The evolution of group-level pathogenic traits

A group-selection model for the evolutionary origin of phase-variation in E. coli is proposed. Populations of commensal strains of E. coli populating mammalian hosts modulate its immune defenses through population-level control of the expression of fimbriae. At any time only a proportion of the population expresses these cell-surface adhesins. Collectively they elicit a host-based nutrient release if the fimbriae expression is low. Too high levels of fimbriation would provoke an inflammatory response and thus intolerable conditions for the cells. The optimal level of fimbriation is a group property and its evolution is difficult to explain by naive individual selection scenarios. This article presents a computational model to simulate the evolution of fimbriae. The two main conclusions of this contribution are: (i) the evolution of this group property requires the population to be partitioned into weakly interacting sub-populations. (ii) Given certain scenarios evolution consistently under-performs, in the sense that it does not find the optimal level of fimbriation.     

Modes of evolution in a parasite-host interaction: dis-entangling factors determining the evolution of regulated fimbriation in E. coli

Escherichia coli expresses type-I fimbriae; these are protrusions from the outer cell wall and have been identified as a virulence factor. They are also expressed by commensal strains of E. coli although (at any one time) only by a small proportion of the population. The orthodox interpretation of this is that fimbriation is regulated so as (i) to trigger a host-based release of nutrients in the form of inflammation signals by slightly activating host defenses and (ii) while avoiding a full scale inflammatory response. This article presents a number of computer simulations of the evolution of fimbriae to scrutinize the evolutionary plausibility of this orthodox view. It turns out that these simulations suggest a revised interpretation of the fimbriae mediated parasite-host interaction. Rather than being a passive victim the host is actively providing a niche that evolutionary favors less virulent parasites. The article closes with a number of testable predictions of this model.     

Grazing protozoa and the evolution of the Escherichia coli O157:H7 Shiga toxin-encoding prophage

Humans play little role in the epidemiology of Escherichia coli O157:H7, a commensal bacterium of cattle. Why then does E. coli O157:H7 code for virulence determinants, like the Shiga toxins (Stxs), responsible for the morbidity and mortality of colonized humans? One possibility is that the virulence of these bacteria to humans is coincidental and these virulence factors evolved for and are maintained for other roles they play in the ecology of these bacteria. Here, we test the hypothesis that the carriage of the Stx-encoding prophage of E. coli O157:H7 increases the rate of survival of E. coli in the presence of grazing protozoa, Tetrahymena pyriformis. In the presence but not the absence of Tetrahymena, the carriage of the Stx-encoding prophage considerably augments the fitness of E. coli K-12 as well as clinical isolates of E. coli O157 by increasing the rate of survival of the bacteria in the food vacuoles of these ciliates. Grazing protozoa in the environment or natural host are likely to play a significant role in the ecology and maintenance of the Stx-encoding prophage of E. coli O157:H7 and may well contribute to the evolution of the virulence of these bacteria to colonize humans.     

Functional analysis of the minor subunits of S fimbrial adhesin (SfaI) in pathogenic Escherichia coli

S fimbrial adhesins I and II (SfaI and II), produced by extraintestinal Escherichia coli pathogens that cause urinary tract infections (UTI) and newborn meningitis (NBM), respectively, mediate bacterial adherence to sialic acid-containing glycoprotein receptors present on host epithelial cells and extracellular matrix. The S fimbrial adhesin complexes consist of four proteins: SfaI-A, the major subunit protein and the minor subunit proteins SfaI-G, SfaI-S and SfaI-H. Sialic acid-specific binding is mediated by the minor subunit protein SfaI-S. In order to determine whether the minor subunit proteins SfaI-G, -S and -H play a role in the modulation of adherence and the degree of fimbriation, a trans-complementation system was developed. A non-adhesive E. coli K-12 derivative, harbouring the sfaI-A gene but lacking sfaI-G, -S and -H, was transformed with sfaI-G, -S or -H. Only SfaI-S was able to increase the degree of fimbriation and to confer adhesion properties on the recombinant E. coli K-12 strains. Amino acid residues in SfaI-S that are involved in modulation of fimbriation as well as in receptor recognition were localized by random and site-directed mutagenesis.     

Type 1 Fimbriae,a Colonization Factor of Uropathogenic Escherichia coli,Are Controlled by the Metabolic Sensor CRP-cAMP

Type 1 fimbriae are a crucial factor for the virulence of uropathogenic Escherichia coli during the first steps of infection by mediating adhesion to epithelial cells. They are also required for the consequent colonization of the tissues and for invasion of the uroepithelium. Here, we studied the role of the specialized signal transduction system CRP-cAMP in the regulation of type 1 fimbriation. Although initially discovered by regulating carbohydrate metabolism, the CRP-cAMP complex controls a major regulatory network in Gram-negative bacteria, including a broad subset of genes spread into different functional categories of the cell. Our results indicate that CRP-cAMP plays a dual role in type 1 fimbriation, affecting both the phase variation process and fimA promoter activity, with an overall repressive outcome on fimbriation. The dissection of the regulatory pathway let us conclude that CRP-cAMP negatively affects FimB-mediated recombination by an indirect mechanism that requires DNA gyrase activity. Moreover, the underlying studies revealed that CRP-cAMP controls the expression of another global regulator in Gram-negative bacteria, the leucine-responsive protein Lrp. CRP-cAMP-mediated repression is limiting the switch from the non-fimbriated to the fimbriated state. Consistently, a drop in the intracellular concentration of cAMP due to altered physiological conditions (e.g. growth in presence of glucose) increases the percentage of fimbriated cells in the bacterial population. We also provide evidence that the repression of type 1 fimbriae by CRP-cAMP occurs during fast growth conditions (logarithmic phase) and is alleviated during slow growth (stationary phase), which is consistent with an involvement of type 1 fimbriae in the adaptation to stress conditions by promoting biofilm growth or entry into host cells. Our work suggests that the metabolic sensor CRP-cAMP plays a role in coupling the expression of type 1 fimbriae to environmental conditions, thereby also affecting subsequent attachment and colonization of host tissues.     

Functional analysis of the minor subunits of S fimbrial adhesion (SfaI) in pathogenic Escherichia coli

S fimbrial adhesins I and II (SfaI and II), produced by extraintestinal Escherichia coli pathogens that cause urinary tract infections (UTI) and newborn meningitis (NBM), respectively, mediate bacterial adherence to sialic acid-containing glycoprotein receptors present on host epithelial cells and extracellular matrix. The S fimbrial adhesin complexes consist of four proteins: SfaI-A, the major subunit protein and the minor subunit proteins SfaI-G, SfaI-S and SfaI-H. Sialic acid-specific binding is mediated by the minor subunit protein SfaI-S. In order to determine whether the minor subunit proteins SfaI-G, -S and -H play a role in the modulation of adherence and the degree of fimbriation, a trans-complementation system was developed. A non-adhesive E. coli K-12 derivative, harbouring the sfaI-A gene but lacking sfaI-G, -S and -H, was transformed with sfaI-G, -S or -H. Only SfaI-S was able to increase the degree of fimbriation and to confer adhesion properties on the recombinant E. coli K-12 strains. Amino acid residues in SfaI-S that are involved in modulation of fimbriation as well as in receptor recognition were localized by random and site-directed mutagenesis. Received: 15 March 1999 / Accepted: 2 November 1999     

Biochemical characteristic of biofilm of uropathogenic Escherichia coli Dr+ strains

Urinary tract infections caused by Escherichia coli are very common health problem in the developed countries. The virulence of the uropathogenic E. coli Dr⁺ IH11128 is determined by Dr fimbriae, which are homopolymeric structures composed of DraE subunits with the DraD protein capping the fiber. In this study, we have analyzed the structural and biochemical properties of biofilms developed by E. coli strains expressing Dr fimbriae with or without the DraD tip subunit and the surface-exposed DraD protein. We have also demonstrated that these E. coli strains form biofilms on an abiotic surface in a nutrient-dependent fashion. We present evidence that Dr fimbriae are necessary during the first stage of bacterial interaction with the abiotic surface. In addition, we reveal that the DraD alone is also sufficient for the initial surface attachment at an even higher level than Dr fimbriae and that chloramphenicol is able to reduce the normal attachment of the analyzed E. coli. The action of chloramphenicol also shows that protein synthesis is required for the early events of biofilm formation. Additionally, we have identified reduced exopolysaccharide coverage in E. coli that express only Dr fimbrial polyadhesins at the cell surface with or without the DraD capping subunit.     

Orientational control is an efficient control mechanism for phase switching in the E. coli fim system

The fim system in E. coli controls the expression of type-1 fimbriae. These are hair-like structures that can be used to attach to host cells. Fimbriation is controlled by a mechanism called "orientational control." We present two families of models for orientational control to understand the details of how it works. We find that the main benefits of orientational control are that (i) it allows rapid adjustment of fimbriation levels in response to a change of environmental conditions while (ii) keeping the overall frequencies with which a cell switches between the fimbriate state and the afimbriate state low. The main reason for the efficiency of orientational control in regulation of fimbriation levels is that it keeps the system far from its steady state.     

Cytolethal Distending Toxin Family Members Are Differentially Affected by Alterations in Host Glycans and Membrane Cholesterol

Cytolethal distending toxins (CDTs) are tripartite protein exotoxins produced by a diverse group of pathogenic Gram-negative bacteria. Based on their ability to induce DNA damage, cell cycle arrest, and apoptosis of cultured cells, CDTs are proposed to enhance virulence by blocking cellular division and/or directly killing epithelial and immune cells. Despite the widespread distribution of CDTs among several important human pathogens, our understanding of how these toxins interact with host cells is limited. Here we demonstrate that CDTs from Haemophilus ducreyi, Aggregatibacter actinomycetemcomitans, Escherichia coli, and Campylobacter jejuni differ in their abilities to intoxicate host cells with defined defects in host factors previously implicated in CDT binding, including glycoproteins, and glycosphingolipids. The absence of cell surface sialic acid sensitized cells to intoxication by three of the four CDTs tested. Surprisingly, fucosylated N-linked glycans and glycolipids, previously implicated in CDT-host interactions, were not required for intoxication by any of the CDTs tested. Finally, altering host-cellular cholesterol, also previously implicated in CDT binding, affected intoxication by only a subset of CDTs tested. The findings presented here provide insight into the molecular and cellular basis of CDT-host interactions.     

Virulence factors of Escherichia coli isolated from urine of diabetic women with asymptomatic bacteriuria: correlation with clinical characteristics

Since Escherichia coli isolated from compromised patients with symptomatic urinary tract infections (UTIs) express fewer virulence factors than those isolated from healthy controls, the question arises whether this is also the case for diabetic patients with asymptomatic bacteriuria (ASB). Polymerase chain reaction (PCR) assays were conducted on 111E. coli strains, isolated from the urine of diabetic women with ASB, using primers for the major subunit A and the G-adhesin (I, II, and III) of P fimbriae, type 1 fimbriae, S fimbriae, afimbrial adhesin, cytotoxic necrotizing factor (CNF), and aerobactin. Phenotypically, hemolysis, mannose-sensitive hemagglutination, mannose-resistant hemagglutination and O:K:H-serotypes were determined. Furthermore, we investigated the associations between virulence factors and patient characteristics (including deterioration of renal function). Type 1 fimbriae were the most prevalent virulence factor (86% by genotyping and 59% phenotypically). Except for a lower prevalence of known uropathogenic O-serotypes, we found the same number of virulence factors in our compromised patient group as listed in the literature in noncompromised patients with ASB. Certain virulence factors (type 1 and S fimbriae and CNF) of the causative E. colicorrelated with the risk of a decline in renal function. In conclusion, the number of virulence factors in E. coli isolated from the urine of diabetic women with ASB are comparable with the results found in other (noncompromised) patients with ASB. Furthermore, certain virulence factors of E. colimight contribute to a decline in renal function.     

Comparison of surface proteomes of enterotoxigenic (ETEC) and commensal Escherichia coli strains

Pathogenesis of enterotoxigenic Escherichia coli (ETEC) infections involves colonization of the small intestine mediated by cell-surface fimbriae (CS) or colonization fimbriae antigens (CFA). However, protection against reinfection of ETEC is also conferred by somatic antigens rather than by virulence factors. To discover ETEC specific somatic antigens, the surface proteome of the ETEC H10406 strain was compared with that of non-pathogenic E. coli K12 strains. In this study, we were using stable isotope labelling with amino acids in cell culture (SILAC) technology for the labelling and relative quantification of surface proteins in order to identify polypeptides that are specifically present on ETEC strains. Outer membrane proteins were isolated, separated by gel electrophoresis, and identified by mass spectrometry. Twenty-three differentially expressed cell-surface polypeptides of ETEC were identified and evaluated by bioinformatics for protein vaccine candidates. The combination of being surface-exposed and present differentially makes these polypeptides highly suitable as targets for antibodies and thus for use in passive or active immunisation/vaccination.     

Survival of Escherichia coli in the environment: fundamental and public health aspects

In this review, our current understanding of the species Escherichia coli and its persistence in the open environment is examined. E. coli consists of six different subgroups, which are separable by genomic analyses. Strains within each subgroup occupy various ecological niches, and can be broadly characterized by either commensalistic or different pathogenic behaviour. In relevant cases, genomic islands can be pinpointed that underpin the behaviour. Thus, genomic islands of, on the one hand, broad environmental significance, and, on the other hand, virulence, are highlighted in the context of E. coli survival in its niches. A focus is further placed on experimental studies on the survival of the different types of E. coli in soil, manure and water. Overall, the data suggest that E. coli can persist, for varying periods of time, in such terrestrial and aquatic habitats. In particular, the considerable persistence of the pathogenic E. coli O157:H7 is of importance, as its acid tolerance may be expected to confer a fitness asset in the more acidic environments. In this context, the extent to which E. coli interacts with its human/animal host and the organism''s survivability in natural environments are compared. In addition, the effect of the diversity and community structure of the indigenous microbiota on the fate of invading E. coli populations in the open environment is discussed. Such a relationship is of importance to our knowledge of both public and environmental health.     

The gene fimU affects expression of Salmonella typhimurium type 1 fimbriae and is related to the Escherichia coli tRNA gene argU

The gene fimU, located on a recombinant plasmid carrying the Salmonella typhimurium type 1 fimbrial gene cluster is closely related to the Escherichia coli tRNA gene argU. The fimU gene complements an E. coli argU mutant that is a P2 lysogen, thereby allowing the phage P4 to grow in this strain but preventing the growth of phage lambda. In addition, fimU was shown to be involved in fimbrial expression since transformants of the E. coli argU mutant could produce fimbriae only in the presence of fimU but not in its absence, whereas in an E. coli argU ⁺ strain fimbriation did not require the fimU gene.     

Role of Type 1 Fimbriae in the Adhesion of Escherichia coli to Salivary Mucin and Secretory Immunoglobulin A

Saliva is known to modulate the adhesion of bacteria in the oral cavity. The present work was performed to assess the effect of salivary components on the adhesion of Escherichia coli to a model oral surface. Several genetically engineered E. coli strains were used to examine the role of type 1 fimbriation in the interaction of these strains with salivary components in solution or adsorbed to hydroxyapatite. High (MG1) and low (MG2) molecular weight salivary mucins, and secretory immunoglobulin A (sIgA), were found to interact with the surface of E. coli, and these interactions were independent of the expression of fimbriae or capsule. In contrast, fimbriated strains of E. coli adhered to a greater extent to saliva-coated synthetic hydroxyapatite (HAP) than did nonfimbriated strains. Testing of salivary components separated by gel filtration chromatography revealed that only high-molecular-weight components promoted adhesion of E. coli to HAP. Additional studies found that purified MG2 and sIgA promoted the adhesion of E. coli to HAP. Expression of type 1 fimbriae enhanced adhesion, while mannose inhibited adhesion of fimbriated strains, to saliva-coated HAP and to HAP coated with MG2 and sIgA. We conclude that salivary MG2 and sIgA may provide receptors for the adhesion of type 1 fimbriated E. coli to oral surfaces. Received: 10 February 1996 / Accepted: 11 March 1996     

Association between the Presence of Class 1 Integrons, Virulence Genes, and Phylogenetic Groups of Escherichia coli Isolates from River Water

Ninety-six class 1 integron-positive and 96 integron-negative Escherichia coli isolates cultured from the water of the Warta River, Poland, were characterized for their phylogenetic group affiliation and for the presence of genes associated with virulence. Most strains belonged to phylogenetic group A, but phylogenetic group affiliation was not related with the presence of integrons. The occurrence of heat-stable toxin gene of enterotoxigenic E. coli, S fimbriae subunit gene sfaS, and siderophore receptor genes, fyuA and iutA, was associated with the presence of class 1 integrons. Moreover, virulence factor score (the total number of virulence-associated genes) was associated with the presence of integrons in groups. The results bring new insight into relations between the presence of integrons in E. coli, virulence traits, as well as phylogenetic group affiliation.