Adhesive properties of enteropathogenic Escherichia coli isolated from infants with acute diarrhea in Africa

Neuroinvasive Infection from O117:K52:H-Escherichia colifollowing Acute Pyelonephritis

Spontaneous or nosocomial Escherichia coli meningitis remains rare in healthy adults but is still carrying a high mortality rate despite adapted antimicrobial treatment for susceptible strains. A 39-year-old woman was admitted to the hospital with severe subarachnoid haemorrhage complicated by acute hydrocephalus. On hospital day 10, she developed Streptococcus anginosus septicaemia and urinary tract infection due to a multisensitive strain of E. coli. This infection was successfully controlled by antimicrobial therapy. As a late complication in the neurosurgical ward (day 39), she developed fever, alteration of consciousness, and shock, leading to the diagnosis of bacterial meningitis. The culture of blood, cerebrospinal fluid, and urine grew positive for a multisensitive E. coli. The strain was identified as O117:K52:H, a serotype that was until now never associated with acute meningitis or brain abscesses. The source appeared to be the urinary tract with the demonstration of acute pyelonephritis. The patient died on day 94 from delayed complications of multiple brain abscesses.

Identification of the two glycosyltransferase genes responsible for the difference between Escherichia coli O107 and O117 O-antigens

The O-antigen is one of the most variable Gram-negative cell constituents, and its specificity is important for bacterial niche adaptation. The observed diversity of O-antigen forms is mainly due to genetic variations in O-antigen gene clusters. Less common is a change of gene function due to nucleotide substitution; a new instance of which is reported here. The O-antigens of E. coli O107 and O117 have similar structures differing only in a single sugar residue (GlcNAc in O107 substituted for Glc in O117). These O-antigen gene clusters contain the same set of 11 genes and share 98.6% overall DNA identity. The function of the genes in the gene clusters have been proposed previously, and a glycosyltransferase gene (wclY) with nucleotide polymorphism in each strain was proposed to transfer different sugars in different strains. To identify the gene responsible for the transfer of different sugars, wclY mutants of E. coli O107 and O117 were constructed, and each mutant was complemented with the wclY genes cloned from both O107 and O117. Structural analysis of the O-antigens of the four recombinant strains identified wclY as a Glc-transferase in O117 and a GlcNAc-transferase in O107. The evolutionary relationship of E. coli O107 and O117 O-antigens is also discussed.

Genetic and structural analyses of Escherichia coli O107 and O117 O-antigens

The O-antigen, consisting of many repeats of an oligosaccharide, is an essential component of the lipopolysaccharide on the surface of Gram-negative bacteria. The O-antigen is one of the most variable cell constituents, and different O-antigen forms are almost entirely due to genetic variations in O-antigen gene clusters. In this paper, we present structural and genetic evidence for a close relationship between Escherichia coli O107 and E. coli O117 O antigens. The O-antigen of E. coli O107 has a pentasaccharide repeating unit with the following structure: -->4)-beta-D-GalpNAc-(1-->3)-alpha-L-Rhap-(1-->4)-alpha-D-GlcpNAc-(1-->4)-beta-D-Galp-(1-->3)-alpha-D-GalpNAc-(1-->, which differs from the known repeating unit of E. coli O117 only in the substitution of D-GlcNAc for D-Glc. The O-antigen gene clusters of E. coli O107 and O117 share 98.6% overall DNA identity and contain the same set of genes in the same organization. It is proposed that one cluster was evolved from another via mutations, and the substitution of a few amino acids residues in predicted glycosyltransferases resulted in the functional change of one such protein for transferring different sugars in O107 (D-GlcNAc) and O117 (D-Glc), leading to different O-antigen structures. This is an example of the O-antigen alteration caused by nucleotide mutations, which is less commonly reported for O-antigen variations.

Phylogenetic background of attaching and effacing Escherichia coli isolates from animals

Detection and distribution of eae gene in forty-four attaching and effacing Escherichia coli (AEEC) strains of animal origin were investigated. Association of distinct intimin alleles with phylogenetic background were assessed among strains in comparison with different serogroups. Phylogenetic analysis showed that 31 EHEC/eae+ STEC strains belong to groups A, B1 and E, 13 EPEC strains segregated in B1 and B2. Moreover, group A possessed the eae gamma2/theta type, group B1 the eae beta1, eae kappa, eae zeta, and eae epsilon types, group B2 the eae alpha1, eae alpha2 and eae iota types, while the group E possessed the eae gamma1 type. The presence of numerous eae-types show that EPEC and EHEC/eae+ STEC tested have a high genetic homology within each phylogenetic group.

Sequencing and analysis of the Escherichia coli serogroup O117, O126, and O146 O-antigen gene clusters and development of PCR assays targeting serogroup O117-, O126-, and O146-specific DNA sequences

The O-antigen gene clusters of Escherichia coli serogroups O117, O126, and O146 were sequenced, and 11, 10, and 11 open reading frames (ORFs) were identified, respectively. Genes required for O-antigen sugar biosynthesis, sugar transfer, and sugar processing were identified. Multiplex polymerase chain reaction (PCR) assays were developed targeting the wzx and wzy genes present in the O-antigen gene cluster of these serogroups. The assays were highly serogroup specific when tested against strains belonging to serogroups that were isolated from food, humans, animals, and environmental sources, as well as against representative strains belonging to ca. 165 different E. coli O serogroups and a number of non-E. coli bacteria. Thus, the results demonstrate that the wzx and wzy gene sequences were specific to E. coli O117, O126, and O146 and can be used as diagnostic markers for rapid identification and detection of these serogroups.

Molecular characterization and distribution of genes encoding members of the type III effector nleA family among pathogenic Escherichia coli strains

In this study, we investigated the occurrence of the previously described gene nleA(4795) and variants of nleA, putatively encoding non-locus-of-enterocyte-effacement-encoded type III effector proteins with functions that are unknown. nleA variants were detected in 150 out of 170 enteropathogenic Escherichia coli strains and enterohemorrhagic E. coli strains, two of them being eae negative. Besides the known variants nleA(4795), Z6024, and the espI-like gene, 11 novel nleA variants with different lengths and sequence identities at the deduced amino acid level (between 71% and 96%) have been identified. Whereas most of the serogroups associated with more severe disease were quite homogenous with respect to the presence of a particular nleA variant, other serogroups were not. Moreover, Southern blot hybridization revealed that certain strains carry two copies of nleA in their chromosome, frequently encoding different variants. In most cases, the open reading frame of one of the copies was disrupted, usually by an insertion element. Furthermore, transmission of the type III effector-encoding gene could be shown by transduction of nleA-carrying bacteriophages to a laboratory E. coli strain.

Genetic diversity of intimin genes of attaching and effacing Escherichia coli strains

In this study, we determined the sequences of four intimin variant genes detected in attaching and effacing Escherichia coli isolates of human origin. Three of them were novel and were designated eae-eta (eta), eae-iota (iota), and eae-kappa (kappa). The fourth was identical to the recently described eae-zeta (zeta), isolated from a bovine E. coli O84:NM isolate. We compared these sequences with those of published intimin-alpha, intimin-beta, intimin-gamma1, intimin-gamma2, intimin- epsilon, and intimin-theta alleles. Sequence analysis of these 10 intimin alleles confirmed extensive genetic diversity within the intimin gene family in E. coli. The genetic diversity was more prominent in the 3' region (starting at bp 2,112), which encodes the binding domain of intimin. Phylogenetic analyses revealed four groups of closely related intimin genes: alpha and zeta; beta and kappa; gamma1 and gamma2/theta; and epsilon and eta. Calculation of homoplasy ratios of sequences of the 5' region of eae (positions 1 to 2,111) revealed evidence for intragenic recombination. Split decomposition analysis also indicates that recombination events have played a role in the evolutionary history of eae. In conclusion, we recommend an eae nomenclature system based on the Greek alphabet and provide an updated PCR scheme for amplification and typing of E. coli eae.

Processing of the AIDA-I precursor: removal of AIDAc and evidence for the outer membrane anchoring as a beta-barrel structure

The AIDA-I adhesin known to be responsible for the diffuse adherence (DA) phenotype of the diarrhoeagenic Escherichia coli (DAEC) strain 2787 has been shown previously to be synthesized as a precursor protein and to undergo additional C-terminal processing. Here, the C-terminal processing of the AIDA-I precursor and the outer membrane topology of the cleaved C-terminal fragment, AIDAc, were investigated. By isolation of the cleaved AIDAc fragment and N-terminal sequencing, the C-terminal cleavage site was identified between Ser-846 and Ala-847 thereby indicating a molecular mass of 47.5 kDa for AIDAc. The correct processing to AIDA-I and AIDAc in OmpT, OmpP and DegP protease-deficient E. coli strains as well as in avirulent salmonellae and shigellae points to an autocatalytic cleavage mechanism. The cleaved AIDAc was localized in the outer membrane. A leader sequence-AIDAc fusion was efficiently routed to the outer membrane. Analysis by protease digestion, secondary-structure prediction and modelling, by comparison with structurally related bacterial proteins like the IgA1 protease from neisseria, the vacuolating toxin from Helicobacter pylori, and the VirG protein of Shigella flexneri, strongly indicates that AIDAc is present in the outer membrane as a beta-barrel structure.

Adhesion and its role in the virulence of enteropathogenic Escherichia coli

Enteropathogenic Escherichia coli (EPEC) organisms are an important cause of diarrheal disease in young children. The virulence of EPEC is a multifactorial process and involves a number of distinct stages. Initial adherence to intestinal mucosa is mediated by fimbriae which bring about a distinct form of adhesion, localized adhesion. Intimate adhesion of the bacterium to the eukaryotic membrane occurs, resulting in the activation of signal transduction pathways. Microvilli are disrupted and effaced from the apical membrane which then cups around the organism to form pedestal structures, the attaching and effacing lesion. Diarrhea may be produced by alteration of the permeability of the apical membrane and also through a malabsorption mechanism. The pathways involved in the production of the attaching and effacing lesion are described. EPEC organisms were originally thought to belong to a number of distinct serogroups; it is now apparent that many isolates belonging to these serogroups are not pathogenic or belong to other pathogenic groups of E. coli. In addition, isolates falling outside of these serogroups are considered to be true EPEC. The definition of EPEC based on serotyping is inaccurate and should be replaced by methods that specifically detect the virulence properties of EPEC.

Diffuse adherence of enteropathogenic Escherichia coli strains--processing of AIDA-I

The adherence of pathogenic Escherichia coli to the mucosa of the small intestine is an important step in the development of diarrhoea. To study the molecular basis of the diffuse adherence (DA) pattern of E. coli strains expressing the classical serotypes of enteropathogenic E. coli (EPEC), strain 2787 (O126:H27) was investigated. By expression cloning, a plasmid-derived 6.0 kb DNA fragment was identified which conferred the DA phenotype on recipient K-12 strains. This fragment encoded the 100 kDa adhesin involved in diffuse adherence (AIDA-I) which by mild heat shock treatment was isolated from the surface of the wild-type and recombinant DA-positive strains. Analysis of the entire DNA fragment revealed two open reading frames coding for proteins of 45 kDa and 132 kDa, respectively. The 132 kDa protein has been identified as the AIDA-I precursor protein which after cleavage of the signal sequence undergoes additional C-terminal processing for maturation to AIDA-I. Though the function of the cytoplasmic 45 kDa protein is not known, preliminary evidence indicates that authentic expression of the protein is a prerequisite for the correct processing of the 132 kDa precursor to AIDA-I. The AIDA-I precursor exhibits significant homology to the virG (icsA) protein of Shigella flexneri which apparently plays a major role in the events leading to the intercellular spread of invasive Shigella organisms.

Bacterial diarrhoea

Diarrhoeal disease caused by enteric bacterial pathogens has become less prevalent in industrialized countries, but remains an important cause of morbidity and mortality in developing countries. Although better management of acute diarrhoeal episodes has led to more favourable outcomes, persistent diarrhoea remains a problem for which risk factors are being recognized and associated bacterial pathogens identified. Unusual or intractable diarrhoea should alert health workers to the possibility of impaired immune function, which is associated with a range of enteric pathogens and opportunistic infections. Improved microbiological methods have resulted in more frequent detection of pathogens in association with diarrhoea, as well as greater understanding of pathogenesis. Clinical features of diarrhoeal disease and mechanisms involved in pathogenesis are discussed in relation to specific bacterial enteric pathogens.

AIDA-I, the adhesin involved in diffuse adherence of the diarrhoeagenic Escherichia coli strain 2787 (O126:H27), is synthesized via a precursor molecule

The adherence mechanisms of enteropathogenic Escherichia coli (EPEC) to epithelial cells are still not understood. To study the molecular basis of the diffuse adherence (DA) phenotype exhibited by diarrhoeagenic E. coli expressing classical EPEC serotypes we investigated strain 2787 (O126:H27) isolated from a case of infantile diarrhoea. A 6.0 kb plasmid-derived DNA fragment mediates the DA phenotype and encodes the 100 kDa adhesin protein AIDA-I (adhesin involved in diffuse adherence). Sequencing of the entire fragment revealed two open reading frames which encoded proteins of 45 kDa and 132 kDa, respectively. The 132 kDa protein has been identified as an AIDA-I precursor protein. After cleavage of the signal sequence further processing at the C-terminus of the 132 kDa precursor leads to the mature approximately 100 kDa AIDA-I. While the exact function of the cytoplasmic 45 kDa protein is not known, preliminary evidence indicates that it is necessary for the correct maturation of AIDA-I. The AIDA-I precursor exhibits significant homology with the virG(icsA) protein of Shigella flexneri which seems to be involved in the intercellular spread of invasive Shigella organisms.

New adhesive factor (antigen 8786) on a human enterotoxigenic Escherichia coli O117:H4 strain isolated in Africa

An enterotoxigenic Escherichia coli strain, E. coli 8786, of serotype O117:H4 produced only heat-stable enterotoxin and gave mannose-resistant hemagglutination with human and bovine erythrocytes. The strain adhered to the brush border of human enterocytes and to enterocytelike cell line Caco-2. Adhesion inhibition assays using Caco-2 cells with different adhesive factor extracts showed that the adhesive factor of E. coli 8786 is different from colonization factor antigen I (CFA/I). CFA/II, CFA/III of Darfeuille et al. (A. Darfeuille, B. Lafeuille, B. Joly, and R. Cluzel, Ann. Microbiol. Inst. Pasteur 134A:53-64, 1983), CS6, and antigen 2230. A bacterial surface protein, designated antigen 8786, with a molecular mass of 16,300 Da was responsible for the adhesion to intestinal cells. It was immunologically different from previously described adhesive factors as determined by immunoblotting. Antigen 8786 was detected on the bacterial cell surface and appeared to be nonfimbrial. NH2-terminal analysis of antigen 8786 showed no homology with the previously described adhesive factors. Nevertheless, antigen 8786 is closely related to the NH2-terminal sequence of Salmonella enteritidis fimbrin. A hybridization experiment using a synthetic oligonucleotide probe based on the NH2-terminal amino acid sequence of antigen 8786 revealed that the coding region was located on a 70-MDa plasmid.