Virulence factors associated with cytotoxic necrotizing factor type two in bovine diarrheic and septicemic strains of Escherichia coli

Cyclomodulins and Hemolysis in E. coli as Potential Low-Cost Non-Invasive Biomarkers for Colorectal Cancer Screening

The frequent occurrence of E. coli positive for cyclomodulins such as colibactin (CLB), the cytotoxic necrotizing factor (CNF), and the cytolethal distending factor (CDT) in colorectal cancer (CRC) patients published so far provides the opportunity to use them as CRC screening markers. We examined the practicability and performance of a low-cost detection approach that relied on culture followed by simplified DNA extraction and PCR in E. coli isolates recovered from 130 CRC patients and 111 controls. Our results showed a statistically significant association between CRC and the presence of colibactin genes clbB and clbN, the cnf gene, and newly, the hemolytic phenotype of E. coli isolates. We also observed a significant increase in the mean number of morphologically distinct E. coli isolates per patient in the CRC cohort compared to controls, indicating that the cyclomodulin-producing E. coli strains may represent potentially preventable harmful newcomers in CRC patients. A colibactin gene assay showed the highest detection rate (45.4%), and males would benefit from the screening more than females. However, because of the high number of false positives, practical use of this marker must be explored. In our opinion, it may serve as an auxiliary marker to increase the specificity and/or sensitivity of the well-established fecal immunochemical test (FIT) in CRC screening.

Animal Enterotoxigenic Escherichia coli

Enterotoxigenic Escherichia coli (ETEC) is the most common cause of E. coli diarrhea in farm animals. ETEC are characterized by the ability to produce two types of virulence factors: adhesins that promote binding to specific enterocyte receptors for intestinal colonization and enterotoxins responsible for fluid secretion. The best-characterized adhesins are expressed in the context of fimbriae, such as the F4 (also designated K88), F5 (K99), F6 (987P), F17, and F18 fimbriae. Once established in the animal small intestine, ETEC produce enterotoxin(s) that lead to diarrhea. The enterotoxins belong to two major classes: heat-labile toxins that consist of one active and five binding subunits (LT), and heat-stable toxins that are small polypeptides (STa, STb, and EAST1). This review describes the disease and pathogenesis of animal ETEC, the corresponding virulence genes and protein products of these bacteria, their regulation and targets in animal hosts, as well as mechanisms of action. Furthermore, vaccines, inhibitors, probiotics, and the identification of potential new targets by genomics are presented in the context of animal ETEC.

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.

Adhesins of Enterotoxigenic Escherichia coli Strains That Infect Animals

The first described adhesive antigen of Escherichia coli strains isolated from animals was the K88 antigen, expressed by strains from diarrheic pigs. The K88 antigen was visible by electron microscopy as a surface-exposed filament that was thin and flexible and had hemagglutinating properties. Many different fimbriae have been identified in animal enterotoxigenic E. coli (ETEC) and have been discussed in this article. The role of these fimbriae in the pathogenesis of ETEC has been best studied with K88, K99, 987P, and F41. Each fimbrial type carries at least one adhesive moiety that is specific for a certain host receptor, determining host species, age, and tissue specificities. ETEC are the most frequently diagnosed pathogens among neonatal and post-weaning piglets that die of diarrhea. Immune electron microscopy of animal ETEC fimbriae usually shows that the minor subunits are located at the fimbrial tips and at discrete sites along the fimbrial threads. Since fimbriae most frequently act like lectins by binding to the carbohydrate moieties of glycoproteins or glycolipids, fimbrial receptors have frequently been studied with red blood cells of various animal species. Identification and characterization of the binding moieties of ETEC fimbrial adhesins should be useful for the design of new prophylactic or therapeutic strategies. Some studies describing potential receptor or adhesin analogues that interfere with fimbria-mediated colonization have been described in the article.

Identification and detection of three new F17 fimbrial variants in Escherichia coli strains isolated from cattle

F17 fimbriae are produced by pathogenic Escherichia coli involved in diarrhea and septicemia outbreaks in calves and lambs. These proteins result from the expression of four different clustered genes, namely f17A, f17D, f17C and f17G, encoding a pilin protein, a periplasmic protein, an anchor protein and an adhesin protein, respectively. Several variants of f17A and f17G genes have been reported and found genetically associated with typical virulence factors of bovine pathogenic E. coli strains. In this study, a new F17e-A variant, closely related to F17b-A, was identified from a collection of 58 E. coli isolates from diarrheic calves in Iran. While highly prevalent in Iranian F17-producing clinical isolates from calves, this variant was rare among E. coli from a French healthy adult bovine population, suggesting a possible association with virulence. The f17Ae gene was also found in the genome of the Shiga-like toxin variant Stx1d-producing bovine E. coli strain MHI813, and belonged to a gene cluster also encoding a new F17-G3 variant, which greatly differed from F17-G1 and F17-G2. This gene cluster was located on a pathogenicity island integrated in the tRNA pheV gene. The gene coding for a third new F17f-A variant corresponding to a combination of F17c-A and F17d-A was also identified on the pVir68 plasmid in the bovine pathogenic E. coli strain 6.0900. In conclusion, we identified three new F17-A and F17-G variants in cattle E. coli, which may also have significant impact on the development of new diagnostics and vaccination tools.

Type II heat-labile enterotoxins from 50 diverse Escherichia coli isolates belong almost exclusively to the LT-IIc family and may be prophage encoded

Some enterotoxigenic Escherichia coli (ETEC) produce a type II heat-labile enterotoxin (LT-II) that activates adenylate cyclase in susceptible cells but is not neutralized by antisera against cholera toxin or type I heat-labile enterotoxin (LT-I). LT-I variants encoded by plasmids in ETEC from humans and pigs have amino acid sequences that are ≥ 95% identical. In contrast, LT-II toxins are chromosomally encoded and are much more diverse. Early studies characterized LT-IIa and LT-IIb variants, but a novel LT-IIc was reported recently. Here we characterized the LT-II encoding loci from 48 additional ETEC isolates. Two encoded LT-IIa, none encoded LT-IIb, and 46 encoded highly related variants of LT-IIc. Phylogenetic analysis indicated that the predicted LT-IIc toxins encoded by these loci could be assigned to 6 subgroups. The loci corresponding to individual toxins within each subgroup had DNA sequences that were more than 99% identical. The LT-IIc subgroups appear to have arisen by multiple recombinational events between progenitor loci encoding LT-IIc1- and LT-IIc3-like variants. All loci from representative isolates encoding the LT-IIa, LT-IIb, and each subgroup of LT-IIc enterotoxins are preceded by highly-related genes that are between 80 and 93% identical to predicted phage lysozyme genes. DNA sequences immediately following the B genes differ considerably between toxin subgroups, but all are most closely related to genomic sequences found in predicted prophages. Together these data suggest that the LT-II loci are inserted into lambdoid type prophages that may or may not be infectious. These findings raise the possibility that production of LT-II enterotoxins by ETEC may be determined by phage conversion and may be activated by induction of prophage, in a manner similar to control of production of Shiga-like toxins by converting phages in isolates of enterohemmorhagic E. coli.

Cyclomodulins in urosepsis strains of Escherichia coli

Determinants of urosepsis in Escherichia coli remain incompletely defined. Cyclomodulins (CMs) are a growing functional family of toxins that hijack the eukaryotic cell cycle. Four cyclomodulin types are actually known in E. coli: cytotoxic necrotizing factors (CNFs), cycle-inhibiting factor (Cif), cytolethal distending toxins (CDTs), and the pks-encoded toxin. In the present study, the distribution of CM-encoding genes and the functionality of these toxins were investigated in 197 E. coli strains isolated from patients with community-acquired urosepsis (n = 146) and from uninfected subjects (n = 51). This distribution was analyzed in relation to the phylogenetic background, clinical origin, and antibiotic resistance of the strains. It emerged from this study that strains harboring the pks island and the cnf1 gene (i) were strongly associated with the B2 phylogroup (P, <0.001), (ii) frequently harbored both toxin-encoded genes in phylogroup B2 (33%), and (iii) were predictive of a urosepsis origin (P, <0.001 to 0.005). However, the prevalences of the pks island among phylogroup B2 strains, in contrast to those of the cnf1 gene, were not significantly different between fecal and urosepsis groups, suggesting that the pks island is more important for the colonization process and the cnf1 gene for virulence. pks- or cnf1-harboring strains were significantly associated with susceptibility to antibiotics (amoxicillin, cotrimoxazole, and quinolones [P, <0.001 to 0.043]). Otherwise, only 6% and 1% of all strains harbored the cdtB and cif genes, respectively, with no particular distribution by phylogenetic background, antimicrobial susceptibility, or clinical origin.

Pyrosequencing of the Vir plasmid of necrotoxigenic Escherichia coli

Necrotoxigenic Escherichia coli, or NTEC, are defined as E. coli producing the toxin known as cytotoxic necrotizing factor, or CNF. NTEC are responsible for various diseases of humans and animals, including urinary tract infection, septicemia, and diarrhea. A subgroup of NTEC known as NTEC-2 produce a variant of CNF (CNF-2) whose gene is located on a plasmid known as Vir. Because of its involvement in NTEC-2 pathogenesis and its broad distribution among production animals, a Vir plasmid from a bovine NTEC-2 strain was sequenced and analyzed. This plasmid was found to belong to the RepFIB and RepFIIA replicon types, and it totaled 138,682 base pairs in size. Within this plasmid was an approximately 60-kb pathogenicity island, defined by its possession of multiple virulence factors within distinct genetic regions of lower G+C content bounded by inverted repeats. Within this PAI were a variety of putative virulence factors, including F17b fimbrial genes, genes of a novel fimbrial operon, tibAC, hemolysins, and the cnf2 and cdt toxin-encoding genes. The occurrence of this plasmid's virulence- and replication-associated genes was sought among a collection of 96 CNF-2-positive isolates. The most prevalent genes among this collection included repA (RepFIB), cnf2, an ompP homolog, and the tib-AC genes encoding for aggregation and biofilm formation. The Vir plasmid has evolved from an IncFIB ancestral backbone, with the RepFIB locus apparently driving the acquisition of its accessory virulence-associated elements via site-specific recombination. Overall, the completed sequence of a Vir plasmid increases our understanding of NTEC-2 pathogenomics and IncFIB plasmid evolution.

Characteristics and virulence genes of Escherichia coli isolated from septicemic calves in southeast of Iran

Virulence factors are associated with the capacity of E. coli strains to cause intestinal and extraintestinal infections. Thirty one E. coli isolates were obtained from heart blood or internal organs of septicemic calves. The O serogroups of isolates were determined. PCR assays were performed to determine the phylogenetic groups and presence of specific virulence genes. Fourteen (45.16%) isolates belonged to seven O serogroups (O8, O15, O20, O45, O78, O101 and O103) and 17 (54.83%) isolates were O-nontypeable. E. coli isolates fall into three phylogenetic groups included 15 isolates belonged to B1, 9 to A and 7 to D phylogenetic groups. Nineteen (61.29%) isolates exhibited at least one of the virulence genes. F17 family (5 isolates f17b, 3 isolates f17c, 1 isolate f17a) genes and aerobactin encoding gene of iucD (5 isolates) were the two most prevalent virulence genes. Three isolates were positive for cnf2 and cdtIII genes in combination and they were O-nontypeable. AfaE-VIII, CS31A gene (clpG) and hemolysin encoding gene (hly) were detected in 3, 4 and 3 isolates respectively. None of the isolates contained the ipaH sequences and the genes encoding fimbria (F5, F41, S, P), AfaI adesin, toxins (LT-I, ST-I, SLT-I, SLT-II, CNF1 and CDT-IV) and intimin.

Occurrence and characteristics of cytotoxic necrotizing factors, cytolethal distending toxins and other virulence factors in Escherichia coli from human blood and faecal samples

Escherichia coli isolates from human blood (n=266) and faecal (n=237) samples were examined for cytotoxic necrotizing factors 1 and 2 (CNF 1 and 2), cytolethal distending toxin (CDT), and putative virulence factors that have been associated with disease conditions in humans and animals. PCR showed that the chromosomally encoded, Rho-activating, CNF1 (68/544, 12.5%) was more common than the transmissible plasmid-borne CNF2 (3/544, 0.6%). The relative risk of having either CNF or CDT toxin genes in blood compared to faecal isolates was 3.88 (95% CI 2.36-6.38). This was highly significant (P<0.0001) and demonstrates the importance of these factors in bloodstream infections. Fifty-one of 65 (78%) E. coli bearing CNF1 and 11 of 21 (52%) of E. coli bearing CDT also carried the pyelonephritis-associated pilus gene, papG. The S fimbrial adhesin gene, sfa, was found in 57 blood (21%) and eight faecal samples (3%). The F17 fimbrial adhesin gene and afimbrial adhesin gene afa did not occur frequently. Haemolysin (hly) was found in all of the isolates tested. Further studies must be designed to identify the clinical significance of these genes and their role in pathogenesis.

Characteristics of cytotoxic necrotizing factor and cytolethal distending toxin producing Escherichia coli strains isolated from meat samples in Northern Ireland

Swabs collected from pig, lamb and beef carcasses and samples of pork, lamb and beef mince were cultured for Escherichia coli strains. Strains harbouring cytotoxic necrotizing factors (CNF1 and 2) and cytolethal distending toxins (CDT-I,-II,-III and -IV) were identified in plate cultures of the isolates by colony hybridization with labelled probes and multiplex PCR assays. Simplex and multiplex PCR assays were used to further characterize the isolates to determine the presence of P, S and F17 fimbriae as well as afimbrial adhesins and haemolysin. The serotype was also determined where possible. Thirty strains with the capacity to code for CNF (4), CDT (24) or both (2) were isolated and characterized, and a wide range of associated factor patterns was observed. The methods utilized were successful in demonstrating the detection of viable strains with potentially significant pathogenic factors from human food sources.

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.

Production of cytolethal distending toxins by pathogenic Escherichia coli strains isolated from human and animal sources: establishment of the existence of a new cdt variant (Type IV)

Three types of cytolethal distending toxin (CDT), namely, CDT-I, CDT-II, and CDT-III, have been described in Escherichia coli. Using primers designed for the detection of sequences common to the cdtB genes, we analyzed by PCR a set of 21 CDT-producing E. coli strains of intestinal and extraintestinal origins isolated from human and different animal species in several European countries and in the United States. On the basis of the existing differences in the cdtB genes, cdt-I-, cdt-II-, and cdt-III-specific primer pairs were designed and used for cdt typing. These new primers successfully differentiated all of the previously described cdt genes. Six strains proved to be cdt-I; eight strains proved to be cdt-III. However, none of the type I-, II-, and III-specific primers generated amplicons from six CDT(+) strains, suggesting the existence of a new cdt variant. Sequence analysis of the amplicons from two untypeable genes confirmed the existence of a new cdt variant that we called cdt-IV. Using the new specific primers, cdt-IV was detected in human, porcine, and poultry strains of intestinal and extraintestinal origins. To validate all sets of cdt specific primers, a group of 353 human E. coli strains isolated in Hungary was then investigated for the presence of cdt genes. This included 190 strains isolated from patients with urinary tract infections (UTI), 51 strains isolated from other (nonurinary) extraintestinal infections, and 112 intestinal strains isolated from healthy individuals. Of 190 UTI strains, 15 (7.9%) had cdt genes. Of 51 non-UTI extraintestinal strains 3 (5.9%) contained the cdt gene, and 1 (0.9%) of 112 healthy intestinal strains was PCR positive. Five strains proved to be cdt-I, and fourteen strains proved to be cdt-IV. The CDT-producing extraintestinal strains belonged to a wide variety of serogroups, including O2, O6, O75, and O170. In conclusion, we have developed a new PCR typing system for CDT able to detect a new CDT variant present in pathogenic E. coli strains obtained from animals and humans.

Multiplex PCRs for identification of necrotoxigenic Escherichia coli

Two multiplex PCRs were developed for the detection of necrotoxigenic Escherichia coli virulence genes. M1 contained the primers for the toxins and the aerobactin, and M2 contained the primers for the adhesins. They were validated by single PCRs performed with reference E. coli strains and by multiplex PCRs with necrotoxigenic E. coli strains isolated from different animal species.

Occurrence and characteristics of CS31A antigen-producing Escherichia coli in calves with diarrhoea and septicaemia in Argentina

CS31A is a K88-related non-fimbrial adhesin first described on Escherichia coli strains isolated from diarrhoeic and septicaemic calves. In this report, CS31A antigen was screened by immunological methods and confirmed by PCR among bovine E. coli isolates. In addition, CS31A-producing strains were characterized with respect to different fimbrial antigens, O-serogroup and other properties related to virulence. Faecal or tissue specimens of 100 diarrhoeic or septicaemic calves and 27 older cattle with different pathologies from 71 outbreaks or individual cases that occurred in Buenos Aires province, Argentina, were examined. CS31A + E. coli strains were isolated from 21 (21.0%) calves from 16 outbreaks or individual cases. No CS31A + E. coli was detected in samples from cattle more than 1 year old. Fimbriae F5, F41, F17a and F17b were not detected among the CS31A-producing strains. Three (14.3%) of the CS31A+ E. coli strains expressed the F17c fimbria. All of the 21 isolates exhibited at least one property of septicaemic strains (resistance to serum, production of aerobactin or colicins) but none of them demonstrated heat-stable enterotoxigenic activity. CS31A + E. coli isolates belonged to 10 serogroups, more commonly O8, O7, O17 and O21. The results obtained here confirm the worldwide distribution of CS31A antigen in bovine E. coli strains. However, CS31A + or CS31A + /F17c + E. coli were less frequently isolated than they were in North hemisphere countries.

Yersinia pseudotuberculosis produces a cytotoxic necrotizing factor

Cell extracts from Yersinia pseudotuberculosis induced multinucleation in HEp-2 cells in a manner similar to the effect caused by Escherichia coli cytotoxic necrotizing factor (CNF). The activity was not dependent on the Yersinia 70-kb virulence plasmid, and the activity was not inhibited by antibodies capable of neutralizing E. coli CNF type 1. The nucleotide sequence of the Yersinia cnf gene was 65.1% identical to the E. coli cnf gene.

Characterization of necrotoxigenic Escherichia coli (NTEC) strains isolated from healthy calves in Poland

Faecal samples from 132 healthy, 4-8-week-old calves from four different farms were examined for necrotoxigenic Escherichia coli (NTEC) producing the cytotoxic necrotizing factors type 1 (CNF1) and type 2 (CNF2). CNF2 genes were detected by polymerase chain reaction in 24 (6.1%) of the 396 E. coli strains tested; these strains were found in 18 (13.6%) calves used in the study. None of the 396 E. coli isolates examined possessed the gene encoding CNF1. Overall, 28.8% of E. coli examined expressed the F17 fimbrial antigen. A strong association between CNF2 toxin and F17 fimbriae was found (62.5% of CNF2-positive strains were F17-positive). Moreover, six out of 24 NTEC strains had the Stx1 or the Stx2 shiga toxin genes, and three additional isolates possessed the eae genetic marker of the intimin protein.

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.

Necrotoxigenic Escherichia coli type-2 invade and cause diarrhoea during experimental infection in colostrum-restricted newborn calves

There exists experimental evidence that necrotoxigenic Escherichia coli (NTEC) strains producing the cytotoxic necrotising factor 1 cause intestinal and extra-intestinal disease in piglets. On the other hand, no experimental model has been developed with NTEC strains producing the cytotoxic necrotising factor 2. In all, 14 colostrum-restricted calves were orally challenged with two strains isolated from the faeces of a diarrheic calf (B20a) or from the heart blood of a septicaemic calf (1404). All calves had diarrhoea which lasted until euthanasia in eight of them. In those calves, diarrhoea was correlated with the faecal excretion of the challenge strains. At necropsy, vascular congestion of the intestinal mucosa, hypertrophy of the mesenteric lymph nodes (MLN) and some congestion of the lungs were observed. Bacteriology confirmed the colonisation of the intestine by the challenge strains which were also recovered from the heart blood, the lungs and/or the liver. Histological sections confirmed enterocolitis, lymphadenitis and limited bronchopneumonia. In the intestinal tissue sections, bacteria testing positive in an in situ DNA hybridisation assay with a CNF2 probe were observed. Those results were confirmed by immunohistochemistry with a polyclonal anti-O78 and a monoclonal anti-F17b antisera. Three of the five control calves receiving either saline or a CNF(-), F17a strain (25KH09) had no clinical signs or lesions. The other two presented a profuse liquid diarrhoea but those calves were positive for the presence of K99(+) E. coli. In this model, both NTEC2 strains were thus, able to colonise the intestine, to cause long-lasting diarrhoea and to invade the blood stream with localisation in various internal organs in colostrum-restricted conventional newborn calves.

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.

Prevalence of virulence genes and clonality in Escherichia coli strains that cause bacteremia in cancer patients

Phenotypic analysis of Escherichia coli strains causing bacteremia in cancer patients suggests that they possess specific virulence properties. To investigate this hypothesis, we compared the frequency of the virulence-related genes cnf1, cnf2, papC, hlyC, and iut in 155 E. coli strains isolated from hospitalized cancer patients with epidemiologically unrelated cases of bacteremia to their frequency in 70 E. coli strains isolated from the feces of healthy unrelated volunteers. Of the blood isolates, 24, 37, and 26% were positive for cnf1, papC, and hlyC, respectively, versus only 6, 17, and 6% of the fecal isolates (P < 0.05 in all instances). By contrast, 47% of both isolates carried the iut gene. The patients' clinical characteristics did not significantly influence these frequencies. The presence on various pathogenicity islands (PAIs) of a combination of the cnf1, papC, and hlyC genes on the chromosome was strongly suggested by Southern blotting of pulsed-field gel electrophoresis (PFGE) patterns with specific DNA probes. The phylogenetic relatedness among 60 strains carrying three, two, one, or no virulence genes and 6 ECOR strains included as references was determined by neighbor joining, the unweighted pair-group method with arithmetic mean, and Wagner analysis of the randomly amplified polymorphic DNA (RAPD) patterns generated by 11 primers. Identification of a major cluster including 96.4% of the strains carrying the cnf1, papC, and hlyC genes and ECOR subgroup B2 strains suggested that the virulent E. coli strains causing bacteremia in cancer patients are closely related to ECOR B2 strains. The presence in the E. coli population surveyed of a strong linkage disequilibrium, and especially of a highly significant correlation between PFGE and RAPD genetic distances, confirms that clonal propagation has a major impact on the E. coli population structure. Nevertheless, low bootstrap values in the phylogenetic tree suggested that frequent genetic exchange inhibits the individualization of discrete genetic lineages, which are stable on an evolutionary scale.

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.

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.

Prevalence and characteristics of necrotoxigenic Escherichia coli (NTEC) strains isolated from diarrhoeic dairy calves

Fecal samples from 246, 1-90-days old diarrhoeic dairy calves in 72 herds were screened for the presence of cytotoxic necrotizing factors (CNF)-producing Escherichia coli (NTEC). NTEC were detected by tissue culture assays and PCR in 39 (15.8%) of the diarrheic calves, and the majority of these animals (34 of 39, ca. 87.2%) were infected by NTEC producing CNF2. Calves were grouped according to their age (1-7 days, 8-14 days, 15-21 days, 22-30 days and 31-90 days) and analyses of prevalence were done by the Mantel-Haenzsel chi2-test for trend. A significant age-associated increase in the prevalence of NTEC producing CNF2 (p<0.0001) was found. Eighty-one (8.4%) of the 958 E. coli isolates from the 246 diarrheic calves were positive for CNF in the tissue culture assays. These strains were analyzed by PCR and this technique showed that three (3.7%) strains were CNF1-positive and 75 (92.6%) were CNF2-positive. Moreover, three of the strains positive in the tissue culture assays were negative by PCR. These strains were subsequently assayed in several biological tests (rabbit skin test, mouse intraperitoneal test and mouse footpad test) which showed that they were really NTEC, probably producing CNF2, but with some different properties to classical strains producing CNF2. NTEC strains producing CNF2 belonged to different serogroups (O2, O7, O9, O14, O15, O41, O43, O45, O55, O76, O86, O88, O109, O115, O123, O128, O153 and O159) than strains producing CNF1 (O11 and O32) or PCR-negative strains (O111). Moreover, a strong association between CNF2 and F17 fimbriae was found (78.6% of CNF2-positive strains were F17-positive, whereas only 22.9% of CNF2-negative strains were F17-positive).

Cytotoxicity of hemolytic, cytotoxic necrotizing factor 1-positive and -negative Escherichia coli to human T24 bladder cells

Approximately one-half of Escherichia coli isolates from patients with cystitis or pyelonephritis produce the pore-forming cytotoxin hemolysin, a molecule with the capacity to lyse erythrocytes and a range of nucleated cell types. A second toxin, cytotoxic necrotizing factor 1 (CNF1), is found in approximately 70% of hemolytic, but rarely in nonhemolytic, isolates. To evaluate the potential interplay of these two toxins, we used epidemiological and molecular biologic techniques to compare the cytotoxicity of hemolytic, CNF1(+), and CNF1(-) cystitis strains toward human T24 bladder epithelial cells in vitro. A total of 29 isolates from two collections of cystitis-associated E. coli were evaluated by using methylene blue staining of bladder monolayers at 1-h intervals after inoculation with each strain. Most (20 of 29) isolates damaged or destroyed the T24 monolayer (less than 50% remaining) within 4 h after inoculation. As a group, CNF1(+) isolates from one collection (11 strains) were less cytotoxic at 4 h than the CNF1(-) strains in that collection (P = 0.009), but this pattern was not observed among isolates from the second collection (18 strains). To directly evaluate the role of CNF1 in cytotoxicity of hemolytic E. coli without the variables present in multiple clinical isolates, we constructed mutants defective in production of CNF1. Compared to the CNF1(+) parental isolates, no change in cytotoxicity was detected in these cnf1 mutants. Our results indicate that CNF1 does not have a detectable effect on the ability of hemolytic E. coli to damage human bladder cell monolayers in vitro.

Characterization of the roles of hemolysin and other toxins in enteropathy caused by alpha-hemolytic Escherichia coli linked to human diarrhea

Escherichia coli strains producing alpha-hemolysin have been associated with diarrhea in several studies, but it has not been clearly demonstrated that these strains are enteropathogens or that alpha-hemolysin is an enteric virulence factor. Such strains are generally regarded as avirulent commensals. We examined a collection of diarrhea-associated hemolytic E. coli (DHEC) strains for virulence factors. No strain produced classic enterotoxins, but they all produced an alpha-hemolysin that was indistinguishable from that of uropathogenic E. coli strains. DHEC strains also produced other toxins including cytotoxic necrotizing factor 1 (CNF1) and novel toxins, including a cell-detaching cytotoxin and a toxin that causes HeLa cell elongation. DHEC strains were enteropathogenic in the RITARD (reversible intestinal tie adult rabbit diarrhea) model of diarrhea, causing characteristic enteropathies, including inflammation, necrosis, and colonic cell hyperplasia in both small and large intestines. Alpha-hemolysin appeared to be a major virulence factor in this model since it conferred virulence to nonpathogenic E. coli strains. Other virulence factors also appear to be contributing to virulence. These findings support the epidemiologic link to diarrhea and suggest that further research into the role of DHEC and alpha-hemolysin in enteric disease is warranted.

Distribution and characterization of faecal necrotoxigenic Escherichia coli CNF1+ and CNF2+ isolated from healthy cows and calves

Faecal swabs obtained from a random sample of 268 cows and 90 calves on 19 Lugo (northwestern Spain) farms were examined for necrotoxigenic Escherichia coli (NTEC) producing the cytotoxic necrotizing factors type 1 (CNF1) and type 2 (CNF2). We found NTEC CNF1+ and CNF2+ on 11% and 95% of the farms, respectively, NTEC producing CNF2 were significantly more frequently isolated from calves (58%) than from cows (17%) (P < 0.001). The proportion of animals colonized with CNF2+ strains on each farm ranged from 0% to 60%. NTEC strains producing CNF2 isolated from healthy cattle belonged to 27 O serogroups; however, 64% were of one of 12 serogroups (O2, O8, O8-O75, O14, O15, O55, O86, O88, O115, O121, O147, and O168). Furthermore, the serogroups determined in CNF2+ strains isolated from cows (O2, O8, and O14) were different from those found in NTEC producing CNF2 isolated from calves (O8-O75, O15, O55, O86, O88, O115 and O147).

Prevalence and characteristics of necrotoxigenic Escherichia coli CNF1+ and CNF2+ in healthy cattle

From February to July of 1994, 328 faecal samples from 32 herds were collected and examined for necrotoxigenic Escherichia coli (NTEC). Strains producing the cytotoxic necrotizing factors type 1 (CNF1) and type 2 (CNF2) were found on 4 and 63% of the farms, respectively. The proportion of animals infected within each herd varied from 0 to 38%. NTEC producing CNF2 were significantly more frequently isolated from calves (24%; 17 of 71) than from cows (4%; 11 of 257) (chi 2corr. 25.088; P < 0.001). Although the bovine CNF2+ strains belonged to 16 different serogroups, 5 (O15, O77, O88, O142 and O153) accounted for 44% of strains. This study confirmed that healthy cattle are a reservoir of NTEC producing CNF2, and revealed that CNF2+ strains are more frequently carried by calves than by adult cows.

Presence of pap-, sfa-, and afa-related sequences in necrotoxigenic Escherichia coli isolates from cattle: evidence for new variants of the AFA family

Necrotoxigenic Escherichia coli (NTEC) are associated with intestinal and extraintestinal diseases in animals and human beings and produce Cytotoxic Necrotizing Factor 1 (CNF1) or 2 (CNF2). Fourty-three NTEC1, 42 NTEC2, and 32 CNF-negative isolates from cattle were tested by colony DNA hybridization, by plasmid DNA hybridization and by PCR assays for the presence of DNA sequences homologous to the operons coding for fimbrial (PAP/PRS, SFA/FIC, and F17) and afimbrial (AFA/Dr) adhesins of extraintestinal E. coli. Most NTEC1 isolates hybridized with the PAP probes and either the SFA probe (37%) or the AFA probes (49%). Most NTEC2 isolates, in contrast, hybridized with the F17 probe (45%), the AFA probes (19%), or the F17 and AFA probes (22%). A probe-positive plasmid was identified in each of the 19 NTEC2 isolates studied. They all hybridized with the CNF2 toxin probe (Vir plasmids) and most of them with the F17 (6 plasmids) or AFA (7 plasmids) probes. PCR amplification was obtained with 6 of the 11 NTEC isolates tested for the papGII/prsG genes; with all 5 NTEC isolates tested for the sfa and related operons; but with none of the 18 NTEC isolates tested for the afa and related operons. pap-, sfa-, and afa-related sequences are thus present in NTEC isolates from cattle in addition to f17-related operons and may code for adhesins corresponding to specific colonization factors. f17- and afa-related sequences can be located on the Vir plasmids along with the cnf2 gene. Existence of new variants of the AFA/Dr family is evident from the negative results of this family-specific PCR assay.

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.

Escherichia coli and virus isolated from "sticky kits"

A total of 121 Escherichia coli strains isolated from 3-week-old mink kits were serotyped and examined for virulence factors. 56 strains were isolated from healthy kits while 65 were from "sticky kits". Among these, 34 different serotypes were detected. No difference in serotypes or the presence of virulence factors could be detected between healthy and diseased kits. By electron microscopy of faecal samples corona-, rota-, and calicivirus were demonstrated among healthy as well as diseased kits.

The GafD protein of the G (F17) fimbrial complex confers adhesiveness of Escherichia coli to laminin

Escherichia coli IHE11088(pRR-5) expressing the G (F17) fimbria adhered to immobilized laminin as well as to reconstituted basement membranes. No adhesion was seen with the plasmidless strain IHE11088 or with the deletion derivative IHE11088(pHUB110), which expresses the G-fimbrial filament with a defective GafD lectin and lacks N-acetyl-D-glucosamine-specific binding. Adhesion of IHE11088(pRR-5) to laminin and to reconstituted basement membranes was specifically inhibited by N-acetyl-D-glucosamine, and adhesion was abolished after N-glycosidase F treatment of laminin. The results show that the GafD lectin binds to laminin carbohydrate and suggest a novel function for the F17 fimbria in binding to mammalian basement membranes.

The Hha protein as a modulator of expression of virulence factors in Escherichia coli

We constructed hha derivatives from both a clinical uropathogenic Escherichia coli isolate (strain FVL4) and a wild E. coli strain causing bovine diarrhea (strain CCB21) and analyzed the effect of the hha allele on the expression of the different virulence factors exhibited by these strains. Expression of hemolysin and of the Vir antigen was altered in hha mutants. Whereas production of hemolysin by strain FVL4 was repressed both at a low temperature and at high osmolarity, the hha allele accounted for a significant increase of hemolysin production under these conditions. Also, the low temperature-sensitive expression of the Vir adhesin was modified in hha mutants, which were able to express this adhesin at a low temperature. Expression of other virulence factors, such as cytotoxic necrotizing factor type 1 and 2 toxins, remained unmodified in hha derivatives of strains FVL4 and CCB21.

CNF producing Escherichia coli isolated from cattle in Northern Ireland

Tissue culture assays were used to investigate the incidence of cytotoxic necrotising factors (CNFs) 1 and 2 in Escherichia coli strains from cattle. E. coli cultures were obtained from faeces collected from 223 cases of diarrhoea and from 113 healthy animals. In addition, strains cultured from 62 cases of mastitis, 66 cases of septicaemia and 68 cases of abortion were also investigated. E. coli producing CNF 1 or 2 were identified in all sample groups except for the abortion cases. Comparable levels of CNF1 strains were present in E. coli from the faces of diarrhoeic (4%) and healthy faeces (4.4%) whereas lower levels of CNF2 were identified in the faeces from diarrhoeic animals (19.3%) in comparison with healthy animals (30.9%). One CNF1 producing strain was identified among the E. coli isolated from mastitis samples, while 3% and 10.6% of septicaemic strains were positive for CNF1 and 2, respectively. Serogrouping of CNF isolates did not reveal the association of any particular serogroups with the different conditions.

Characterization of Escherichia coli isolated from foals

Serotype, biotype, antibiogram, hemolysin production, fimbrial hemagglutinins, select toxin genes (STb, STaP, LT, slt1 and slt2) and the attaching effacing (eae) gene were determined for 99 foal strains of E. coli. E. coli from diarrheic and normal foals could not be distinguished by serotype, biotype, or antibiogram. Differences (P < or = 0.05) were observed in hemolysin production (11.5% vs 0%) and the expression of mannose-resistant hemagglutinins (23% vs 13%) among E. coli from diarrheic and healthy foals, respectively. Three of the E. coli strains from diarrheic foals were positive with probes for slt genes and one was positive for STb and LT genes. One strain from a healthy foal possessed the STb gene. As determined by the polymerase chain reaction, 8 strains possessed the eae gene. Seven of the 8 strains were from diarrheic foals and one eae-positive strain was from a healthy foal. The slt-positive strains did not possess eae genes and the eae-positive strains did not possess slt genes. These results indicate that enterotoxigenic strains of E. coli are not implicated in any substantial degree in sporadic foal diarrhea. However, the identification of slt-positive and eae-positive strains in foal feces indicate the presence of potentially virulent strains among foals.

Escherichia coli and virus isolated from "sticky kits"

A total of 121 Escherichia coli strains isolated from 3-week-old mink kits were serotyped and examined for virulence factors. 56 strains were isolated from healthy kits while 65 were from "sticky kits". Among these, 34 different serotypes were detected. No difference in serotypes or the presence of virulence factors could be detected between healthy and diseased kits. By electron microscopy of faecal samples corona-, rota-, and calicivirus were demonstrated among healthy as well as diseased kits.

Serotypes of CNF1-producing Escherichia coli strains that cause extraintestinal infections in humans

The O:K:H serotypes of 137 necrotoxigenic Escherichia coli (NTEC) producing the cytotoxic necrotizing factor type 1 (CNF1) isolated from human extraintestinal infection were determined. Although NTEC producing CNF1 belonged to 58 different serotypes, only 10 of them accounted for 54% of strains. The most common serotypes, in order of frequency, were: O4:K?:H5, O6:K13:H1, O83:K1:H31, O75:K95:H5, O2:K1:H6, O2:K7:H-, O75:K1:H7, O2:K?:H1, O4:K12:H1 and O22:K13:H1. CNF1 strains of serotypes O2:K7:H- and O4:K12:H1 express P-fimbriae, whereas CNF1 strains of serotypes O2:K?:H1, O2:K1:H6 and O75:K95:H5 possess the adhesin responsible for MRHA type III. Among CNF1 strains of serotype O4:K?:H5 there exist some that express P-fimbriae and others that possess MRHA type III. Lastly, the majority of CNF1 strains of serotypes O6:K13:H1, O22:K13:H1, O75:K1:H7 and O83:K1:H31 do not express P-fimbriae nor the adhesin responsible to MRHA type III. Our results show that extraintestinal infections are caused by a limited number of virulent clones, as suggested by the theory of special pathogenicity.

F17-like fimbriae from an invasive Escherichia coli strain producing cytotoxic necrotizing factor type 2 toxin

The F17b fimbriae encoded by the transmissible virulence plasmid Vir, also coding for cytotoxic necrotizing factor type 2, were characterized. A 5.7-kb region of Vir mediates in vitro N-acetylglucosamine-sensitive adhesion to calf intestinal villi. Sequence analysis revealed that this region codes for a structural subunit and an adhesin closely related to the F17-A and F17-G proteins encoded by the F17 fimbrial gene cluster. The F17b-A gene presents an open reading frame of 540 bp encoding a polypeptide of 180 amino acids with a putative signal peptide of 21 residues. The mature protein shows an identity of 74% with the F17-A structural subunit. This 20-kDa protein is recognized by antiserum directed against F17 fimbriae. The F17b-G gene shows an open reading frame of 1,029 bp encoding a polypeptide of 343 amino acids with a putative signal peptide of 22 residues. The F17b-G polypeptide exhibits 95% identity with the F17-G adhesin. The functional homology of the gene products was further confirmed by demonstrating that mutants in the F17-A gene can be complemented by the F17b-A gene and vice versa. These results prove that fimbriae belonging to the F17 family are also found on pathogenic Escherichia coli strains other than enterotoxigenic isolates producing heat-labile or heat-stable enterotoxin.

Cytotoxic necrotizing factor type 2 produced by virulent Escherichia coli modifies the small GTP-binding proteins Rho involved in assembly of actin stress fibers

Cytotoxic necrotizing factor type 2 (CNF2) produced by Escherichia coli strains isolated from intestinal and extraintestinal infections is a dermonecrotic toxin of 110 kDa. We cloned the CNF2 gene from a large plasmid carried by an Escherichia coli strain isolated from a lamb with septicemia. Hydropathy analysis of the deduced amino acid sequence revealed a largely hydrophilic protein with two potential hydrophobic transmembrane domains. The N-terminal half of CNF2 showed striking homology (27% identity and 80% conserved residues) to the N-terminal portion of Pasteurella multocida toxin. Methylamine protection experiments and immunofluorescence studies suggested that CNF2 enters the cytosol of the target cell through an acidic compartment and induces the reorganization of actin into stress fibers. Since the formation of stress fibers in eukaryotic cells involves Rho proteins, we radiolabeled these small GTP-binding proteins from CNF2-treated and control cells with a Rho-specific ADP-ribosyltransferase. The [32P]ADP-ribosylated Rho proteins from CNF2-treated cells migrated slightly more slowly in SDS/PAGE than did the labeled proteins from the control cells. This shift in mobility of Rho proteins in SDS/PAGE was also observed when CNF2 and the RhoA protein were coexpressed in E. coli. We propose that Rho proteins are the targets of CNF2 in mammalian cells.

Serotypes of bovine Escherichia coli producing cytotoxic necrotizing factor type 2 (CNF2)

The serotypes of 101 faecal bovine necrotoxigenic Escherichia coli (NTEC) producing the cytotoxic necrotizing factor type 2 (CNF2) were determined. Although, NTEC producing CNF2 belonged to 48 different O:K:H serotypes, only eleven of them accounted for 54% of strains. The most common serotypes in order of frequency were: O123:K-:H16, O3:K-:H21, O88:K-:H8, O15:K14:H21, O1:K-:H12, O1:K1:H46, O2:K1:H5, O55:H21, O88:K?:H25, O117:K?:H21 and O123:K-:H-. The serotypes of CNF2 NTEC were different from those found in NTEC producing CNF1 and in enterotoxigenic, verotoxigenic, enteropathogenic, enteroinvasive and enteroadherent E. coli strains that cause infections in humans and animals.

Isolation and nucleotide sequence of the gene encoding cytotoxic necrotizing factor 1 of Escherichia coli

Cytotoxic necrotizing factors (CNFs) are dermonecrotic protein toxins produced by human and animal clinical isolates of Escherichia coli. In this study, the CNF1 determinant was isolated and sequenced, showing that expression of biologically active toxin is governed by a unique open reading frame encoding a protein of 1,014 amino acids with a predicted molecular mass of 113.7 kDa. Nucleotide and protein data base searches showed significant homology between CNF1 and the dermonecrotic toxin of Pasteurella multocida. In particular, the two toxins were found to share a hydrophobic region of about 220 amino acids which is a potential membrane-spanning domain.

Comparison of chemical assay, bioassay, enzyme-linked immunosorbent assay, and dot blot hybridization for detection of aerobactin in members of the family Enterobacteriaceae

In order to determine the best strategy for detection of aerobactin in members of the family Enterobacteriaceae, we compared the results of three phenotypic assays, including a chemical assay, a cross-feeding bioassay, and an enzyme-linked immunosorbent assay (ELISA), with the results of a dot blot hybridization assay using a specific probe for the aerobactin genes. The sensitivity and specificity of the ELISA were better than those of the chemical and cross-feeding assays, but the results of dot blot hybridization were the most reproducible. However, none of the Serratia and Enterobacter cloacae strains which produced aerobactin hybridized with the probe. We concluded that the best strategy for aerobactin detection is a two-step procedure that combines screening by dot blot hybridization with an ELISA for negative strains.