Cytotoxic effect of multinucleation in HeLa cell cultures associated with the presence of Vir plasmid inEscherichia colistrains

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.

Insight Into the Virulence Related Secretion Systems, Fimbriae, and Toxins in O2:K1 Escherichia coli Isolated From Bovine Mastitis

Mastitis remains a major infection of dairy cows and an important issue for the dairy farmers, and Escherichia coli (E. coli) bovine mastitis is a disease of significant economic importance in the dairy industry. Our study identified six isolates belong to phylogroup B2 from 69 bovine mastitis E. coli strains. Except for one serotype O1 strain, all group B2 isolates were identified into serotype O2 and showed significantly higher mortality in the mouse infection than other phylogroups' strains. Genomic analyses and further tests were performed to examine the role of secretion systems, fimbriae, and toxins during the systemic infection of O2:K1 strain BCE049. Two integral T6SS loci and three predicted effectors clusters were found to assemble the functional T6SS complex and deliver diverse toxic effectors to modulate bacterial virulence in the mouse infection model. A total of four T4SS loci were harbored in the BCE049 genome, three of them are encoded in different plasmids, respectively, whereas the last one locates within the bacterial chromosome at FQU84_16715 to FQU84_16760, and was significantly involved in the bacterial pathogenicity. Numerous predicted pilus biosynthesis gene loci were found in the BCE049 genome, whereas most of them lost long fragments encoding key genes for the pili assembly. Unexpectedly, a type IV pilus gene locus locating at FQU84_01405 to FQU84_01335 in the plasmid 2, was found to be required for the full virulence of mastitis strain BCE049. It should be noted that a genetic neighborhood inserted with diverse genes is encoded by the plasmid 1, which harbors three prominent toxins including β-hemolysin, cytotoxic necrotizing factor 2 and cytolethal distending toxin type III. Consequent studies verified that these toxins significantly contributed to the bacterial pathogenicity. These findings provide a molecular blueprint for understanding the underlying mechanisms employed by the bovine mastitis E. coli to colonize in host and cause systemic infection.

CNF1-like deamidase domains: common Lego bricks among cancer-promoting immunomodulatory bacterial virulence factors

Alterations of the cellular proteome over time due to spontaneous or toxin-mediated enzymatic deamidation of glutamine (Gln) and asparagine (Asn) residues contribute to bacterial infection and might represent a source of aging-related diseases. Here, we put into perspective what is known about the mode of action of the CNF1 toxin from pathogenic Escherichia coli, a paradigm of bacterial deamidases that activate Rho GTPases, to illustrate the importance of determining whether exposure to these factors are risk factors in the etiology age-related diseases, such as cancer. In particular, through in silico analysis of the distribution of the CNF1-like deamidase active site Gly-Cys-(Xaa)n-His sequence motif in bacterial genomes, we unveil the wide distribution of the super-family of CNF-like toxins and CNF-like deamidase domains among members of the Enterobacteriacae and in association with a large variety of toxin delivery systems. We extent our discussion with recent findings concerning cellular systems that control activated Rac1 GTPase stability and provide protection against cancer. These findings point to the urgency for developing holistic approaches toward personalized medicine that include monitoring for asymptomatic carriage of pathogenic toxin-producing bacteria and that ultimately might lead to improved public health and increased lifespans.

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.

Divergent Evolution of the repFII Replicon of IncF Plasmids Carrying Cytotoxic Necrotizing Factor cnf2, Cytolethal Distending Toxin cdtIII, and f17Ae Fimbrial Variant Genes in Type 2 Necrotoxigenic Escherichia coli Isolates from Calves

Among the pathovars of Escherichia coli in cattle, necrotoxigenic E. coli (NTEC) is defined by the production of cytotoxic necrotizing factors (CNFs). In particular, type 2 NTEC (NTEC2) strains are frequent in diarrheic and septicemic calves and usually coproduce CNF type 2 (CNF2), cytolethal distending toxin type III (CDTIII), and fimbrial adhesins of the F17 family, whose genetic determinants have frequently been reported on the same Vir-like plasmid. In this study, we investigated the genetic environment of the cnf2, f17Ae, and cdtIII genes in a collection of fecal E. coli isolates recovered from 484 French and 58 Iranian calves. In particular, we highlighted the spread of cnf2, f17Ae, and cdtIII on similar 150-kb IncF plasmids harboring the newly assigned repFII replicon allele F74 in NTEC2 isolates. Interestingly, this 150-kb IncF plasmid differed from the 140-kb IncF plasmid harboring the newly assigned repFII replicon allele F75 and carrying cnf2 alone. These results suggest two divergent lineages of cnf2-carrying IncF plasmids depending on the presence of the f17Ae and cdtIII genes. This partition was observed in E. coli strains of unrelated backgrounds, suggesting two different evolutionary paths of cnf2-carrying IncF plasmids rather than divergent evolutions of NTEC2 clones. The driving forces for such divergent evolutions are not known, and further studies are required to clarify the selection of plasmid subtypes spreading virulence determinants in E. coli, in particular, plasmids of the IncF family.

What a difference a Dalton makes: bacterial virulence factors modulate eukaryotic host cell signaling systems via deamidation

Pathogenic bacteria commonly deploy enzymes to promote virulence. These enzymes can modulate the functions of host cell targets. While the actions of some enzymes can be very obvious (e.g., digesting plant cell walls), others have more subtle activities. Depending on the lifestyle of the bacteria, these subtle modifications can be crucially important for pathogenesis. In particular, if bacteria rely on a living host, subtle mechanisms to alter host cellular function are likely to dominate. Several bacterial virulence factors have evolved to use enzymatic deamidation as a subtle posttranslational mechanism to modify the functions of host protein targets. Deamidation is the irreversible conversion of the amino acids glutamine and asparagine to glutamic acid and aspartic acid, respectively. Interestingly, all currently characterized bacterial deamidases affect the function of the target protein by modifying a single glutamine residue in the sequence. Deamidation of target host proteins can disrupt host signaling and downstream processes by either activating or inactivating the target. Despite the subtlety of this modification, it has been shown to cause dramatic, context-dependent effects on host cells. Several crystal structures of bacterial deamidases have been solved. All are members of the papain-like superfamily and display a cysteine-based catalytic triad. However, these proteins form distinct structural subfamilies and feature combinations of modular domains of various functions. Based on the diverse pathogens that use deamidation as a mechanism to promote virulence and the recent identification of multiple deamidases, it is clear that this enzymatic activity is emerging as an important and widespread feature in bacterial pathogenesis.

Bacterial protein toxins that modify host regulatory GTPases

Many bacterial pathogens produce protein toxins to outmanoeuvre the immune system of the host. Some of these proteins target regulatory GTPases such as those belonging to the RHO family, which control the actin cytoskeleton of the host cell. In this Review, I discuss a diversity of mechanisms that are used by bacterial effectors and toxins to modulate the activity of host GTPases, with a focus on covalent modifications such as ADP-ribosylation, glucosylation, adenylylation, proteolysis, deamidation and transglutamination.

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.

Characterization of cytotoxic necrotizing factor 1-producing Escherichia coli strains from faeces of healthy macaques

Twenty-five (27 %) of 92 clinically normal macaques were found to have beta-haemolytic Escherichia coli isolated from their faeces. Five of six isolates chosen for further characterization had multiple antibiotic resistance and were PCR-positive for cytotoxic necrotizing factor 1 (cnf1) with a demonstrated cytopathic effect in vitro. By repetitive element sequence-based PCR genotyping, genetic similarity was established for selected isolates. We believe this to be the first report of E. coli strains producing CNF1 in non-human primates.

Molecular characteristics of Escherichia coli serogroup O78 strains isolated from diarrheal cases in bovines urge further investigations on their zoonotic potential

We investigated the virulence properties and clonal relationship of 21 Escherichia coli strains of serogroup O78 isolated from diarrhoeic cattle and calves. Isolates were screened for 18 genes representing virulence features of different Escherichia coli pathotypes. None of the strains harboured enterotoxin-genes estIa/Ib, eltIa/Ib, or Shiga toxin (stx) genes, genes involved in adhesion (eae, f5, f41) hemolysin gene hlyA or invasion gene ipaC. With a high prevalence we detected enterotoxin astA (61.9%), genes involved in iron acquisition, like fyuA, irp (each 57.1%) and iucD (81.0%), and the operon sequence of Colicin V plasmids (38.1%). Some strains possessed toxin genes cdt-IIIB and cnf1/2 (both 14.3%), the invasion gene tia (23.8%), and the serine protease encoding gene espP (23.8%). Moreover, we could show that E. coli O78 strains under investigation were able to adhere to and invade MDBK-cells with varying efficiencies. The results indicate that the closely related O78 strains, constituting two major PFGE-clusters, harbor various virulence features for bovine intestinal disease but cannot be grouped into one of the common E. coli intestinal pathogenic or other pathotypes according to their virulence gene pattern. Nevertheless, the ability to adhere, invade or harbor toxin genes lets us suggest that O78 strains isolated from diarrheal cases in bovines urges further investigations on the zoonotic potential of these strains.

Cytotoxic Necrotizing Factors: Rho-Activating Toxins from Escherichia coli

This article reviews the Escherichia coli toxins called cytotoxic necrotizing factors (CNFs), which cause activation of Rho GTPases. It describes their modes of action, structure-function relationships, and roles in disease. Rho GTPases, the targets of CNFs, belong to the Ras superfamily of low molecular mass GTPases and act as molecular switches in various signaling pathways. Low molecular mass GTPases of the Rho family are known as master regulators of the actin cytoskeleton. Moreover, they are involved in various signal transduction processes, from transcriptional activation, cell cycle progression, and cell transformation to apoptosis. CNFs are cytotoxic for a wide variety of cells, including 3T3 fibroblasts, Chinese hamster ovary cells, Vero cells, HeLa cells, and cell lines of neuronal origin. This implies that a commonly expressed receptor is responsible for the uptake of CNF1. Cultured mammalian cells treated with CNFs are characterized by dramatic changes in actin-containing structures, including stress fibers, lamellipodia, and filopodia. Most striking is the formation of multinucleation in these cells. Rho GTPases are increasingly recognized as essential factors in the development of cancer and metastasis. This fact has initiated a discussion as to whether activation of Rho proteins by CNFs might be involved in tumorigenesis. Moreover, CNF1 increases the expression of the cyclooxygenase 2 (Cox2) gene in fibroblasts. Increased expression of Cox2 is observed in some types of tumors, e.g., colon carcinoma. Lipid-mediators produced by the enzyme are suggested to be responsible for tumor progression.

Genomic subtraction for the identification of putative new virulence factors of an avian pathogenic Escherichia coli strain of O2 serogroup

To identify putative new virulence factors of avian pathogenicEscherichia coli(APEC) strains, a genomic subtraction was performed between the APEC strain MT512 and the non-pathogenicE. colistrain of avian origin EC79. Seventeen DNA fragments were cloned that were specific for the APEC strain. Among them, nine were identified that were more frequent among pathogenic than non-pathogenic isolates in a collection of 67 avianE. coli. Chromosome or plasmid location, and the nucleotide sequence of these nine fragments were characterized. Four fragments were plasmid-located. The nucleotide sequence of two of them exhibited identity with the sequence of the RepF1B replicon ofE. coliplasmids, and the amino-acid deduced sequences from the two other fragments exhibited similarity to the products of genessitAofSalmonellaTyphimurium andiroDofE. coli, which are involved in iron metabolism. Of the five chromosome-located fragments, three were predicted to encode parts of proteins that were significantly homologous to previously described proteins: TktA (transketolase) ofHaemophilus influenzae, a FruA (fructokinase) homologue ofListeria innocuaand Gp2 (large terminal subunit) of phage 21. The putative products of the two other chromosome-located fragments were homologous to proteins with unknown functions: Z0255 ofE. colistrain EDL933 (EHEC) and RatA ofSalmonellaTyphimurium strain LT2. Both these chromosomal fragments, whose presence is correlated with serogroups O1 and O2 and to the virulence of APEC strains belonging to these serogroups, are good candidates for being part of novel virulence determinants of APEC. Moreover, several fragments were shown to be located close to tRNAselC,asnTorthrW, which suggests they could be part of pathogenicity islands. Six fragments that were shown to be part of whole ORFs present in the APEC strain MT 512 were also present in extra-intestinal pathogenicE. coli(ExPEC) strains of human and animal origin. Thus, the putative novel virulence factors identified in this study could be shared by ExPEC strains of different origins.

CNF and DNT

The actin cytoskeleton of mammalian cells is involved in many processes that affect the growth and colonization of bacteria, such as migration of immune cells, phagocytosis by macrophages, secretion of cytokines, maintenance of epithelial barrier functions and others. With respect to these functions, it is not surprising that many bacterial protein toxins, which are important virulence factors and causative agents of human and/or animal diseases, target the actin cytoskeleton of the host. Some toxins target actin directly, such as the C2 toxin produced by Clostridium botulinum. Moreover, bacterial toxins target the cytoskeleton indirectly by modifying actin regulators such as the low-molecular-mass guanosine triphosphate (GTP)-binding proteins of the Rho family. Remarkably, toxins affect these GTPases in a bidirectional manner. Some toxins inhibit and some activate the GTPases. Here we review the Rho-activating toxins CNF1 and CNF2 (cytotoxic necrotizing factors) from Escherichia coli, the Yersinia CNF(Y) and the dermonecrotic toxin (DNT) from Bordetella species. We describe and compare their uptake into mammalian cells, mode of action, structure-function relationship, substrate specificity and role in diseases.

Putative roles of the CNF2 and CDTIII toxins in experimental infections with necrotoxigenic Escherichia coli type 2 (NTEC2) strains in calves

Newborn colostrum-restricted calves were orally inoculated with an Escherichia coli strain, identified originally as non-pathogenic, and into which the plasmid pVir was conjugally transferred. This resulted in diarrhea, intestinal lesions and extra-intestinal invasion, suggesting that factors affecting these pathogenic properties are located on pVir. In order to analyze the respective roles of the toxins CNF2 and CDTIII in the pathogenesis, colostrum-restricted calves were inoculated with isogenic mutants in the cnf2 and the cdt-III genes. The loss of cnf2 is associated with a reduction in the pathogenicity, since diarrhea does not occur in calves challenged, in spite of successful colonization of the intestine. Nevertheless, the mutant strain remains able to invade the bloodstream and to localize in the internal organs. Conversely, the calves inoculated with mutant in the cdt-III gene evolved in the same way as wild-type strain-inoculated calves with regard to clinical signs and macroscopic or microscopic lesions.

Isolation from a sheep of an attaching and effacing Escherichia coli O115:H- with a novel combination of virulence factors

Attaching and effacing (AE) lesions were observed in the caecum, proximal colon and rectum of one of four lambs experimentally inoculated at 6 weeks of age with Escherichia coli O157:H7. However, the attached bacteria did not immunostain with O157-specific antiserum. Subsequent bacteriological analysis of samples from this animal yielded two E. coli O115:H(-) strains, one from the colon (CO) and one from the rectum (RC), and those bacteria forming the AE lesions were shown to be of the O115 serogroup by immunostaining. The O115:H(-)isolates formed microcolonies and attaching and effacing lesions, as demonstrated by the fluorescence actin staining test, on HEp-2 tissue culture cells. Both isolates were confirmed by PCR to encode the epsilon (epsilon) subtype of intimin. Supernates of both O115:H(-) isolates induced cytopathic effects on Vero cell monolayers, and PCR analysis verified that both isolates encoded EAST1, CNF1 and CNF2 toxins but not Shiga-like toxins. Both isolates harboured similar sized plasmids but PCR analysis indicated that only one of the O115:H(-) isolates (CO) possessed the plasmid-associated virulence determinants ehxA and etpD. Neither strain possessed the espP, katP or bfpA plasmid-associated virulence determinants. These E. coli O115:H(-) strains exhibited a novel combination of virulence determinants and are the first isolates found to possess both CNF1 and CNF2.

Escherichia coli cytotoxic necrotizing factors and Bordetella dermonecrotic toxin: the dermonecrosis-inducing toxins activating Rho small GTPases

Escherichia coli cytotoxic necrotizing factors (CNFs) and Bordetella dermonecrotic toxin (DNT) have been recently found to comprise a novel family of dermonecrosis-inducing toxins which activate the small GTPases of the Rho family. They are single chain polypeptides consisting of an N-terminal domain responsible for binding to target cells and a C-terminal catalytic domain. CNFs (CNF1 and 2) and DNT share in the catalytic domain about 30% identical residues and a consensus sequence where the catalytically active center Cys resides. Both toxins deamidate Rho and other members of the Rho family, Rac and Cdc42, at Gln in the switch II region, which plays an important role in their GTPase activity. DNT, in addition, catalyzes a cross-link of the Gln of the GTPases with ubiquitous polyamines such as putrescine, spermidine, and spermine. The deamidation and the polyamination result in abrogation of the GTPase activity, and in addition, the polyamination endows Rho with the ability to interact with a downstream effector, ROCK, in a GTP-independent manner. These effects render the GTPases constitutively active, which underlies the toxicities of CNFs and DNT.

Characteristics of necrotoxigenic Escherichia coli isolated from septicemic and diarrheic calves between 1958 and 1970

A total of 434 Escherichia coli isolated from septicemic calves between 1958 and 1965 and 430 E. coli isolated from diarrheic calves between 1967 and 1970 were studied by colony hybridisation and PCR assays for the presence of the cnf1- and the cnf2-like genes. They were also studied for the presence of genes coding for putative virulence factors associated with the CNF toxins including F17-, Pap- and Sfa-fimbrial adhesins and the recently described CDT-III toxin and AfaVIII-afimbrial adhesin. Thirty (7%) of the 434 septicemic strains were positive for CNF by colony hybridisation. Twenty-six were confirmed as necrotoxigenic E. coli type 2 (NTEC2) and four as NTEC1 by PCR. Thirty-five (8%) of the 430 diarrheic strains were positive for CNF by colony hybridisation. Five of them were studied by PCR and confirmed as NTEC1. The 26 septicemic NTEC2 strains and 20 of the 35 diarrheic NTEC including three of the five NTEC1 were positive for CDT-III. All adhesins studied were present in NTEC as well as in non-NTEC. NTEC1 were mainly Pap-, Sfa- and/or Afa8-positive, whereas NTEC2 were mainly F17- and/or Afa8-positive. This study shows that necrotoxigenic E. coli with their associated adhesins and toxins were present in calves as early as 1958, but their prevalence seems to have increased since that time.

The afa-related gene cluster in necrotoxigenic and other Escherichia coli from animals belongs to the afa-8 variant

Six hundred and nine necrotoxigenic Escherichia coli type 1 and 2 (NTEC1 and NTEC2) and non-NTEC isolated in Western and Southern Europe, North Africa and Canada from diseased calves, pigs, humans, poultry, and 55 isolated from asymptomatic calves were studied for the identification of afa-related sequences to the recently described afa-7 and afa-8 gene cluster variants from two bovine Escherichia coli (Lalioui et al., 1999). Colony hybridization and PCR assays for the afaD-7, afaE-7, afaD-8 and afaE-8 identified the afa-related sequences to the afa-8 gene cluster in most (67/79; 85%) of the E. coli positive with the Afa-f family probe and in 14 additional strains negative with the Afa-f probe. No E. coli was positive for the afa-7 gene cluster. The existence of afa-8 positive strains was thus confirmed among bovine E. coli and for the first time among porcine, poultry and human E. coli. Sequencing of the afaE-8 amplicon of nine strains from the different host species showed a high degree of conservation (>95% at the DNA level; >92% at the amino-acid level). The afa-8 gene cluster was more frequent in E. coli from diseased calves (18%) than from piglets (12%), humans (6%) and poultry (5%). Bovine NTEC2 (26%) were more frequently positive than NTEC 1 (20%) and non-NTEC (11%). E. coli isolated from asymptomatic calves were rarely positive: one NTEC2 (3%) and no non-NTEC. The afa-8 gene cluster was located on the Vir plasmid in 11/23 NTEC2, but no plasmid localization was detected in NTEC1 or non-NTEC.

Identification of the F17 fimbrial subunit- and adhesin-encoding (f17A and f17G) gene variants in necrotoxigenic Escherichia coli from cattle, pigs and humans

Putative colonization factors of the F17 family of fimbrial adhesins have been identified in necrotoxigenic Escherichia coli Type 1 and Type 2 (NTEC1 and NTEC2) from calves, pigs, and humans. The f17A and f17G gene variants, coding respectively for the major subunit and for the adhesin of the F17 fimbriae, were typed in 70 E. coli carrying f17-related sequences (15 NTEC1, 51 NTEC2, and four non-NTEC) by colony hybridisation with gene probes derived from the different f17A gene variants (a, b, c, and d) and by PCRs specific for each f17A and f17G (I and II) gene variants. Typing of f17A genes was not possible by colony hybridisation, as most 70 E. coli were positive with more than one gene probe. On the other hand, the PCRs allowed the typing of the f17A gene in 37 E. coli and of the f17G gene in all 70 E. coli. The f17Ab gene variant was detected in 13 NTEC2; the f17Ac, in all 15 NTEC1, six NTEC2 and two non-NTEC; and the f17Ad, in one non-NTEC. Seven additional NTEC2 were positive with the PCRs for two variants: f17Ab and f17Ac in three of them; f17Ac and f17Ad in four of them. Either these seven NTEC2 harbour two variants or the variant present can be detected by two PCRs. The remaining 25 NTEC2 and one non-NTEC tested negative with the PCRs for the four f17A gene variants, suggesting the existence of other variant(s). In contrast, all 70 E. coli were positive with the PCR for the f17GII gene variant and none with the PCR for the f17GI gene variant. The f17-related sequences were present on the CNF2/Vir plasmids in 27 out of the 46 NTEC2 from which plasmid DNA could be extracted: all but one of those positive for the f17Ab gene variant and various proportions of those positive for other variants. In contrast, no plasmid carried f17-related sequences in NTEC1 and non-NTEC.

Reproduction of lesions and clinical signs with a CNF2-producing Escherichia coli in neonatal calves

CNF2-producing necrotoxigenic E. coli (NTEC2) are associated with diarrhoea and septicaemia in calves. We orally inoculated neonatal calves with a NTEC2 strain in order to reproduce clinical signs and lesions. We observed diarrhoea in each inoculated calf, bacteraemia (80%), the presence of CNF2+ bacteria in the lungs (80%) and in the liver (20%). The observed lesions were inflammation of the entire gut, hypertrophy of the mesenteric lymph nodes and hepatisation of the lungs. We were unable to detect characteristic lesions that are classical signs of septicaemia.

Cytotoxic necrotizing factor type 2 produced by pathogenic Escherichia coli deamidates a gln residue in the conserved G-3 domain of the rho family and preferentially inhibits the GTPase activity of RhoA and rac1

Cytotoxic necrotizing factor types 1 and 2 (CNF1 and -2) produced by pathogenic Escherichia coli strains have 90% conserved residues over 1,014-amino-acid sequences. Both CNFs are able to provoke a remarkable increase in F-actin structures in cultured cells and covalently modify the RhoA small GTPases. In this study, we demonstrated that CNF2 reduced RhoA GTPase activity in the presence and absence of P122(RhoGAP). Subsequently, peptide mapping and amino acid sequencing of CNF2-modified FLAG-RhoA produced in E. coli revealed that CNF2 deamidates Q63 of RhoA-like CNF1. In vitro incubation of the C-terminal domain of CNF2 with FLAG-RhoA resulted also in deamidation of the FLAG-RhoA, suggesting that this region contains the enzymatic domain of CNF2. An oligopeptide antibody (anti-E63) which specifically recognized the altered G-3 domain of the Rho family reacted with glutathione S-transferase (GST)-RhoA and GST-Rac1 but not with GST-Cdc42 when coexpressed with CNF2. In addition, CNF2 selectively induced accumulation of GTP form of FLAG-RhoA and FLAG-Rac1 but not of FLAG-Cdc42 in Cos-7 cells. Taken together, these results indicate that CNF2 preferentially deamidates RhoA Q63 and Rac1 Q61 and constitutively activates these small GTPases in cultured cells. In contrast, anti-E63 reacted with GST-RhoA and GST-Cdc42 but not with GST-Rac1 when coexpressed with CNF1. These results indicate that CNF2 and CNF1 share the same catalytic activity but have distinct substrate specificities, which may reflect their differences in toxic activity in vivo.

Comparison of necrotoxigenic Escherichia coli isolates from farm animals and from humans

Necrotoxigenic Escherichia coli (NTEC) isolated from animals and humans can belong to the same serogroups/types and produce or carry the genes coding for fimbrial and afimbrial adhesins of the same family, P, S, F17, and/or AFA, raising the question of a potential zoonotic source of human infection. The main purpose of this study was to compare 239 NTEC1 strains (45 from cattle, 65 from humans and 129 from piglets) and 98 NTEC2 strains from cattle, using a uniform and standardized typing scheme. The O serogroups and the biotypes recognized amongst NTEC1 and NTEC2 strains were quite varied, although some were more frequently observed (serogroups O2, O4, O6, O8, O18, O78, and O83 and biotypes 1, 2, 5, 6, and 9). Hybridization, results with gene probes for the P family (PAP probe), S family (SFA probe), AFA family (AFA probe), F17 family (F17 probe) of fimbrial and afimbrial adhesins, could differentiate most NTEC1 strains, which are PAP-, SFA- and/or AFA-positive, from NTEC2 strains, which are mainly F17- and/or AFA-positive, but were of no help in differentiating between NTEC1 strains from cattle, humans, and piglets. All but seven (98%) NTEC1 and NTEC2 strains were serum resistant, 199 (59%) produced an aerobactin, and colicin (I, V, or unidentified) was produced by 22-34% of them. On the other hand, more than 90% of the NTEC1 strains were haemolytic on sheep blood agar compared with only 40% of the NTEC2 strains. Production of a classical haemolysin, active on sheep erythrocytes, and hybridization with the PAP probe were associated in a majority of NTEC1 strains (63-81%), but very rarely in NTEC2 strains (3%). Production of enterohaemolysin and hybridization with the PAP probe were much less frequently associated in NTEC strains (1-9%). It was thus possible neither to completely differentiate NTEC1 strains from cattle, humans, and pigs, nor to define a signature for the NTEC strains. Necrotoxigenic E. coli must still be identified on the basis of the production of the Cytotoxic Necrotizing Factors 1 or 2 (or of their encoding genes) and complete differentiation of NTEC1 strains from cattle, humans, and piglets, use additionnal methods.

Virulence plasmids of enterotoxigenic Escherichia coli isolates from piglets

Virulence plasmids of 68 ETEC isolates from piglets belonging to different pathotypes and six ETEC isolates from calves with pathotypes typical of porcine ETEC were identified with seven virulence probes for the heat-stable (STa and STb) and heat-labile (LT) enterotoxins, for the F4, F5, F6, and F41 fimbrial adhesion subunit, and also with five Rep probes for the RepFIA and RepFIB basic replicons, and the RepFIC family of basic replicons. With the exception of the F41 probe, the other virulence probes hybridized with at least one plasmid band of a size range from 65 to more than 100 Mda. Common associations of virulence factor-encoding genes on plasmid bands were: STb/LT, STa/F5, STa/F6, STa/STb. Other associations, STa/F4, STa/F4/F6, and STa/STb/LT/F6, were rarer. On the other hand the F4 adhesin-encoding genes were isolated on one plasmid band in all but three F4+ isolates. All but one of the 92 virulence plasmids which were studied have Rep probe hybridization profiles and replicon types typical of the uni- or multireplicon plasmids belonging to the various incompatibility groups of the F incompatibility complex.

Expression of P, S, and F1C adhesins by cytotoxic necrotizing factor 1-producing Escherichia coli from septicemic and diarrheic pigs

Nineteen papC-positive cytotoxic necrotizing factor 1 (CNF1)-producing Escherichia coli isolates from pigs with septicemia or diarrhea were tested for the presence of pap-, sfa-, and afa-related sequences encoding P/Prs, S/F1C, and Dr/AFA adhesins respectively. Production of adhesins by isolates was tested by mannose-resistant hemagglutination (MRHA), sialidase treatment of erythrocytes and particle agglutination tests. Production of P, S, and F1C fimbriae by isolates was also examined by immunofluorescence. All isolates were pap+ by PCR. Eighteen isolates (95%) were MRHA for ovine and human A erythrocytes and exhibited GalNac-GalNac receptor specificity associated with class III P(Prs) adhesins. Fifteen (79%) of the 19 isolates reacted with antisera specific for one or more different P fimbrial serotypes on immunofluorescence. Three of these isolates also demonstrated Gal-Gal receptor specificity associated with class I or II P fimbrial adhesins. Fifteen (79%) of the isolates were sfa+ by PCR. Seven of these isolates exhibited sialidase-sensitive MRHA of bovine and human O erythrocytes and reacted with serum specific for S fimbriae on immunofluorescence. Seven of the 8 sfa+ isolates which were MRHA-negative for bovine erythrocytes reacted with serum specific for F1C fimbriae on immunofluorescence. All isolates produced type 1 fimbriae as determined by mannose-sensitive agglutination of yeast cells. None of the isolates were afa+ by PCR or colony hybridization. Results suggest that most pap+ porcine CNF1-producing E. coli isolates express P fimbriae bearing class III (Prs) type adhesins. In addition, most of these isolates also produce S or F1C fimbriae.

Uncoupling of S-phase and mitosis by recombinant cytotoxic necrotizing factor 2 (CNF2)

Cytotoxic necrotizing factor 2 (CNF2) is an exotoxin identified from virulent clinical isolates of Escherichia coli. It has been characterized in adherent cell lines as an inducer of cellular death, hyperploidy (multinucleation), and cytoskeletal reorganization. The molecular mechanism of these actions is unclear. Two cellular mechanisms can be hypothesized to explain the DNA content increase (hyperploidy) induced by the toxin. The first is that the toxin interferes with cytoplasmic division without interfering with normal nuclear cycling, such that DNA is replicated in the absence of cell division. The second is that the toxin drives the nuclear machinery to replicate the DNA multiple times within one cell cycle, without interfering with cytoplasmic division. In order to investigate these phenomena, we have constructed a recombinant CNF2 gene that expresses a toxin with both an epitope tag and a polyhistidine tag. Extracts made from E. coli that express this gene have a high multinucleating activity that colocalizes with the recombinant 115-kDa protein. To distinguish between these hypotheses, we used recombinant CNF2 and several growth conditions (time, partial differentiation, and stage of growth) to establish a relationship between cellular divisions and generation of hyperploidy. It was also determined that the toxin had no effect upon in vitro DNA replication using a Xenopus egg extract system. In aggregate, these data are consistent with the hypothesis that CNF2 is affecting cytoplasmic division and thereby removing the requirement for a completed mitosis before the initiation of another S-phase. These data are discussed in relation to the generation of polyploid cells during megakaryopoeisis and the generation of aneuploid cells during tumorigenesis.

Rapid and specific detection of F17-related pilin and adhesin genes in diarrheic and septicemic Escherichia coli strains by multiplex PCR

The F17-related adhesins are prevalent in Escherichia coli strains isolated from calves with diarrhea or septicemia and from lambs with nephropathy. The F17 family includes the F17a, F17b, F17c, and F111 fimbriae produced by bovine E. coli strains and the G agglutinin produced by human uropathogenic E. coli strains. An easy and inexpensive multiplex PCR method was developed to detect all the F17-related fimbriae and to identify four subtypes of structural subunit genes and two distinct subfamilies of adhesin genes by only two runs of amplification. A strict correlation was observed between the phenotypic assays and the multiplex PCR method when 166 pathogenic E. coli strains isolated from intestinal content of calves or lambs were tested. Genes encoding the F17c structural subunit and the subfamily II adhesins were prominent among the bovine and ovine isolates, and the capsule-like CS31A antigen was strictly associated with the F17c fimbriae. The F17b subtype fimbriae were prominent among the bovine isolates producing the CNF2 toxin, whereas the F17a subtype fimbriae were associated with the bovine isolates producing neither the CS31A antigen nor the CNF2 toxin. Five bacterial strains possessed two distinct and complete F17-related fimbrial gene clusters, and two of them produced two F17-related fimbriae at the bacterial cell surface. The related fimbrial gene clusters are probably organized in mosaic operons consisting of F17-related pilin and adhesin genes, and horizontal gene transfer may occur among E. coli strains isolated form different animal species.

Mitotic block and delayed lethality in HeLa epithelial cells exposed to Escherichia coli BM2-1 producing cytotoxic necrotizing factor type 1

The cytopathic effect (CPE) of Escherichia coli producing cytotoxic necrotizing factor type 1 (CNF1) was investigated by using a human epithelial cell (HeLa) model of infection with CNF1-producing E. coli BM2-1. This strain was shown to bind loosely, but massively, to HeLa cells. A 4-h interaction between bacteria and eukaryotic cells triggered the delayed appearance of a progressive dose-dependent CPE characterized by (i) intense swelling of cells accompanied by the formation of a dense network of actin stress fibers, (ii) inhibition of cell division due to a complete block in the G2 phase of the cell cycle, and (iii) nucleus swelling and chromatin fragmentation. These alterations resulted in cell death starting about 5 days after interaction. The absence of multinucleation clearly distinguished the CPE from the effect produced by cell-free culture supernatants of infected cells nor prevented by a CNF1-neutralizing antiserum. Pathogenicity was completely abolished after Tn5::phoA insertion mutagenesis in the cnf-1 structural gene but not restored by trans complementation with a recombinant plasmid containing intact cnf-1 and its promoter. These results suggest that a gene downstream of cnf-1, essential to the induction of the CPE, was affected by the mutation. On the other hand, transformation of the wild-type strain BM2-1 with the same recombinant plasmid leads to a significant increase in both CNF1 activity and CPE, demonstrating the direct contribution of CNF1 to the CPE. In conclusion, the pathogenicity of E. coli BM2-1 for HeLa cells results from a complex interaction involving cnf-1 and associated genes and possibly requiring a preliminary step of binding of bacterial organisms to target cells.

Detection of Escherichia coli strains producing cytotoxic necrotizing factor type two (CNF2) by enzyme-linked immunosorbent assay

Sheep and rabbit antisera were produced against lysates of E. coli strain 711 (pVir). This K-12 strain carries the Vir plasmid which codes for Cytotoxic Necrotizing Factor type 2 (CNF2). Immunoglobulin G (IgG) fractions of both immune sera were subsequently purified by a two-step precipitation method. To increase the specificity for CNF2, the sheep IgG preparation was extensively adsorbed against both a sonicated extract of isogenic K-12 strain 711 and intact phenol-treated cells of vaccine strain 711 (pVir). An enzyme-linked immunosorbent assay (ELISA) was developed to detect clinical isolates of E. coli producing CNF2, using the final preparations of rabbit and sheep IgG in a double sandwich technique. The results obtained with this CNF2-ELISA were compared to those obtained with the conventional HeLa cell cytotoxicity assay. The testing of 133 E. coli strains (49 CNF2 positive strains and 84 negative strains) resulted in no false-negative and no false-positive. Therefore, the CNF2-ELISA offers a good alternative to the HeLa cell culture assay for the detection of CNF2-producing strains where facilities for and experience with cell cultures is lacking.

Production of cytotoxic necrotizing factor, verocytotoxin and haemolysin by pyelonephritogenic Escherichia coli

Two hundred and thirty-two strains of Escherichia coli isolated from children with non-obstructive acute pyelonephritis (n = 65), women with non-obstructive acute pyelonephritis (n = 63) and the faecal flora of healthy children (n = 33) and adults (n = 71) were examined for cytotoxic necrotizing factor production, haemolysin synthesis, verocytotoxin production and expression of mannose-resistant haemaglutination of human erythrocytes. Forty-eight per cent of the pyelonephritogenic Escherichia coli strains produced cytotoxic necrotizing factor and 61% produced haemolysin compared to 25% and 27% of faecal control strains (p less than 0.001 and p less than 0.001 respectively). Cytotoxic necrotizing factor production did not occur among the non-haemolytic Escherichia coli strains which confirms the close association between these two toxic factors. The bacterial phenotypes producing both haemolysin and cytotoxic necrotizing factor, and the phenotype expressing both these toxic factors and mannose-resistant haemagglutination occurred significantly more often in pyelonephritogenic strains than in faecal isolates (p less than 0.001). Haemolytic strains without the ability to produce cytotoxic necrotizing factor were more common in faecal isolates than in uropathogenic strains (p = 0.05). Strains lacking the ability to synthesize both these toxins were also over-represented in faecal isolates (p less than 0.01).