Contribution of FliC to epithelial cell invasion by enterohemorrhagic Escherichia coli O113:H21

Pathogenomes and variations in Shiga toxin production among geographically distinct clones of Escherichia coli O113:H21

Infections with globally disseminated Shiga toxin-producing Escherichia coli (STEC) of the O113:H21 serotype can progress to severe clinical complications, such as hemolytic uremic syndrome (HUS). Two phylogeographically distinct clonal complexes have been established by multi locus sequence typing (MLST). Infections with ST-820 isolates circulating exclusively in Australia have caused severe human disease, such as HUS. Conversely, ST-223 isolates prevalent in the US and outside Australia seem to rarely cause severe human disease but are frequent contaminants. Following a genomic epidemiology approach, we wanted to gain insights into the underlying cause for this disparity. We examined the plasticity in the genome make-up and Shiga toxin production in a collection of 20 ST-820 and ST-223 strains isolated from produce, the bovine reservoir, and clinical cases. STEC are notorious for assembly into fragmented draft sequences when using short-read sequencing technologies due to the extensive and partly homologous phage complement. The application of long-read technology (LRT) sequencing yielded closed reference chromosomes and plasmids for two representative ST-820 and ST-223 strains. The established high-resolution framework, based on whole genome alignments, single nucleotide polymorphism (SNP)-typing and MLST, includes the chromosomes and plasmids of other publicly available O113:H21 sequences and allowed us to refine the phylogeographical boundaries of ST-820 and ST-223 complex isolates and to further identify a historic non-shigatoxigenic strain from Mexico as a quasi-intermediate. Plasmid comparison revealed strong correlations between the strains' featured pO113 plasmid genotypes and chromosomally inferred ST, which suggests coevolution of the chromosome and virulence plasmids. Our pathogenicity assessment revealed statistically significant differences in the Stx_2a-production capabilities of ST-820 as compared to ST-223 strains under RecA-induced Stx phage mobilization, a condition that mimics Stx-phage induction. These observations suggest that ST-820 strains may confer an increased pathogenic potential in line with the strain-associated epidemiological metadata. Still, some of the tested ST-223 cultures sourced from contaminated produce or the bovine reservoir also produced Stx at levels comparable to those of ST-820 isolates, which calls for awareness and for continued surveillance of this lineage.

Epidemic HI2 Plasmids Mobilising the Carbapenemase Gene blaIMP-4 in Australian Clinical Samples Identified in Multiple Sublineages of Escherichia coli ST216 Colonising Silver Gulls

Escherichia coli ST216, including those that carry bla_KPC-2, bla_FOX-5, bla_CTX-M-15 and mcr-1, have been linked to wild and urban-adapted birds and the colonisation of hospital environments causing recalcitrant, carbapenem-resistant human infections. Here we sequenced 22 multiple-drug resistant ST216 isolates from Australian silver gull chicks sampled from Five Islands, of which 21 carried nine or more antibiotic resistance genes including bla_IMP-4 (n = 21), bla_TEM-1b (n = 21), aac(3)-IId (n = 20), mph(A) (n = 20), catB3 (n = 20), sul1 (n = 20), aph(3”)-Ib (n = 18) and aph(6)-Id (n = 18) on FIB(K) (n = 20), HI2-ST1 (n = 11) and HI2-ST3 (n = 10) plasmids. We show that (i) all HI2 plasmids harbour bla_IMP-4 in resistance regions containing In809 flanked by IS26 (HI2-ST1) or IS15DI (HI2-ST3) and diverse metal resistance genes; (ii) HI2-ST1 plasmids are highly related to plasmids reported in diverse Enterobacteriaceae sourced from humans, companion animals and wildlife; (iii) HI2 were a feature of the Australian gull isolates and were not observed in international ST216 isolates. Phylogenetic analyses identified close relationships between ST216 from Australian gull and clinical isolates from overseas. E. coli ST216 from Australian gulls harbour HI2 plasmids encoding resistance to clinically important antibiotics and metals. Our studies underscore the importance of adopting a one health approach to AMR and pathogen surveillance.

Molecular determinants of surface colonisation in diarrhoeagenic Escherichia coli (DEC): from bacterial adhesion to biofilm formation

Escherichia coli is primarily known as a commensal colonising the gastrointestinal tract of infants very early in life but some strains being responsible for diarrhoea, which can be especially severe in young children. Intestinal pathogenic E. coli include six pathotypes of diarrhoeagenic E. coli (DEC), namely, the (i) enterotoxigenic E. coli, (ii) enteroaggregative E. coli, (iii) enteropathogenic E. coli, (iv) enterohemorragic E. coli, (v) enteroinvasive E. coli and (vi) diffusely adherent E. coli. Prior to human infection, DEC can be found in natural environments, animal reservoirs, food processing environments and contaminated food matrices. From an ecophysiological point of view, DEC thus deal with very different biotopes and biocoenoses all along the food chain. In this context, this review focuses on the wide range of surface molecular determinants acting as surface colonisation factors (SCFs) in DEC. In the first instance, SCFs can be broadly discriminated into (i) extracellular polysaccharides, (ii) extracellular DNA and (iii) surface proteins. Surface proteins constitute the most diverse group of SCFs broadly discriminated into (i) monomeric SCFs, such as autotransporter (AT) adhesins, inverted ATs, heat-resistant agglutinins or some moonlighting proteins, (ii) oligomeric SCFs, namely, the trimeric ATs and (iii) supramolecular SCFs, including flagella and numerous pili, e.g. the injectisome, type 4 pili, curli chaperone-usher pili or conjugative pili. This review also details the gene regulatory network of these numerous SCFs at the various stages as it occurs from pre-transcriptional to post-translocational levels, which remains to be fully elucidated in many cases.

Plasmids from Shiga Toxin-Producing Escherichia coli Strains with Rare Enterohemolysin Gene (ehxA) Subtypes Reveal Pathogenicity Potential and Display a Novel Evolutionary Path

Most Shiga toxin-producing Escherichia coli (STEC) strains associated with severe disease, such as hemolytic-uremic syndrome (HUS), carry large enterohemolysin-encoding (ehxA) plasmids, e.g., pO157 and pO103, that contribute to STEC clinical manifestations. Six ehxA subtypes (A through F) exist that phylogenetically cluster into eae-positive (B, C, F), a mix of eae-positive (E) and eae-negative (A), and a third, more distantly related, cluster of eae-negative (D) STEC strains. While subtype B, C, and F plasmids share a number of virulence traits that are distinct from those of subtype A, sequence data have not been available for subtype D and E plasmids. Here, we determined and compared the genetic composition of four subtype D and two subtype E plasmids to establish their evolutionary relatedness among ehxA subtypes and define their potential role in pathogenicity. We found that subtype D strains carry one exceptionally large plasmid (>200 kbp) that carries a variety of virulence genes that are associated with enterotoxigenic and enterohemorrhagic E. coli, which, quite possibly, enables these strains to cause disease despite being food isolates. Our data offer further support for the hypothesis that this subtype D plasmid represents a novel virulence plasmid, sharing very few genetic features with other plasmids; we conclude that these plasmids have evolved from a different evolutionary lineage than the plasmids carrying the other ehxA subtypes. In contrast, the 50-kbp plasmids of subtype E (pO145), although isolated from HUS outbreak strains, carried only few virulence-associated determinants, suggesting that the clinical presentation of subtype E strains is largely a result of chromosomally encoded virulence factors.

IMPORTANCE

Bacterial plasmids are known to be key agents of change in microbial populations, promoting the dissemination of various traits, such as drug resistance and virulence. This study determined the genetic makeup of virulence plasmids from rare enterohemolysin subtype D and E Shiga toxin-producing E. coli strains. We demonstrated that ehxA subtype D plasmids represent a novel E. coli virulence plasmid, and although subtype D plasmids were derived from nonclinical isolates, they encoded a variety of virulence determinants that are associated with pathogenic E. coli In contrast, subtype E plasmids, isolated from strains recovered from severely ill patients, carry only a few virulence determinants. The results of this study reemphasize the plasticity and vast diversity among E. coli plasmids. This work demonstrates that, although E. coli strains of certain serogroups may not be frequently associated with disease, they should not be underestimated in protecting human health and food safety.

Flagellar Cap Protein FliD Mediates Adherence of Atypical Enteropathogenic Escherichia coli to Enterocyte Microvilli

The expression of flagella correlates with different aspects of bacterial pathogenicity, ranging from adherence to host cells to activation of inflammatory responses by the innate immune system. In the present study, we investigated the role of flagella in the adherence of an atypical enteropathogenic Escherichia coli (aEPEC) strain (serotype O51:H40) to human enterocytes. Accordingly, isogenic mutants deficient in flagellin (FliC), the flagellar structural subunit; the flagellar cap protein (FliD); or the MotAB proteins, involved in the control of flagellar motion, were generated and tested for binding to differentiated Caco-2 cells. Binding of the aEPEC strain to enterocytes was significantly impaired in strains with the fliCa nd fliD genes deleted, both of which could not form flagella on the bacterial surface. A nonmotile but flagellated MotAB mutant also showed impaired adhesion to Caco-2 cells. In accordance with these observations, adhesion of a EPEC strain 1711-4 to Caco-2 cells was drastically reduced after the treatment of Caco-2 cells with purified FliD. In addition, incubation of a EPEC bacteria with specific anti-FliD serum impaired binding to Caco-2 cells. Finally, incubation of Caco-2 cells with purified FliD, followed by immunolabeling, showed that the protein was specifically bound to the microvillus tips of differentiated Caco-2 cells. The a EPEC FliD or anti-FliD serum also reduced the adherence of prototype typical enteropathogenic, enterohemorrhagic, and enterotoxigenic E. coli strains to Caco-2 cells. In conclusion, our findings further strengthened the role of flagella in the adherence of a EPEC to human enterocytes and disclosed the relevant structural and functional involvement of FliD in the adhesion process.

The role of the bacterial flagellum in adhesion and virulence

The bacterial flagellum is a complex apparatus assembled of more than 20 different proteins. The flagellar basal body traverses the cell wall, whereas the curved hook connects the basal body to the whip-like flagellar filament that protrudes several µm from the bacterial cell. The flagellum has traditionally been regarded only as a motility organelle, but more recently it has become evident that flagella have a number of other biological functions. The major subunit, flagellin or FliC, of the flagellum plays a well-documented role in innate immunity and as a dominant antigen of the adaptive immune response. Importantly, flagella have also been reported to function as adhesins. Whole flagella have been indicated as significant in bacterial adhesion to and invasion into host cells. In various pathogens, e.g., Escherichia coli, Pseudomonas aeruginosa and Clostridium difficile, flagellin and/or the distally located flagellar cap protein have been reported to function as adhesins. Recently, FliC of Shiga-toxigenic E. coli was shown to be involved in cellular invasion via lipid rafts. Here, we examine the latest or most important findings regarding flagellar adhesive and invasive properties, especially focusing on the flagellum as a potential virulence factor.

Recent advances in understanding enteric pathogenic Escherichia coli

Although Escherichia coli can be an innocuous resident of the gastrointestinal tract, it also has the pathogenic capacity to cause significant diarrheal and extraintestinal diseases. Pathogenic variants of E. coli (pathovars or pathotypes) cause much morbidity and mortality worldwide. Consequently, pathogenic E. coli is widely studied in humans, animals, food, and the environment. While there are many common features that these pathotypes employ to colonize the intestinal mucosa and cause disease, the course, onset, and complications vary significantly. Outbreaks are common in developed and developing countries, and they sometimes have fatal consequences. Many of these pathotypes are a major public health concern as they have low infectious doses and are transmitted through ubiquitous mediums, including food and water. The seriousness of pathogenic E. coli is exemplified by dedicated national and international surveillance programs that monitor and track outbreaks; unfortunately, this surveillance is often lacking in developing countries. While not all pathotypes carry the same public health profile, they all carry an enormous potential to cause disease and continue to present challenges to human health. This comprehensive review highlights recent advances in our understanding of the intestinal pathotypes of E. coli.

Contribution of flagellin subunit FliC to piglet epithelial cells invasion by F18ab E. coli

Flagellar structures contribute to the virulence of multiple gastrointestinal pathogens either as the effectors of motility, as adhesins, or as a secretion apparatus for virulence factors. Escherichia coli F18ab variant strains are associated with edema disease (ED) in pig industries worldwide. These strains use flagella to increase the efficiency of epithelial cell invasion. In this study, we aimed to elucidate the mechanism by which flagella contribute to F18ab E. coli invasion. To explore the role of flagella in the invasion process, we performed invasion assays with either flagellated and motile, flagellated but non-motile, or non-flagellated non-motile bacteria. We observed that flagellated but non-motile bacteria invade piglet epithelial cells even more efficiently than the parent wild-type (WT) strain in vitro. By contrast, the non-flagellated bacteria have significantly reduced invasion as compared with the parent strain. These results demonstrate that flagella function mainly as adhesins to enhance the ability of F18ab E. coli to target piglet epithelial cells.

Distribution of pathogenicity islands OI-122, OI-43/48, and OI-57 and a high-pathogenicity island in Shiga toxin-producing Escherichia coli

Pathogenicity islands (PAIs) play an important role in Shiga toxin-producing Escherichia coli (STEC) pathogenicity. The distribution of PAIs OI-122, OI-43/48, and OI-57 and a high-pathogenicity island (HPI) were determined among 98 STEC strains assigned to seropathotypes (SPTs) A to E. PCR and PCR-restriction fragment length polymorphism assays were used to identify 14 virulence genes that belonged to the four PAIs and to subtype eae and stx genes, respectively. Phylogenetic trees were constructed based on the sequences of pagC among 34 STEC strains and iha among 67 diverse pathogenic E. coli, respectively. Statistical analysis demonstrated that the prevalences of OI-122 (55.82%) and OI-57 (82.35%) were significantly greater in SPTs (i.e., SPTs A, B, and C) that are frequently associated with severe disease than in other SPTs. terC (62.5%) and ureC (62.5%) in OI-43/48 were also significantly more prevalent in SPTs A, B, and C than in SPTs D and E. In addition, OI-122, OI-57, and OI-43/48 and their associated virulence genes (except iha) were found to be primarily associated with eae-positive STEC, whereas HPI occurred independently of the eae presence. The strong association of OI-122, OI-43/48, and OI-57 with eae-positive STEC suggests in part that different pathogenic mechanisms exist between eae-positive and eae-negative STEC strains. Virulence genes in PAIs that are associated with severe diseases can be used as potential markers to aid in identifying highly virulent STEC.

Unique secreted-surface protein complex of Lactobacillus rhamnosus, identified by phage display

Proteins are the most diverse structures on bacterial surfaces; hence, they are candidates for species- and strain-specific interactions of bacteria with the host, environment, and other microorganisms. Genomics has decoded thousands of bacterial surface and secreted proteins, yet the function of most cannot be predicted because of the enormous variability and a lack of experimental data that would allow deduction of function through homology. Here, we used phage display to identify a pair of interacting extracellular proteins in the probiotic bacterium Lactobacillus rhamnosus HN001. A secreted protein, SpcA, containing two bacterial immunoglobulin-like domains type 3 (Big-3) and a domain distantly related to plant pathogen response domain 1 (PR-1-like) was identified by screening of an L. rhamnosus HN001 library using HN001 cells as bait. The SpcA-"docking" protein, SpcB, was in turn detected by another phage display library screening, using purified SpcA as bait. SpcB is a 3275-residue cell-surface protein that contains general features of large glycosylated Serine-rich adhesins/fibrils from gram-positive bacteria, including the hallmark signal sequence motif KxYKxGKxW. Both proteins are encoded by genes within a L. rhamnosus-unique gene cluster that distinguishes this species from other lactobacilli. To our knowledge, this is the first example of a secreted-docking protein pair identified in lactobacilli.

Shiga toxin-producing Escherichia coli serotype O78:H(-) in family, Finland, 2009

STEC carrying stx_1c and hlyA genes can invade the human bloodstream.

Shiga toxin–producing Escherichia coli (STEC) is a pathogen that causes gastroenteritis and bloody diarrhea but can lead to severe disease, such as hemolytic uremic syndrome (HUS). STEC serotype O78:H^– is rare among humans, and infections are often asymptomatic. We detected a sorbitol-fermenting STEC O78:H^–stx_1c:hlyA in blood and fecal samples of a 2-week-old boy who had bacteremia and HUS and in fecal samples of his asymptomatic family members. The phenotypic and genotypic characteristics and the virulence properties of this invasive STEC were investigated. Our findings demonstrate that contrary to earlier suggestions, STEC under certain conditions can invade the human bloodstream. Moreover, this study highlights the need to implement appropriate diagnostic methods for identifying the whole spectrum of STEC strains associated with HUS.

Role of lipid rafts and flagellin in invasion of colonic epithelial cells by Shiga-toxigenic Escherichia coli O113:H21

Shiga-toxigenic Escherichia coli (STEC) O113:H21 strains that lack the locus of enterocyte effacement (LEE) efficiently invade eukaryotic cells in vitro, unlike LEE-positive O157:H7 strains. We used a fliC deletion mutant of the O113:H21 STEC strain 98NK2 (98NK2ΔfliC) to show that invasion of colonic epithelial (HCT-8) cells is heavily dependent on production of flagellin, even though adherence to the cells was actually enhanced in the mutant. Flagellin binds and signals through Toll-like receptor 5 (TLR5), but there was no evidence that either TLR5, the adaptor protein myeloid differentiation primary response gene 88 (MyD88), or the serine kinase interleukin-1 receptor-associated kinase (IRAK) were required for invasion of HCT-8 cells by strain 98NK2, as judged by transfection, RNA knockdown, or inhibitor studies. However, pretreatment of cells with anti-asialo-GM1 significantly decreased 98NK2 invasion (by 40.8%), while neuraminidase treatment (which cleaves terminal sialic acid residues, thus converting GM1 into asialo-GM1) significantly increased invasion (by 70.7%). Pretreatment of HCT-8 cells with either the cholesterol-depleting agent methyl-β-cyclodextrin (MβCD) or the tyrosine kinase inhibitor genistein significantly decreased invasion by 98NK2, indicating a potential role for lipid rafts in the invasion mechanism. Confocal microscopy also showed invading 98NK2 colocalized with lipid raft markers caveolin-1 and GM1. Interestingly, anti-asialo-GM1, neuraminidase, MβCD, and genistein have similar effects on the vestigial level of STEC invasion seen for STEC strain 98NK2ΔfliC, indicating that lipid rafts mediate a common step in flagellin-dependent and flagellin-independent cellular invasion.

The fimbriae activator MatA switches off motility in Escherichia coli by repression of the flagellar master operon flhDC

Flagella provide advantages to Escherichia coli by facilitating taxis towards nutrients and away from unfavourable niches. On the other hand, flagellation is an energy sink to the bacterial cell, and flagella also stimulate host innate inflammatory responses against infecting bacteria. The flagellar assembly pathway is ordered and under a complex regulatory circuit that involves three classes of temporally regulated promoters as well as the flagellar master regulator FlhD(4)C(2). We report here that transcription of the flhDC operon from the class 1 promoter is under negative regulation by MatA, a key activator of the common mat (or ecp) fimbria operon that enhances biofilm formation by E. coli. Ectopic expression of MatA completely precluded motility and flagellar synthesis in the meningitis-associated E. coli isolate IHE 3034. Northern blotting, analysis of chromosomal promoter-lacZ fusions and electrophoretic mobility shift assays revealed an interaction between MatA and the flhDC promoter region that apparently repressed flagellum biosynthesis. However, inactivation of matA in the chromosome of IHE 3034 had only a minor effect on flagellation, which underlines the complexity of regulatory signals that promote flagellation in E. coli. We propose that the opposite regulatory actions of MatA on mat and on flhDC promoters advance the adaptation of E. coli from a planktonic to an adhesive lifestyle.

Flagella and bacterial pathogenicity

As locomotive organelles, flagella allow bacteria to move toward favorable environments. A flagellum consists of three parts: the basal structure (rotary motor), the hook (universal joint), and the filament (helical propeller). For ages, flagella have been generally regarded as important virulence factors, mainly because of their motility property. However, flagella are getting recognized to play multiple roles with more functions besides motility and chemotaxis. Recent evidence has pinpointed that the bacterial flagella participate in many additional processes including adhesion, biofilm formation, virulence factor secretion, and modulation of the immune system of eukaryotic cells. This mini-review summarizes data from recent studies that elucidated how flagella, as a virulence factor, contribute to bacterial pathogenicity.

Distinct Interaction of Two Atypical Enteropathogenic Escherichia coli Strains with Enterocytes In Vitro

Typical and atypical Enteropathogenic Escherichia coli (EPEC) promote attaching-effacing lesions in intestinal cells but only typical EPEC carry the EPEC adherence factor plasmid. Atypical EPEC (aEPEC) are emerging agents of acute and persistent diarrhea worldwide. We aimed at comparing the ability of two aEPEC strains, 1711-4 (serotype O51:H40) and 3991-1 (serotype O non-typeable:non-motile) to invade, persist inside Caco-2 and T84 cells, and to induce IL-8 production. Typical EPEC strain E2348/69 was used for comparisons. The strains associated more significantly with T84 than with Caco-2 cells, with 3991-1 being the most adherent (P < 0.001). In contrast, aEPEC 1711-4 was significantly more invasive than the other strains in both cell lines, and was found within vacuoles near the basolateral cell surfaces. Strains persisted within both cell lines for at least 48 hours, but the persistence index was higher for 3991-1 in Caco-2 cells. IL-8 production was significantly higher from Caco-2 cells infected with 1711-4 for at least 48 hours (P < 0.001), and from T84 cells after 24 and 48 h than with the other strains (P = 0.001). We demonstrated that aEPEC are heterogeneous in various aspects of their interaction with enterocytes in vitro.

Shiga toxin 2 and flagellin from shiga-toxigenic Escherichia coli superinduce interleukin-8 through synergistic effects on host stress-activated protein kinase activation

Shiga toxins expressed in the intestinal lumen during infection with Shiga-toxigenic Escherichia coli must translocate across the epithelium and enter the systemic circulation to cause systemic (pathological) effects, including hemolytic uremic syndrome. The transepithelial migration of polymorphonuclear leukocytes in response to chemokine expression by intestinal epithelial cells is thought to promote uptake of Stx from the intestinal lumen by compromising the epithelial barrier. In the present study, we investigated the hypothesis that flagellin acts in conjunction with Shiga toxin to augment this chemokine expression. We investigated the relative contributions of nuclear factor kappaB (NF-kappaB) and mitogen-activated protein kinase (MAPK) signaling to transcription and translation of interleukin-8. Using reporter gene constructs, we showed that flagellin-mediated interleukin-8 gene transcription is heavily dependent on both NF-kappaB and extracellular signal-regulated kinase 1 and 2 (ERK-1/2) activation. In contrast, inhibition of p38 has no detectable effect on interleukin-8 gene transcription, even though flagellin-mediated activation of host p38 is critical for maximal interleukin-8 protein expression. Inhibition of MAPK-interacting kinase 1 suggests that p38 signaling affects the posttranscriptional regulation of interleukin-8 protein expression induced by flagellin. Cotreatment with Stx2 and flagellin results in a synergistic upregulation of c-Jun N-terminal protein kinases (JNKs), p38 activation, and a superinduction of interleukin-8 mRNA. This synergism was also evident at the protein level, with increased interleukin-8 protein detectable following cotreatment with flagellin and Stx2. We propose that flagellin, in conjunction with Shiga toxin, synergistically upregulates stress-activated protein kinases, resulting in superinduction of interleukin-8 and, ultimately, absorption of Stx into the systemic circulation.

Identification of the long polar fimbriae gene variants in the locus of enterocyte effacement-negative Shiga toxin-producing Escherichia coli strains isolated from humans and cattle in Argentina

The long polar fimbriae (Lpf) is one of few adhesive factors of Shiga toxin-producing Escherichia coli (STEC) and it is associated with colonization of the intestine. Studies have demonstrated the presence of lpf genes in several pathogenic E. coli strains, and classification of variants based on polymorphisms in the lpfA1 and lpfA2 genes has been adopted. Using a collection of Argentinean locus of enterocyte effacement (LEE)-negative STEC strains, we determined that the different lpfA types were present in a wide variety of serotypes with no apparent association between the types of lpfA1 or lpfA2 genes and the severity of human disease. The lpfA2-1 was the most prevalent variant identified, which was present in 95.8% of the isolates, and lpfA1-3 and lpfA2-2, proposed as specific biomarkers of E. coli O157:H7, were not found in any of the serotypes studied. The prevalence of lpf genes in a large number of strains is useful to understand the genetic diversity of LEE-negative STEC and to define the association of some of these isolates carrying specific lpf-variants with disease.

Pathogenicity islands PAPI-1 and PAPI-2 contribute individually and synergistically to the virulence of Pseudomonas aeruginosa strain PA14

Pseudomonas aeruginosa is a leading cause of hospital-acquired pneumonia and severe chronic lung infections in cystic fibrosis patients. The reference strains PA14 and PAO1 have been studied extensively, revealing that PA14 is more virulent than PAO1 in diverse infection models. Among other factors, this may be due to two pathogenicity islands, PAPI-1 and PAPI-2, both present in PA14 but not in PAO1. We compared the global contributions to virulence of PAPI-1 and PAPI-2, rather than that of individual island-borne genes, using murine models of acute pneumonia and bacteremia. Three isogenic island-minus mutants (PAPI-1-minus, PAPI-2-minus, and PAPI-1-minus, PAPI-2-minus mutants) were compared with the wild-type parent strain PA14 and with PAO1. Our results showed that both islands contributed significantly to the virulence of PA14 in acute pneumonia and bacteremia models. However, in contrast to the results for the bacteremia model, where each island was found to contribute individually, loss of the 108-kb PAPI-1 island alone was insufficient to measurably attenuate the mutant in the acute pneumonia model. Nevertheless, the double mutant was substantially more attenuated, and exhibited a lesser degree of virulence, than even PAO1 in the acute pneumonia model. In particular, its ability to disseminate from the lungs to the bloodstream was markedly inhibited. We conclude that both PAPI-1 and PAPI-2 contribute directly and synergistically in a major way to the virulence of PA14, and we suggest that analysis of island-minus strains may be a more appropriate way than individual gene knockouts to assess the contributions to virulence of large, horizontally acquired segments of DNA.

The flagella of an atypical enteropathogenic Escherichia coli strain are required for efficient interaction with and stimulation of interleukin-8 production by enterocytes in vitro

The ability of some typical enteropathogenic Escherichia coli (EPEC) strains to adhere to, invade, and increase interleukin-8 (IL-8) production in intestinal epithelial cells in vitro has been demonstrated. However, few studies regarding these aspects have been performed with atypical EPEC (aEPEC) strains, which are emerging enteropathogens in Brazil. In this study, we evaluated a selected aEPEC strain (1711-4) of serotype O51:H40, the most prevalent aEPEC serotype in Brazil, in regard to its ability to adhere to and invade Caco-2 and T84 cells and to elicit IL-8 production in Caco-2 cells. The role of flagella in aEPEC 1711-4 adhesion, invasion, and IL-8 production was investigated by performing the same experiments with an isogenic aEPEC mutant unable to produce flagellin (FliC), the flagellum protein subunit. We demonstrated that this mutant (fliC mutant) had a marked decrease in the ability to adhere to T84 cells and invade both T84 and Caco-2 cells in gentamicin protection assays and by transmission electron microscopy. In addition, the aEPEC 1711-4 fliC mutant had a reduced ability to stimulate IL-8 production by Caco-2 cells in early (3-h) but not in late (24-h) infections. Our findings demonstrate that flagella of aEPEC 1711-4 are required for efficient adhesion, invasion, and early but not late IL-8 production in intestinal epithelial cells in vitro.

Interaction of Escherichia coli O157:H7 with leafy green produce

Enterohemorrhagic Escherichia coli (EHEC) is a foodborne pathogen responsible for human diarrheal disease. EHEC lives in the intestinal tract of cattle and other farm and wild animals, which may be the source of environmental contamination particularly of agricultural fields. Human infections are associated with consumption of tainted animal products and fresh produce. How the bacteria interact with the plant phyllosphere and withstand industrial decontamination remain to be elucidated. The goals of the present study were to investigate the environmental conditions and surface structures that influence the interaction of EHEC O157:H7 with baby spinach and lettuce leaves in vitro. Independently of the production of Shiga toxin, EHEC O157:H7 colonizes the leaf surface via flagella and the type 3 secretion system (T3SS). Ultrastructural analysis of EHEC-infected leafy greens revealed the presence of flagellated bacteria, and mutation of the fliC flagellin gene in EHEC EDL933 rendered the bacteria significantly less adherent, suggesting the involvement of flagella in the bacteria-leaf interaction. EDL933 mutated in the escN (ATPase) gene associated with the function of the T3SS but not in the eae (intimin adhesin) gene required for adherence to host intestinal cells had significantly reduced adherence compared with that of the parental strain. The data suggest a compelling role of flagella and the T3SS in colonization of leafy green produce. Colonization of salad leaves by EHEC strains may be a strategy that ensures survival of these bacteria in the environment and allows transmission to the human host.

Shiga toxin-producing Escherichia coli strains negative for locus of enterocyte effacement

Most Shiga toxin-producing Escherichia coli (STEC) infections that are associated with severe sequelae such as hemolytic uremic syndrome (HUS) are caused by attaching and effacing pathogens that carry the locus of enterocyte effacement (LEE). However, a proportion of STEC isolates that do not carry LEE have been associated with HUS. To clarify the emergence of LEE-negative STEC, we compared the genetic composition of the virulence plasmids pO113 and pO157 from LEE-negative and LEE-positive STEC, respectively. The complete nucleotide sequence of pO113 showed that several plasmid genes were shared by STEC O157:H7. In addition, allelic profiling of the ehxA gene demonstrated that pO113 belongs to a different evolutionary lineage than pO157 and that the virulence plasmids of LEE-negative STEC strains were highly related. In contrast, multilocus sequence typing of 17 LEE-negative STEC isolates showed several clonal groups, suggesting that pathogenic LEE-negative STEC has emerged several times throughout its evolution.

Shiga toxin-producing Escherichia coli strains negative for locus of enterocyte effacement

Most Shiga toxin-producing Escherichia coli (STEC) infections that are associated with severe sequelae such as hemolytic uremic syndrome (HUS) are caused by attaching and effacing pathogens that carry the locus of enterocyte effacement (LEE). However, a proportion of STEC isolates that do not carry LEE have been associated with HUS. To clarify the emergence of LEE-negative STEC, we compared the genetic composition of the virulence plasmids pO113 and pO157 from LEE-negative and LEE-positive STEC, respectively. The complete nucleotide sequence of pO113 showed that several plasmid genes were shared by STEC O157:H7. In addition, allelic profiling of the ehxA gene demonstrated that pO113 belongs to a different evolutionary lineage than pO157 and that the virulence plasmids of LEE-negative STEC strains were highly related. In contrast, multilocus sequence typing of 17 LEE-negative STEC isolates showed several clonal groups, suggesting that pathogenic LEE-negative STEC has emerged several times throughout its evolution.

An investigation of the expression and adhesin function of H7 flagella in the interaction of Escherichia coli O157 : H7 with bovine intestinal epithelium

Enterohaemorrhagic Escherichia coli O157 : H7 is a bacterial pathogen that can cause haemorrhagic colitis and haemolytic uremic syndrome. In the primary reservoir host, cattle, the terminal rectum is the principal site of E. coli O157 colonization. In this study, bovine terminal rectal primary epithelial cells were used to examine the role of H7 flagella in epithelial adherence. Binding of a fliC(H7) mutant O157 strain to rectal epithelium was significantly reduced as was binding of the flagellated wild-type strain following incubation with H7-specific antibodies. Complementation of fliC(H7) mutant O157 strain with fliC(H7) restored the adherence to wild-type levels; however, complementation with fliC(H6) did not restore it. High-resolution ultrastructural and imunofluorescence studies demonstrated the presence of abundant flagella forming physical contact points with the rectal epithelium. Binding to terminal rectal epithelium was specific to H7 by comparison with other flagellin types tested. In-cell Western assays confirmed temporal expression of flagella during O157 interaction with epithelium, early expression was suppressed during the later stages of microcolony and attaching and effacing lesion formation. H7 flagella are expressed in vivo by individual bacteria in contact with rectal mucosa. Our data demonstrate that the H7 flagellum acts as an adhesin to bovine intestinal epithelium and its involvement in this crucial initiating step for colonization indicates that H7 flagella could be an important target in intervention strategies.

The type 4 pili of enterohemorrhagic Escherichia coli O157:H7 are multipurpose structures with pathogenic attributes

Enterohemorrhagic Escherichia coli (EHEC) O157:H7 produces long bundles of polar type 4 pili (T4P) called HCP (for hemorrhagic coli pili) that form physical bridges between bacteria associating with human and animal epithelial cells. Here, we sought to further investigate whether HCP possessed other pathogenicity attributes associated with T4P production. Comparative studies performed with wild-type EHEC EDL933 and an isogenic hcpA mutant revealed that HCP play different roles in the biology of this organism. We found that in addition to promoting bacterial attachment to host cells, HCP mediate (i) invasion of epithelial cells, (ii) hemagglutination of rabbit erythrocytes, (iii) interbacterial connections conducive to biofilm formation, (iv) specific binding to host extracellular matrix proteins laminin and fibronectin but not collagen, and (v) twitching motility. Nonadherent laboratory E. coli strain HB101 complemented with hcpABC genes on plasmid pJX22, which specifies for HCP overproduction in EDL933, became hyperadherent and invasive and produced a thick biofilm, suggesting that the presence of HCP confers HB101(pJX22) new attributes otherwise not exhibited by HB101. Analogous to other bacteria in which T4P are involved in the pathogenesis of several infectious diseases, our data strongly suggest that HCP display multiple functions that may contribute to EHEC colonization of different hosts and to virulence, survival, and transmission of this food-borne pathogen.