations into the molecular mechanisms of these effector domains are providing insight into how the function of some effectors overlap and regulate one another during infection. Coordinated crosstalk of effector function suggests that MARTX toxins are not simply a sum of all their parts. Instead, modulation of cell function by effector domains may depend on which other effector domain are co-delivered. Future studies will elucidate how these effectors interact with each other to modulate the bacterial host interaction.

Uropathogenic Escherichia coli-Associated Exotoxins

Escherichia coli are a common cause of infectious disease outside of the gastrointestinal tract. Several independently evolved E. coli clades are common causes of urinary tract and bloodstream infections. There is ample epidemiological and in vitro evidence that several different protein toxins common to many, but not all, of these strains are likely to aid the colonization and immune-evasion ability of these bacteria. This review discusses our current knowledge and areas of ignorance concerning the contribution of the hemolysin; cytotoxic-necrotizing factor-1; and the autotransporters, Sat, Pic, and Vat, to extraintestinal human disease.

Secretion of Alpha-Hemolysin by Escherichia coli Disrupts Tight Junctions in Ulcerative Colitis Patients

Objectives:

The potential of Escherichia coli (E. coli) isolated from inflammatory bowel disease (IBD) patients to damage the integrity of the intestinal epithelium was investigated.

Methods:

E. coli strains isolated from patients with ulcerative colitis (UC) and healthy controls were tested for virulence capacity by molecular techniques and cytotoxic assays and transepithelial electric resistance (TER). E. coli isolate p19A was selected, and deletion mutants were created for alpha-hemolysin (α-hemolysin) (hly) clusters and cytotoxic necrotizing factor type 1 (cnf1). Probiotic E. coli Nissle and pathogenic E. coli LF82 were used as controls.

Results:

E. coli strains from patients with active UC completely disrupted epithelial cell tight junctions shortly after inoculation. These strains belong to phylogenetic group B2 and are all α-hemolysin positive. In contrast, probiotic E. coli Nissle, pathogenic E. coli LF82, four E. coli from patients with inactive UC and three E. coli strains from healthy controls did not disrupt tight junctions. E. coli p19A WT as well as cnf1, and single loci of hly mutants from cluster I and II were all able to damage Caco-2 (Heterogeneous human epithelial colorectal adenocarcinoma) cell tight junctions. However, this phenotype was lost in a mutant with knockout (Δ) of both hly loci (P<0.001).

Conclusions:

UC-associated E. coli producing α-hemolysin can cause rapid loss of tight junction integrity in differentiated Caco-2 cell monolayers. This effect was abolished in a mutant unable to express α-hemolysin. These results suggest that high Hly expression may be a mechanism by which specific strains of E. coli pathobionts can contribute to epithelial barrier dysfunction and pathophysiology of disease in IBD.

Switching Rho GTPase activation into effective antibacterial defenses requires the caspase-1/IL-1beta signaling axis

The monitoring of the activation state of Rho GTPases has emerged as a potent innate immune mechanism for detecting pathogens. In the March issue of PLOS Pathogens, we show that the activation of Rho GTPases by the CNF1 toxin during E. coli-triggered bacteremia leads to a GR1(+)cell-mediated efficient bacterial clearing and improves host survival. Host alarm requires the Caspase-1/IL-1beta signaling axis. Furthermore, we discover that pathogenic bacteria have the capacity to block immune responses via the expression of the α-hemolysin pore-forming toxin. In this commentary, we will comment on these findings and highlight the questions raised by this example of attack-defense mechanisms used alternatively by the pathogen and the host during blood infection.

Virulence Gene Regulation in Escherichia coli

Escherichia colicauses three types of illnesses in humans: diarrhea, urinary tract infections, and meningitis in newborns. The acquisition of virulence-associated genes and the ability to properly regulate these, often horizontally transferred, loci distinguishes pathogens from the normally harmless commensal E. coli found within the human intestine. This review addresses our current understanding of virulence gene regulation in several important diarrhea-causing pathotypes, including enteropathogenic, enterohemorrhagic,enterotoxigenic, and enteroaggregativeE. coli-EPEC, EHEC, ETEC and EAEC, respectively. The intensely studied regulatory circuitry controlling virulence of uropathogenicE. coli, or UPEC, is also reviewed, as is that of MNEC, a common cause of meningitis in neonates. Specific topics covered include the regulation of initial attachment events necessary for infection, environmental cues affecting virulence gene expression, control of attaching and effacing lesionformation, and control of effector molecule expression and secretion via the type III secretion systems by EPEC and EHEC. How phage control virulence and the expression of the Stx toxins of EHEC, phase variation, quorum sensing, and posttranscriptional regulation of virulence determinants are also addressed. A number of important virulence regulators are described, including the AraC-like molecules PerA of EPEC, CfaR and Rns of ETEC, and AggR of EAEC;the Ler protein of EPEC and EHEC;RfaH of UPEC;and the H-NS molecule that acts to silence gene expression. The regulatory circuitry controlling virulence of these greatly varied E. colipathotypes is complex, but common themes offerinsight into the signals and regulators necessary forE. coli disease progression.

Pathogenic potential of Escherichia coli clinical strains from orthopedic implant infections towards human osteoblastic cells

Escherichia coli is one of the first causes of Gram-negative orthopedic implant infections (OII), but little is known about the pathogenicity of this species in such infections that are increasing due to the ageing of the population. We report how this pathogen interacts with human osteoblastic MG-63 cells in vitro, by comparing 20 OII E. coli strains to two Staphylococcus aureus and two Pseudomonas aeruginosa strains. LDH release assay revealed that 6/20 (30%) OII E. coli induced MG-63 cell lysis whereas none of the four control strains was cytotoxic after 4 h of coculture. This high cytotoxicity was associated with hemolytic properties and linked to hlyA gene expression. We further showed by gentamicin protection assay and confocal microscopy that the non-cytotoxic E. coli were not able to invade MG-63 cells unlike S. aureus strains (internalization rate <0.01% for the non-cytotoxic E. coli versus 8.88 ± 2.31% and 4.60 ± 0.42% for both S. aureus). The non-cytotoxic E. coli also demonstrated low adherence rates (<7%), the most adherent E. coli eliciting higher IL-6 and TNF-α mRNA expression in the osteoblastic cells. Either highly cytotoxic or slightly invasive OII E. coli do not show the same infection strategies as S. aureus towards osteoblasts.

Escherichia coli α-hemolysin counteracts the anti-virulence innate immune response triggered by the Rho GTPase activating toxin CNF1 during bacteremia

The detection of the activities of pathogen-encoded virulence factors by the innate immune system has emerged as a new paradigm of pathogen recognition. Much remains to be determined with regard to the molecular and cellular components contributing to this defense mechanism in mammals and importance during infection. Here, we reveal the central role of the IL-1β signaling axis and Gr1+ cells in controlling the Escherichia coli burden in the blood in response to the sensing of the Rho GTPase-activating toxin CNF1. Consistently, this innate immune response is abrogated in caspase-1/11-impaired mice or following the treatment of infected mice with an IL-1β antagonist. In vitro experiments further revealed the synergistic effects of CNF1 and LPS in promoting the maturation/secretion of IL-1β and establishing the roles of Rac, ASC and caspase-1 in this pathway. Furthermore, we found that the α-hemolysin toxin inhibits IL-1β secretion without affecting the recruitment of Gr1+ cells. Here, we report the first example of anti-virulence-triggered immunity counteracted by a pore-forming toxin during bacteremia.

The pathogenic potentials of most microbes depend on a repertoire of virulence factors. Despite major progress in the understanding of the molecular mechanisms underlying the activities of bacterial effectors, little is known about how they cooperate during infection to overcome host immune defenses and promote microbial persistence. Here, we investigated the roles of two uropathogenic Escherichia coli (UPEC) effectors that are co-ordinately expressed, α-hemolysin (HlyA) and cytotoxic necrotizing factor 1 (CNF1). We demonstrated that the HlyA toxin is critical for bacterial stability in the blood and showed that one important role of HlyA is to inhibit the CNF1-induced host response. Collectively, these findings reveal why the coordinated activities of HlyA and CNF1 are necessary for the full virulence of UPEC. Moreover, they unravel a HlyA-driven counter-defense mechanism used by bacteria to facilitate their survival.

Virulence factors of uropathogenic E. coli and their interaction with the host

Urinary tract infections (UTIs) belong to the most common infectious diseases worldwide. The most frequently isolated pathogen from uncomplicated UTIs is Escherichia coli. To establish infection in the urinary tract, E. coli has to overcome several defence strategies of the host, including the urine flow, exfoliation of urothelial cells, endogenous antimicrobial factors and invading neutrophils. Thus, uropathogenic E. coli (UPEC) harbour a number of virulence and fitness factors enabling the bacterium to resist and overcome these different defence mechanisms. There is no particular factor which allows the identification of UPEC among the commensal faecal flora apart from the ability to enter the urinary tract and cause an infection. Many of potential virulence or fitness factors occur moreover with high redundancy. Fimbriae are inevitable for adherence to and invasion into the host cells; the type 1 pilus is an established virulence factor in UPEC and indispensable for successful infection of the urinary tract. Flagella and toxins promote bacterial dissemination, while different iron-acquisition systems allow bacterial survival in the iron-limited environment of the urinary tract. The immune response to UPEC is primarily mediated by toll-like receptors recognising lipopolysaccharide, flagella and other structures on the bacterial surface. UPEC have the capacity to subvert this immune response of the host by means of actively impacting on pro-inflammatory signalling pathways, or by physical masking of immunogenic structures. The large repertoire of bacterial virulence and fitness factors in combination with host-related differences results in a complex interaction between host and pathogen in the urinary tract.

The Type 1 secretion pathway - the hemolysin system and beyond

Type 1 secretion systems (T1SS) are wide-spread among Gram-negative bacteria. An important example is the secretion of the hemolytic toxin HlyA from uropathogenic strains. Secretion is achieved in a single step directly from the cytosol to the extracellular space. The translocation machinery is composed of three indispensable membrane proteins, two in the inner membrane, and the third in the outer membrane. The inner membrane proteins belong to the ABC transporter and membrane fusion protein families (MFPs), respectively, while the outer membrane component is a porin-like protein. Assembly of the three proteins is triggered by accumulation of the transport substrate (HlyA) in the cytoplasm, to form a continuous channel from the inner membrane, bridging the periplasm and finally to the exterior. Interestingly, the majority of substrates of T1SS contain all the information necessary for targeting the polypeptide to the translocation channel - a specific sequence at the extreme C-terminus. Here, we summarize our current knowledge of regulation, channel assembly, translocation of substrates, and in the case of the HlyA toxin, its interaction with host membranes. We try to provide a complete picture of structure function of the components of the translocation channel and their interaction with the substrate. Although we will place the emphasis on the paradigm of Type 1 secretion systems, the hemolysin A secretion machinery from E. coli, we also cover as completely as possible current knowledge of other examples of these fascinating translocation systems. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey.

RfaH promotes the ability of the avian pathogenic Escherichia coli O2 strain E058 to cause avian colibacillosis

Avian pathogenic Escherichia coli (APEC) infection causes avian colibacillosis, which refers to any localized or systemic infection, such as acute fatal septicemia or subacute pericarditis and airsacculitis. The RfaH transcriptional regulator in E. coli is known to regulate a number of phenotypic traits. The direct effect of RfaH on the virulence of APEC has not been investigated yet. Our results showed that the inactivation of rfaH significantly decreased the virulence of APEC E058. The attenuation was assessed by in vivo and in vitro assays, including chicken infection assays, an ingestion and intracellular survival assay, and a bactericidal assay with serum complement. The virulence phenotype was restored to resemble that of the wild type by complementation of the rfaH gene in trans. The results of the quantitative real-time reverse transcription-PCR (qRT-PCR) analysis and animal system infection experiments indicated that the deletion of rfaH correlated with decreased virulence of the APEC E058 strain.

Cytotoxic necrotizing factor 1 and hemolysin from uropathogenic Escherichia coli elicit different host responses in the murine bladder

Cytotoxic necrotizing factor 1 (CNF1) and hemolysin (HlyA1) are toxins produced by uropathogenic Escherichia coli (UPEC). We previously showed that these toxins contribute to the inflammation and tissue damage seen in a mouse model of ascending urinary tract infection. CNF1 constitutively activates small Rho GTPases by deamidation of a conserved glutamine residue, and HlyA1 forms pores in eukaryotic cell membranes. In this study, we used cDNA microarrays of bladder tissue isolated from mice infected intraurethrally with wild-type CP9, CP9cnf1, or CP9ΔhlyA to further evaluate the role that each toxin plays in the host response to UPEC. Regardless of the strain used, we found that UPEC itself elicited a significant change in host gene expression 24 h after inoculation. The largest numbers of upregulated genes were in the cytokine and chemokine signaling and Toll-like receptor signaling pathways. CNF1 exerted a strong positive influence on expression of genes involved in innate immunity and signal transduction and a negative impact on metabolism- and transport-associated genes. HlyA1 evoked an increase in expression of genes that encode innate immunity factors and a decrease in expression of genes involved in cytoskeletal and metabolic processes. Multiplex cytokine and myeloperoxidase assays corroborated our finding that a strong proinflammatory response was elicited by all strains tested. Bladders challenged intraurethrally with purified CNF1 displayed pathology similar to but significantly less intense than the pathology that we observed in CP9-challenged mice. Our data demonstrate substantial roles for CNF1 and HlyA1 in initiation of a strong proinflammatory response to UPEC in the bladder.

Pathogenicity island markers, virulence determinants malX and usp, and the capacity of Escherichia coli to persist in infants' commensal microbiotas

Virulence-associated genes in bacteria are often located on chromosomal regions, termed pathogenicity islands (PAIs). Several PAIs are found in Escherichia coli strains that cause extraintestinal infections, but their role in commensal bowel colonization is unknown. Resident strains are enriched in adhesins (P fimbriae and type 1 fimbriae), capsular antigens (K1 and K5), hemolysin, and aerobactin and mostly belong to phylogenetic group B2. Here, we investigated whether six pathogenicity islands and the virulence determinants malX and usp are associated with fitness of E. coli in the infant bowel microbiota. E. coli strains isolated from stools of 130 Swedish infants during the first year of life were examined for their carriage of PAI markers, malX, and usp by PCR. Carriage was related to strain persistence: long-term colonizers (≥12 months) carried significantly more of PAI II from strain CFT703 (II(CFT703)), IV(536,) and II(J96) and malX and usp than intermediate colonizers (1 to 11 months) and transient strains (<3 weeks). The accumulation of PAI markers in each individual strain correlated positively with its time of persistence in the colon. Phylogenetic group B2 accounted for 69% of long-term colonizers, 46% of intermediate colonizers and 14% of transient strains. These results support the hypothesis that some bacterial traits contributing to extraintestinal infections have in fact evolved primarily because they increase the fitness of E. coli in its natural niche, the colon; accordingly, they may be regarded as fitness islands in the gut.

Genome dynamics and its impact on evolution of Escherichia coli

The Escherichia coli genome consists of a conserved part, the so-called core genome, which encodes essential cellular functions and of a flexible, strain-specific part. Genes that belong to the flexible genome code for factors involved in bacterial fitness and adaptation to different environments. Adaptation includes increase in fitness and colonization capacity. Pathogenic as well as non-pathogenic bacteria carry mobile and accessory genetic elements such as plasmids, bacteriophages, genomic islands and others, which code for functions required for proper adaptation. Escherichia coli is a very good example to study the interdependency of genome architecture and lifestyle of bacteria. Thus, these species include pathogenic variants as well as commensal bacteria adapted to different host organisms. In Escherichia coli, various genetic elements encode for pathogenicity factors as well as factors, which increase the fitness of non-pathogenic bacteria. The processes of genome dynamics, such as gene transfer, genome reduction, rearrangements as well as point mutations contribute to the adaptation of the bacteria into particular environments. Using Escherichia coli model organisms, such as uropathogenic strain 536 or commensal strain Nissle 1917, we studied mechanisms of genome dynamics and discuss these processes in the light of the evolution of microbes.

Use of zebrafish to probe the divergent virulence potentials and toxin requirements of extraintestinal pathogenic Escherichia coli

Extraintestinal pathogenic E. coli (ExPEC) cause an array of diseases, including sepsis, neonatal meningitis, and urinary tract infections. Many putative virulence factors that might modulate ExPEC pathogenesis have been identified through sequencing efforts, epidemiology, and gene expression profiling, but few of these genes have been assigned clearly defined functional roles during infection. Using zebrafish embryos as surrogate hosts, we have developed a model system with the ability to resolve diverse virulence phenotypes and niche-specific restrictions among closely related ExPEC isolates during either localized or systemic infections. In side-by-side comparisons of prototypic ExPEC isolates, we observed an unexpectedly high degree of phenotypic diversity that is not readily apparent using more traditional animal hosts. In particular, the capacity of different ExPEC isolates to persist and multiply within the zebrafish host and cause disease was shown to be variably dependent upon two secreted toxins, α-hemolysin and cytotoxic necrotizing factor. Both of these toxins appear to function primarily in the neutralization of phagocytes, which are recruited in high numbers to sites of infection where they act as an essential host defense against ExPEC as well as less virulent E. coli strains. These results establish zebrafish as a valuable tool for the elucidation and functional analysis of both ExPEC virulence factors and host defense mechanisms.

Escherichia coli can exist among the normal intestinal microbiota without causing any overt problems for the human host. However, humans as well as other animals can often acquire less-mild mannered variants of E. coli strains known as extraintestinal pathogenic E. coli (ExPEC) that can colonize sites outside of the intestinal tract and cause a range of serious illnesses, including sepsis, meningitis, and urinary tract infections. Despite many advances over the years using cell culture and rodent infection models, the spectrum of genes that control the ability of different ExPEC strains to colonize and grow within specific host niches and cause disease remain, for the most part, elusive. Here, we report the development of a new model system that uses zebrafish as surrogate hosts for ExPEC and related isolates. Using zebrafish to model both localized and systemic infections, we found that closely related ExPEC isolates display an unexpected array of virulence characteristics and toxin requirements that are not readily apparent from genomic information alone. This model system is amenable to high-throughput genetic and pharmacological screens and should prove useful in the development of more efficacious therapeutics.

Uropathogenic Escherichia coli

The urinary tract is among the most common sites of bacterial infection, and Escherichia coli is by far the most common species infecting this site. Individuals at high risk for symptomatic urinary tract infection (UTI) include neonates, preschool girls, sexually active women, and elderly women and men. E. coli that cause the majority of UTIs are thought to represent only a subset of the strains that colonize the colon. E. coli strains that cause UTIs are termed uropathogenic E. coli (UPEC). In general, UPEC strains differ from commensal E. coli strains in that the former possess extragenetic material, often on pathogenicity-associated islands (PAIs), which code for gene products that may contribute to bacterial pathogenesis. Some of these genes allow UPEC to express determinants that are proposed to play roles in disease. These factors include hemolysins, secreted proteins, specific lipopolysaccharide and capsule types, iron acquisition systems, and fimbrial adhesions. The current dogma of bacterial pathogenesis identifies adherence, colonization, avoidance of host defenses, and damage to host tissues as events vital for achieving bacterial virulence. These considerations, along with analysis of the E. coli CFT073, UTI89, and 536 genomes and efforts to identify novel virulence genes should advance the field significantly and allow for the development of a comprehensive model of pathogenesis for uropathogenic E. coli.Further study of the adaptive immune response to UTI will be especially critical to refine our understanding and treatment of recurrent infections and to develop vaccines.

Hemolysin of uropathogenic Escherichia coli evokes extensive shedding of the uroepithelium and hemorrhage in bladder tissue within the first 24 hours after intraurethral inoculation of mice

Many uropathogenic Escherichia coli (UPEC) strains produce both hemolysin (Hly) and cytotoxic necrotizing factor type 1 (CNF1), and the loci for these toxins are often linked. The conclusion that Hly and CNF1 contribute to urovirulence is supported by the results of epidemiological studies associating the severity of urinary tract infections (UTIs) with toxin production by UPEC isolates. Additionally, we previously reported that mouse bladders and rat prostates infected with UPEC strain CP9 exhibit a more profound inflammatory response than the organs from animals challenged with CP9cnf(1) and that CNF1 decreases the antimicrobial activities of polymorphonuclear leukocytes. More recently, we created an Hly mutant, CP9Delta hlyA(1)::cat, and showed that it was less hemolytic and destructive for cultured bladder cells than CP9 was. Here we evaluated the relative effects of mutations in hlyA(1) or cnf(1) alone or together on the pathogenicity of CP9 in a mouse model of ascending UTI. To do this, we constructed an hlyA(1)-complemented clone of CP9Delta hlyA(1)::cat and an hlyA(1) cnf(1) CP9 double mutant. We found that Hly had no influence on bacterial colonization of the bladder or kidneys in single or mixed infections with the wild type and CP9Delta hlyA(1)::cat but that it did provoke sloughing of the uroepithelium and bladder hemorrhage within the first 24 h after challenge. Finally, we confirmed that CNF1 expression induces bladder inflammation and, in particular, as shown in this study, submucosal edema. From these data, we speculate that Hly and CNF1 may be largely responsible for the signs and symptoms of cystitis in humans infected with toxigenic UPEC.

Rho GTPase-activating bacterial toxins: from bacterial virulence regulation to eukaryotic cell biology

Studies on the interactions of bacterial pathogens with their host have provided an invaluable source of information on the major functions of eukaryotic and prokaryotic cell biology. In addition, this expanding field of research, known as cellular microbiology, has revealed fascinating examples of trans-kingdom functional interplay. Bacterial factors actually exploit eukaryotic cell machineries using refined molecular strategies to promote invasion and proliferation within their host. Here, we review a family of bacterial toxins that modulate their activity in eukaryotic cells by activating Rho GTPases and exploiting the ubiquitin/proteasome machineries. This family, found in human and animal pathogenic Gram-negative bacteria, encompasses the cytotoxic necrotizing factors (CNFs) from Escherichia coli and Yersinia species as well as dermonecrotic toxins from Bordetella species. We survey the genetics, biochemistry, molecular and cellular biology of these bacterial factors from the standpoint of the CNF1 toxin, the paradigm of Rho GTPase-activating toxins produced by urinary tract infections causing pathogenic Escherichia coli. Because it reveals important connections between bacterial invasion and the host inflammatory response, the mode of action of CNF1 and its related Rho GTPase-targetting toxins addresses major issues of basic and medical research and constitutes a privileged experimental model for host-pathogen interaction.

Specificity of immunomodulator secretion in urinary samples in response to infection by alpha-hemolysin and CNF1 bearing uropathogenic Escherichia coli

Escherichia coli are the most common etiological agents of urinary tract infections (UTIs). Uropathogenic E. coli (UPECs) produce specific toxins including the cytotoxic necrotizing factor-1 (CNF1) and the alpha-hemolysin (alpha-Hly). CNF1 triggers, through Rho protein activation, a specific gene response of host cells, which results in the production for instance of interleukin-8 (IL-8), monocyte chemoattractant protein-1 (MCP-1) and the macrophage inflammatory protein-3alpha (MIP-3alpha). The alpha hemolysin alpha-Hly also triggers the production of inflammatory mediators. Cnf1 is always associated with alpha-hly in a pathogenicity island conserved among UPECs. Using two complementary approaches we have investigated whether alpha-hly and cnf1 bearing UPECs are associated with a specific type of UTI both in term of pathology and host response. Here we report that UPECs bearing alpha-hly/cnf1 have a prevalence of 50% in UPECs isolated from hemorrhagic UTIs, as compared to 30% in the overall UPEC population. In addition, we observed that MCP-1, and IL-8 to a lower extent, is produced in urine at higher concentrations in UTIs caused by UPECs carrying alpha-hly/cnf1.

HlyA knock out yields a saferEscherichia coliA0 34/86 variant with unaffected colonization capacity in piglets

Escherichia coli A0 34/86 (O83:K24:H31) has been successfully used for prophylactic and therapeutic intestinal colonization of premature and newborn infants, with the aim of preventing nosocomial infections. Although E. coli A0 34/86 was described as a nonpathogenic commensal, partial sequencing revealed that its genome harbours gene clusters highly homologous to virulence determinants of different types of E. coli, including closely linked genes of the alpha-haemolysin operon (hlyCABD) and for the cytotoxic necrotizing factor (cnf1). A haemolysin-deficient mutant (Delta hlyA) of E. coli A0 34/86 was generated and its colonization capacity was determined. The results show that a single dose of the A0 34/86 wild-type or Delta hlyA strains resulted in efficient intestinal colonization of newborn conventional piglets, and that this was still considerable after several weeks. No difference was observed between the wild-type and the mutant strains, showing that haemolysin expression does not contribute to intestinal colonization capacity of E. coli A0 34/86. Safety experiments revealed that survival of colostrum-deprived gnotobiotic newborn piglets was substantially higher upon colonization by the nonhaemolytic strain than following inoculation by its wild-type ancestor. We suggest that the E. coli A0 34/86 Delta hlyA mutant may represent a safer prophylactic and/or immunomodulatory tool with unaffected colonization capacity.

Pathogenicity island markers in commensal and uropathogenic Escherichia coli isolates

Uropathogenic isolates of Escherichia coli (UPEC) contain blocks of DNA, termed pathogenicity islands (PAIs), that contribute to their virulence. Two multiplex PCR assays were developed to detect eight PAI markers among 50 commensal E. coli and 100 UPEC isolates. In total, 40% of commensal isolates and 93% of UPEC carried PAIs. Despite this difference, the distribution of various PAIs showed the same pattern in both groups, with the most prevalent being PAI IV(536) (38% commensal vs. 89% UPEC), followed by PAI I(CFT073) (26% vs. 73%), PAI II(CFT073) (14% vs. 46%), PAI II(J96) (8% vs. 34%), PAI I(536) (8% vs. 33%) and PAI II(536) (4% vs. 20%). PAI III(536) was detected only in UPEC (2%), while PAI I(J96) was not detected in any isolate. Although the mean number of PAIs per isolate was higher among UPEC (2.97) than in commensal (0.98) isolates, there were no statistical differences among group B2 E. coli from the two origins; however, commensal isolates from groups D and B1 appeared to be less virulent than pathogenic isolates. Regardless of their phylogenetic group, nearly all the commensal and UPEC isolates with the same number of PAIs had the same PAI combinations. Although group B2 E. coli are uncommon among commensal intestinal flora, they are highly virulent when present, suggesting that the intestinal flora may act as a reservoir for bacteria that can cause urinary tract infection.

Active cytotoxic necrotizing factor 1 associated with outer membrane vesicles from uropathogenic Escherichia coli

Cytotoxic necrotizing factor type 1 (CNF1) is one of the virulence factors produced by uropathogenic Escherichia coli (UPEC). How this toxin is translocated from the bacterial cytoplasm to the surrounding environment is not well understood. Our data suggest that CNF1 may be regarded as a secreted protein, since it could be detected in culture supernatants. Furthermore, we found that CNF1 was tightly associated to outer membrane vesicles, suggesting that such vesicles play a role in the secretion of this protein. Interestingly, vesicle samples containing CNF1 could exert the effects known for this protein on HeLa cell cultures, showing that CNF1 is transported by vesicles in its active form. Taken together, our results strongly suggest that outer membrane vesicles could be a means for the bacteria to deliver CNF1 to the environment and to the infected tissue. In addition, our results indicate that the histone-like nucleoid structuring protein H-NS has a role in the downregulation of CNF1 production and that it affects the outer membrane vesicle release in UPEC strain J96.

The transcriptional antiterminator RfaH represses biofilm formation in Escherichia coli

We investigated the influence of regulatory and pathogenicity island-associated factors (Hha, RpoS, LuxS, EvgA, RfaH, and tRNA5Leu) on biofilm formation by uropathogenic Escherichia coli (UPEC) strain 536. Only inactivation of rfaH, which encodes a transcriptional antiterminator, resulted in increased initial adhesion and biofilm formation by E. coli 536. rfaH inactivation in nonpathogenic E. coli K-12 isolate MG1655 resulted in the same phenotype. Transcriptome analysis of wild-type strain 536 and an rfaH mutant of this strain revealed that deletion of rfaH correlated with increased expression of flu orthologs. flu encodes antigen 43 (Ag43), which mediates autoaggregation and biofilm formation. We confirmed that deletion of rfaH leads to increased levels of flu and flu-like transcripts in E. coli K-12 and UPEC. Supporting the hypothesis that RfaH represses biofilm formation through reduction of the Ag43 level, the increased-biofilm phenotype of E. coli MG1655rfaH was reversed upon inactivation of flu. Deletion of the two flu orthologs, however, did not modify the behavior of mutant 536rfaH. Our results demonstrate that the strong initial adhesion and biofilm formation capacities of strain MG1655rfaH are mediated by both increased steady-state production of Ag43 and likely increased Ag43 presentation due to null rfaH-dependent lipopolysaccharide depletion. Although the roles of rfaH in the biofilm phenotype are different in UPEC strain 536 and K-12 strain MG1655, this study shows that RfaH, in addition to affecting the expression of bacterial virulence factors, also negatively controls expression and surface presentation of Ag43 and possibly another Ag43-independent factor(s) that mediates cell-cell interactions and biofilm formation.

Microarray based comparison of two Escherichia coli O157:H7 lineages

BACKGROUND

Previous research has identified the potential for the existence of two separate lineages of Escherichia coli O157:H7. Clinical isolates tended to cluster primarily within one of these two lineages. To determine if there are virulence related genes differentially expressed between the two lineages we chose to utilize microarray technology to perform an initial screening.

RESULTS

Using a 610 gene microarray, designed against the E. coli O157 EDL 933 transcriptome, targeting primarily virulence systems, we chose 3 representative Lineage I isolates (LI groups mostly clinical isolates) and 3 representative Lineage II isolates (LII groups mostly bovine isolates). Using standard dye swap experimental designs, statistically different expression (P < 0.05) of 73 genes between the two lineages was revealed. Result highlights indicate that under in vitro anaerobic growth conditions, there is up-regulation of stx2b, ureD, curli (csgAFEG), and stress related genes (hslJ, cspG, ibpB, ibpA) in Lineage I, which may contribute to enhanced virulence or transmission potential. Lineage II exhibits significant up-regulation of type III secretion apparatus, LPS, and flagella related transcripts.

CONCLUSION

These results give insight into comparative regulation of virulence genes as well as providing directions for future research. Ultimately, evaluating the expression of key virulence factors among different E. coli O157 isolates has inherent value and the interpretation of such expression data will continue to evolve as our understanding of virulence, pathogenesis and transmission improves.

Novel three-dimensional organoid model for evaluation of the interaction of uropathogenic Escherichia coli with terminally differentiated human urothelial cells

Human bladder 5637 cells cultivated under microgravity conditions formed organoids that displayed characteristics of in vivo tissue-specific differentiation. Uropathogenic Escherichia coli (UPEC) strain CP9 colonized and penetrated the organoids and induced alpha-hemolysin-mediated exfoliation of uroepithelial cells. We propose these uro-organoids as models that simulate the interactions between UPEC and terminally differentiated human urothelium.

Bacterial toxins activating Rho GTPases

The CNF1 toxin is produced by some uropathogenic (UPECs) andmeningitis-causing Escherichia coli strains. It belongs to a large family of bacterial virulence factors and toxins modifying cellular regulators of the actin cytoskeleton, namely the Rho GTPases. CNF1 autonomously enters the host cell cytosol, where it catalyzes the constitutive activation of Rho GTPases by deamidation. This activation is, however, attenuated because of activated Rho protein ubiquitin-mediated proteasomal degradation. Both Rho protein activation and deactivation confer phagocytic properties on epithelial and endothelial cells, as well as epithelial cell motility and cell-cell junction dynamics. Transcriptome analysis using DNA microarray revealed that endothelial cells respond to high doses of CNF1 by launching a genetic program of host alarm. This host cell reaction to CNF1 intoxication also indicates that degradation of activated Rho proteins by the proteasome may lead to a lowering of the threshold of the intoxicated cell inflammatory response. These results are consistent with growing evidence that Rho proteins control the cell inflammatory responses. It is tempting to assume that Rho deregulation may participate in various immunological disorders also involved in cancer.

Multiple insertional events, restricted by the genetic background, have led to acquisition of pathogenicity island IIJ96-like domains among Escherichia coli strains of different clinical origins

We investigated the dissemination of pathogenicity island (PAI) II(J96)-like elements (hra, hly, cnf1, and pap) among 455 Escherichia coli isolates from children and adults with urinary tract infection (UTI), neonates with meningitis or colonized healthy neonates, and 74 reference strains by means of PCR phylogenetic grouping, ribotyping, and PCR analysis of virulence genes. Colocalization of these genes was documented by pulsed-field gel electrophoresis followed by Southern hybridization and long-range PCR (LRPCR) between the hra and the papG alleles. Site-specific insertion of the PAI was determined by LRPCR between hra and tRNA flanking sequences. hra, hly, and cnf1 were found in 113 isolates and consistently colocalized, constituting the backbone of PAI II(J96)-like domains. The prevalence of PAI II(J96)-like domains was significantly higher among UTI isolates than among neonatal meningitis and commensal isolates. These domains were restricted to a few ribotypes of group B2. In contrast to the consistent colocalization of hra, hly, and cnf1, the pap operon was varied: 12% of strains exhibited an allelic exchange of the papG class III allele (papGIII) for the papG class II allele (papGII) (only UTI isolates), and the pap operon was deleted in 23% of strains. No strains harbored papGIII outside the PAI, which appears to be the only source of this allele. PAI II(J96)-like domains were inserted in the vicinities of three different tRNAs--pheU (54%), leuX (29%), and pheV (15%)--depending on the genetic backgrounds and origins of the isolates. Multiple insertional events restricted by the genetic background have thus led to PAI II(J96) acquisition. Specific genetic backgrounds and insertion sites may have played a role in additional recombination processes for E. coli adaptation to different ecological niches.

Transcriptional regulation through RfaH contributes to intestinal colonization byEscherichia coli

The Escherichia coli regulatory protein RfaH contributes to efficient colonization of the mouse gut. Extraintestinal pathogenic (ExPEC) as well as non-pathogenic probiotic E. coli strains rapidly outcompeted their isogenic rfaH mutants following oral mixed infections. LPS-core and O-antigen side-chain as well as capsular polysaccharide synthesis are among the E. coli virulence factors affected by RfaH. In respect of colonization, deep-rough LPS mutants (waaG) but not capsular (kps) mutants were shown to behave similarly to rfaH mutants. Furthermore, alteration in the length of O-antigen side-chains did not modify colonization ability either indicating that it was the regulatory effect of RfaH on LPS-core synthesis, which affected intestinal colonization. Loss of RfaH did not significantly influence adhesion of bacteria to cultured colon epithelial cells. Increased susceptibility of rfaH mutants to bile salts, on the other hand, suggested that impaired in vivo survival could be responsible for the reduced colonization capacity.

Oral immunization with an rfaH mutant elicits protection against salmonellosis in mice

Loss of the transcriptional antiterminator RfaH results in virulence attenuation (>10(4)-fold increase in 50% lethal dose) of the archetypal Salmonella enterica serovar Typhimurium strain SL1344 by both orogastric and intraperitoneal routes of infection in BALB/c mice. Oral immunization with the mutant efficiently protects mice against a subsequent oral infection with the wild-type strain. Interestingly, in vitro immunoreactivity is not confined to strain SL1344; rather, it is directed also towards other serovars of S. enterica and even Salmonella bongori strains.

Pathogenomics of mobile genetic elements of toxigenic bacteria

The growing knowledge of genetic diversity and whole genome organization in bacteria shows that pathogenicity islands (PAIs) represent a subtype of a more general genetic element, termed genomic island (GEI), which is widespread among pathogenic and non-pathogenic microbes. These findings mirror the importance of horizontal gene transfer, genome reduction and recombination events as fundamental mechanisms involved in evolution of bacterial variants. GEIs are part of the flexible gene pool and carry selfish genes, but also determinants which may be beneficial under certain conditions thus increasing bacterial fitness and consequently their survival or transmission. In this review, we focus on the role of mobile genetic elements that may also contain toxin-encoding genes for genome variability and evolution of bacteria.

E. coli CNF1 toxin: a two-in-one system for host-cell invasion

The cytotoxic necrotizing factor-1 (CNF1), a bacterial toxin of uropathogenic bacteria (UPEC), hijacks cellular Rho proteins of the Ras GTPase super-family. Recently, we have made three important findings. First, we have established that, following Rho protein activation by deamidation, these cellular proteins are ubiquitylated and degraded by the proteasome. Second, the low level of activated Rho proteins which results from the dual molecular mechanism of action of CNF1 (Rho protein activation followed by their degradation), confers high invasive properties to UPECs. Finally, we have reported that ubiquitylation and degradation of Rac is lost in HEp-2 carcinoma cells, thereby suggesting a possible link between Rho protein ubiquitylation and tumor progression.